Doc: Fix documentation of BGP gateway option

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<book>

<title>BIRD 2.0 User's Guide
<author>
Ondrej Filip <it/&lt;feela@network.cz&gt;/,
Pavel Machek <it/&lt;pavel@ucw.cz&gt;/,
Martin Mares <it/&lt;mj@ucw.cz&gt;/,
Maria Matejka <it/&lt;mq@jmq.cz&gt;/,
Ondrej Zajicek <it/&lt;santiago@crfreenet.org&gt;/
</author>

<abstract>
This document contains user documentation for the BIRD Internet Routing Daemon project.
</abstract>

<!-- Table of contents -->
<toc>

<!-- Begin the document -->


<chapt>Introduction
<label id="intro">

<sect>What is BIRD
<label id="what-is-bird">

<p>The name `BIRD' is actually an acronym standing for `BIRD Internet Routing
Daemon'. Let's take a closer look at the meaning of the name:

<p><em/BIRD/: Well, we think we have already explained that. It's an acronym
standing for `BIRD Internet Routing Daemon', you remember, don't you? :-)

<p><em/Internet Routing/: It's a program (well, a daemon, as you are going to
discover in a moment) which works as a dynamic router in an Internet type
network (that is, in a network running either the IPv4 or the IPv6 protocol).
Routers are devices which forward packets between interconnected networks in
order to allow hosts not connected directly to the same local area network to
communicate with each other. They also communicate with the other routers in the
Internet to discover the topology of the network which allows them to find
optimal (in terms of some metric) rules for forwarding of packets (which are
called routing tables) and to adapt themselves to the changing conditions such
as outages of network links, building of new connections and so on. Most of
these routers are costly dedicated devices running obscure firmware which is
hard to configure and not open to any changes (on the other hand, their special
hardware design allows them to keep up with lots of high-speed network
interfaces, better than general-purpose computer does). Fortunately, most
operating systems of the UNIX family allow an ordinary computer to act as a
router and forward packets belonging to the other hosts, but only according to a
statically configured table.

<p>A <em/Routing Daemon/ is in UNIX terminology a non-interactive program
running on background which does the dynamic part of Internet routing, that is
it communicates with the other routers, calculates routing tables and sends them
to the OS kernel which does the actual packet forwarding. There already exist
other such routing daemons: routed (RIP only), GateD (non-free),
<HTMLURL URL="http://www.zebra.org" name="Zebra"> and
<HTMLURL URL="http://sourceforge.net/projects/mrt" name="MRTD">,
but their capabilities are limited and they are relatively hard to configure
and maintain.

<p>BIRD is an Internet Routing Daemon designed to avoid all of these shortcomings,
to support all the routing technology used in the today's Internet or planned to
be used in near future and to have a clean extensible architecture allowing new
routing protocols to be incorporated easily. Among other features, BIRD
supports:

<itemize>
        <item>both IPv4 and IPv6 protocols
        <item>multiple routing tables
        <item>the Border Gateway Protocol (BGPv4)
        <item>the Routing Information Protocol (RIPv2, RIPng)
        <item>the Open Shortest Path First protocol (OSPFv2, OSPFv3)
        <item>the Babel Routing Protocol
        <item>the Router Advertisements for IPv6 hosts
        <item>a virtual protocol for exchange of routes between different
                routing tables on a single host
        <item>a command-line interface allowing on-line control and inspection
                of status of the daemon
        <item>soft reconfiguration (no need to use complex online commands to
                change the configuration, just edit the configuration file and
                notify BIRD to re-read it and it will smoothly switch itself to
                the new configuration, not disturbing routing protocols unless
                they are affected by the configuration changes)
        <item>a powerful language for route filtering
</itemize>

<p>BIRD has been developed at the Faculty of Math and Physics, Charles
University, Prague, Czech Republic as a student project. It can be freely
distributed under the terms of the GNU General Public License.

<p>BIRD has been designed to work on all UNIX-like systems. It has been
developed and tested under Linux 2.0 to 2.6, and then ported to FreeBSD, NetBSD
and OpenBSD, porting to other systems (even non-UNIX ones) should be relatively
easy due to its highly modular architecture.

<p>BIRD 1.x supported either IPv4 or IPv6 protocol, but had to be compiled separately
for each one. BIRD~2 supports both of them with a possibility of further extension.
BIRD~2 supports Linux at least 3.16, FreeBSD 10, NetBSD 7.0, and OpenBSD 5.8.
Anyway, it will probably work well also on older systems.

<sect>Installing BIRD
<label id="install">

<p>On a recent UNIX system with GNU development tools (GCC, binutils, m4, make)
and Perl, installing BIRD should be as easy as:

<code>
        ./configure
        make
        make install
        vi /usr/local/etc/bird.conf
        bird
</code>

<p>You can use <tt>./configure --help</tt> to get a list of configure
options. The most important ones are: <tt/--with-protocols=/ to produce a slightly smaller
BIRD executable by configuring out routing protocols you don't use, and
<tt/--prefix=/ to install BIRD to a place different from <file>/usr/local</file>.


<sect>Running BIRD
<label id="argv">

<p>You can pass several command-line options to bird:

<descrip>
        <tag><label id="argv-config">-c <m/config name/</tag>
        use given configuration file instead of <it/prefix/<file>/etc/bird.conf</file>.

        <tag><label id="argv-debug">-d</tag>
        enable debug messages to stderr, and run bird in foreground.

        <tag><label id="argv-debug-file">-D <m/filename of debug log/</tag>
        enable debug messages to given file.

        <tag><label id="argv-foreground">-f</tag>
        run bird in foreground.

        <tag><label id="argv-group">-g <m/group/</tag>
        use that group ID, see the next section for details.

        <tag><label id="argv-help">-h, --help</tag>
        display command-line options to bird.

        <tag><label id="argv-local">-l</tag>
        look for a configuration file and a communication socket in the current
        working directory instead of in default system locations. However, paths
        specified by options <cf/-c/, <cf/-s/ have higher priority.

        <tag><label id="argv-parse">-p</tag>
        just parse the config file and exit. Return value is zero if the config
        file is valid, nonzero if there are some errors.

        <tag><label id="argv-pid">-P <m/name of PID file/</tag>
        create a PID file with given filename.

        <tag><label id="argv-recovery">-R</tag>
        apply graceful restart recovery after start.

        <tag><label id="argv-socket">-s <m/name of communication socket/</tag>
        use given filename for a socket for communications with the client,
        default is <it/prefix/<file>/var/run/bird.ctl</file>.

        <tag><label id="argv-user">-u <m/user/</tag>
        drop privileges and use that user ID, see the next section for details.

        <tag><label id="argv-version">--version</tag>
        display bird version.
</descrip>

<p>BIRD writes messages about its work to log files or syslog (according to config).


<sect>Privileges
<label id="privileges">

<p>BIRD, as a routing daemon, uses several privileged operations (like setting
routing table and using raw sockets). Traditionally, BIRD is executed and runs
with root privileges, which may be prone to security problems. The recommended
way is to use a privilege restriction (options <cf/-u/, <cf/-g/). In that case
BIRD is executed with root privileges, but it changes its user and group ID to
an unprivileged ones, while using Linux capabilities to retain just required
privileges (capabilities CAP_NET_*). Note that the control socket is created
before the privileges are dropped, but the config file is read after that. The
privilege restriction is not implemented in BSD port of BIRD.

<p>An unprivileged user (as an argument to <cf/-u/ options) may be the user
<cf/nobody/, but it is suggested to use a new dedicated user account (like
<cf/bird/). The similar considerations apply for the group option, but there is
one more condition -- the users in the same group can use <file/birdc/ to
control BIRD.

<p>Finally, there is a possibility to use external tools to run BIRD in an
environment with restricted privileges. This may need some configuration, but it
is generally easy -- BIRD needs just the standard library, privileges to read
the config file and create the control socket and the CAP_NET_* capabilities.


<chapt>Architecture
<label id="architecture">

<sect>Routing tables
<label id="routing-tables">

<p>The heart of BIRD is a routing table. BIRD has several independent routing tables;
each of them contains routes of exactly one <m/nettype/ (see below). There are two
default tables -- <cf/master4/ for IPv4 routes and <cf/master6/ for IPv6 routes.
Other tables must be explicitly configured.

<p>
These routing tables are not kernel forwarding tables. No forwarding is done by
BIRD. If you want to forward packets using the routes in BIRD tables, you may
use the Kernel protocol (see below) to synchronize them with kernel FIBs.

<p>
Every nettype defines a (kind of) primary key on routes. Every route source can
supply one route for every possible primary key; new route announcement replaces
the old route from the same source, keeping other routes intact. BIRD always
chooses the best route for each primary key among the known routes and keeps the
others as suboptimal. When the best route is retracted, BIRD re-runs the best
route selection algorithm to find the current best route.

<p>
The global best route selection algorithm is (roughly) as follows:

<itemize>
        <item>Preferences of the routes are compared.
        <item>Source protocol instance preferences are compared.
        <item>If source protocols are the same (e.g. BGP vs. BGP), the protocol's route selection algorithm is invoked.
        <item>If source protocols are different (e.g. BGP vs. OSPF), result of the algorithm is undefined.
</itemize>

<p><label id="dsc-table-sorted">Usually, a routing table just chooses a selected
route from a list of entries for one network. But if the <cf/sorted/ option is
activated, these lists of entries are kept completely sorted (according to
preference or some protocol-dependent metric). This is needed for some features
of some protocols (e.g. <cf/secondary/ option of BGP protocol, which allows to
accept not just a selected route, but the first route (in the sorted list) that
is accepted by filters), but it is incompatible with some other features (e.g.
<cf/deterministic med/ option of BGP protocol, which activates a way of choosing
selected route that cannot be described using comparison and ordering). Minor
advantage is that routes are shown sorted in <cf/show route/, minor disadvantage
is that it is slightly more computationally expensive.

<sect>Routes and network types
<label id="routes">

<p>BIRD works with several types of routes. Some of them are typical IP routes,
others are better described as forwarding rules. We call them all routes,
regardless of this difference.

<p>Every route consists of several attributes (read more about them in the
<ref id="route-attributes" name="Route attributes"> section); the common for all
routes are:

<itemize>
        <item>IP address of router which told us about this route
        <item>Source protocol instance
        <item>Route preference
        <item>Optional attributes defined by protocols
</itemize>

<p>Other attributes depend on nettypes. Some of them are part of the primary key, these are marked (PK).

<sect1>IPv4 and IPv6 routes
<label id="ip-routes">

<p>The traditional routes. Configuration keywords are <cf/ipv4/ and <cf/ipv6/.

<itemize>
        <item>(PK) Route destination (IP prefix together with its length)
        <item>Route next hops (see below)
</itemize>

<sect1>IPv6 source-specific routes
<label id="ip-sadr-routes">

<p>The IPv6 routes containing both destination and source prefix. They are used
for source-specific routing (SSR), also called source-address dependent routing
(SADR), see <rfc id="8043">. Currently limited mostly to the Babel protocol.
Configuration keyword is <cf/ipv6 sadr/.

<itemize>
        <item>(PK) Route destination (IP prefix together with its length)
        <item>(PK) Route source (IP prefix together with its length)
        <item>Route next hops (see below)
</itemize>

<sect1>VPN IPv4 and IPv6 routes
<label id="vpn-routes">

<p>Routes for IPv4 and IPv6 with VPN Route Distinguisher (<rfc id="4364">).
Configuration keywords are <cf/vpn4/ and <cf/vpn6/.

<itemize>
        <item>(PK) Route destination (IP prefix together with its length)
        <item>(PK) Route distinguisher (according to <rfc id="4364">)
        <item>Route next hops
</itemize>

<sect1>Route Origin Authorization for IPv4 and IPv6
<label id="roa-routes">

<p>These entries can be used to validate route origination of BGP routes.
A ROA entry specifies prefixes which could be originated by an AS number.
Their keywords are <cf/roa4/ and <cf/roa6/.

<itemize>
        <item>(PK) IP prefix together with its length
        <item>(PK) Matching prefix maximal length
        <item>(PK) AS number
</itemize>

<sect1>Flowspec for IPv4 and IPv6
<label id="flow-routes">

<p>Flowspec rules are a form of firewall and traffic flow control rules
distributed mostly via BGP. These rules may help the operators stop various
network attacks in the beginning before eating up the whole bandwidth.
Configuration keywords are <cf/flow4/ and <cf/flow6/.

<itemize>
        <item>(PK) IP prefix together with its length
        <item>(PK) Flow definition data
        <item>Flow action (encoded internally as BGP communities according to <rfc id="5575">)
</itemize>

<sect1>MPLS switching rules
<label id="mpls-routes">

<p>This nettype is currently a stub before implementing more support of <rfc id="3031">.
BIRD currently does not support any label distribution protocol nor any label assignment method.
Only the Kernel, Pipe and Static protocols can use MPLS tables.
Configuration keyword is <cf/mpls/.

<itemize>
        <item>(PK) MPLS label
        <item>Route next hops
</itemize>

<sect1>Route next hops
<label id="route-next-hop">

<p>This is not a nettype. The route next hop is a complex attribute common for many
nettypes as you can see before. Every next hop has its assigned device
(either assumed from its IP address or set explicitly). It may have also
an IP address and an MPLS stack (one or both independently).
Maximal MPLS stack depth is set (in compile time) to 8 labels.

<p>Every route (when eligible to have a next hop) can have more than one next hop.
In that case, every next hop has also its weight.

<sect>Protocols and channels
<label id="protocols-concept">

<p>BIRD protocol is an abstract class of producers and consumers of the routes.
Each protocol may run in multiple instances and bind on one side to route
tables via channels, on the other side to specified listen sockets (BGP),
interfaces (Babel, OSPF, RIP), APIs (Kernel, Direct), or nothing (Static, Pipe).

<p>There are also two protocols that do not have any channels -- BFD and Device.
Both of them are kind of service for other protocols.

<p>Each protocol is connected to a routing table through a channel. Some protocols
support only one channel (OSPF, RIP), some protocols support more channels (BGP, Direct).
Each channel has two filters which can accept, reject and modify the routes.
An <it/export/ filter is applied to routes passed from the routing table to the protocol,
an <it/import/ filter is applied to routes in the opposite direction.

<sect>Graceful restart
<label id="graceful-restart">

<p>When BIRD is started after restart or crash, it repopulates routing tables in
an uncoordinated manner, like after clean start. This may be impractical in some
cases, because if the forwarding plane (i.e. kernel routing tables) remains
intact, then its synchronization with BIRD would temporarily disrupt packet
forwarding until protocols converge. Graceful restart is a mechanism that could
help with this issue. Generally, it works by starting protocols and letting them
repopulate routing tables while deferring route propagation until protocols
acknowledge their convergence. Note that graceful restart behavior have to be
configured for all relevant protocols and requires protocol-specific support
(currently implemented for Kernel and BGP protocols), it is activated for
particular boot by option <cf/-R/.

<p>Some protocols (e.g. BGP) could be restarted gracefully after both
intentional outage and crash, while others (e.g. OSPF) after intentional outage
only. For planned graceful restart, BIRD must be shut down by
<ref id="cli-graceful-restart" name="graceful restart"> command instead of
regular <ref id="cli-down" name="down"> command. In this way routing neighbors
are notified about planned graceful restart and routes are kept in kernel table
after shutdown.


<chapt>Configuration
<label id="config">

<sect>Introduction
<label id="config-intro">

<p>BIRD is configured using a text configuration file. Upon startup, BIRD reads
<it/prefix/<file>/etc/bird.conf</file> (unless the <tt/-c/ command line option
is given). Configuration may be changed at user's request: if you modify the
config file and then signal BIRD with <tt/SIGHUP/, it will adjust to the new
config. Then there's the client which allows you to talk with BIRD in an
extensive way.

<p>In the config, everything on a line after <cf/#/ or inside <cf>/* */</cf> is
a comment, whitespace characters are treated as a single space. If there's a
variable number of options, they are grouped using the <cf/{ }/ brackets. Each
option is terminated by a <cf/;/. Configuration is case sensitive. There are two
ways how to name symbols (like protocol names, filter names, constants etc.).
You can either use a simple string starting with a letter (or underscore)
followed by any combination of letters, numbers and underscores (e.g. <cf/R123/,
<cf/my_filter/, <cf/bgp5/) or you can enclose the name into apostrophes (<cf/'/)
and than you can use any combination of numbers, letters, underscores, hyphens,
dots and colons (e.g.  <cf/'1:strange-name'/, <cf/'-NAME-'/, <cf/'cool::name'/).

<p>Here is an example of a simple config file. It enables synchronization of
routing tables with OS kernel, learns network interfaces and runs RIP on all
network interfaces found.

<code>
protocol kernel {
        ipv4 {
                export all;     # Default is export none
        };
        persist;                # Don't remove routes on BIRD shutdown
}

protocol device {
}

protocol rip {
        ipv4 {
                import all;
                export all;
        };
        interface "*";
}
</code>


<sect>Global options
<label id="global-opts">

<p><descrip>
        <tag><label id="opt-include">include "<m/filename/";</tag>
        This statement causes inclusion of a new file. The <m/filename/ could
        also be a wildcard, in that case matching files are included in
        alphabetic order. The maximal depth is 8. Note that this statement can
        be used anywhere in the config file, even inside other options, but
        always on the beginning of line. In the following example, the first
        semicolon belongs to the <cf/include/, the second to <cf/ipv6 table/.
        If the <file/tablename.conf/ contains exactly one token (the name of the
        table), this construction is correct:
<code>
ipv6 table
include "tablename.conf";;
</code>

        <tag><label id="opt-log">log "<m/filename/" [<m/limit/ "<m/backup/"] | syslog [name <m/name/] | stderr all|{ <m/list of classes/ }</tag>
        Set logging of messages having the given class (either <cf/all/ or <cf>{
        error|trace [, <m/.../] }</cf> etc.) into selected destination - a file
        specified as a filename string (with optional log rotation information),
        syslog (with optional name argument), or the stderr output.

        Classes are:
        <cf/info/, <cf/warning/, <cf/error/ and <cf/fatal/ for messages about local problems,
        <cf/debug/ for debugging messages,
        <cf/trace/ when you want to know what happens in the network,
        <cf/remote/ for messages about misbehavior of remote machines,
        <cf/auth/ about authentication failures,
        <cf/bug/ for internal BIRD bugs.

        Logging directly to file supports basic log rotation -- there is an
        optional log file limit and a backup filename, when log file reaches the
        limit, the current log file is renamed to the backup filename and a new
        log file is created.

        You may specify more than one <cf/log/ line to establish logging to
        multiple destinations. Default: log everything to the system log, or
        to the debug output if debugging is enabled by <cf/-d//<cf/-D/
        command-line option.

        <tag><label id="opt-debug-protocols">debug protocols all|off|{ states|routes|filters|interfaces|events|packets [, <m/.../] }</tag>
        Set global defaults of protocol debugging options. See <cf/debug/ in the
        following section. Default: off.

        <tag><label id="opt-debug-commands">debug commands <m/number/</tag>
        Control logging of client connections (0 for no logging, 1 for logging
        of connects and disconnects, 2 and higher for logging of all client
        commands). Default: 0.

        <tag><label id="opt-debug-latency">debug latency <m/switch/</tag>
        Activate tracking of elapsed time for internal events. Recent events
        could be examined using <cf/dump events/ command. Default: off.

        <tag><label id="opt-debug-latency-limit">debug latency limit <m/time/</tag>
        If <cf/debug latency/ is enabled, this option allows to specify a limit
        for elapsed time. Events exceeding the limit are logged. Default: 1 s.

        <tag><label id="opt-watchdog-warn">watchdog warning <m/time/</tag>
        Set time limit for I/O loop cycle. If one iteration took more time to
        complete, a warning is logged. Default: 5 s.

        <tag><label id="opt-watchdog-timeout">watchdog timeout <m/time/</tag>
        Set time limit for I/O loop cycle. If the limit is breached, BIRD is
        killed by abort signal. The timeout has effective granularity of
        seconds, zero means disabled. Default: disabled (0).

        <tag><label id="opt-mrtdump">mrtdump "<m/filename/"</tag>
        Set MRTdump file name. This option must be specified to allow MRTdump
        feature. Default: no dump file.

        <tag><label id="opt-mrtdump-protocols">mrtdump protocols all|off|{ states|messages [, <m/.../] }</tag>
        Set global defaults of MRTdump options. See <cf/mrtdump/ in the
        following section. Default: off.

        <tag><label id="opt-filter">filter <m/name local variables/{ <m/commands/ }</tag>
        Define a filter. You can learn more about filters in the following
        chapter.

        <tag><label id="opt-function">function <m/name/ (<m/parameters/) <m/local variables/ { <m/commands/ }</tag>
        Define a function. You can learn more about functions in the following chapter.

        <tag><label id="opt-protocol">protocol rip|ospf|bgp|<m/.../ [<m/name/ [from <m/name2/]] { <m>protocol options</m> }</tag>
        Define a protocol instance called <cf><m/name/</cf> (or with a name like
        "rip5" generated automatically if you don't specify any
        <cf><m/name/</cf>). You can learn more about configuring protocols in
        their own chapters. When <cf>from <m/name2/</cf> expression is used,
        initial protocol options are taken from protocol or template
        <cf><m/name2/</cf> You can run more than one instance of most protocols
        (like RIP or BGP). By default, no instances are configured.

        <tag><label id="opt-template">template rip|ospf|bgp|<m/.../ [<m/name/ [from <m/name2/]] { <m>protocol options</m> }</tag>
        Define a protocol template instance called <m/name/ (or with a name like
        "bgp1" generated automatically if you don't specify any <m/name/).
        Protocol templates can be used to group common options when many
        similarly configured protocol instances are to be defined. Protocol
        instances (and other templates) can use templates by using <cf/from/
        expression and the name of the template. At the moment templates (and
        <cf/from/ expression) are not implemented for OSPF protocol.

        <tag><label id="opt-define">define <m/constant/ = <m/expression/</tag>
        Define a constant. You can use it later in every place you could use a
        value of the same type. Besides, there are some predefined numeric
        constants based on /etc/iproute2/rt_* files. A list of defined constants
        can be seen (together with other symbols) using 'show symbols' command.

        <tag><label id="opt-attribute">attribute <m/type/ <m/name/</tag>
        Declare a custom route attribute. You can set and get it in filters like
        any other route attribute. This feature is intended for marking routes
        in import filters for export filtering purposes instead of locally
        assigned BGP communities which have to be deleted in export filters.

        <tag><label id="opt-router-id">router id <m/IPv4 address/</tag>
        Set BIRD's router ID. It's a world-wide unique identification of your
        router, usually one of router's IPv4 addresses. Default: the lowest
        IPv4 address of a non-loopback interface.

        <tag><label id="opt-router-id-from">router id from [-] [ "<m/mask/" ] [ <m/prefix/ ] [, <m/.../]</tag>
        Set BIRD's router ID based on an IPv4 address of an interface specified by
        an interface pattern.
        See <ref id="proto-iface" name="interface"> section for detailed
        description of interface patterns with extended clauses.

        <tag><label id="opt-graceful-restart">graceful restart wait <m/number/</tag>
        During graceful restart recovery, BIRD waits for convergence of routing
        protocols. This option allows to specify a timeout for the recovery to
        prevent waiting indefinitely if some protocols cannot converge. Default:
        240 seconds.

        <tag><label id="opt-timeformat">timeformat route|protocol|base|log "<m/format1/" [<m/limit/ "<m/format2/"]</tag>
        This option allows to specify a format of date/time used by BIRD. The
        first argument specifies for which purpose such format is used.
        <cf/route/ is a format used in 'show route' command output,
        <cf/protocol/ is used in 'show protocols' command output, <cf/base/ is
        used for other commands and <cf/log/ is used in a log file.

        "<m/format1/" is a format string using <it/strftime(3)/ notation (see
        <it/man strftime/ for details). It is extended to support sub-second
        time part with variable precision (up to microseconds) using "%f"
        conversion code (e.g., "%T.%3f" is hh:mm:ss.sss time). <m/limit/ and
        "<m/format2/" allow to specify the second format string for times in
        past deeper than <m/limit/ seconds.

        There are several shorthands: <cf/iso long/ is a ISO 8601 date/time
        format (YYYY-MM-DD hh:mm:ss) that can be also specified using <cf/"%F
        %T"/. Similarly, <cf/iso long ms/ and <cf/iso long us/ are ISO 8601
        date/time formats with millisecond or microsecond precision.
        <cf/iso short/ is a variant of ISO 8601 that uses just the time format
        (hh:mm:ss) for near times (up to 20 hours in the past) and the date
        format (YYYY-MM-DD) for far times. This is a shorthand for <cf/"%T"
        72000 "%F"/. And there are also <cf/iso short ms/ and <cf/iso short us/
        high-precision variants of that.

        By default, BIRD uses the <cf/iso short ms/ format for <cf/route/ and
        <cf/protocol/ times, and the <cf/iso long ms/ format for <cf/base/ and
        <cf/log/ times.

        <tag><label id="opt-table"><m/nettype/ table <m/name/ [sorted]</tag>
        Create a new routing table. The default routing tables <cf/master4/ and
        <cf/master6/ are created implicitly, other routing tables have to be
        added by this command.  Option <cf/sorted/ can be used to enable sorting
        of routes, see <ref id="dsc-table-sorted" name="sorted table">
        description for details.

        <tag><label id="opt-eval">eval <m/expr/</tag>
        Evaluates given filter expression. It is used by the developers for testing of filters.
</descrip>


<sect>Protocol options
<label id="protocol-opts">

<p>For each protocol instance, you can configure a bunch of options. Some of
them (those described in this section) are generic, some are specific to the
protocol (see sections talking about the protocols).

<p>Several options use a <m/switch/ argument. It can be either <cf/on/,
<cf/yes/ or a numeric expression with a non-zero value for the option to be
enabled or <cf/off/, <cf/no/ or a numeric expression evaluating to zero to
disable it. An empty <m/switch/ is equivalent to <cf/on/ ("silence means
agreement").

<descrip>
        <tag><label id="proto-disabled">disabled <m/switch/</tag>
        Disables the protocol. You can change the disable/enable status from the
        command line interface without needing to touch the configuration.
        Disabled protocols are not activated. Default: protocol is enabled.

        <tag><label id="proto-debug">debug all|off|{ states|routes|filters|interfaces|events|packets [, <m/.../] }</tag>
        Set protocol debugging options. If asked, each protocol is capable of
        writing trace messages about its work to the log (with category
        <cf/trace/). You can either request printing of <cf/all/ trace messages
        or only of the types selected: <cf/states/ for protocol state changes
        (protocol going up, down, starting, stopping etc.), <cf/routes/ for
        routes exchanged with the routing table, <cf/filters/ for details on
        route filtering, <cf/interfaces/ for interface change events sent to the
        protocol, <cf/events/ for events internal to the protocol and <cf/packets/
        for packets sent and received by the protocol. Default: off.

        <tag><label id="proto-mrtdump">mrtdump all|off|{ states|messages [, <m/.../] }</tag>
        Set protocol MRTdump flags. MRTdump is a standard binary format for
        logging information from routing protocols and daemons. These flags
        control what kind of information is logged from the protocol to the
        MRTdump file (which must be specified by global <cf/mrtdump/ option, see
        the previous section). Although these flags are similar to flags of
        <cf/debug/ option, their meaning is different and protocol-specific. For
        BGP protocol, <cf/states/ logs BGP state changes and <cf/messages/ logs
        received BGP messages. Other protocols does not support MRTdump yet.

        <tag><label id="proto-router-id">router id <m/IPv4 address/</tag>
        This option can be used to override global router id for a given
        protocol. Default: uses global router id.

        <tag><label id="proto-description">description "<m/text/"</tag>
        This is an optional description of the protocol. It is displayed as a
        part of the output of 'show protocols all' command.

        <tag><label id="proto-vrf">vrf "<m/text/"|default</tag>
        Associate the protocol with specific VRF. The protocol will be
        restricted to interfaces assigned to the VRF and will use sockets bound
        to the VRF. A corresponding VRF interface must exist on OS level. For
        kernel protocol, an appropriate table still must be explicitly selected
        by <cf/table/ option.

        By selecting <cf/default/, the protocol is associated with the default
        VRF; i.e., it will be restricted to interfaces not assigned to any
        regular VRF. That is different from not specifying <cf/vrf/ at all, in
        which case the protocol may use any interface regardless of its VRF
        status.

        Note that for proper VRF support it is necessary to use Linux kernel
        version at least 4.14, older versions have limited VRF implementation.
        Before Linux kernel 5.0, a socket bound to a port in default VRF collide
        with others in regular VRFs. In BGP, this can be avoided by using
        <ref id="bgp-strict-bind" name="strict bind"> option.

        <tag><label id="proto-channel"><m/channel name/ [{<m/channel config/}]</tag>
        Every channel must be explicitly stated. See the protocol-specific
        configuration for the list of supported channel names. See the
        <ref id="channel-opts" name="channel configuration section"> for channel
        definition.
</descrip>

<p>There are several options that give sense only with certain protocols:

<descrip>
        <tag><label id="proto-iface">interface [-] [ "<m/mask/" ] [ <m/prefix/ ] [, <m/.../] [ { <m/option/; [<m/.../] } ]</tag>
        Specifies a set of interfaces on which the protocol is activated with
        given interface-specific options. A set of interfaces specified by one
        interface option is described using an interface pattern. The interface
        pattern consists of a sequence of clauses (separated by commas), each
        clause is a mask specified as a shell-like pattern. Interfaces are
        matched by their name.

        An interface matches the pattern if it matches any of its clauses. If
        the clause begins with <cf/-/, matching interfaces are excluded. Patterns
        are processed left-to-right, thus <cf/interface "eth0", -"eth*", "*";/
        means eth0 and all non-ethernets.

        Some protocols (namely OSPFv2 and Direct) support extended clauses that
        may contain a mask, a prefix, or both of them. An interface matches such
        clause if its name matches the mask (if specified) and its address
        matches the prefix (if specified). Extended clauses are used when the
        protocol handles multiple addresses on an interface independently.

        An interface option can be used more times with different interface-specific
        options, in that case for given interface the first matching interface
        option is used.

        This option is allowed in Babel, BFD, Device, Direct, OSPF, RAdv and RIP
        protocols. In OSPF protocol it is used in the <cf/area/ subsection.

        Default: none.

        Examples:

        <cf>interface "*" { type broadcast; };</cf> - start the protocol on all
        interfaces with <cf>type broadcast</cf> option.

        <cf>interface "eth1", "eth4", "eth5" { type ptp; };</cf> - start the
        protocol on enumerated interfaces with <cf>type ptp</cf> option.

        <cf>interface -192.168.1.0/24, 192.168.0.0/16;</cf> - start the protocol
        on all interfaces that have address from 192.168.0.0/16, but not from
        192.168.1.0/24.

        <cf>interface "eth*" 192.168.1.0/24;</cf> - start the protocol on all
        ethernet interfaces that have address from 192.168.1.0/24.

        <tag><label id="proto-tx-class">tx class|dscp <m/num/</tag>
        This option specifies the value of ToS/DS/Class field in IP headers of
        the outgoing protocol packets. This may affect how the protocol packets
        are processed by the network relative to the other network traffic. With
        <cf/class/ keyword, the value (0-255) is used for the whole ToS/Class
        octet (but two bits reserved for ECN are ignored). With <cf/dscp/
        keyword, the value (0-63) is used just for the DS field in the octet.
        Default value is 0xc0 (DSCP 0x30 - CS6).

        <tag><label id="proto-tx-priority">tx priority <m/num/</tag>
        This option specifies the local packet priority. This may affect how the
        protocol packets are processed in the local TX queues. This option is
        Linux specific. Default value is 7 (highest priority, privileged traffic).

        <tag><label id="proto-pass">password "<m/password/" [ { <m>password options</m> } ]</tag>
        Specifies a password that can be used by the protocol as a shared secret
        key. Password option can be used more times to specify more passwords.
        If more passwords are specified, it is a protocol-dependent decision
        which one is really used. Specifying passwords does not mean that
        authentication is enabled, authentication can be enabled by separate,
        protocol-dependent <cf/authentication/ option.

        This option is allowed in BFD, OSPF and RIP protocols. BGP has also
        <cf/password/ option, but it is slightly different and described
        separately.
        Default: none.
</descrip>

<p>Password option can contain section with some (not necessary all) password sub-options:

<descrip>
        <tag><label id="proto-pass-id">id <M>num</M></tag>
        ID of the password, (1-255). If it is not used, BIRD will choose ID based
        on an order of the password item in the interface. For example, second
        password item in one interface will have default ID 2. ID is used by
        some routing protocols to identify which password was used to
        authenticate protocol packets.

        <tag><label id="proto-pass-gen-from">generate from "<m/time/"</tag>
        The start time of the usage of the password for packet signing.
        The format of <cf><m/time/</cf> is <tt>dd-mm-yyyy HH:MM:SS</tt>.

        <tag><label id="proto-pass-gen-to">generate to "<m/time/"</tag>
        The last time of the usage of the password for packet signing.

        <tag><label id="proto-pass-accept-from">accept from "<m/time/"</tag>
        The start time of the usage of the password for packet verification.

        <tag><label id="proto-pass-accept-to">accept to "<m/time/"</tag>
        The last time of the usage of the password for packet verification.

        <tag><label id="proto-pass-from">from "<m/time/"</tag>
        Shorthand for setting both <cf/generate from/ and <cf/accept from/.

        <tag><label id="proto-pass-to">to "<m/time/"</tag>
        Shorthand for setting both <cf/generate to/ and <cf/accept to/.

        <tag><label id="proto-pass-algorithm">algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 )</tag>
        The message authentication algorithm for the password when cryptographic
        authentication is enabled. The default value depends on the protocol.
        For RIP and OSPFv2 it is Keyed-MD5 (for compatibility), for OSPFv3
        protocol it is HMAC-SHA-256.

</descrip>


<sect>Channel options
<label id="channel-opts">

<p>Every channel belongs to a protocol and is configured inside its block. The
minimal channel config is empty, then it uses default values. The name of the
channel implies its nettype. Channel definitions can be inherited from protocol
templates. Multiple definitions of the same channel are forbidden, but channels
inherited from templates can be updated by new definitions.

<descrip>
        <tag><label id="proto-table">table <m/name/</tag>
        Specify a table to which the channel is connected. Default: the first
        table of given nettype.

        <tag><label id="proto-preference">preference <m/expr/</tag>
        Sets the preference of routes generated by the protocol and imported
        through this channel. Default: protocol dependent.

        <tag><label id="proto-import">import all | none | filter <m/name/ | filter { <m/filter commands/ } | where <m/boolean filter expression/</tag>
        Specify a filter to be used for filtering routes coming from the
        protocol to the routing table. <cf/all/ is for keeping all routes,
        <cf/none/ is for dropping all routes. Default: <cf/all/ (except for
        EBGP).

        <tag><label id="proto-export">export <m/filter/</tag>
        This is similar to the <cf>import</cf> keyword, except that it works in
        the direction from the routing table to the protocol. Default: <cf/none/
        (except for EBGP).

        <tag><label id="proto-import-keep-filtered">import keep filtered <m/switch/</tag>
        Usually, if an import filter rejects a route, the route is forgotten.
        When this option is active, these routes are kept in the routing table,
        but they are hidden and not propagated to other protocols. But it is
        possible to show them using <cf/show route filtered/. Note that this
        option does not work for the pipe protocol. Default: off.

        <tag><label id="proto-import-limit">import limit [<m/number/ | off ] [action warn | block | restart | disable]</tag>
        Specify an import route limit (a maximum number of routes imported from
        the protocol) and optionally the action to be taken when the limit is
        hit. Warn action just prints warning log message. Block action discards
        new routes coming from the protocol. Restart and disable actions shut
        the protocol down like appropriate commands. Disable is the default
        action if an action is not explicitly specified. Note that limits are
        reset during protocol reconfigure, reload or restart. Default: <cf/off/.

        <tag><label id="proto-receive-limit">receive limit [<m/number/ | off ] [action warn | block | restart | disable]</tag>
        Specify an receive route limit (a maximum number of routes received from
        the protocol and remembered). It works almost identically to <cf>import
        limit</cf> option, the only difference is that if <cf/import keep
        filtered/ option is active, filtered routes are counted towards the
        limit and blocked routes are forgotten, as the main purpose of the
        receive limit is to protect routing tables from overflow. Import limit,
        on the contrary, counts accepted routes only and routes blocked by the
        limit are handled like filtered routes. Default: <cf/off/.

        <tag><label id="proto-export-limit">export limit [ <m/number/ | off ] [action warn | block | restart | disable]</tag>
        Specify an export route limit, works similarly to the <cf>import
        limit</cf> option, but for the routes exported to the protocol. This
        option is experimental, there are some problems in details of its
        behavior -- the number of exported routes can temporarily exceed the
        limit without triggering it during protocol reload, exported routes
        counter ignores route blocking and block action also blocks route
        updates of already accepted routes -- and these details will probably
        change in the future. Default: <cf/off/.
</descrip>

<p>This is a trivial example of RIP configured for IPv6 on all interfaces:
<code>
protocol rip ng {
        ipv6;
        interface "*";
}
</code>

<p>This is a non-trivial example.
<code>
protocol rip ng {
        ipv6 {
                table mytable6;
                import filter { ... };
                export filter { ... };
                import limit 50;
        };
        interface "*";
}
</code>

<p>And this is even more complicated example using templates.
<code>
template bgp {
        local 198.51.100.14 as 65000;

        ipv4 {
                table mytable4;
                import filter { ... };
                export none;
        };
        ipv6 {
                table mytable6;
                import filter { ... };
                export none;
        };
}

protocol bgp from  {
        neighbor 198.51.100.130 as 64496;

        # IPv4 channel is inherited as-is, while IPv6
        # channel is adjusted by export filter option
        ipv6 {
                export filter { ... };
        };
}
</code>


<chapt>Remote control
<label id="remote-control">

<p>You can use the command-line client <file>birdc</file> to talk with a running
BIRD. Communication is done using a <file/bird.ctl/ UNIX domain socket (unless
changed with the <tt/-s/ option given to both the server and the client). The
commands can perform simple actions such as enabling/disabling of protocols,
telling BIRD to show various information, telling it to show routing table
filtered by filter, or asking BIRD to reconfigure. Press <tt/?/ at any time to
get online help. Option <tt/-r/ can be used to enable a restricted mode of BIRD
client, which allows just read-only commands (<cf/show .../). Option <tt/-v/ can
be passed to the client, to make it dump numeric return codes along with the
messages. You do not necessarily need to use <file/birdc/ to talk to BIRD, your
own applications could do that, too -- the format of communication between BIRD
and <file/birdc/ is stable (see the programmer's documentation).

<p>There is also lightweight variant of BIRD client called <file/birdcl/, which
does not support command line editing and history and has minimal dependencies.
This is useful for running BIRD in resource constrained environments, where
Readline library (required for regular BIRD client) is not available.

<p>Many commands have the <m/name/ of the protocol instance as an argument.
This argument can be omitted if there exists only a single instance.

<p>Here is a brief list of supported functions:

<descrip>
        <tag><label id="cli-show-status">show status</tag>
        Show router status, that is BIRD version, uptime and time from last
        reconfiguration.

        <tag><label id="cli-show-interfaces">show interfaces [summary]</tag>
        Show the list of interfaces. For each interface, print its type, state,
        MTU and addresses assigned.

        <tag><label id="cli-show-protocols">show protocols [all]</tag>
        Show list of protocol instances along with tables they are connected to
        and protocol status, possibly giving verbose information, if <cf/all/ is
        specified.

        <!-- TODO: Move these protocol-specific remote control commands to the protocol sections -->
        <tag><label id="cli-show-ospf-iface">show ospf interface [<m/name/] ["<m/interface/"]</tag>
        Show detailed information about OSPF interfaces.

        <tag><label id="cli-show-ospf-neighbors">show ospf neighbors [<m/name/] ["<m/interface/"]</tag>
        Show a list of OSPF neighbors and a state of adjacency to them.

        <tag><label id="cli-show-ospf-state">show ospf state [all] [<m/name/]</tag>
        Show detailed information about OSPF areas based on a content of the
        link-state database. It shows network topology, stub networks,
        aggregated networks and routers from other areas and external routes.
        The command shows information about reachable network nodes, use option
        <cf/all/ to show information about all network nodes in the link-state
        database.

        <tag><label id="cli-show-ospf-topology">show ospf topology [all] [<m/name/]</tag>
        Show a topology of OSPF areas based on a content of the link-state
        database. It is just a stripped-down version of 'show ospf state'.

        <tag><label id="cli-show-ospf-lsadb">show ospf lsadb [global | area <m/id/ | link] [type <m/num/] [lsid <m/id/] [self | router <m/id/] [<m/name/] </tag>
        Show contents of an OSPF LSA database. Options could be used to filter
        entries.

        <tag><label id="cli-show-rip-interfaces">show rip interfaces [<m/name/] ["<m/interface/"]</tag>
        Show detailed information about RIP interfaces.

        <tag><label id="cli-show-rip-neighbors">show rip neighbors [<m/name/] ["<m/interface/"]</tag>
        Show a list of RIP neighbors and associated state.

        <tag><label id="cli-show-static">show static [<m/name/]</tag>
        Show detailed information about static routes.

        <tag><label id="cli-show-bfd-sessions">show bfd sessions [<m/name/]</tag>
        Show information about BFD sessions.

        <tag><label id="cli-show-symbols">show symbols [table|filter|function|protocol|template|roa|<m/symbol/]</tag>
        Show the list of symbols defined in the configuration (names of
        protocols, routing tables etc.).

        <tag><label id="cli-show-route">show route [[for] <m/prefix/|<m/IP/] [table (<m/t/ | all)] [filter <m/f/|where <m/c/] [(export|preexport|noexport) <m/p/] [protocol <m/p/] [(stats|count)] [<m/options/]</tag>
        Show contents of specified routing tables, that is routes, their metrics
        and (in case the <cf/all/ switch is given) all their attributes.

        <p>You can specify a <m/prefix/ if you want to print routes for a
        specific network. If you use <cf>for <m/prefix or IP/</cf>, you'll get
        the entry which will be used for forwarding of packets to the given
        destination. By default, all routes for each network are printed with
        the selected one at the top, unless <cf/primary/ is given in which case
        only the selected route is shown.

        <p>The <cf/show route/ command can process one or multiple routing
        tables. The set of selected tables is determined on three levels: First,
        tables can be explicitly selected by <cf/table/ switch, which could be
        used multiple times, all tables are specified by <cf/table all/. Second,
        tables can be implicitly selected by channels or protocols that are
        arguments of several other switches (e.g., <cf/export/, <cf/protocol/).
        Last, the set of default tables is used: <cf/master4/, <cf/master6/ and
        each first table of any other network type.

        <p>You can also ask for printing only routes processed and accepted by
        a given filter (<cf>filter <m/name/</cf> or <cf>filter { <m/filter/ }
        </cf> or matching a given condition (<cf>where <m/condition/</cf>).

        The <cf/export/, <cf/preexport/ and <cf/noexport/ switches ask for
        printing of routes that are exported to the specified protocol or
        channel. With <cf/preexport/, the export filter of the channel is
        skipped. With <cf/noexport/, routes rejected by the export filter are
        printed instead. Note that routes not exported for other reasons
        (e.g. secondary routes or routes imported from that protocol) are not
        printed even with <cf/noexport/. These switches also imply that
        associated routing tables are selected instead of default ones.

        <p>You can also select just routes added by a specific protocol.
        <cf>protocol <m/p/</cf>. This switch also implies that associated
        routing tables are selected instead of default ones.

        <p>If BIRD is configured to keep filtered routes (see <cf/import keep
        filtered/ option), you can show them instead of routes by using
        <cf/filtered/ switch.

        <p>The <cf/stats/ switch requests showing of route statistics (the
        number of networks, number of routes before and after filtering). If
        you use <cf/count/ instead, only the statistics will be printed.

        <tag><label id="cli-mrt-dump">mrt dump table <m/name/|"<m/pattern/" to "<m/filename/" [filter <m/f/|where <m/c/]</tag>
        Dump content of a routing table to a specified file in MRT table dump
        format. See <ref id="mrt" name="MRT protocol"> for details.

        <tag><label id="cli-configure">configure [soft] ["<m/config file/"] [timeout [<m/num/]]</tag>
        Reload configuration from a given file. BIRD will smoothly switch itself
        to the new configuration, protocols are reconfigured if possible,
        restarted otherwise. Changes in filters usually lead to restart of
        affected protocols.

        If <cf/soft/ option is used, changes in filters does not cause BIRD to
        restart affected protocols, therefore already accepted routes (according
        to old filters) would be still propagated, but new routes would be
        processed according to the new filters.

        If <cf/timeout/ option is used, config timer is activated. The new
        configuration could be either confirmed using <cf/configure confirm/
        command, or it will be reverted to the old one when the config timer
        expires. This is useful for cases when reconfiguration breaks current
        routing and a router becomes inaccessible for an administrator. The
        config timeout expiration is equivalent to <cf/configure undo/
        command. The timeout duration could be specified, default is 300 s.

        <tag><label id="cli-configure-confirm">configure confirm</tag>
        Deactivate the config undo timer and therefore confirm the current
        configuration.

        <tag><label id="cli-configure-undo">configure undo</tag>
        Undo the last configuration change and smoothly switch back to the
        previous (stored) configuration. If the last configuration change was
        soft, the undo change is also soft. There is only one level of undo, but
        in some specific cases when several reconfiguration requests are given
        immediately in a row and the intermediate ones are skipped then the undo
        also skips them back.

        <tag><label id="cli-configure-check">configure check ["<m/config file/"]</tag>
        Read and parse given config file, but do not use it. useful for checking
        syntactic and some semantic validity of an config file.

        <tag><label id="cli-enable-disable-restart">enable|disable|restart <m/name/|"<m/pattern/"|all</tag>
        Enable, disable or restart a given protocol instance, instances matching
        the <cf><m/pattern/</cf> or <cf/all/ instances.

        <tag><label id="cli-reload">reload [in|out] <m/name/|"<m/pattern/"|all</tag>
        Reload a given protocol instance, that means re-import routes from the
        protocol instance and re-export preferred routes to the instance. If
        <cf/in/ or <cf/out/ options are used, the command is restricted to one
        direction (re-import or re-export).

        This command is useful if appropriate filters have changed but the
        protocol instance was not restarted (or reloaded), therefore it still
        propagates the old set of routes. For example when <cf/configure soft/
        command was used to change filters.

        Re-export always succeeds, but re-import is protocol-dependent and might
        fail (for example, if BGP neighbor does not support route-refresh
        extension). In that case, re-export is also skipped. Note that for the
        pipe protocol, both directions are always reloaded together (<cf/in/ or
        <cf/out/ options are ignored in that case).

        <tag><label id="cli-down">down</tag>
        Shut BIRD down.

        <tag><label id="cli-graceful-restart">graceful restart</tag>
        Shut BIRD down for graceful restart. See <ref id="graceful-restart"
        name="graceful restart"> section for details.

        <tag><label id="cli-debug">debug <m/protocol/|<m/pattern/|all all|off|{ states|routes|filters|events|packets [, <m/.../] }</tag>
        Control protocol debugging.

        <tag><label id="cli-dump">dump resources|sockets|interfaces|neighbors|attributes|routes|protocols</tag>
        Dump contents of internal data structures to the debugging output.

        <tag><label id="cli-echo">echo all|off|{ <m/list of log classes/ } [ <m/buffer-size/ ]</tag>
        Control echoing of log messages to the command-line output.
        See <ref id="opt-log" name="log option"> for a list of log classes.

        <tag><label id="cli-eval">eval <m/expr/</tag>
        Evaluate given expression.
</descrip>


<chapt>Filters
<label id="filters">

<sect>Introduction
<label id="filters-intro">

<p>BIRD contains a simple programming language. (No, it can't yet read mail :-).
There are two objects in this language: filters and functions. Filters are
interpreted by BIRD core when a route is being passed between protocols and
routing tables. The filter language contains control structures such as if's and
switches, but it allows no loops. An example of a filter using many features can
be found in <file>filter/test.conf</file>.

<p>Filter gets the route, looks at its attributes and modifies some of them if
it wishes. At the end, it decides whether to pass the changed route through
(using <cf/accept/) or whether to <cf/reject/ it. A simple filter looks like
this:

<code>
filter not_too_far
int var;
{
        if defined( rip_metric ) then
                var = rip_metric;
        else {
                var = 1;
                rip_metric = 1;
        }
        if rip_metric &gt; 10 then
                reject "RIP metric is too big";
        else
                accept "ok";
}
</code>

<p>As you can see, a filter has a header, a list of local variables, and a body.
The header consists of the <cf/filter/ keyword followed by a (unique) name of
filter. The list of local variables consists of <cf><M>type name</M>;</cf>
pairs where each pair declares one local variable. The body consists of <cf>
{ <M>statements</M> }</cf>. Each <m/statement/ is terminated by a <cf/;/. You
can group several statements to a single compound statement by using braces
(<cf>{ <M>statements</M> }</cf>) which is useful if you want to make a bigger
block of code conditional.

<p>BIRD supports functions, so that you don't have to repeat the same blocks of
code over and over. Functions can have zero or more parameters and they can have
local variables. Recursion is not allowed. Function definitions look like this:

<code>
function name ()
int local_variable;
{
        local_variable = 5;
}

function with_parameters (int parameter)
{
        print parameter;
}
</code>

<p>Unlike in C, variables are declared after the <cf/function/ line, but before
the first <cf/{/. You can't declare variables in nested blocks. Functions are
called like in C: <cf>name(); with_parameters(5);</cf>. Function may return
values using the <cf>return <m/[expr]/</cf> command. Returning a value exits
from current function (this is similar to C).

<p>Filters are defined in a way similar to functions except they can't have
explicit parameters. They get a route table entry as an implicit parameter, it
is also passed automatically to any functions called. The filter must terminate
with either <cf/accept/ or <cf/reject/ statement. If there's a runtime error in
filter, the route is rejected.

<p>A nice trick to debug filters is to use <cf>show route filter <m/name/</cf>
from the command line client. An example session might look like:

<code>
pavel@bug:~/bird$ ./birdc -s bird.ctl
BIRD 0.0.0 ready.
bird> show route
10.0.0.0/8         dev eth0 [direct1 23:21] (240)
195.113.30.2/32    dev tunl1 [direct1 23:21] (240)
127.0.0.0/8        dev lo [direct1 23:21] (240)
bird> show route ?
show route [<prefix>] [table <t>] [filter <f>] [all] [primary]...
bird> show route filter { if 127.0.0.5 &tilde; net then accept; }
127.0.0.0/8        dev lo [direct1 23:21] (240)
bird>
</code>


<sect>Data types
<label id="data-types">

<p>Each variable and each value has certain type. Booleans, integers and enums
are incompatible with each other (that is to prevent you from shooting oneself
in the foot).

<descrip>
        <tag><label id="type-bool">bool</tag>
        This is a boolean type, it can have only two values, <cf/true/ and
        <cf/false/. Boolean is the only type you can use in <cf/if/ statements.

        <tag><label id="type-int">int</tag>
        This is a general integer type. It is an unsigned 32bit type; i.e., you
        can expect it to store values from 0 to 4294967295. Overflows are not
        checked. You can use <cf/0x1234/ syntax to write hexadecimal values.

        <tag><label id="type-pair">pair</tag>
        This is a pair of two short integers. Each component can have values
        from 0 to 65535. Literals of this type are written as <cf/(1234,5678)/.
        The same syntax can also be used to construct a pair from two arbitrary
        integer expressions (for example <cf/(1+2,a)/).

        <tag><label id="type-quad">quad</tag>
        This is a dotted quad of numbers used to represent router IDs (and
        others). Each component can have a value from 0 to 255. Literals of
        this type are written like IPv4 addresses.

        <tag><label id="type-string">string</tag>
        This is a string of characters. There are no ways to modify strings in
        filters. You can pass them between functions, assign them to variables
        of type <cf/string/, print such variables, use standard string
        comparison operations (e.g. <cf/=, !=, &lt;, &gt;, &lt;=, &gt;=/), but
        you can't concatenate two strings. String literals are written as
        <cf/"This is a string constant"/. Additionally matching (<cf/&tilde;,
        !&tilde;/) operators could be used to match a string value against
        a shell pattern (represented also as a string).

        <tag><label id="type-ip">ip</tag>
        This type can hold a single IP address. The IPv4 addresses are stored as
        IPv4-Mapped IPv6 addresses so one data type for both of them is used.
        Whether the address is IPv4 or not may be checked by <cf>.is_ip4</cf>
        which returns a <cf/bool/. IP addresses are written in the standard
        notation (<cf/10.20.30.40/ or <cf/fec0:3:4::1/). You can apply special
        operator <cf>.mask(<M>num</M>)</cf> on values of type ip. It masks out
        all but first <cf><M>num</M></cf> bits from the IP address. So
        <cf/1.2.3.4.mask(8) = 1.0.0.0/ is true.

        <tag><label id="type-prefix">prefix</tag>
        This type can hold a network prefix consisting of IP address, prefix
        length and several other values. This is the key in route tables.

        Prefixes may be of several types, which can be determined by the special
        operator <cf/.type/. The type may be:

        <cf/NET_IP4/ and <cf/NET_IP6/ prefixes hold an IP prefix. The literals
        are written as <cf><m/ipaddress//<m/pxlen/</cf>. There are two special
        operators on these: <cf/.ip/ which extracts the IP address from the
        pair, and <cf/.len/, which separates prefix length from the pair.
        So <cf>1.2.0.0/16.len = 16</cf> is true.

        <cf/NET_IP6_SADR/ nettype holds both destination and source IPv6
        prefix. The literals are written as <cf><m/ipaddress//<m/pxlen/ from
        <m/ipaddress//<m/pxlen/</cf>, where the first part is the destination
        prefix and the second art is the source prefix. They support the same
        operators as IP prefixes, but just for the destination part. They also
        support <cf/.src/ and <cf/.dst/ operators to get respective parts of the
        address as separate <cf/NET_IP6/ values.

        <cf/NET_VPN4/ and <cf/NET_VPN6/ prefixes hold an IP prefix with VPN
        Route Distinguisher (<rfc id="4364">). They support the same special
        operators as IP prefixes, and also <cf/.rd/ which extracts the Route
        Distinguisher. Their literals are written
        as <cf><m/vpnrd/ <m/ipprefix/</cf>

        <cf/NET_ROA4/ and <cf/NET_ROA6/ prefixes hold an IP prefix range
        together with an ASN. They support the same special operators as IP
        prefixes, and also <cf/.maxlen/ which extracts maximal prefix length,
        and <cf/.asn/ which extracts the ASN.

        <cf/NET_FLOW4/ and <cf/NET_FLOW6/ hold an IP prefix together with a
        flowspec rule. Filters currently do not support much flowspec parsing,
        only <cf/.src/ and <cf/.dst/ operators to get source and destination
        parts of the flowspec as separate <cf/NET_IP4/ / <cf/NET_IP6/ values.

        <cf/NET_MPLS/ holds a single MPLS label and its handling is currently
        not implemented.

        <tag><label id="type-vpnrd">vpnrd</tag>
        This is a route distinguisher according to <rfc id="4364">. There are
        three kinds of RD's: <cf><m/asn/:<m/32bit int/</cf>, <cf><m/asn4/:<m/16bit int/</cf>
        and <cf><m/IPv4 address/:<m/32bit int/</cf>

        <tag><label id="type-ec">ec</tag>
        This is a specialized type used to represent BGP extended community
        values. It is essentially a 64bit value, literals of this type are
        usually written as <cf>(<m/kind/, <m/key/, <m/value/)</cf>, where
        <cf/kind/ is a kind of extended community (e.g. <cf/rt/ / <cf/ro/ for a
        route target / route origin communities), the format and possible values
        of <cf/key/ and <cf/value/ are usually integers, but it depends on the
        used kind. Similarly to pairs, ECs can be constructed using expressions
        for <cf/key/ and <cf/value/ parts, (e.g. <cf/(ro, myas, 3*10)/, where
        <cf/myas/ is an integer variable).

        <tag><label id="type-lc">lc</tag>
        This is a specialized type used to represent BGP large community
        values. It is essentially a triplet of 32bit values, where the first
        value is reserved for the AS number of the issuer, while meaning of
        remaining parts is defined by the issuer. Literals of this type are
        written as <cf/(123, 456, 789)/, with any integer values. Similarly to
        pairs, LCs can be constructed using expressions for its parts, (e.g.
        <cf/(myas, 10+20, 3*10)/, where <cf/myas/ is an integer variable).

        <tag><label id="type-set">int|pair|quad|ip|prefix|ec|lc|enum set</tag>
        Filters recognize four types of sets. Sets are similar to strings: you
        can pass them around but you can't modify them. Literals of type <cf>int
        set</cf> look like <cf> [ 1, 2, 5..7 ]</cf>. As you can see, both simple
        values and ranges are permitted in sets.

        For pair sets, expressions like <cf/(123,*)/ can be used to denote
        ranges (in that case <cf/(123,0)..(123,65535)/). You can also use
        <cf/(123,5..100)/ for range <cf/(123,5)..(123,100)/. You can also use
        <cf/*/ and <cf/a..b/ expressions in the first part of a pair, note that
        such expressions are translated to a set of intervals, which may be
        memory intensive. E.g. <cf/(*,4..20)/ is translated to <cf/(0,4..20),
        (1,4..20), (2,4..20), ... (65535, 4..20)/.

        EC sets use similar expressions like pair sets, e.g. <cf/(rt, 123,
        10..20)/ or <cf/(ro, 123, *)/. Expressions requiring the translation
        (like <cf/(rt, *, 3)/) are not allowed (as they usually have 4B range
        for ASNs).

        Also LC sets use similar expressions like pair sets. You can use ranges
        and wildcards, but if one field uses that, more specific (later) fields
        must be wildcards. E.g., <cf/(10, 20..30, *)/ or <cf/(10, 20, 30..40)/
        is valid, while <cf/(10, *, 20..30)/ or <cf/(10, 20..30, 40)/ is not
        valid.

        You can also use expressions for int, pair, EC and LC set values.
        However, it must be possible to evaluate these expressions before daemon
        boots. So you can use only constants inside them. E.g.

        <code>
         define one=1;
         define myas=64500;
         int set odds;
         pair set ps;
         ec set es;

         odds = [ one, 2+1, 6-one, 2*2*2-1, 9, 11 ];
         ps = [ (1,one+one), (3,4)..(4,8), (5,*), (6,3..6), (7..9,*) ];
         es = [ (rt, myas, 3*10), (rt, myas+one, 0..16*16*16-1), (ro, myas+2, *) ];
        </code>

        Sets of prefixes are special: their literals does not allow ranges, but
        allows prefix patterns that are written
        as <cf><M>ipaddress</M>/<M>pxlen</M>{<M>low</M>,<M>high</M>}</cf>.
        Prefix <cf><m>ip1</m>/<m>len1</m></cf> matches prefix
        pattern <cf><m>ip2</m>/<m>len2</m>{<m>l</m>,<m>h</m>}</cf> if the
        first <cf>min(len1, len2)</cf> bits of <cf/ip1/ and <cf/ip2/ are
        identical and <cf>len1 &lt;= ip1 &lt;= len2</cf>. A valid prefix pattern
        has to satisfy <cf>low &lt;= high</cf>, but <cf/pxlen/ is not
        constrained by <cf/low/ or <cf/high/. Obviously, a prefix matches a
        prefix set literal if it matches any prefix pattern in the prefix set
        literal.

        There are also two shorthands for prefix patterns: <cf><m/address//<m/len/+</cf>
        is a shorthand for <cf><m/address//<m/len/{<m/len/,<m/maxlen/}</cf>
        (where <cf><m/maxlen/</cf> is 32 for IPv4 and 128 for IPv6), that means
        network prefix <cf><m/address//<m/len/</cf> and all its subnets.
        <cf><m/address//<m/len/-</cf> is a shorthand for
        <cf><m/address//<m/len/{0,<m/len/}</cf>, that means network prefix
        <cf><m/address//<m/len/</cf> and all its supernets (network prefixes
        that contain it).

        For example, <cf>[ 1.0.0.0/8, 2.0.0.0/8+, 3.0.0.0/8-, 4.0.0.0/8{16,24}
        ]</cf> matches prefix <cf>1.0.0.0/8</cf>, all subprefixes of
        <cf>2.0.0.0/8</cf>, all superprefixes of <cf>3.0.0.0/8</cf> and prefixes
        <cf/4.X.X.X/ whose prefix length is 16 to 24. <cf>[ 0.0.0.0/0{20,24} ]</cf>
        matches all prefixes (regardless of IP address) whose prefix length is
        20 to 24, <cf>[ 1.2.3.4/32- ]</cf> matches any prefix that contains IP
        address <cf>1.2.3.4</cf>. <cf>1.2.0.0/16 &tilde; [ 1.0.0.0/8{15,17} ]</cf>
        is true, but <cf>1.0.0.0/16 &tilde; [ 1.0.0.0/8- ]</cf> is false.

        Cisco-style patterns like <cf>10.0.0.0/8 ge 16 le 24</cf> can be expressed
        in BIRD as <cf>10.0.0.0/8{16,24}</cf>, <cf>192.168.0.0/16 le 24</cf> as
        <cf>192.168.0.0/16{16,24}</cf> and <cf>192.168.0.0/16 ge 24</cf> as
        <cf>192.168.0.0/16{24,32}</cf>.

        It is possible to mix IPv4 and IPv6 prefixes/addresses in a prefix/ip set
        but its behavior may change between versions without any warning; don't do
        it unless you are more than sure what you are doing. (Really, don't do it.)

        <tag><label id="type-enum">enum</tag>
        Enumeration types are fixed sets of possibilities. You can't define your
        own variables of such type, but some route attributes are of enumeration
        type. Enumeration types are incompatible with each other.

        <tag><label id="type-bgppath">bgppath</tag>
        BGP path is a list of autonomous system numbers. You can't write
        literals of this type. There are several special operators on bgppaths:

        <cf><m/P/.first</cf> returns the first ASN (the neighbor ASN) in path <m/P/.

        <cf><m/P/.last</cf> returns the last ASN (the source ASN) in path <m/P/.

        <cf><m/P/.last_nonaggregated</cf> returns the last ASN in the non-aggregated part of the path <m/P/.

        Both <cf/first/ and <cf/last/ return zero if there is no appropriate
        ASN, for example if the path contains an AS set element as the first (or
        the last) part. If the path ends with an AS set, <cf/last_nonaggregated/
        may be used to get last ASN before any AS set.

        <cf><m/P/.len</cf> returns the length of path <m/P/.

        <cf><m/P/.empty</cf> makes the path <m/P/ empty.

        <cf>prepend(<m/P/,<m/A/)</cf> prepends ASN <m/A/ to path <m/P/ and
        returns the result.

        <cf>delete(<m/P/,<m/A/)</cf> deletes all instances of ASN <m/A/ from
        from path <m/P/ and returns the result. <m/A/ may also be an integer
        set, in that case the operator deletes all ASNs from path <m/P/ that are
        also members of set <m/A/.

        <cf>filter(<m/P/,<m/A/)</cf> deletes all ASNs from path <m/P/ that are
        not members of integer set <m/A/. I.e., <cf/filter/ do the same as
        <cf/delete/ with inverted set <m/A/.

        Statement <cf><m/P/ = prepend(<m/P/, <m/A/);</cf> can be shortened to
        <cf><m/P/.prepend(<m/A/);</cf> if <m/P/ is appropriate route attribute
        (for example <cf/bgp_path/). Similarly for <cf/delete/ and <cf/filter/.

        <tag><label id="type-bgpmask">bgpmask</tag>
        BGP masks are patterns used for BGP path matching (using <cf>path
        &tilde; [= 2 3 5 * =]</cf> syntax). The masks resemble wildcard patterns
        as used by UNIX shells. Autonomous system numbers match themselves,
        <cf/*/ matches any (even empty) sequence of arbitrary AS numbers and
        <cf/?/ matches one arbitrary AS number. For example, if <cf>bgp_path</cf>
        is 4 3 2 1, then: <tt>bgp_path &tilde; [= * 4 3 * =]</tt> is true,
        but <tt>bgp_path &tilde; [= * 4 5 * =]</tt> is false. BGP mask
        expressions can also contain integer expressions enclosed in parenthesis
        and integer variables, for example <tt>[= * 4 (1+2) a =]</tt>. You can
        also use ranges (e.g. <tt>[= * 3..5 2 100..200 * =]</tt>) and sets
        (e.g. <tt>[= 1 2 [3, 5, 7] * =]</tt>).

        <tag><label id="type-clist">clist</tag>
        Clist is similar to a set, except that unlike other sets, it can be
        modified. The type is used for community list (a set of pairs) and for
        cluster list (a set of quads). There exist no literals of this type.
        There are three special operators on clists:

        <cf><m/C/.len</cf> returns the length of clist <m/C/.

        <cf><m/C/.empty</cf> makes the list <m/C/ empty.

        <cf>add(<m/C/,<m/P/)</cf> adds pair (or quad) <m/P/ to clist <m/C/ and
        returns the result. If item <m/P/ is already in clist <m/C/, it does
        nothing. <m/P/ may also be a clist, in that case all its members are
        added; i.e., it works as clist union.

        <cf>delete(<m/C/,<m/P/)</cf> deletes pair (or quad) <m/P/ from clist
        <m/C/ and returns the result. If clist <m/C/ does not contain item
        <m/P/, it does nothing. <m/P/ may also be a pair (or quad) set, in that
        case the operator deletes all items from clist <m/C/ that are also
        members of set <m/P/. Moreover, <m/P/ may also be a clist, which works
        analogously; i.e., it works as clist difference.

        <cf>filter(<m/C/,<m/P/)</cf> deletes all items from clist <m/C/ that are
        not members of pair (or quad) set <m/P/. I.e., <cf/filter/ do the same
        as <cf/delete/ with inverted set <m/P/. <m/P/ may also be a clist, which
        works analogously; i.e., it works as clist intersection.

        Statement <cf><m/C/ = add(<m/C/, <m/P/);</cf> can be shortened to
        <cf><m/C/.add(<m/P/);</cf> if <m/C/ is appropriate route attribute (for
        example <cf/bgp_community/). Similarly for <cf/delete/ and <cf/filter/.

        <tag><label id="type-eclist">eclist</tag>
        Eclist is a data type used for BGP extended community lists. Eclists
        are very similar to clists, but they are sets of ECs instead of pairs.
        The same operations (like <cf/add/, <cf/delete/ or <cf/&tilde;/ and
        <cf/!&tilde;/ membership operators) can be used to modify or test
        eclists, with ECs instead of pairs as arguments.

        <tag><label id="type-lclist">lclist</tag>
        Lclist is a data type used for BGP large community lists. Like eclists,
        lclists are very similar to clists, but they are sets of LCs instead of
        pairs. The same operations (like <cf/add/, <cf/delete/ or <cf/&tilde;/
        and <cf/!&tilde;/ membership operators) can be used to modify or test
        lclists, with LCs instead of pairs as arguments.
</descrip>


<sect>Operators
<label id="operators">

<p>The filter language supports common integer operators <cf>(+,-,*,/)</cf>,
parentheses <cf/(a*(b+c))/, comparison <cf/(a=b, a!=b, a&lt;b, a&gt;=b)/.
Logical operations include unary not (<cf/!/), and (<cf/&amp;&amp;/), and or
(<cf/&verbar;&verbar;/). Special operators include (<cf/&tilde;/,
<cf/!&tilde;/) for "is (not) element of a set" operation - it can be used on
element and set of elements of the same type (returning true if element is
contained in the given set), or on two strings (returning true if first string
matches a shell-like pattern stored in second string) or on IP and prefix
(returning true if IP is within the range defined by that prefix), or on prefix
and prefix (returning true if first prefix is more specific than second one) or
on bgppath and bgpmask (returning true if the path matches the mask) or on
number and bgppath (returning true if the number is in the path) or on bgppath
and int (number) set (returning true if any ASN from the path is in the set) or
on pair/quad and clist (returning true if the pair/quad is element of the
clist) or on clist and pair/quad set (returning true if there is an element of
the clist that is also a member of the pair/quad set).

<p>There is one operator related to ROA infrastructure - <cf/roa_check()/. It
examines a ROA table and does <rfc id="6483"> route origin validation for a
given network prefix. The basic usage is <cf>roa_check(<m/table/)</cf>, which
checks the current route (which should be from BGP to have AS_PATH argument) in
the specified ROA table and returns ROA_UNKNOWN if there is no relevant ROA,
ROA_VALID if there is a matching ROA, or ROA_INVALID if there are some relevant
ROAs but none of them match. There is also an extended variant
<cf>roa_check(<m/table/, <m/prefix/, <m/asn/)</cf>, which allows to specify a
prefix and an ASN as arguments.


<sect>Control structures
<label id="control-structures">

<p>Filters support two control structures: conditions and case switches.

<p>Syntax of a condition is: <cf>if <M>boolean expression</M> then <m/commandT/;
else <m/commandF/;</cf> and you can use <cf>{ <m/command1/; <m/command2/;
<M>...</M> }</cf> instead of either command. The <cf>else</cf> clause may be
omitted. If the <cf><m>boolean expression</m></cf> is true, <m/commandT/ is
executed, otherwise <m/commandF/ is executed.

<p>The <cf>case</cf> is similar to case from Pascal. Syntax is <cf>case
<m/expr/ { else: | <m/num_or_prefix [ .. num_or_prefix]/: <m/statement/ ; [
... ] }</cf>. The expression after <cf>case</cf> can be of any type which can be
on the left side of the &tilde; operator and anything that could be a member of
a set is allowed before <cf/:/. Multiple commands are allowed without <cf/{}/
grouping. If <cf><m/expr/</cf> matches one of the <cf/:/ clauses, statements
between it and next <cf/:/ statement are executed. If <cf><m/expr/</cf> matches
neither of the <cf/:/ clauses, the statements after <cf/else:/ are executed.

<p>Here is example that uses <cf/if/ and <cf/case/ structures:

<code>
case arg1 {
        2: print "two"; print "I can do more commands without {}";
        3 .. 5: print "three to five";
        else: print "something else";
}

if 1234 = i then printn "."; else {
  print "not 1234";
  print "You need {} around multiple commands";
}
</code>


<sect>Route attributes
<label id="route-attributes">

<p>A filter is implicitly passed a route, and it can access its attributes just
like it accesses variables. There are common route attributes, protocol-specific
route attributes and custom route attributes. Most common attributes are
mandatory (always defined), while remaining are optional.  Attempts to access
undefined attribute result in a runtime error; you can check if an attribute is
defined by using the <cf>defined( <m>attribute</m> )</cf> operator. One notable
exception to this rule are attributes of bgppath and *clist types, where
undefined value is regarded as empty bgppath/*clist for most purposes.

Attributes can be defined by just setting them in filters. Custom attributes
have to be first declared by <ref id="opt-attribute" name="attribute"> global
option. You can also undefine optional attribute back to non-existence by using
the <cf>unset( <m/attribute/ )</cf> operator.

Common route attributes are:

<descrip>
        <tag><label id="rta-net"><m/prefix/ net</tag>
        The network prefix or anything else the route is talking about. The
        primary key of the routing table. Read-only. (See the <ref id="routes"
        name="chapter about routes">.)

        <tag><label id="rta-scope"><m/enum/ scope</tag>
        The scope of the route. Possible values: <cf/SCOPE_HOST/ for routes
        local to this host, <cf/SCOPE_LINK/ for those specific for a physical
        link, <cf/SCOPE_SITE/ and <cf/SCOPE_ORGANIZATION/ for private routes and
        <cf/SCOPE_UNIVERSE/ for globally visible routes. This attribute is not
        interpreted by BIRD and can be used to mark routes in filters. The
        default value for new routes is <cf/SCOPE_UNIVERSE/.

        <tag><label id="rta-preference"><m/int/ preference</tag>
        Preference of the route. Valid values are 0-65535. (See the chapter
        about routing tables.)

        <tag><label id="rta-from"><m/ip/ from</tag>
        The router which the route has originated from.

        <tag><label id="rta-gw"><m/ip/ gw</tag>
        Next hop packets routed using this route should be forwarded to.

        <tag><label id="rta-proto"><m/string/ proto</tag>
        The name of the protocol which the route has been imported from.
        Read-only.

        <tag><label id="rta-source"><m/enum/ source</tag>
        what protocol has told me about this route. Possible values:
        <cf/RTS_DUMMY/, <cf/RTS_STATIC/, <cf/RTS_INHERIT/, <cf/RTS_DEVICE/,
        <cf/RTS_STATIC_DEVICE/, <cf/RTS_REDIRECT/, <cf/RTS_RIP/, <cf/RTS_OSPF/,
        <cf/RTS_OSPF_IA/, <cf/RTS_OSPF_EXT1/, <cf/RTS_OSPF_EXT2/, <cf/RTS_BGP/,
        <cf/RTS_PIPE/, <cf/RTS_BABEL/.

        <tag><label id="rta-dest"><m/enum/ dest</tag>
        Type of destination the packets should be sent to
        (<cf/RTD_ROUTER/ for forwarding to a neighboring router,
        <cf/RTD_DEVICE/ for routing to a directly-connected network,
        <cf/RTD_MULTIPATH/ for multipath destinations,
        <cf/RTD_BLACKHOLE/ for packets to be silently discarded,
        <cf/RTD_UNREACHABLE/, <cf/RTD_PROHIBIT/ for packets that should be
        returned with ICMP host unreachable / ICMP administratively prohibited
        messages). Can be changed, but only to <cf/RTD_BLACKHOLE/,
        <cf/RTD_UNREACHABLE/ or <cf/RTD_PROHIBIT/.

        <tag><label id="rta-ifname"><m/string/ ifname</tag>
        Name of the outgoing interface. Sink routes (like blackhole, unreachable
        or prohibit) and multipath routes have no interface associated with
        them, so <cf/ifname/ returns an empty string for such routes. Setting it
        would also change route to a direct one (remove gateway).

        <tag><label id="rta-ifindex"><m/int/ ifindex</tag>
        Index of the outgoing interface. System wide index of the interface. May
        be used for interface matching, however indexes might change on interface
        creation/removal. Zero is returned for routes with undefined outgoing
        interfaces. Read-only.

        <tag><label id="rta-igp-metric"><m/int/ igp_metric</tag>
        The optional attribute that can be used to specify a distance to the
        network for routes that do not have a native protocol metric attribute
        (like <cf/ospf_metric1/ for OSPF routes). It is used mainly by BGP to
        compare internal distances to boundary routers (see below).
</descrip>

<p>Protocol-specific route attributes are described in the corresponding
protocol sections.


<sect>Other statements
<label id="other-statements">

<p>The following statements are available:

<descrip>
        <tag><label id="assignment"><m/variable/ = <m/expr/</tag>
        Set variable (or route attribute) to a given value.

        <tag><label id="filter-accept-reject">accept|reject [ <m/expr/ ]</tag>
        Accept or reject the route, possibly printing <cf><m>expr</m></cf>.

        <tag><label id="return">return <m/expr/</tag>
        Return <cf><m>expr</m></cf> from the current function, the function ends
        at this point.

        <tag><label id="print">print|printn <m/expr/ [<m/, expr.../]</tag>
        Prints given expressions; useful mainly while debugging filters. The
        <cf/printn/ variant does not terminate the line.

        <tag><label id="quitbird">quitbird</tag>
        Terminates BIRD. Useful when debugging the filter interpreter.
</descrip>


<chapt>Protocols
<label id="protocols">

<sect>Babel
<label id="babel">

<sect1>Introduction
<label id="babel-intro">

<p>The Babel protocol
(<rfc id="6126">) is a loop-avoiding distance-vector routing protocol that is
robust and efficient both in ordinary wired networks and in wireless mesh
networks. Babel is conceptually very simple in its operation and "just works"
in its default configuration, though some configuration is possible and in some
cases desirable.

<p>The Babel protocol is dual stack; i.e., it can carry both IPv4 and IPv6
routes over the same IPv6 transport. For sending and receiving Babel packets,
only a link-local IPv6 address is needed.

<p>BIRD implements an extension for IPv6 source-specific routing (SSR or SADR),
but must be configured accordingly to use it. SADR-enabled Babel router can
interoperate with non-SADR Babel router, but the later would ignore routes
with specific (non-zero) source prefix.

<sect1>Configuration
<label id="babel-config">

<p>The Babel protocol support both IPv4 and IPv6 channels; both can be
configured simultaneously. It can also be configured with <ref
id="ip-sadr-routes" name="IPv6 SADR"> channel instead of regular IPv6
channel, in such case SADR support is enabled. Babel supports no global
configuration options apart from those common to all other protocols, but
supports the following per-interface configuration options:

<code>
protocol babel [<name>] {
        ipv4 { <channel config> };
        ipv6 [sadr] { <channel config> };
        randomize router id <switch>;
        interface <interface pattern> {
                type <wired|wireless>;
                rxcost <number>;
                limit <number>;
                hello interval <time>;
                update interval <time>;
                port <number>;
                tx class|dscp <number>;
                tx priority <number>;
                rx buffer <number>;
                tx length <number>;
                check link <switch>;
                next hop ipv4 <address>;
                next hop ipv6 <address>;
        };
}
</code>

<descrip>
      <tag><label id="babel-channel">ipv4 | ipv6 [sadr] <m/channel config/</tag>
      The supported channels are IPv4, IPv6, and IPv6 SADR.

      <tag><label id="babel-random-router-id">randomize router id <m/switch/</tag>
      If enabled, Bird will randomize the top 32 bits of its router ID whenever
      the protocol instance starts up. If a Babel node restarts, it loses its
      sequence number, which can cause its routes to be rejected by peers until
      the state is cleared out by other nodes in the network (which can take on
      the order of minutes). Enabling this option causes Bird to pick a random
      router ID every time it starts up, which avoids this problem at the cost
      of not having stable router IDs in the network. Default: no.

      <tag><label id="babel-type">type wired|wireless </tag>
      This option specifies the interface type: Wired or wireless. On wired
      interfaces a neighbor is considered unreachable after a small number of
      Hello packets are lost, as described by <cf/limit/ option. On wireless
      interfaces the ETX link quality estimation technique is used to compute
      the metrics of routes discovered over this interface. This technique will
      gradually degrade the metric of routes when packets are lost rather than
      the more binary up/down mechanism of wired type links. Default:
      <cf/wired/.

      <tag><label id="babel-rxcost">rxcost <m/num/</tag>
      This option specifies the nominal RX cost of the interface. The effective
      neighbor costs for route metrics will be computed from this value with a
      mechanism determined by the interface <cf/type/. Note that in contrast to
      other routing protocols like RIP or OSPF, the <cf/rxcost/ specifies the
      cost of RX instead of TX, so it affects primarily neighbors' route
      selection and not local route selection. Default: 96 for wired interfaces,
      256 for wireless.

      <tag><label id="babel-limit">limit <m/num/</tag>
      BIRD keeps track of received Hello messages from each neighbor to
      establish neighbor reachability. For wired type interfaces, this option
      specifies how many of last 16 hellos have to be correctly received in
      order to neighbor is assumed to be up. The option is ignored on wireless
      type interfaces, where gradual cost degradation is used instead of sharp
      limit. Default: 12.

      <tag><label id="babel-hello">hello interval <m/time/ s|ms</tag>
      Interval at which periodic Hello messages are sent on this interface,
      with time units. Default: 4 seconds.

      <tag><label id="babel-update">update interval <m/time/ s|ms</tag>
      Interval at which periodic (full) updates are sent, with time
      units. Default: 4 times the hello interval.

      <tag><label id="babel-port">port <m/number/</tag>
      This option selects an UDP port to operate on. The default is to operate
      on port 6696 as specified in the Babel RFC.

      <tag><label id="babel-tx-class">tx class|dscp|priority <m/number/</tag>
      These options specify the ToS/DiffServ/Traffic class/Priority of the
      outgoing Babel packets. See <ref id="proto-tx-class" name="tx class"> common
      option for detailed description.

      <tag><label id="babel-rx-buffer">rx buffer <m/number/</tag>
      This option specifies the size of buffers used for packet processing.
      The buffer size should be bigger than maximal size of received packets.
      The default value is the interface MTU, and the value will be clamped to a
      minimum of 512 bytes + IP packet overhead.

      <tag><label id="babel-tx-length">tx length <m/number/</tag>
      This option specifies the maximum length of generated Babel packets. To
      avoid IP fragmentation, it should not exceed the interface MTU value.
      The default value is the interface MTU value, and the value will be
      clamped to a minimum of 512 bytes + IP packet overhead.

      <tag><label id="babel-check-link">check link <m/switch/</tag>
      If set, the hardware link state (as reported by OS) is taken into
      consideration. When the link disappears (e.g. an ethernet cable is
      unplugged), neighbors are immediately considered unreachable and all
      routes received from them are withdrawn. It is possible that some
      hardware drivers or platforms do not implement this feature. Default:
      yes.

      <tag><label id="babel-next-hop-ipv4">next hop ipv4 <m/address/</tag>
      Set the next hop address advertised for IPv4 routes advertised on this
      interface. Default: the preferred IPv4 address of the interface.

      <tag><label id="babel-next-hop-ipv6">next hop ipv6 <m/address/</tag>
      Set the next hop address advertised for IPv6 routes advertised on this
      interface. If not set, the same link-local address that is used as the
      source for Babel packets will be used. In normal operation, it should not
      be necessary to set this option.
</descrip>

<sect1>Attributes
<label id="babel-attr">

<p>Babel defines just one attribute: the internal babel metric of the route. It
is exposed as the <cf/babel_metric/ attribute and has range from 1 to infinity
(65535).

<sect1>Example
<label id="babel-exam">

<p><code>
protocol babel {
        interface "eth*" {
                type wired;
        };
        interface "wlan0", "wlan1" {
                type wireless;
                hello interval 1;
                rxcost 512;
        };
        interface "tap0";

        # This matches the default of babeld: redistribute all addresses
        # configured on local interfaces, plus re-distribute all routes received
        # from other babel peers.

        ipv4 {
                export where (source = RTS_DEVICE) || (source = RTS_BABEL);
        };
        ipv6 {
                export where (source = RTS_DEVICE) || (source = RTS_BABEL);
        };
}
</code>

<sect1>Known issues
<label id="babel-issues">

<p>When retracting a route, Babel generates an unreachable route for a little
while (according to RFC). The interaction of this behavior with other protocols
is not well tested and strange things may happen.


<sect>BFD
<label id="bfd">

<sect1>Introduction
<label id="bfd-intro">

<p>Bidirectional Forwarding Detection (BFD) is not a routing protocol itself, it
is an independent tool providing liveness and failure detection. Routing
protocols like OSPF and BGP use integrated periodic "hello" messages to monitor
liveness of neighbors, but detection times of these mechanisms are high (e.g. 40
seconds by default in OSPF, could be set down to several seconds). BFD offers
universal, fast and low-overhead mechanism for failure detection, which could be
attached to any routing protocol in an advisory role.

<p>BFD consists of mostly independent BFD sessions. Each session monitors an
unicast bidirectional path between two BFD-enabled routers. This is done by
periodically sending control packets in both directions. BFD does not handle
neighbor discovery, BFD sessions are created on demand by request of other
protocols (like OSPF or BGP), which supply appropriate information like IP
addresses and associated interfaces. When a session changes its state, these
protocols are notified and act accordingly (e.g. break an OSPF adjacency when
the BFD session went down).

<p>BIRD implements basic BFD behavior as defined in <rfc id="5880"> (some
advanced features like the echo mode or authentication are not implemented), IP
transport for BFD as defined in <rfc id="5881"> and <rfc id="5883"> and
interaction with client protocols as defined in <rfc id="5882">.

<p>BFD packets are sent with a dynamic source port number. Linux systems use by
default a bit different dynamic port range than the IANA approved one
(49152-65535). If you experience problems with compatibility, please adjust
<cf>/proc/sys/net/ipv4/ip_local_port_range</cf>.

<sect1>Configuration
<label id="bfd-config">

<p>BFD configuration consists mainly of multiple definitions of interfaces.
Most BFD config options are session specific. When a new session is requested
and dynamically created, it is configured from one of these definitions. For
sessions to directly connected neighbors, <cf/interface/ definitions are chosen
based on the interface associated with the session, while <cf/multihop/
definition is used for multihop sessions. If no definition is relevant, the
session is just created with the default configuration. Therefore, an empty BFD
configuration is often sufficient.

<p>Note that to use BFD for other protocols like OSPF or BGP, these protocols
also have to be configured to request BFD sessions, usually by <cf/bfd/ option.

<p>A BFD instance not associated with any VRF handles session requests from all
other protocols, even ones associated with a VRF. Such setup would work for
single-hop BFD sessions if <cf/net.ipv4.udp_l3mdev_accept/ sysctl is enabled,
but does not currently work for multihop sessions. Another approach is to
configure multiple BFD instances, one for each VRF (including the default VRF).
Each BFD instance associated with a VRF (regular or default) only handles
session requests from protocols in the same VRF.

<p>Some of BFD session options require <m/time/ value, which has to be specified
with the appropriate unit: <m/num/ <cf/s/|<cf/ms/|<cf/us/. Although microseconds
are allowed as units, practical minimum values are usually in order of tens of
milliseconds.

<code>
protocol bfd [&lt;name&gt;] {
        interface &lt;interface pattern&gt; {
                interval &lt;time&gt;;
                min rx interval &lt;time&gt;;
                min tx interval &lt;time&gt;;
                idle tx interval &lt;time&gt;;
                multiplier &lt;num&gt;;
                passive &lt;switch&gt;;
                authentication none;
                authentication simple;
                authentication [meticulous] keyed md5|sha1;
                password "&lt;text&gt;";
                password "&lt;text&gt;" {
                        id &lt;num&gt;;
                        generate from "&lt;date&gt;";
                        generate to "&lt;date&gt;";
                        accept from "&lt;date&gt;";
                        accept to "&lt;date&gt;";
                        from "&lt;date&gt;";
                        to "&lt;date&gt;";
                };
        };
        multihop {
                interval &lt;time&gt;;
                min rx interval &lt;time&gt;;
                min tx interval &lt;time&gt;;
                idle tx interval &lt;time&gt;;
                multiplier &lt;num&gt;;
                passive &lt;switch&gt;;
        };
        neighbor &lt;ip&gt; [dev "&lt;interface&gt;"] [local &lt;ip&gt;] [multihop &lt;switch&gt;];
}
</code>

<descrip>
        <tag><label id="bfd-iface">interface <m/pattern/ [, <m/.../] { <m/options/ }</tag>
        Interface definitions allow to specify options for sessions associated
        with such interfaces and also may contain interface specific options.
        See <ref id="proto-iface" name="interface"> common option for a detailed
        description of interface patterns. Note that contrary to the behavior of
        <cf/interface/ definitions of other protocols, BFD protocol would accept
        sessions (in default configuration) even on interfaces not covered by
        such definitions.

        <tag><label id="bfd-multihop">multihop { <m/options/ }</tag>
        Multihop definitions allow to specify options for multihop BFD sessions,
        in the same manner as <cf/interface/ definitions are used for directly
        connected sessions. Currently only one such definition (for all multihop
        sessions) could be used.

        <tag><label id="bfd-neighbor">neighbor <m/ip/ [dev "<m/interface/"] [local <m/ip/] [multihop <m/switch/]</tag>
        BFD sessions are usually created on demand as requested by other
        protocols (like OSPF or BGP). This option allows to explicitly add
        a BFD session to the specified neighbor regardless of such requests.

        The session is identified by the IP address of the neighbor, with
        optional specification of used interface and local IP. By default
        the neighbor must be directly connected, unless the session is
        configured as multihop. Note that local IP must be specified for
        multihop sessions.
</descrip>

<p>Session specific options (part of <cf/interface/ and <cf/multihop/ definitions):

<descrip>
        <tag><label id="bfd-interval">interval <m/time/</tag>
        BFD ensures availability of the forwarding path associated with the
        session by periodically sending BFD control packets in both
        directions. The rate of such packets is controlled by two options,
        <cf/min rx interval/ and <cf/min tx interval/ (see below). This option
        is just a shorthand to set both of these options together.

        <tag><label id="bfd-min-rx-interval">min rx interval <m/time/</tag>
        This option specifies the minimum RX interval, which is announced to the
        neighbor and used there to limit the neighbor's rate of generated BFD
        control packets. Default: 10 ms.

        <tag><label id="bfd-min-tx-interval">min tx interval <m/time/</tag>
        This option specifies the desired TX interval, which controls the rate
        of generated BFD control packets (together with <cf/min rx interval/
        announced by the neighbor). Note that this value is used only if the BFD
        session is up, otherwise the value of <cf/idle tx interval/ is used
        instead. Default: 100 ms.

        <tag><label id="bfd-idle-tx-interval">idle tx interval <m/time/</tag>
        In order to limit unnecessary traffic in cases where a neighbor is not
        available or not running BFD, the rate of generated BFD control packets
        is lower when the BFD session is not up. This option specifies the
        desired TX interval in such cases instead of <cf/min tx interval/.
        Default: 1 s.

        <tag><label id="bfd-multiplier">multiplier <m/num/</tag>
        Failure detection time for BFD sessions is based on established rate of
        BFD control packets (<cf>min rx/tx interval</cf>) multiplied by this
        multiplier, which is essentially (ignoring jitter) a number of missed
        packets after which the session is declared down. Note that rates and
        multipliers could be different in each direction of a BFD session.
        Default: 5.

        <tag><label id="bfd-passive">passive <m/switch/</tag>
        Generally, both BFD session endpoints try to establish the session by
        sending control packets to the other side. This option allows to enable
        passive mode, which means that the router does not send BFD packets
        until it has received one from the other side. Default: disabled.

        <tag>authentication none</tag>
        No passwords are sent in BFD packets. This is the default value.

        <tag>authentication simple</tag>
        Every packet carries 16 bytes of password. Received packets lacking this
        password are ignored. This authentication mechanism is very weak.

        <tag>authentication [meticulous] keyed md5|sha1</tag>
        An authentication code is appended to each packet. The cryptographic
        algorithm is keyed MD5 or keyed SHA-1. Note that the algorithm is common
        for all keys (on one interface), in contrast to OSPF or RIP, where it
        is a per-key option. Passwords (keys) are not sent open via network.

        The <cf/meticulous/ variant means that cryptographic sequence numbers
        are increased for each sent packet, while in the basic variant they are
        increased about once per second. Generally, the <cf/meticulous/ variant
        offers better resistance to replay attacks but may require more
        computation.

        <tag>password "<M>text</M>"</tag>
        Specifies a password used for authentication. See <ref id="proto-pass"
        name="password"> common option for detailed description. Note that
        password option <cf/algorithm/ is not available in BFD protocol. The
        algorithm is selected by <cf/authentication/ option for all passwords.

</descrip>

<sect1>Example
<label id="bfd-exam">

<p><code>
protocol bfd {
        interface "eth*" {
                min rx interval 20 ms;
                min tx interval 50 ms;
                idle tx interval 300 ms;
        };
        interface "gre*" {
                interval 200 ms;
                multiplier 10;
                passive;
        };
        multihop {
                interval 200 ms;
                multiplier 10;
        };

        neighbor 192.168.1.10;
        neighbor 192.168.2.2 dev "eth2";
        neighbor 192.168.10.1 local 192.168.1.1 multihop;
}
</code>


<sect>BGP
<label id="bgp">

<p>The Border Gateway Protocol is the routing protocol used for backbone level
routing in the today's Internet. Contrary to other protocols, its convergence
does not rely on all routers following the same rules for route selection,
making it possible to implement any routing policy at any router in the network,
the only restriction being that if a router advertises a route, it must accept
and forward packets according to it.

<p>BGP works in terms of autonomous systems (often abbreviated as AS). Each AS
is a part of the network with common management and common routing policy. It is
identified by a unique 16-bit number (ASN). Routers within each AS usually
exchange AS-internal routing information with each other using an interior
gateway protocol (IGP, such as OSPF or RIP). Boundary routers at the border of
the AS communicate global (inter-AS) network reachability information with their
neighbors in the neighboring AS'es via exterior BGP (eBGP) and redistribute
received information to other routers in the AS via interior BGP (iBGP).

<p>Each BGP router sends to its neighbors updates of the parts of its routing
table it wishes to export along with complete path information (a list of AS'es
the packet will travel through if it uses the particular route) in order to
avoid routing loops.

<sect1>Supported standards
<label id="bgp-standards">

<p>
<itemize>
<item> <rfc id="4271"> - Border Gateway Protocol 4 (BGP)
<item> <rfc id="1997"> - BGP Communities Attribute
<item> <rfc id="2385"> - Protection of BGP Sessions via TCP MD5 Signature
<item> <rfc id="2545"> - Use of BGP Multiprotocol Extensions for IPv6
<item> <rfc id="2918"> - Route Refresh Capability
<item> <rfc id="3107"> - Carrying Label Information in BGP
<item> <rfc id="4360"> - BGP Extended Communities Attribute
<item> <rfc id="4364"> - BGP/MPLS IPv4 Virtual Private Networks
<item> <rfc id="4456"> - BGP Route Reflection
<item> <rfc id="4486"> - Subcodes for BGP Cease Notification Message
<item> <rfc id="4659"> - BGP/MPLS IPv6 Virtual Private Networks
<item> <rfc id="4724"> - Graceful Restart Mechanism for BGP
<item> <rfc id="4760"> - Multiprotocol extensions for BGP
<item> <rfc id="4798"> - Connecting IPv6 Islands over IPv4 MPLS
<item> <rfc id="5065"> - AS confederations for BGP
<item> <rfc id="5082"> - Generalized TTL Security Mechanism
<item> <rfc id="5492"> - Capabilities Advertisement with BGP
<item> <rfc id="5549"> - Advertising IPv4 NLRI with an IPv6 Next Hop
<item> <rfc id="5575"> - Dissemination of Flow Specification Rules
<item> <rfc id="5668"> - 4-Octet AS Specific BGP Extended Community
<item> <rfc id="6286"> - AS-Wide Unique BGP Identifier
<item> <rfc id="6608"> - Subcodes for BGP Finite State Machine Error
<item> <rfc id="6793"> - BGP Support for 4-Octet AS Numbers
<item> <rfc id="7311"> - Accumulated IGP Metric Attribute for BGP
<item> <rfc id="7313"> - Enhanced Route Refresh Capability for BGP
<item> <rfc id="7606"> - Revised Error Handling for BGP UPDATE Messages
<item> <rfc id="7911"> - Advertisement of Multiple Paths in BGP
<item> <rfc id="7947"> - Internet Exchange BGP Route Server
<item> <rfc id="8092"> - BGP Large Communities Attribute
<item> <rfc id="8203"> - BGP Administrative Shutdown Communication
<item> <rfc id="8212"> - Default EBGP Route Propagation Behavior without Policies
</itemize>

<sect1>Route selection rules
<label id="bgp-route-select-rules">

<p>BGP doesn't have any simple metric, so the rules for selection of an optimal
route among multiple BGP routes with the same preference are a bit more complex
and they are implemented according to the following algorithm. It starts the
first rule, if there are more "best" routes, then it uses the second rule to
choose among them and so on.

<itemize>
        <item>Prefer route with the highest Local Preference attribute.
        <item>Prefer route with the shortest AS path.
        <item>Prefer IGP origin over EGP and EGP origin over incomplete.
        <item>Prefer the lowest value of the Multiple Exit Discriminator.
        <item>Prefer routes received via eBGP over ones received via iBGP.
        <item>Prefer routes with lower internal distance to a boundary router.
        <item>Prefer the route with the lowest value of router ID of the
        advertising router.
</itemize>

<sect1>IGP routing table
<label id="bgp-igp-routing-table">

<p>BGP is mainly concerned with global network reachability and with routes to
other autonomous systems. When such routes are redistributed to routers in the
AS via BGP, they contain IP addresses of a boundary routers (in route attribute
NEXT_HOP). BGP depends on existing IGP routing table with AS-internal routes to
determine immediate next hops for routes and to know their internal distances to
boundary routers for the purpose of BGP route selection. In BIRD, there is
usually one routing table used for both IGP routes and BGP routes.

<sect1>Protocol configuration
<label id="bgp-proto-config">

<p>Each instance of the BGP corresponds to one neighboring router. This allows
to set routing policy and all the other parameters differently for each neighbor
using the following configuration parameters:

<descrip>
        <tag><label id="bgp-local">local [<m/ip/] [port <m/number/] [as <m/number/]</tag>
        Define which AS we are part of. (Note that contrary to other IP routers,
        BIRD is able to act as a router located in multiple AS'es simultaneously,
        but in such cases you need to tweak the BGP paths manually in the filters
        to get consistent behavior.) Optional <cf/ip/ argument specifies a source
        address, equivalent to the <cf/source address/ option (see below).
        Optional <cf/port/ argument specifies the local BGP port instead of
        standard port 179. The parameter may be used multiple times with
        different sub-options (e.g., both <cf/local 10.0.0.1 as 65000;/ and
        <cf/local 10.0.0.1; local as 65000;/ are valid). This parameter is
        mandatory.

        <tag><label id="bgp-neighbor">neighbor [<m/ip/ | range <m/prefix/] [port <m/number/] [as <m/number/] [internal|external]</tag>
        Define neighboring router this instance will be talking to and what AS
        it is located in. In case the neighbor is in the same AS as we are, we
        automatically switch to IBGP. Alternatively, it is possible to specify
        just <cf/internal/ or <cf/external/ instead of AS number, in that case
        either local AS number, or any external AS number is accepted.
        Optionally, the remote port may also be specified. Like <cf/local/
        parameter, this parameter may also be used multiple times with different
        sub-options. This parameter is mandatory.

        It is possible to specify network prefix (with <cf/range/ keyword)
        instead of explicit neighbor IP address. This enables dynamic BGP
        behavior, where the BGP instance listens on BGP port, but new BGP
        instances are spawned for incoming BGP connections (if source address
        matches the network prefix). It is possible to mix regular BGP instances
        with dynamic BGP instances and have multiple dynamic BGP instances with
        different ranges.

        <tag><label id="bgp-iface">interface <m/string/</tag>
        Define interface we should use for link-local BGP IPv6 sessions.
        Interface can also be specified as a part of <cf/neighbor address/
        (e.g., <cf/neighbor fe80::1234%eth0 as 65000;/). The option may also be
        used for non link-local sessions when it is necessary to explicitly
        specify an interface, but only for direct (not multihop) sessions.

        <tag><label id="bgp-direct">direct</tag>
        Specify that the neighbor is directly connected. The IP address of the
        neighbor must be from a directly reachable IP range (i.e. associated
        with one of your router's interfaces), otherwise the BGP session
        wouldn't start but it would wait for such interface to appear. The
        alternative is the <cf/multihop/ option. Default: enabled for eBGP.

        <tag><label id="bgp-multihop">multihop [<m/number/]</tag>
        Configure multihop BGP session to a neighbor that isn't directly
        connected. Accurately, this option should be used if the configured
        neighbor IP address does not match with any local network subnets. Such
        IP address have to be reachable through system routing table. The
        alternative is the <cf/direct/ option. For multihop BGP it is
        recommended to explicitly configure the source address to have it
        stable. Optional <cf/number/ argument can be used to specify the number
        of hops (used for TTL). Note that the number of networks (edges) in a
        path is counted; i.e., if two BGP speakers are separated by one router,
        the number of hops is 2. Default: enabled for iBGP.

        <tag><label id="bgp-source-address">source address <m/ip/</tag>
        Define local address we should use as a source address for the BGP
        session. Default: the address of the local end of the interface our
        neighbor is connected to.

        <tag><label id="bgp-dynamic-name">dynamic name "<m/text/"</tag>
        Define common prefix of names used for new BGP instances spawned when
        dynamic BGP behavior is active. Actual names also contain numeric
        index to distinguish individual instances.  Default: "dynbgp".

        <tag><label id="bgp-dynamic-name-digits">dynamic name digits <m/number/</tag>
        Define minimum number of digits for index in names of spawned dynamic
        BGP instances. E.g., if set to 2, then the first name would be
        "dynbgp01". Default: 0.

        <tag><label id="bgp-strict-bind">strict bind <m/switch/</tag>
        Specify whether BGP listening socket should be bound to a specific local
        address (the same as the <cf/source address/) and associated interface,
        or to all addresses. Binding to a specific address could be useful in
        cases like running multiple BIRD instances on a machine, each using its
        IP address. Note that listening sockets bound to a specific address and
        to all addresses collide, therefore either all BGP protocols (of the
        same address family and using the same local port) should have set
        <cf/strict bind/, or none of them. Default: disabled.

        <tag><label id="bgp-check-link">check link <M>switch</M></tag>
        BGP could use hardware link state into consideration.  If enabled,
        BIRD tracks the link state of the associated interface and when link
        disappears (e.g. an ethernet cable is unplugged), the BGP session is
        immediately shut down. Note that this option cannot be used with
        multihop BGP. Default: enabled for direct BGP, disabled otherwise.

        <tag><label id="bgp-bfd">bfd <M>switch</M>|graceful</tag>
        BGP could use BFD protocol as an advisory mechanism for neighbor
        liveness and failure detection. If enabled, BIRD setups a BFD session
        for the BGP neighbor and tracks its liveness by it. This has an
        advantage of an order of magnitude lower detection times in case of
        failure. When a neighbor failure is detected, the BGP session is
        restarted. Optionally, it can be configured (by <cf/graceful/ argument)
        to trigger graceful restart instead of regular restart. Note that BFD
        protocol also has to be configured, see <ref id="bfd" name="BFD">
        section for details. Default: disabled.

        <tag><label id="bgp-ttl-security">ttl security <m/switch/</tag>
        Use GTSM (<rfc id="5082"> - the generalized TTL security mechanism). GTSM
        protects against spoofed packets by ignoring received packets with a
        smaller than expected TTL. To work properly, GTSM have to be enabled on
        both sides of a BGP session. If both <cf/ttl security/ and
        <cf/multihop/ options are enabled, <cf/multihop/ option should specify
        proper hop value to compute expected TTL. Kernel support required:
        Linux: 2.6.34+ (IPv4), 2.6.35+ (IPv6), BSD: since long ago, IPv4 only.
        Note that full (ICMP protection, for example) <rfc id="5082"> support is
        provided by Linux only. Default: disabled.

        <tag><label id="bgp-password">password <m/string/</tag>
        Use this password for MD5 authentication of BGP sessions (<rfc id="2385">). When
        used on BSD systems, see also <cf/setkey/ option below. Default: no
        authentication.

        <tag><label id="bgp-setkey">setkey <m/switch/</tag>
        On BSD systems, keys for TCP MD5 authentication are stored in the global
        SA/SP database, which can be accessed by external utilities (e.g.
        setkey(8)). BIRD configures security associations in the SA/SP database
        automatically based on <cf/password/ options (see above), this option
        allows to disable automatic updates by BIRD when manual configuration by
        external utilities is preferred. Note that automatic SA/SP database
        updates are currently implemented only for FreeBSD. Passwords have to be
        set manually by an external utility on NetBSD and OpenBSD. Default:
        enabled (ignored on non-FreeBSD).

        <tag><label id="bgp-passive">passive <m/switch/</tag>
        Standard BGP behavior is both initiating outgoing connections and
        accepting incoming connections. In passive mode, outgoing connections
        are not initiated. Default: off.

        <tag><label id="bgp-confederation">confederation <m/number/</tag>
        BGP confederations (<rfc id="5065">) are collections of autonomous
        systems that act as one entity to external systems, represented by one
        confederation identifier (instead of AS numbers). This option allows to
        enable BGP confederation behavior and to specify the local confederation
        identifier. When BGP confederations are used, all BGP speakers that are
        members of the BGP confederation should have the same confederation
        identifier configured. Default: 0 (no confederation).

        <tag><label id="bgp-confederation-member">confederation member <m/switch/</tag>
        When BGP confederations are used, this option allows to specify whether
        the BGP neighbor is a member of the same confederation as the local BGP
        speaker. The option is unnecessary (and ignored) for IBGP sessions, as
        the same AS number implies the same confederation. Default: no.

        <tag><label id="bgp-rr-client">rr client</tag>
        Be a route reflector and treat the neighbor as a route reflection
        client. Default: disabled.

        <tag><label id="bgp-rr-cluster-id">rr cluster id <m/IPv4 address/</tag>
        Route reflectors use cluster id to avoid route reflection loops. When
        there is one route reflector in a cluster it usually uses its router id
        as a cluster id, but when there are more route reflectors in a cluster,
        these need to be configured (using this option) to use a common cluster
        id. Clients in a cluster need not know their cluster id and this option
        is not allowed for them. Default: the same as router id.

        <tag><label id="bgp-rs-client">rs client</tag>
        Be a route server and treat the neighbor as a route server client.
        A route server is used as a replacement for full mesh EBGP routing in
        Internet exchange points in a similar way to route reflectors used in
        IBGP routing. BIRD does not implement obsoleted <rfc id="1863">, but
        uses ad-hoc implementation, which behaves like plain EBGP but reduces
        modifications to advertised route attributes to be transparent (for
        example does not prepend its AS number to AS PATH attribute and
        keeps MED attribute). Default: disabled.

        <tag><label id="bgp-allow-local-pref">allow bgp_local_pref <m/switch/</tag>
        A standard BGP implementation do not send the Local Preference attribute
        to eBGP neighbors and ignore this attribute if received from eBGP
        neighbors, as per <rfc id="4271">.  When this option is enabled on an
        eBGP session, this attribute will be sent to and accepted from the peer,
        which is useful for example if you have a setup like in <rfc id="7938">.
        The option does not affect iBGP sessions. Default: off.

        <tag><label id="bgp-allow-local-as">allow local as [<m/number/]</tag>
        BGP prevents routing loops by rejecting received routes with the local
        AS number in the AS path. This option allows to loose or disable the
        check. Optional <cf/number/ argument can be used to specify the maximum
        number of local ASNs in the AS path that is allowed for received
        routes. When the option is used without the argument, the check is
        completely disabled and you should ensure loop-free behavior by some
        other means. Default: 0 (no local AS number allowed).

        <tag><label id="bgp-allow-as-sets">allow as sets [<m/switch/]</tag>
        AS path attribute received with BGP routes may contain not only
        sequences of AS numbers, but also sets of AS numbers. These rarely used
        artifacts are results of inter-AS route aggregation. AS sets are
        deprecated (<rfc id="6472">), and likely to be rejected in the future,
        as they complicate security features like RPKI validation. When this
        option is disabled, then received AS paths with AS sets are rejected as
        malformed and corresponding BGP updates are treated as withdraws.
        Default: on.

        <tag><label id="bgp-enforce-first-as">enforce first as [<m/switch/]</tag>
        Routes received from an EBGP neighbor are generally expected to have the
        first (leftmost) AS number in their AS path equal to the neighbor AS
        number. This is not enforced by default as there are legitimate cases
        where it is not true, e.g. connections to route servers. When this
        option is enabled, routes with non-matching first AS number are rejected
        and corresponding updates are treated as withdraws. The option is valid
        on EBGP sessions only. Default: off.

        <tag><label id="bgp-enable-route-refresh">enable route refresh <m/switch/</tag>
        After the initial route exchange, BGP protocol uses incremental updates
        to keep BGP speakers synchronized. Sometimes (e.g., if BGP speaker
        changes its import filter, or if there is suspicion of inconsistency) it
        is necessary to do a new complete route exchange. BGP protocol extension
        Route Refresh (<rfc id="2918">) allows BGP speaker to request
        re-advertisement of all routes from its neighbor. BGP protocol
        extension Enhanced Route Refresh (<rfc id="7313">) specifies explicit
        begin and end for such exchanges, therefore the receiver can remove
        stale routes that were not advertised during the exchange. This option
        specifies whether BIRD advertises these capabilities and supports
        related procedures. Note that even when disabled, BIRD can send route
        refresh requests.  Default: on.

        <tag><label id="bgp-graceful-restart">graceful restart <m/switch/|aware</tag>
        When a BGP speaker restarts or crashes, neighbors will discard all
        received paths from the speaker, which disrupts packet forwarding even
        when the forwarding plane of the speaker remains intact. <rfc id="4724">
        specifies an optional graceful restart mechanism to alleviate this
        issue. This option controls the mechanism. It has three states:
        Disabled, when no support is provided. Aware, when the graceful restart
        support is announced and the support for restarting neighbors is
        provided, but no local graceful restart is allowed (i.e. receiving-only
        role). Enabled, when the full graceful restart support is provided
        (i.e. both restarting and receiving role). Restarting role could be also
        configured per-channel. Note that proper support for local graceful
        restart requires also configuration of other protocols. Default: aware.

        <tag><label id="bgp-graceful-restart-time">graceful restart time <m/number/</tag>
        The restart time is announced in the BGP graceful restart capability
        and specifies how long the neighbor would wait for the BGP session to
        re-establish after a restart before deleting stale routes. Default:
        120 seconds.

        <tag><label id="bgp-long-lived-graceful-restart">long lived graceful restart <m/switch/|aware</tag>
        The long-lived graceful restart is an extension of the traditional
        <ref id="bgp-graceful-restart" name="BGP graceful restart">, where stale
        routes are kept even after the <ref id="bgp-graceful-restart-time"
        name="restart time"> expires for additional long-lived stale time, but
        they are marked with the LLGR_STALE community, depreferenced, and
        withdrawn from routers not supporting LLGR. Like traditional BGP
        graceful restart, it has three states: disabled, aware (receiving-only),
        and enabled. Note that long-lived graceful restart requires at least
        aware level of traditional BGP graceful restart. Default: aware, unless
        graceful restart is disabled.

        <tag><label id="bgp-long-lived-stale-time">long lived stale time <m/number/</tag>
        The long-lived stale time is announced in the BGP long-lived graceful
        restart capability and specifies how long the neighbor would keep stale
        routes depreferenced during long-lived graceful restart until either the
        session is re-stablished and synchronized or the stale time expires and
        routes are removed. Default: 3600 seconds.

        <tag><label id="bgp-interpret-communities">interpret communities <m/switch/</tag>
        <rfc id="1997"> demands that BGP speaker should process well-known
        communities like no-export (65535, 65281) or no-advertise (65535,
        65282). For example, received route carrying a no-adverise community
        should not be advertised to any of its neighbors. If this option is
        enabled (which is by default), BIRD has such behavior automatically (it
        is evaluated when a route is exported to the BGP protocol just before
        the export filter).  Otherwise, this integrated processing of
        well-known communities is disabled. In that case, similar behavior can
        be implemented in the export filter.  Default: on.

        <tag><label id="bgp-enable-as4">enable as4 <m/switch/</tag>
        BGP protocol was designed to use 2B AS numbers and was extended later to
        allow 4B AS number. BIRD supports 4B AS extension, but by disabling this
        option it can be persuaded not to advertise it and to maintain old-style
        sessions with its neighbors. This might be useful for circumventing bugs
        in neighbor's implementation of 4B AS extension. Even when disabled
        (off), BIRD behaves internally as AS4-aware BGP router. Default: on.

        <tag><label id="bgp-enable-extended-messages">enable extended messages <m/switch/</tag>
        The BGP protocol uses maximum message length of 4096 bytes. This option
        provides an extension (<rfc id="8654">) to allow extended messages with
        length up to 65535 bytes. Default: off.

        <tag><label id="bgp-capabilities">capabilities <m/switch/</tag>
        Use capability advertisement to advertise optional capabilities. This is
        standard behavior for newer BGP implementations, but there might be some
        older BGP implementations that reject such connection attempts. When
        disabled (off), features that request it (4B AS support) are also
        disabled. Default: on, with automatic fallback to off when received
        capability-related error.

        <tag><label id="bgp-advertise-ipv4">advertise ipv4 <m/switch/</tag>
        Advertise IPv4 multiprotocol capability. This is not a correct behavior
        according to the strict interpretation of <rfc id="4760">, but it is
        widespread and required by some BGP implementations (Cisco and Quagga).
        This option is relevant to IPv4 mode with enabled capability
        advertisement only. Default: on.

        <tag><label id="bgp-disable-after-error">disable after error <m/switch/</tag>
        When an error is encountered (either locally or by the other side),
        disable the instance automatically and wait for an administrator to fix
        the problem manually. Default: off.

        <tag><label id="bgp-disable-after-cease">disable after cease <m/switch/|<m/set-of-flags/</tag>
        When a Cease notification is received, disable the instance
        automatically and wait for an administrator to fix the problem manually.
        When used with <m/switch/ argument, it means handle every Cease subtype
        with the exception of <cf/connection collision/. Default: off.

        The <m/set-of-flags/ allows to narrow down relevant Cease subtypes. The
        syntax is <cf>{<m/flag/ [, <m/.../] }</cf>, where flags are: <cf/cease/,
        <cf/prefix limit hit/, <cf/administrative shutdown/,
        <cf/peer deconfigured/, <cf/administrative reset/,
        <cf/connection rejected/, <cf/configuration change/,
        <cf/connection collision/, <cf/out of resources/.

        <tag><label id="bgp-hold-time">hold time <m/number/</tag>
        Time in seconds to wait for a Keepalive message from the other side
        before considering the connection stale. Default: depends on agreement
        with the neighboring router, we prefer 240 seconds if the other side is
        willing to accept it.

        <tag><label id="bgp-startup-hold-time">startup hold time <m/number/</tag>
        Value of the hold timer used before the routers have a chance to exchange
        open messages and agree on the real value. Default: 240 seconds.

        <tag><label id="bgp-keepalive-time">keepalive time <m/number/</tag>
        Delay in seconds between sending of two consecutive Keepalive messages.
        Default: One third of the hold time.

        <tag><label id="bgp-connect-delay-time">connect delay time <m/number/</tag>
        Delay in seconds between protocol startup and the first attempt to
        connect. Default: 5 seconds.

        <tag><label id="bgp-connect-retry-time">connect retry time <m/number/</tag>
        Time in seconds to wait before retrying a failed attempt to connect.
        Default: 120 seconds.

        <tag><label id="bgp-error-wait-time">error wait time <m/number/,<m/number/</tag>
        Minimum and maximum delay in seconds between a protocol failure (either
        local or reported by the peer) and automatic restart. Doesn't apply
        when <cf/disable after error/ is configured. If consecutive errors
        happen, the delay is increased exponentially until it reaches the
        maximum. Default: 60, 300.

        <tag><label id="bgp-error-forget-time">error forget time <m/number/</tag>
        Maximum time in seconds between two protocol failures to treat them as a
        error sequence which makes <cf/error wait time/ increase exponentially.
        Default: 300 seconds.

        <tag><label id="bgp-path-metric">path metric <m/switch/</tag>
        Enable comparison of path lengths when deciding which BGP route is the
        best one. Default: on.

        <tag><label id="bgp-med-metric">med metric <m/switch/</tag>
        Enable comparison of MED attributes (during best route selection) even
        between routes received from different ASes. This may be useful if all
        MED attributes contain some consistent metric, perhaps enforced in
        import filters of AS boundary routers. If this option is disabled, MED
        attributes are compared only if routes are received from the same AS
        (which is the standard behavior). Default: off.

        <tag><label id="bgp-deterministic-med">deterministic med <m/switch/</tag>
        BGP route selection algorithm is often viewed as a comparison between
        individual routes (e.g. if a new route appears and is better than the
        current best one, it is chosen as the new best one). But the proper
        route selection, as specified by <rfc id="4271">, cannot be fully
        implemented in that way. The problem is mainly in handling the MED
        attribute. BIRD, by default, uses an simplification based on individual
        route comparison, which in some cases may lead to temporally dependent
        behavior (i.e. the selection is dependent on the order in which routes
        appeared). This option enables a different (and slower) algorithm
        implementing proper <rfc id="4271"> route selection, which is
        deterministic. Alternative way how to get deterministic behavior is to
        use <cf/med metric/ option. This option is incompatible with <ref
        id="dsc-table-sorted" name="sorted tables">.  Default: off.

        <tag><label id="bgp-igp-metric">igp metric <m/switch/</tag>
        Enable comparison of internal distances to boundary routers during best
        route selection. Default: on.

        <tag><label id="bgp-prefer-older">prefer older <m/switch/</tag>
        Standard route selection algorithm breaks ties by comparing router IDs.
        This changes the behavior to prefer older routes (when both are external
        and from different peer). For details, see <rfc id="5004">. Default: off.

        <tag><label id="bgp-default-med">default bgp_med <m/number/</tag>
        Value of the Multiple Exit Discriminator to be used during route
        selection when the MED attribute is missing. Default: 0.

        <tag><label id="bgp-default-local-pref">default bgp_local_pref <m/number/</tag>
        A default value for the Local Preference attribute. It is used when
        a new Local Preference attribute is attached to a route by the BGP
        protocol itself (for example, if a route is received through eBGP and
        therefore does not have such attribute). Default: 100 (0 in pre-1.2.0
        versions of BIRD).
</descrip>

<sect1>Channel configuration
<label id="bgp-channel-config">

<p>BGP supports several AFIs and SAFIs over one connection. Every AFI/SAFI
announced to the peer corresponds to one channel. The table of supported AFI/SAFIs
together with their appropriate channels follows.

<table loc="h">
<tabular ca="l|l|l|r|r">
  <bf/Channel name/   | <bf/Table nettype/ | <bf/IGP table allowed/  | <bf/AFI/ | <bf/SAFI/
@<hline>
  <cf/ipv4/           | <cf/ipv4/          | <cf/ipv4/ and <cf/ipv6/ | 1        | 1
@ <cf/ipv6/           | <cf/ipv6/          | <cf/ipv4/ and <cf/ipv6/ | 2        | 1
@ <cf/ipv4 multicast/ | <cf/ipv4/          | <cf/ipv4/ and <cf/ipv6/ | 1        | 2
@ <cf/ipv6 multicast/ | <cf/ipv6/          | <cf/ipv4/ and <cf/ipv6/ | 2        | 2
@ <cf/ipv4 mpls/      | <cf/ipv4/          | <cf/ipv4/ and <cf/ipv6/ | 1        | 4
@ <cf/ipv6 mpls/      | <cf/ipv6/          | <cf/ipv4/ and <cf/ipv6/ | 2        | 4
@ <cf/vpn4 mpls/      | <cf/vpn4/          | <cf/ipv4/ and <cf/ipv6/ | 1        | 128
@ <cf/vpn6 mpls/      | <cf/vpn6/          | <cf/ipv4/ and <cf/ipv6/ | 2        | 128
@ <cf/vpn4 multicast/ | <cf/vpn4/          | <cf/ipv4/ and <cf/ipv6/ | 1        | 129
@ <cf/vpn6 multicast/ | <cf/vpn6/          | <cf/ipv4/ and <cf/ipv6/ | 2        | 129
@ <cf/flow4/          | <cf/flow4/         | ---                     | 1        | 133
@ <cf/flow6/          | <cf/flow6/         | ---                     | 2        | 133
</tabular>
</table>

<p>Due to <rfc id="8212">, external BGP protocol requires explicit configuration
of import and export policies (in contrast to other protocols, where default
policies of <cf/import all/ and <cf/export none/ are used in absence of explicit
configuration). Note that blanket policies like <cf/all/ or <cf/none/ can still
be used in explicit configuration.

<p>BGP channels have additional config options (together with the common ones):

<descrip>
        <tag><label id="bgp-mandatory">mandatory <m/switch/</tag>
        When local and neighbor sets of configured AFI/SAFI pairs differ,
        capability negotiation ensures that a common subset is used. For
        mandatory channels their associated AFI/SAFI must be negotiated
        (i.e., also announced by the neighbor), otherwise BGP session
        negotiation fails with <it/'Required capability missing'/ error.
        Regardless, at least one AFI/SAFI must be negotiated in order to BGP
        session be successfully established. Default: off.

        <tag><label id="bgp-next-hop-keep">next hop keep <m/switch/|ibgp|ebgp</tag>
        Do not modify the Next Hop attribute and advertise the current one
        unchanged even in cases where our own local address should be used
        instead. This is necessary when the BGP speaker does not forward network
        traffic (route servers and some route reflectors) and also can be useful
        in some other cases (e.g. multihop EBGP sessions). Can be enabled for
        all routes, or just for routes received from IBGP / EBGP neighbors.
        Default: disabled for regular BGP, enabled for route servers,
        <cf/ibgp/ for route reflectors.

        <tag><label id="bgp-next-hop-self">next hop self <m/switch/|ibgp|ebgp</tag>
        Always advertise our own local address as a next hop, even in cases
        where the current Next Hop attribute should be used unchanged. This is
        sometimes used for routes propagated from EBGP to IBGP when IGP routing
        does not cover inter-AS links, therefore IP addreses of EBGP neighbors
        are not resolvable through IGP. Can be enabled for all routes, or just
        for routes received from IBGP / EBGP neighbors. Default: disabled.

        <tag><label id="bgp-next-hop-address">next hop address <m/ip/</tag>
        Specify which address to use when our own local address should be
        announced in the Next Hop attribute. Default: the source address of the
        BGP session (if acceptable), or the preferred address of an associated
        interface.

        <tag><label id="bgp-missing-lladdr">missing lladdr self|drop|ignore</tag>
        Next Hop attribute in BGP-IPv6 sometimes contains just the global IPv6
        address, but sometimes it has to contain both global and link-local IPv6
        addresses. This option specifies what to do if BIRD have to send both
        addresses but does not know link-local address. This situation might
        happen when routes from other protocols are exported to BGP, or when
        improper updates are received from BGP peers. <cf/self/ means that BIRD
        advertises its own local address instead. <cf/drop/ means that BIRD
        skips that prefixes and logs error. <cf/ignore/ means that BIRD ignores
        the problem and sends just the global address (and therefore forms
        improper BGP update). Default: <cf/self/, unless BIRD is configured as a
        route server (option <cf/rs client/), in that case default is <cf/ignore/,
        because route servers usually do not forward packets themselves.

        <tag><label id="bgp-gateway">gateway direct|recursive</tag>
        For received routes, their <cf/gw/ (immediate next hop) attribute is
        computed from received <cf/bgp_next_hop/ attribute. This option
        specifies how it is computed. Direct mode means that the IP address from
        <cf/bgp_next_hop/ is used and must be directly reachable. Recursive mode
        means that the gateway is computed by an IGP routing table lookup for
        the IP address from <cf/bgp_next_hop/. Note that there is just one level
        of indirection in recursive mode - the route obtained by the lookup must
        not be recursive itself, to prevent mutually recursive routes.

        Recursive mode is the behavior specified by the BGP
        standard. Direct mode is simpler, does not require any routes in a
        routing table, and was used in older versions of BIRD, but does not
        handle well nontrivial iBGP setups and multihop. Recursive mode is
        incompatible with <ref id="dsc-table-sorted" name="sorted tables">. Default:
        <cf/direct/ for direct sessions, <cf/recursive/ for multihop sessions.

        <tag><label id="bgp-igp-table">igp table <m/name/</tag>
        Specifies a table that is used as an IGP routing table. The type of this
        table must be as allowed in the table above. This option is allowed once
        for every allowed table type. Default: the same as the main table
        the channel is connected to (if eligible).

        <tag><label id="bgp-import-table">import table <m/switch/</tag>
        A BGP import table contains all received routes from given BGP neighbor,
        before application of import filters. It is also called <em/Adj-RIB-In/
        in BGP terminology. BIRD BGP by default operates without import tables,
        in which case received routes are just processed by import filters,
        accepted ones are stored in the master table, and the rest is forgotten.
        Enabling <cf/import table/ allows to store unprocessed routes, which can
        be examined later by <cf/show route/, and can be used to reconfigure
        import filters without full route refresh. Default: off.

        <tag><label id="bgp-export-table">export table <m/switch/</tag>
        A BGP export table contains all routes sent to given BGP neighbor, after
        application of export filters. It is also called <em/Adj-RIB-Out/ in BGP
        terminology. BIRD BGP by default operates without export tables, in
        which case routes from master table are just processed by export filters
        and then announced by BGP. Enabling <cf/export table/ allows to store
        routes after export filter processing, so they can be examined later by
        <cf/show route/, and can be used to eliminate unnecessary updates or
        withdraws. Default: off.

        <tag><label id="bgp-secondary">secondary <m/switch/</tag>
        Usually, if an export filter rejects a selected route, no other route is
        propagated for that network. This option allows to try the next route in
        order until one that is accepted is found or all routes for that network
        are rejected. This can be used for route servers that need to propagate
        different tables to each client but do not want to have these tables
        explicitly (to conserve memory). This option requires that the connected
        routing table is <ref id="dsc-table-sorted" name="sorted">. Default: off.

        <tag><label id="bgp-extended-next-hop">extended next hop <m/switch/</tag>
        BGP expects that announced next hops have the same address family as
        associated network prefixes. This option provides an extension to use
        IPv4 next hops with IPv6 prefixes and vice versa. For IPv4 / VPNv4
        channels, the behavior is controlled by the Extended Next Hop Encoding
        capability, as described in <rfc id="5549">. For IPv6 / VPNv6 channels,
        just IPv4-mapped IPv6 addresses are used, as described in
        <rfc id="4798"> and <rfc id="4659">. Default: off.

        <tag><label id="bgp-add-paths">add paths <m/switch/|rx|tx</tag>
        Standard BGP can propagate only one path (route) per destination network
        (usually the selected one). This option controls the add-path protocol
        extension, which allows to advertise any number of paths to a
        destination. Note that to be active, add-path has to be enabled on both
        sides of the BGP session, but it could be enabled separately for RX and
        TX direction. When active, all available routes accepted by the export
        filter are advertised to the neighbor. Default: off.

        <tag><label id="bgp-aigp">aigp <m/switch/|originate</tag>
        The BGP protocol does not use a common metric like other routing
        protocols, instead it uses a set of criteria for route selection
        consisting both overall AS path length and a distance to the nearest AS
        boundary router. Assuming that metrics of different autonomous systems
        are incomparable, once a route is propagated from an AS to a next one,
        the distance in the old AS does not matter.

        The AIGP extension (<rfc id="7311">) allows to propagate accumulated
        IGP metric (in the AIGP attribute) through both IBGP and EBGP links,
        computing total distance through multiple autonomous systems (assuming
        they use comparable IGP metric). The total AIGP metric is compared in
        the route selection process just after Local Preference comparison (and
        before AS path length comparison).

        This option controls whether AIGP attribute propagation is allowed on
        the session. Optionally, it can be set to <cf/originate/, which not only
        allows AIGP attribute propagation, but also new AIGP attributes are
        automatically attached to non-BGP routes with valid IGP metric (e.g.
        <cf/ospf_metric1/) as they are exported to the BGP session. Default:
        enabled for IBGP (and intra-confederation EBGP), disabled for regular
        EBGP.

        <tag><label id="bgp-cost">cost <m/number/</tag>
        When BGP <ref id="bgp-gateway" name="gateway mode"> is <cf/recursive/
        (mainly multihop IBGP sessions), then the distance to BGP next hop is
        based on underlying IGP metric. This option specifies the distance to
        BGP next hop for BGP sessions in direct gateway mode (mainly direct
        EBGP sessions).

        <tag><label id="bgp-graceful-restart-c">graceful restart <m/switch/</tag>
        Although BGP graceful restart is configured mainly by protocol-wide
        <ref id="bgp-graceful-restart" name="options">, it is possible to
        configure restarting role per AFI/SAFI pair by this channel option.
        The option is ignored if graceful restart is disabled by protocol-wide
        option. Default: off in aware mode, on in full mode.

        <tag><label id="bgp-long-lived-graceful-restart-c">long lived graceful restart <m/switch/</tag>
        BGP long-lived graceful restart is configured mainly by protocol-wide
        <ref id="bgp-long-lived-graceful-restart" name="options">, but the
        restarting role can be set per AFI/SAFI pair by this channel option.
        The option is ignored if long-lived graceful restart is disabled by
        protocol-wide option. Default: off in aware mode, on in full mode.

        <tag><label id="bgp-long-lived-stale-time-c">long lived stale time <m/number/</tag>
        Like previous graceful restart channel options, this option allows to
        set <ref id="bgp-long-lived-stale-time" name="long lived stale time">
        per AFI/SAFI pair instead of per protocol. Default: set by protocol-wide
        option.
</descrip>

<sect1>Attributes
<label id="bgp-attr">

<p>BGP defines several route attributes. Some of them (those marked with
`<tt/I/' in the table below) are available on internal BGP connections only,
some of them (marked with `<tt/O/') are optional.

<descrip>
        <tag><label id="rta-bgp-path">bgppath bgp_path</tag>
        Sequence of AS numbers describing the AS path the packet will travel
        through when forwarded according to the particular route. In case of
        internal BGP it doesn't contain the number of the local AS.

        <tag><label id="rta-bgp-local-pref">int bgp_local_pref [I]</tag>
        Local preference value used for selection among multiple BGP routes (see
        the selection rules above). It's used as an additional metric which is
        propagated through the whole local AS.

        <tag><label id="rta-bgp-med">int bgp_med [O]</tag>
        The Multiple Exit Discriminator of the route is an optional attribute
        which is used on external (inter-AS) links to convey to an adjacent AS
        the optimal entry point into the local AS. The received attribute is
        also propagated over internal BGP links. The attribute value is zeroed
        when a route is exported to an external BGP instance to ensure that the
        attribute received from a neighboring AS is not propagated to other
        neighboring ASes. A new value might be set in the export filter of an
        external BGP instance. See <rfc id="4451"> for further discussion of
        BGP MED attribute.

        <tag><label id="rta-bgp-origin">enum bgp_origin</tag>
        Origin of the route: either <cf/ORIGIN_IGP/ if the route has originated
        in an interior routing protocol or <cf/ORIGIN_EGP/ if it's been imported
        from the <tt>EGP</tt> protocol (nowadays it seems to be obsolete) or
        <cf/ORIGIN_INCOMPLETE/ if the origin is unknown.

        <tag><label id="rta-bgp-next-hop">ip bgp_next_hop</tag>
        Next hop to be used for forwarding of packets to this destination. On
        internal BGP connections, it's an address of the originating router if
        it's inside the local AS or a boundary router the packet will leave the
        AS through if it's an exterior route, so each BGP speaker within the AS
        has a chance to use the shortest interior path possible to this point.

        <tag><label id="rta-bgp-atomic-aggr">void bgp_atomic_aggr [O]</tag>
        This is an optional attribute which carries no value, but the sole
        presence of which indicates that the route has been aggregated from
        multiple routes by some router on the path from the originator.

        <tag><label id="rta-bgp-aggregator">void bgp_aggregator [O]</tag>
        This is an optional attribute specifying AS number and IP address of the
        BGP router that created the route by aggregating multiple BGP routes.
        Currently, the attribute is not accessible from filters.

        <tag><label id="rta-bgp-community">clist bgp_community [O]</tag>
        List of community values associated with the route. Each such value is a
        pair (represented as a <cf/pair/ data type inside the filters) of 16-bit
        integers, the first of them containing the number of the AS which
        defines the community and the second one being a per-AS identifier.
        There are lots of uses of the community mechanism, but generally they
        are used to carry policy information like "don't export to USA peers".
        As each AS can define its own routing policy, it also has a complete
        freedom about which community attributes it defines and what will their
        semantics be.

        <tag><label id="rta-bgp-ext-community">eclist bgp_ext_community [O]</tag>
        List of extended community values associated with the route. Extended
        communities have similar usage as plain communities, but they have an
        extended range (to allow 4B ASNs) and a nontrivial structure with a type
        field. Individual community values are represented using an <cf/ec/ data
        type inside the filters.

        <tag><label id="rta-bgp-large-community">lclist bgp_large_community [O]</tag>
        List of large community values associated with the route. Large BGP
        communities is another variant of communities, but contrary to extended
        communities they behave very much the same way as regular communities,
        just larger -- they are uniform untyped triplets of 32bit numbers.
        Individual community values are represented using an <cf/lc/ data type
        inside the filters.

        <tag><label id="rta-bgp-originator-id">quad bgp_originator_id [I, O]</tag>
        This attribute is created by the route reflector when reflecting the
        route and contains the router ID of the originator of the route in the
        local AS.

        <tag><label id="rta-bgp-cluster-list">clist bgp_cluster_list [I, O]</tag>
        This attribute contains a list of cluster IDs of route reflectors. Each
        route reflector prepends its cluster ID when reflecting the route.

        <tag><label id="rta-bgp-aigp">void bgp_aigp [O]</tag>
        This attribute contains accumulated IGP metric, which is a total
        distance to the destination through multiple autonomous systems.
        Currently, the attribute is not accessible from filters.
</descrip>

<sect1>Example
<label id="bgp-exam">

<p><code>
protocol bgp {
        local 198.51.100.14 as 65000;        # Use a private AS number
        neighbor 198.51.100.130 as 64496;    # Our neighbor ...
        multihop;                            # ... which is connected indirectly
        ipv4 {
                export filter {                      # We use non-trivial export rules
                        if source = RTS_STATIC then { # Export only static routes
                                # Assign our community
                                bgp_community.add((65000,64501));
                                # Artificially increase path length
                                # by advertising local AS number twice
                                if bgp_path ~ [= 65000 =] then
                                        bgp_path.prepend(65000);
                                accept;
                        }
                        reject;
                };
                import all;
                next hop self; # advertise this router as next hop
                igp table myigptable4; # IGP table for routes with IPv4 nexthops
                igp table myigptable6; # IGP table for routes with IPv6 nexthops
        };
        ipv6 {
                export filter mylargefilter; # We use a named filter
                import all;
                missing lladdr self;
                igp table myigptable4; # IGP table for routes with IPv4 nexthops
                igp table myigptable6; # IGP table for routes with IPv6 nexthops
        };
        ipv4 multicast {
                import all;
                export filter someotherfilter;
                table mymulticasttable4; # Another IPv4 table, dedicated for multicast
                igp table myigptable4;
        };
}
</code>


<sect>Device
<label id="device">

<p>The Device protocol is not a real routing protocol. It doesn't generate any
routes and it only serves as a module for getting information about network
interfaces from the kernel. This protocol supports no channel.

<p>Except for very unusual circumstances, you probably should include this
protocol in the configuration since almost all other protocols require network
interfaces to be defined for them to work with.

<sect1>Configuration
<label id="device-config">

<p><descrip>
        <tag><label id="device-scan-time">scan time <m/number/</tag>
        Time in seconds between two scans of the network interface list. On
        systems where we are notified about interface status changes
        asynchronously (such as newer versions of Linux), we need to scan the
        list only in order to avoid confusion by lost notification messages,
        so the default time is set to a large value.

        <tag><label id="device-iface">interface <m/pattern/ [, <m/.../]</tag>
        By default, the Device protocol handles all interfaces without any
        configuration. Interface definitions allow to specify optional
        parameters for specific interfaces. See <ref id="proto-iface"
        name="interface"> common option for detailed description. Currently only
        one interface option is available:

        <tag><label id="device-preferred">preferred <m/ip/</tag>
        If a network interface has more than one IP address, BIRD chooses one of
        them as a preferred one. Preferred IP address is used as source address
        for packets or announced next hop by routing protocols. Precisely, BIRD
        chooses one preferred IPv4 address, one preferred IPv6 address and one
        preferred link-local IPv6 address. By default, BIRD chooses the first
        found IP address as the preferred one.

        This option allows to specify which IP address should be preferred. May
        be used multiple times for different address classes (IPv4, IPv6, IPv6
        link-local). In all cases, an address marked by operating system as
        secondary cannot be chosen as the primary one.
</descrip>

<p>As the Device protocol doesn't generate any routes, it cannot have
any attributes. Example configuration looks like this:

<p><code>
protocol device {
        scan time 10;           # Scan the interfaces often
        interface "eth0" {
                preferred 192.168.1.1;
                preferred 2001:db8:1:10::1;
        };
}
</code>


<sect>Direct
<label id="direct">

<p>The Direct protocol is a simple generator of device routes for all the
directly connected networks according to the list of interfaces provided by the
kernel via the Device protocol. The Direct protocol supports both IPv4 and IPv6
channels; both can be configured simultaneously. It can also be configured with
<ref id="ip-sadr-routes" name="IPv6 SADR"> channel instead of regular IPv6
channel in order to be used together with SADR-enabled Babel protocol.

<p>The question is whether it is a good idea to have such device routes in BIRD
routing table. OS kernel usually handles device routes for directly connected
networks by itself so we don't need (and don't want) to export these routes to
the kernel protocol. OSPF protocol creates device routes for its interfaces
itself and BGP protocol is usually used for exporting aggregate routes. But the
Direct protocol is necessary for distance-vector protocols like RIP or Babel to
announce local networks.

<p>There are just few configuration options for the Direct protocol:

<p><descrip>
        <tag><label id="direct-iface">interface <m/pattern/ [, <m/.../]</tag>
        By default, the Direct protocol will generate device routes for all the
        interfaces available. If you want to restrict it to some subset of
        interfaces or addresses (e.g. if you're using multiple routing tables
        for policy routing and some of the policy domains don't contain all
        interfaces), just use this clause. See <ref id="proto-iface" name="interface">
        common option for detailed description. The Direct protocol uses
        extended interface clauses.

        <tag><label id="direct-check-link">check link <m/switch/</tag>
        If enabled, a hardware link state (reported by OS) is taken into
        consideration. Routes for directly connected networks are generated only
        if link up is reported and they are withdrawn when link disappears
        (e.g., an ethernet cable is unplugged). Default value is no.
</descrip>

<p>Direct device routes don't contain any specific attributes.

<p>Example config might look like this:

<p><code>
protocol direct {
        ipv4;
        ipv6;
        interface "-arc*", "*";         # Exclude the ARCnets
}
</code>


<sect>Kernel
<label id="krt">

<p>The Kernel protocol is not a real routing protocol. Instead of communicating
with other routers in the network, it performs synchronization of BIRD's routing
tables with the OS kernel. Basically, it sends all routing table updates to the
kernel and from time to time it scans the kernel tables to see whether some
routes have disappeared (for example due to unnoticed up/down transition of an
interface) or whether an `alien' route has been added by someone else (depending
on the <cf/learn/ switch, such routes are either ignored or accepted to our
table).

<p>Note that routes created by OS kernel itself, namely direct routes
representing IP subnets of associated interfaces, are not imported even with
<cf/learn/ enabled. You can use <ref id="direct" name="Direct protocol"> to
generate these direct routes.

<p>If your OS supports only a single routing table, you can configure only one
instance of the Kernel protocol. If it supports multiple tables (in order to
allow policy routing; such an OS is for example Linux), you can run as many
instances as you want, but each of them must be connected to a different BIRD
routing table and to a different kernel table.

<p>Because the kernel protocol is partially integrated with the connected
routing table, there are two limitations - it is not possible to connect more
kernel protocols to the same routing table and changing route destination
(gateway) in an export filter of a kernel protocol does not work. Both
limitations can be overcome using another routing table and the pipe protocol.

<p>The Kernel protocol supports both IPv4 and IPv6 channels; only one channel
can be configured in each protocol instance. On Linux, it also supports <ref
id="ip-sadr-routes" name="IPv6 SADR"> and <ref id="mpls-routes" name="MPLS">
channels.

<sect1>Configuration
<label id="krt-config">

<p><descrip>
        <tag><label id="krt-persist">persist <m/switch/</tag>
        Tell BIRD to leave all its routes in the routing tables when it exits
        (instead of cleaning them up).

        <tag><label id="krt-scan-time">scan time <m/number/</tag>
        Time in seconds between two consecutive scans of the kernel routing
        table.

        <tag><label id="krt-learn">learn <m/switch/</tag>
        Enable learning of routes added to the kernel routing tables by other
        routing daemons or by the system administrator. This is possible only on
        systems which support identification of route authorship.

        <tag><label id="krt-kernel-table">kernel table <m/number/</tag>
        Select which kernel table should this particular instance of the Kernel
        protocol work with. Available only on systems supporting multiple
        routing tables.

        <tag><label id="krt-metric">metric <m/number/</tag> (Linux)
        Use specified value as a kernel metric (priority) for all routes sent to
        the kernel. When multiple routes for the same network are in the kernel
        routing table, the Linux kernel chooses one with lower metric. Also,
        routes with different metrics do not clash with each other, therefore
        using dedicated metric value is a reliable way to avoid overwriting
        routes from other sources (e.g. kernel device routes). Metric 0 has a
        special meaning of undefined metric, in which either OS default is used,
        or per-route metric can be set using <cf/krt_metric/ attribute. Default:
        32.

        <tag><label id="krt-graceful-restart">graceful restart <m/switch/</tag>
        Participate in graceful restart recovery. If this option is enabled and
        a graceful restart recovery is active, the Kernel protocol will defer
        synchronization of routing tables until the end of the recovery. Note
        that import of kernel routes to BIRD is not affected.

        <tag><label id="krt-merge-paths">merge paths <M>switch</M> [limit <M>number</M>]</tag>
        Usually, only best routes are exported to the kernel protocol. With path
        merging enabled, both best routes and equivalent non-best routes are
        merged during export to generate one ECMP (equal-cost multipath) route
        for each network. This is useful e.g. for BGP multipath. Note that best
        routes are still pivotal for route export (responsible for most
        properties of resulting ECMP routes), while exported non-best routes are
        responsible just for additional multipath next hops. This option also
        allows to specify a limit on maximal number of nexthops in one route. By
        default, multipath merging is disabled. If enabled, default value of the
        limit is 16.
</descrip>

<sect1>Attributes
<label id="krt-attr">

<p>The Kernel protocol defines several attributes. These attributes are
translated to appropriate system (and OS-specific) route attributes. We support
these attributes:

<descrip>
        <tag><label id="rta-krt-source">int krt_source</tag>
        The original source of the imported kernel route. The value is
        system-dependent. On Linux, it is a value of the protocol field of the
        route. See /etc/iproute2/rt_protos for common values. On BSD, it is
        based on STATIC and PROTOx flags. The attribute is read-only.

        <tag><label id="rta-krt-metric">int krt_metric</tag> (Linux)
        The kernel metric of the route. When multiple same routes are in a
        kernel routing table, the Linux kernel chooses one with lower metric.
        Note that preferred way to set kernel metric is to use protocol option
        <cf/metric/, unless per-route metric values are needed.

        <tag><label id="rta-krt-prefsrc">ip krt_prefsrc</tag> (Linux)
        The preferred source address. Used in source address selection for
        outgoing packets. Has to be one of the IP addresses of the router.

        <tag><label id="rta-krt-realm">int krt_realm</tag> (Linux)
        The realm of the route. Can be used for traffic classification.

        <tag><label id="rta-krt-scope">int krt_scope</tag> (Linux IPv4)
        The scope of the route. Valid values are 0-254, although Linux kernel
        may reject some values depending on route type and nexthop. It is
        supposed to represent `indirectness' of the route, where nexthops of
        routes are resolved through routes with a higher scope, but in current
        kernels anything below <it/link/ (253) is treated as <it/global/ (0).
        When not present, global scope is implied for all routes except device
        routes, where link scope is used by default.
</descrip>

<p>In Linux, there is also a plenty of obscure route attributes mostly focused
on tuning TCP performance of local connections. BIRD supports most of these
attributes, see Linux or iproute2 documentation for their meaning. Attributes
<cf/krt_lock_*/ and <cf/krt_feature_*/ have type bool, others have type int.
Supported attributes are:

<cf/krt_mtu/, <cf/krt_lock_mtu/, <cf/krt_window/, <cf/krt_lock_window/,
<cf/krt_rtt/, <cf/krt_lock_rtt/, <cf/krt_rttvar/, <cf/krt_lock_rttvar/,
<cf/krt_sstresh/, <cf/krt_lock_sstresh/, <cf/krt_cwnd/, <cf/krt_lock_cwnd/,
<cf/krt_advmss/, <cf/krt_lock_advmss/, <cf/krt_reordering/, <cf/krt_lock_reordering/,
<cf/krt_hoplimit/, <cf/krt_lock_hoplimit/, <cf/krt_rto_min/, <cf/krt_lock_rto_min/,
<cf/krt_initcwnd/, <cf/krt_initrwnd/, <cf/krt_quickack/,
<cf/krt_feature_ecn/, <cf/krt_feature_allfrag/

<sect1>Example
<label id="krt-exam">

<p>A simple configuration can look this way:

<p><code>
protocol kernel {
        export all;
}
</code>

<p>Or for a system with two routing tables:

<p><code>
protocol kernel {               # Primary routing table
        learn;                  # Learn alien routes from the kernel
        persist;                # Don't remove routes on bird shutdown
        scan time 10;           # Scan kernel routing table every 10 seconds
        ipv4 {
                import all;
                export all;
        };
}

protocol kernel {               # Secondary routing table
        kernel table 100;
        ipv4 {
                table auxtable;
                export all;
        };
}
</code>


<sect>MRT
<label id="mrt">

<sect1>Introduction
<label id="mrt-intro">

<p>The MRT protocol is a component responsible for handling the Multi-Threaded
Routing Toolkit (MRT) routing information export format, which is mainly used
for collecting and analyzing of routing information from BGP routers. The MRT
protocol can be configured to do periodic dumps of routing tables, created MRT
files can be analyzed later by other tools. Independent MRT table dumps can also
be requested from BIRD client. There is also a feature to save incoming BGP
messages in MRT files, but it is controlled by <ref id="proto-mrtdump"
name="mrtdump"> options independently of MRT protocol, although that might
change in the future.

BIRD implements the main MRT format specification as defined in <rfc id="6396">
and the ADD_PATH extension (<rfc id="8050">).

<sect1>Configuration
<label id="mrt-config">

<p>MRT configuration consists of several statements describing routing table
dumps. Multiple independent periodic dumps can be done as multiple MRT protocol
instances. The MRT protocol does not use channels. There are two mandatory
statements: <cf/filename/ and <cf/period/.

The behavior can be modified by following configuration parameters:

<descrip>
        <tag><label id="mrt-table">table <m/name/ | "<m/pattern/"</tag>
        Specify a routing table (or a set of routing tables described by a
        wildcard pattern) that are to be dumped by the MRT protocol instance.
        Default: the master table.

        <tag><label id="mrt-filter">filter { <m/filter commands/ }</tag>
        The MRT protocol allows to specify a filter that is applied to routes as
        they are dumped. Rejected routes are ignored and not saved to the MRT
        dump file. Default: no filter.

        <tag><label id="mrt-where">where <m/filter expression/</tag>
        An alternative way to specify a filter for the MRT protocol.

        <tag><label id="mrt-filename">filename "<m/filename/"</tag>
        Specify a filename for MRT dump files. The filename may contain time
        format sequences with <it/strftime(3)/ notation (see <it/man strftime/
        for details), there is also a sequence "%N" that is expanded to the name
        of dumped table. Therefore, each periodic dump of each table can be
        saved to a different file. Mandatory, see example below.

        <tag><label id="mrt-period">period <m/number/</tag>
        Specify the time interval (in seconds) between periodic dumps.
        Mandatory.

        <tag><label id="mrt-always-add-path">always add path <m/switch/</tag>
        The MRT format uses special records (specified in <rfc id="8050">) for
        routes received using BGP ADD_PATH extension to keep Path ID, while
        other routes use regular records. This has advantage of better
        compatibility with tools that do not know special records, but it loses
        information about which route is the best route. When this option is
        enabled, both ADD_PATH and non-ADD_PATH routes are stored in ADD_PATH
        records and order of routes for network is preserved. Default: disabled.
</descrip>

<sect1>Example
<label id="mrt-exam">

<p><code>
protocol mrt {
        table "tab*";
        where source = RTS_BGP;
        filename "/var/log/bird/%N_%F_%T.mrt";
        period 300;
}
</code>


<sect>OSPF
<label id="ospf">

<sect1>Introduction
<label id="ospf-intro">

<p>Open Shortest Path First (OSPF) is a quite complex interior gateway
protocol. The current IPv4 version (OSPFv2) is defined in <rfc id="2328"> and
the current IPv6 version (OSPFv3) is defined in <rfc id="5340"> It's a link
state (a.k.a. shortest path first) protocol -- each router maintains a database
describing the autonomous system's topology. Each participating router has an
identical copy of the database and all routers run the same algorithm
calculating a shortest path tree with themselves as a root. OSPF chooses the
least cost path as the best path.

<p>In OSPF, the autonomous system can be split to several areas in order to
reduce the amount of resources consumed for exchanging the routing information
and to protect the other areas from incorrect routing data. Topology of the area
is hidden to the rest of the autonomous system.

<p>Another very important feature of OSPF is that it can keep routing information
from other protocols (like Static or BGP) in its link state database as external
routes. Each external route can be tagged by the advertising router, making it
possible to pass additional information between routers on the boundary of the
autonomous system.

<p>OSPF quickly detects topological changes in the autonomous system (such as
router interface failures) and calculates new loop-free routes after a short
period of convergence. Only a minimal amount of routing traffic is involved.

<p>Each router participating in OSPF routing periodically sends Hello messages
to all its interfaces. This allows neighbors to be discovered dynamically. Then
the neighbors exchange theirs parts of the link state database and keep it
identical by flooding updates. The flooding process is reliable and ensures that
each router detects all changes.

<sect1>Configuration
<label id="ospf-config">

<p>First, the desired OSPF version can be specified by using <cf/ospf v2/ or
<cf/ospf v3/ as a protocol type. By default, OSPFv2 is used. In the main part of
configuration, there can be multiple definitions of OSPF areas, each with a
different id. These definitions includes many other switches and multiple
definitions of interfaces. Definition of interface may contain many switches and
constant definitions and list of neighbors on nonbroadcast networks.

<p>OSPFv2 needs one IPv4 channel. OSPFv3 needs either one IPv6 channel, or one
IPv4 channel (<rfc id="5838">). Therefore, it is possible to use OSPFv3 for both
IPv4 and Pv6 routing, but it is necessary to have two protocol instances anyway.
If no channel is configured, appropriate channel is defined with default
parameters.

<code>
protocol ospf [v2|v3] &lt;name&gt; {
        rfc1583compat &lt;switch&gt;;
        rfc5838 &lt;switch&gt;;
        instance id &lt;num&gt;;
        stub router &lt;switch&gt;;
        tick &lt;num&gt;;
        ecmp &lt;switch&gt; [limit &lt;num&gt;];
        merge external &lt;switch&gt;;
        graceful restart &lt;switch&gt;|aware;
        graceful restart time &lt;num&gt;;
        area &lt;id&gt; {
                stub;
                nssa;
                summary &lt;switch&gt;;
                default nssa &lt;switch&gt;;
                default cost &lt;num&gt;;
                default cost2 &lt;num&gt;;
                translator &lt;switch&gt;;
                translator stability &lt;num&gt;;

                networks {
                        &lt;prefix&gt;;
                        &lt;prefix&gt; hidden;
                }
                external {
                        &lt;prefix&gt;;
                        &lt;prefix&gt; hidden;
                        &lt;prefix&gt; tag &lt;num&gt;;
                }
                stubnet &lt;prefix&gt;;
                stubnet &lt;prefix&gt; {
                        hidden &lt;switch&gt;;
                        summary &lt;switch&gt;;
                        cost &lt;num&gt;;
                }
                interface &lt;interface pattern&gt; [instance &lt;num&gt;] {
                        cost &lt;num&gt;;
                        stub &lt;switch&gt;;
                        hello &lt;num&gt;;
                        poll &lt;num&gt;;
                        retransmit &lt;num&gt;;
                        priority &lt;num&gt;;
                        wait &lt;num&gt;;
                        dead count &lt;num&gt;;
                        dead &lt;num&gt;;
                        secondary &lt;switch&gt;;
                        rx buffer [normal|large|&lt;num&gt;];
                        tx length &lt;num&gt;;
                        type [broadcast|bcast|pointopoint|ptp|
                                nonbroadcast|nbma|pointomultipoint|ptmp];
                        link lsa suppression &lt;switch&gt;;
                        strict nonbroadcast &lt;switch&gt;;
                        real broadcast &lt;switch&gt;;
                        ptp netmask &lt;switch&gt;;
                        check link &lt;switch&gt;;
                        bfd &lt;switch&gt;;
                        ecmp weight &lt;num&gt;;
                        ttl security [&lt;switch&gt;; | tx only]
                        tx class|dscp &lt;num&gt;;
                        tx priority &lt;num&gt;;
                        authentication none|simple|cryptographic;
                        password "&lt;text&gt;";
                        password "&lt;text&gt;" {
                                id &lt;num&gt;;
                                generate from "&lt;date&gt;";
                                generate to "&lt;date&gt;";
                                accept from "&lt;date&gt;";
                                accept to "&lt;date&gt;";
                                from "&lt;date&gt;";
                                to "&lt;date&gt;";
                                algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
                        };
                        neighbors {
                                &lt;ip&gt;;
                                &lt;ip&gt; eligible;
                        };
                };
                virtual link &lt;id&gt; [instance &lt;num&gt;] {
                        hello &lt;num&gt;;
                        retransmit &lt;num&gt;;
                        wait &lt;num&gt;;
                        dead count &lt;num&gt;;
                        dead &lt;num&gt;;
                        authentication none|simple|cryptographic;
                        password "&lt;text&gt;";
                        password "&lt;text&gt;" {
                                id &lt;num&gt;;
                                generate from "&lt;date&gt;";
                                generate to "&lt;date&gt;";
                                accept from "&lt;date&gt;";
                                accept to "&lt;date&gt;";
                                from "&lt;date&gt;";
                                to "&lt;date&gt;";
                                algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
                        };
                };
        };
}
</code>

<descrip>
        <tag><label id="ospf-rfc1583compat">rfc1583compat <M>switch</M></tag>
        This option controls compatibility of routing table calculation with
        <rfc id="1583">. Default value is no.

        <tag><label id="ospf-rfc5838">rfc5838 <m/switch/</tag>
        Basic OSPFv3 is limited to IPv6 unicast routing. The <rfc id="5838">
        extension defines support for more address families (IPv4, IPv6, both
        unicast and multicast). The extension is enabled by default, but can be
        disabled if necessary, as it restricts the range of available instance
        IDs. Default value is yes.

        <tag><label id="ospf-instance-id">instance id <m/num/</tag>
        When multiple OSPF protocol instances are active on the same links, they
        should use different instance IDs to distinguish their packets. Although
        it could be done on per-interface basis, it is often preferred to set
        one instance ID to whole OSPF domain/topology (e.g., when multiple
        instances are used to represent separate logical topologies on the same
        physical network). This option specifies the instance ID for all
        interfaces of the OSPF instance, but can be overridden by
        <cf/interface/ option. Default value is 0 unless OSPFv3-AF extended
        address families are used, see <rfc id="5838"> for that case.

        <tag><label id="ospf-stub-router">stub router <M>switch</M></tag>
        This option configures the router to be a stub router, i.e., a router
        that participates in the OSPF topology but does not allow transit
        traffic. In OSPFv2, this is implemented by advertising maximum metric
        for outgoing links. In OSPFv3, the stub router behavior is announced by
        clearing the R-bit in the router LSA. See <rfc id="6987"> for details.
        Default value is no.

        <tag><label id="ospf-tick">tick <M>num</M></tag>
        The routing table calculation and clean-up of areas' databases is not
        performed when a single link state change arrives. To lower the CPU
        utilization, it's processed later at periodical intervals of <m/num/
        seconds. The default value is 1.

        <tag><label id="ospf-ecmp">ecmp <M>switch</M> [limit <M>number</M>]</tag>
        This option specifies whether OSPF is allowed to generate ECMP
        (equal-cost multipath) routes. Such routes are used when there are
        several directions to the destination, each with the same (computed)
        cost. This option also allows to specify a limit on maximum number of
        nexthops in one route. By default, ECMP is enabled if supported by
        Kernel. Default value of the limit is 16.

        <tag><label id="ospf-merge-external">merge external <M>switch</M></tag>
        This option specifies whether OSPF should merge external routes from
        different routers/LSAs for the same destination. When enabled together
        with <cf/ecmp/, equal-cost external routes will be combined to multipath
        routes in the same way as regular routes. When disabled, external routes
        from different LSAs are treated as separate even if they represents the
        same destination. Default value is no.

        <tag><label id="ospf-graceful-restart">graceful restart <m/switch/|aware</tag>
        When an OSPF instance is restarted, neighbors break adjacencies and
        recalculate their routing tables, which disrupts packet forwarding even
        when the forwarding plane of the restarting router remains intact.
        <rfc id="3623"> specifies a graceful restart mechanism to alleviate this
        issue. For OSPF graceful restart, restarting router originates
        Grace-LSAs, announcing intent to do graceful restart. Neighbors
        receiving these LSAs enter helper mode, in which they ignore breakdown
        of adjacencies, behave as if nothing is happening and keep old routes.
        When adjacencies are reestablished, the restarting router flushes
        Grace-LSAs and graceful restart is ended.

        This option controls the graceful restart mechanism. It has three
        states: Disabled, when no support is provided. Aware, when graceful
        restart helper mode is supported, but no local graceful restart is
        allowed (i.e. helper-only role). Enabled, when the full graceful restart
        support is provided (i.e. both restarting and helper role). Note that
        proper support for local graceful restart requires also configuration of
        other protocols. Default: aware.

        <tag><label id="ospf-graceful-restart-time">graceful restart time <m/num/</tag>
        The restart time is announced in the Grace-LSA and specifies how long
        neighbors should wait for proper end of the graceful restart before
        exiting helper mode prematurely. Default: 120 seconds.

        <tag><label id="ospf-area">area <M>id</M></tag>
        This defines an OSPF area with given area ID (an integer or an IPv4
        address, similarly to a router ID). The most important area is the
        backbone (ID 0) to which every other area must be connected.

        <tag><label id="ospf-stub">stub</tag>
        This option configures the area to be a stub area. External routes are
        not flooded into stub areas. Also summary LSAs can be limited in stub
        areas (see option <cf/summary/). By default, the area is not a stub
        area.

        <tag><label id="ospf-nssa">nssa</tag>
        This option configures the area to be a NSSA (Not-So-Stubby Area). NSSA
        is a variant of a stub area which allows a limited way of external route
        propagation. Global external routes are not propagated into a NSSA, but
        an external route can be imported into NSSA as a (area-wide) NSSA-LSA
        (and possibly translated and/or aggregated on area boundary). By
        default, the area is not NSSA.

        <tag><label id="ospf-summary">summary <M>switch</M></tag>
        This option controls propagation of summary LSAs into stub or NSSA
        areas. If enabled, summary LSAs are propagated as usual, otherwise just
        the default summary route (0.0.0.0/0) is propagated (this is sometimes
        called totally stubby area). If a stub area has more area boundary
        routers, propagating summary LSAs could lead to more efficient routing
        at the cost of larger link state database. Default value is no.

        <tag><label id="ospf-default-nssa">default nssa <M>switch</M></tag>
        When <cf/summary/ option is enabled, default summary route is no longer
        propagated to the NSSA. In that case, this option allows to originate
        default route as NSSA-LSA to the NSSA. Default value is no.

        <tag><label id="ospf-default-cost">default cost <M>num</M></tag>
        This option controls the cost of a default route propagated to stub and
        NSSA areas. Default value is 1000.

        <tag><label id="ospf-default-cost2">default cost2 <M>num</M></tag>
        When a default route is originated as NSSA-LSA, its cost can use either
        type 1 or type 2 metric. This option allows to specify the cost of a
        default route in type 2 metric. By default, type 1 metric (option
        <cf/default cost/) is used.

        <tag><label id="ospf-translator">translator <M>switch</M></tag>
        This option controls translation of NSSA-LSAs into external LSAs. By
        default, one translator per NSSA is automatically elected from area
        boundary routers. If enabled, this area boundary router would
        unconditionally translate all NSSA-LSAs regardless of translator
        election. Default value is no.

        <tag><label id="ospf-translator-stability">translator stability <M>num</M></tag>
        This option controls the translator stability interval (in seconds).
        When the new translator is elected, the old one keeps translating until
        the interval is over. Default value is 40.

        <tag><label id="ospf-networks">networks { <m/set/ }</tag>
        Definition of area IP ranges. This is used in summary LSA origination.
        Hidden networks are not propagated into other areas.

        <tag><label id="ospf-external">external { <m/set/ }</tag>
        Definition of external area IP ranges for NSSAs. This is used for
        NSSA-LSA translation. Hidden networks are not translated into external
        LSAs. Networks can have configured route tag.

        <tag><label id="ospf-stubnet">stubnet <m/prefix/ { <m/options/ }</tag>
        Stub networks are networks that are not transit networks between OSPF
        routers. They are also propagated through an OSPF area as a part of a
        link state database. By default, BIRD generates a stub network record
        for each primary network address on each OSPF interface that does not
        have any OSPF neighbors, and also for each non-primary network address
        on each OSPF interface. This option allows to alter a set of stub
        networks propagated by this router.

        Each instance of this option adds a stub network with given network
        prefix to the set of propagated stub network, unless option <cf/hidden/
        is used. It also suppresses default stub networks for given network
        prefix. When option <cf/summary/ is used, also default stub networks
        that are subnetworks of given stub network are suppressed. This might be
        used, for example, to aggregate generated stub networks.

        <tag><label id="ospf-iface">interface <M>pattern</M> [instance <m/num/]</tag>
        Defines that the specified interfaces belong to the area being defined.
        See <ref id="proto-iface" name="interface"> common option for detailed
        description. In OSPFv2, extended interface clauses are used, because
        each network prefix is handled as a separate virtual interface.

        You can specify alternative instance ID for the interface definition,
        therefore it is possible to have several instances of that interface
        with different options or even in different areas. For OSPFv2, instance
        ID support is an extension (<rfc id="6549">) and is supposed to be set
        per-protocol. For OSPFv3, it is an integral feature.

        <tag><label id="ospf-virtual-link">virtual link <M>id</M> [instance <m/num/]</tag>
        Virtual link to router with the router id. Virtual link acts as a
        point-to-point interface belonging to backbone. The actual area is used
        as a transport area. This item cannot be in the backbone. Like with
        <cf/interface/ option, you could also use several virtual links to one
        destination with different instance IDs.

        <tag><label id="ospf-cost">cost <M>num</M></tag>
        Specifies output cost (metric) of an interface. Default value is 10.

        <tag><label id="ospf-stub-iface">stub <M>switch</M></tag>
        If set to interface it does not listen to any packet and does not send
        any hello. Default value is no.

        <tag><label id="ospf-hello">hello <M>num</M></tag>
        Specifies interval in seconds between sending of Hello messages. Beware,
        all routers on the same network need to have the same hello interval.
        Default value is 10.

        <tag><label id="ospf-poll">poll <M>num</M></tag>
        Specifies interval in seconds between sending of Hello messages for some
        neighbors on NBMA network. Default value is 20.

        <tag><label id="ospf-retransmit">retransmit <M>num</M></tag>
        Specifies interval in seconds between retransmissions of unacknowledged
        updates. Default value is 5.

        <tag><label id="ospf-transmit-delay">transmit delay <M>num</M></tag>
        Specifies estimated transmission delay of link state updates send over
        the interface. The value is added to LSA age of LSAs propagated through
        it. Default value is 1.

        <tag><label id="ospf-priority">priority <M>num</M></tag>
        On every multiple access network (e.g., the Ethernet) Designated Router
        and Backup Designated router are elected. These routers have some special
        functions in the flooding process. Higher priority increases preferences
        in this election. Routers with priority 0 are not eligible. Default
        value is 1.

        <tag><label id="ospf-wait">wait <M>num</M></tag>
        After start, router waits for the specified number of seconds between
        starting election and building adjacency. Default value is 4*<m/hello/.

        <tag><label id="ospf-dead-count">dead count <M>num</M></tag>
        When the router does not receive any messages from a neighbor in
        <m/dead count/*<m/hello/ seconds, it will consider the neighbor down.

        <tag><label id="ospf-dead">dead <M>num</M></tag>
        When the router does not receive any messages from a neighbor in
        <m/dead/ seconds, it will consider the neighbor down. If both directives
        <cf/dead count/ and <cf/dead/ are used, <cf/dead/ has precedence.

        <tag><label id="ospf-rx-buffer">rx buffer <M>num</M></tag>
        This option allows to specify the size of buffers used for packet
        processing. The buffer size should be bigger than maximal size of any
        packets. By default, buffers are dynamically resized as needed, but a
        fixed value could be specified. Value <cf/large/ means maximal allowed
        packet size - 65535.

        <tag><label id="ospf-tx-length">tx length <M>num</M></tag>
        Transmitted OSPF messages that contain large amount of information are
        segmented to separate OSPF packets to avoid IP fragmentation. This
        option specifies the soft ceiling for the length of generated OSPF
        packets. Default value is the MTU of the network interface. Note that
        larger OSPF packets may still be generated if underlying OSPF messages
        cannot be splitted (e.g. when one large LSA is propagated).

        <tag><label id="ospf-type-bcast">type broadcast|bcast</tag>
        BIRD detects a type of a connected network automatically, but sometimes
        it's convenient to force use of a different type manually. On broadcast
        networks (like ethernet), flooding and Hello messages are sent using
        multicasts (a single packet for all the neighbors). A designated router
        is elected and it is responsible for synchronizing the link-state
        databases and originating network LSAs. This network type cannot be used
        on physically NBMA networks and on unnumbered networks (networks without
        proper IP prefix).

        <tag><label id="ospf-type-ptp">type pointopoint|ptp</tag>
        Point-to-point networks connect just 2 routers together. No election is
        performed and no network LSA is originated, which makes it simpler and
        faster to establish. This network type is useful not only for physically
        PtP ifaces (like PPP or tunnels), but also for broadcast networks used
        as PtP links. This network type cannot be used on physically NBMA
        networks.

        <tag><label id="ospf-type-nbma">type nonbroadcast|nbma</tag>
        On NBMA networks, the packets are sent to each neighbor separately
        because of lack of multicast capabilities. Like on broadcast networks,
        a designated router is elected, which plays a central role in propagation
        of LSAs. This network type cannot be used on unnumbered networks.

        <tag><label id="ospf-type-ptmp">type pointomultipoint|ptmp</tag>
        This is another network type designed to handle NBMA networks. In this
        case the NBMA network is treated as a collection of PtP links. This is
        useful if not every pair of routers on the NBMA network has direct
        communication, or if the NBMA network is used as an (possibly
        unnumbered) PtP link.

        <tag><label id="ospf-link-lsa-suppression">link lsa suppression <m/switch/</tag>
        In OSPFv3, link LSAs are generated for each link, announcing link-local
        IPv6 address of the router to its local neighbors. These are useless on
        PtP or PtMP networks and this option allows to suppress the link LSA
        origination for such interfaces. The option is ignored on other than PtP
        or PtMP interfaces. Default value is no.

        <tag><label id="ospf-strict-nonbroadcast">strict nonbroadcast <m/switch/</tag>
        If set, don't send hello to any undefined neighbor. This switch is
        ignored on other than NBMA or PtMP interfaces. Default value is no.

        <tag><label id="ospf-real-broadcast">real broadcast <m/switch/</tag>
        In <cf/type broadcast/ or <cf/type ptp/ network configuration, OSPF
        packets are sent as IP multicast packets. This option changes the
        behavior to using old-fashioned IP broadcast packets. This may be useful
        as a workaround if IP multicast for some reason does not work or does
        not work reliably. This is a non-standard option and probably is not
        interoperable with other OSPF implementations. Default value is no.

        <tag><label id="ospf-ptp-netmask">ptp netmask <m/switch/</tag>
        In <cf/type ptp/ network configurations, OSPFv2 implementations should
        ignore received netmask field in hello packets and should send hello
        packets with zero netmask field on unnumbered PtP links. But some OSPFv2
        implementations perform netmask checking even for PtP links. This option
        specifies whether real netmask will be used in hello packets on <cf/type
        ptp/ interfaces. You should ignore this option unless you meet some
        compatibility problems related to this issue. Default value is no for
        unnumbered PtP links, yes otherwise.

        <tag><label id="ospf-check-link">check link <M>switch</M></tag>
        If set, a hardware link state (reported by OS) is taken into consideration.
        When a link disappears (e.g. an ethernet cable is unplugged), neighbors
        are immediately considered unreachable and only the address of the iface
        (instead of whole network prefix) is propagated. It is possible that
        some hardware drivers or platforms do not implement this feature.
        Default value is yes.

        <tag><label id="ospf-bfd">bfd <M>switch</M></tag>
        OSPF could use BFD protocol as an advisory mechanism for neighbor
        liveness and failure detection. If enabled, BIRD setups a BFD session
        for each OSPF neighbor and tracks its liveness by it. This has an
        advantage of an order of magnitude lower detection times in case of
        failure. Note that BFD protocol also has to be configured, see
        <ref id="bfd" name="BFD"> section for details. Default value is no.

        <tag><label id="ospf-ttl-security">ttl security [<m/switch/ | tx only]</tag>
        TTL security is a feature that protects routing protocols from remote
        spoofed packets by using TTL 255 instead of TTL 1 for protocol packets
        destined to neighbors. Because TTL is decremented when packets are
        forwarded, it is non-trivial to spoof packets with TTL 255 from remote
        locations. Note that this option would interfere with OSPF virtual
        links.

        If this option is enabled, the router will send OSPF packets with TTL
        255 and drop received packets with TTL less than 255. If this option si
        set to <cf/tx only/, TTL 255 is used for sent packets, but is not
        checked for received packets. Default value is no.

        <tag><label id="ospf-tx-class">tx class|dscp|priority <m/num/</tag>
        These options specify the ToS/DiffServ/Traffic class/Priority of the
        outgoing OSPF packets. See <ref id="proto-tx-class" name="tx class"> common
        option for detailed description.

        <tag><label id="ospf-ecmp-weight">ecmp weight <M>num</M></tag>
        When ECMP (multipath) routes are allowed, this value specifies a
        relative weight used for nexthops going through the iface. Allowed
        values are 1-256. Default value is 1.

        <tag><label id="ospf-auth-none">authentication none</tag>
        No passwords are sent in OSPF packets. This is the default value.

        <tag><label id="ospf-auth-simple">authentication simple</tag>
        Every packet carries 8 bytes of password. Received packets lacking this
        password are ignored. This authentication mechanism is very weak.
        This option is not available in OSPFv3.

        <tag><label id="ospf-auth-cryptographic">authentication cryptographic</tag>
        An authentication code is appended to every packet. The specific
        cryptographic algorithm is selected by option <cf/algorithm/ for each
        key. The default cryptographic algorithm for OSPFv2 keys is Keyed-MD5
        and for OSPFv3 keys is HMAC-SHA-256. Passwords are not sent open via
        network, so this mechanism is quite secure. Packets can still be read by
        an attacker.

        <tag><label id="ospf-pass">password "<M>text</M>"</tag>
        Specifies a password used for authentication. See
        <ref id="proto-pass" name="password"> common option for detailed
        description.

        <tag><label id="ospf-neighbors">neighbors { <m/set/ } </tag>
        A set of neighbors to which Hello messages on NBMA or PtMP networks are
        to be sent. For NBMA networks, some of them could be marked as eligible.
        In OSPFv3, link-local addresses should be used, using global ones is
        possible, but it is nonstandard and might be problematic. And definitely,
        link-local and global addresses should not be mixed.
</descrip>

<sect1>Attributes
<label id="ospf-attr">

<p>OSPF defines four route attributes. Each internal route has a <cf/metric/.

<p>Metric is ranging from 1 to infinity (65535). External routes use
<cf/metric type 1/ or <cf/metric type 2/. A <cf/metric of type 1/ is comparable
with internal <cf/metric/, a <cf/metric of type 2/ is always longer than any
<cf/metric of type 1/ or any <cf/internal metric/. <cf/Internal metric/ or
<cf/metric of type 1/ is stored in attribute <cf/ospf_metric1/, <cf/metric type
2/ is stored in attribute <cf/ospf_metric2/.

When both metrics are specified then <cf/metric of type 2/ is used. This is
relevant e.g. when a type 2 external route is propagated from one OSPF domain to
another and <cf/ospf_metric1/ is an internal distance to the original ASBR,
while <cf/ospf_metric2/ stores the type 2 metric. Note that in such cases if
<cf/ospf_metric1/ is non-zero then <cf/ospf_metric2/ is increased by one to
ensure monotonicity of metric, as internal distance is reset to zero when an
external route is announced.

<p>Each external route can also carry attribute <cf/ospf_tag/ which is a 32-bit
integer which is used when exporting routes to other protocols; otherwise, it
doesn't affect routing inside the OSPF domain at all. The fourth attribute
<cf/ospf_router_id/ is a router ID of the router advertising that route /
network. This attribute is read-only. Default is <cf/ospf_metric2 = 10000/ and
<cf/ospf_tag = 0/.

<sect1>Example
<label id="ospf-exam">

<p><code>
protocol ospf MyOSPF {
        ipv4 {
                export filter {
                        if source = RTS_BGP then {
                                ospf_metric1 = 100;
                                accept;
                        }
                        reject;
                };
        };
        area 0.0.0.0 {
                interface "eth*" {
                        cost 11;
                        hello 15;
                        priority 100;
                        retransmit 7;
                        authentication simple;
                        password "aaa";
                };
                interface "ppp*" {
                        cost 100;
                        authentication cryptographic;
                        password "abc" {
                                id 1;
                                generate to "22-04-2003 11:00:06";
                                accept from "17-01-2001 12:01:05";
                                algorithm hmac sha384;
                        };
                        password "def" {
                                id 2;
                                generate to "22-07-2005 17:03:21";
                                accept from "22-02-2001 11:34:06";
                                algorithm hmac sha512;
                        };
                };
                interface "arc0" {
                        cost 10;
                        stub yes;
                };
                interface "arc1";
        };
        area 120 {
                stub yes;
                networks {
                        172.16.1.0/24;
                        172.16.2.0/24 hidden;
                }
                interface "-arc0" , "arc*" {
                        type nonbroadcast;
                        authentication none;
                        strict nonbroadcast yes;
                        wait 120;
                        poll 40;
                        dead count 8;
                        neighbors {
                                192.168.120.1 eligible;
                                192.168.120.2;
                                192.168.120.10;
                        };
                };
        };
}
</code>

<sect>Perf
<label id="perf">

<sect1>Introduction
<label id="perf-intro">

<p>The Perf protocol is a generator of fake routes together with a time measurement
framework. Its purpose is to check BIRD performance and to benchmark filters.

<p>Import mode of this protocol runs in several steps. In each step, it generates 2^x routes,
imports them into the appropriate table and withdraws them. The exponent x is configurable.
It runs the benchmark several times for the same x, then it increases x by one
until it gets too high, then it stops.

<p>Export mode of this protocol repeats route refresh from table and measures how long it takes.

<p>Output data is logged on info level. There is a Perl script <cf>proto/perf/parse.pl</cf>
which may be handy to parse the data and draw some plots.

<p>Implementation of this protocol is experimental. Use with caution and do not keep
any instance of Perf in production configs for long time. The config interface is also unstable
and may change in future versions without warning.

<sect1>Configuration
<label id="perf-config">

<p><descrip>
        <tag><label id="perf-mode">mode import|export</tag>
        Set perf mode. Default: import

        <tag><label id="perf-repeat">repeat <m/number/</tag>
        Run this amount of iterations of the benchmark for every amount step. Default: 4

        <tag><label id="perf-from">exp from <m/number/</tag>
        Begin benchmarking on this exponent for number of generated routes in one step.
        Default: 10

        <tag><label id="perf-to">exp to <m/number/</tag>
        Stop benchmarking on this exponent. Default: 20

        <tag><label id="perf-threshold-min">threshold min <m/time/</tag>
        If a run for the given exponent took less than this time for route import,
        increase the exponent immediately. Default: 1 ms

        <tag><label id="perf-threshold-max">threshold max <m/time/</tag>
        If every run for the given exponent took at least this time for route import,
        stop benchmarking. Default: 500 ms
</descrip>

<sect>Pipe
<label id="pipe">

<sect1>Introduction
<label id="pipe-intro">

<p>The Pipe protocol serves as a link between two routing tables, allowing
routes to be passed from a table declared as primary (i.e., the one the pipe is
connected to using the <cf/table/ configuration keyword) to the secondary one
(declared using <cf/peer table/) and vice versa, depending on what's allowed by
the filters. Export filters control export of routes from the primary table to
the secondary one, import filters control the opposite direction. Both tables
must be of the same nettype.

<p>The Pipe protocol retransmits all routes from one table to the other table,
retaining their original source and attributes. If import and export filters
are set to accept, then both tables would have the same content.

<p>The primary use of multiple routing tables and the Pipe protocol is for
policy routing, where handling of a single packet doesn't depend only on its
destination address, but also on its source address, source interface, protocol
type and other similar parameters. In many systems (Linux being a good example),
the kernel allows to enforce routing policies by defining routing rules which
choose one of several routing tables to be used for a packet according to its
parameters. Setting of these rules is outside the scope of BIRD's work (on
Linux, you can use the <tt/ip/ command), but you can create several routing
tables in BIRD, connect them to the kernel ones, use filters to control which
routes appear in which tables and also you can employ the Pipe protocol for
exporting a selected subset of one table to another one.

<sect1>Configuration
<label id="pipe-config">

<p>Essentially, the Pipe protocol is just a channel connected to a table on both
sides. Therefore, the configuration block for <cf/protocol pipe/ shall directly
include standard channel config options; see the example below.

<p><descrip>
        <tag><label id="pipe-peer-table">peer table <m/table/</tag>
        Defines secondary routing table to connect to. The primary one is
        selected by the <cf/table/ keyword.
</descrip>

<sect1>Attributes
<label id="pipe-attr">

<p>The Pipe protocol doesn't define any route attributes.

<sect1>Example
<label id="pipe-exam">

<p>Let's consider a router which serves as a boundary router of two different
autonomous systems, each of them connected to a subset of interfaces of the
router, having its own exterior connectivity and wishing to use the other AS as
a backup connectivity in case of outage of its own exterior line.

<p>Probably the simplest solution to this situation is to use two routing tables
(we'll call them <cf/as1/ and <cf/as2/) and set up kernel routing rules, so that
packets having arrived from interfaces belonging to the first AS will be routed
according to <cf/as1/ and similarly for the second AS. Thus we have split our
router to two logical routers, each one acting on its own routing table, having
its own routing protocols on its own interfaces. In order to use the other AS's
routes for backup purposes, we can pass the routes between the tables through a
Pipe protocol while decreasing their preferences and correcting their BGP paths
to reflect the AS boundary crossing.

<code>
ipv4 table as1;                         # Define the tables
ipv4 table as2;

protocol kernel kern1 {                 # Synchronize them with the kernel
        ipv4 { table as1; export all; };
        kernel table 1;
}

protocol kernel kern2 {
        ipv4 { table as2; export all; };
        kernel table 2;
}

protocol bgp bgp1 {                     # The outside connections
        ipv4 { table as1; import all; export all; };
        local as 1;
        neighbor 192.168.0.1 as 1001;
}

protocol bgp bgp2 {
        ipv4 { table as2; import all; export all; };
        local as 2;
        neighbor 10.0.0.1 as 1002;
}

protocol pipe {                         # The Pipe
        table as1;
        peer table as2;
        export filter {
                if net ~ [ 1.0.0.0/8+] then {   # Only AS1 networks
                        if preference>10 then preference = preference-10;
                        if source=RTS_BGP then bgp_path.prepend(1);
                        accept;
                }
                reject;
        };
        import filter {
                if net ~ [ 2.0.0.0/8+] then {   # Only AS2 networks
                        if preference>10 then preference = preference-10;
                        if source=RTS_BGP then bgp_path.prepend(2);
                        accept;
                }
                reject;
        };
}
</code>


<sect>RAdv
<label id="radv">

<sect1>Introduction
<label id="radv-intro">

<p>The RAdv protocol is an implementation of Router Advertisements, which are
used in the IPv6 stateless autoconfiguration. IPv6 routers send (in irregular
time intervals or as an answer to a request) advertisement packets to connected
networks. These packets contain basic information about a local network (e.g. a
list of network prefixes), which allows network hosts to autoconfigure network
addresses and choose a default route. BIRD implements router behavior as defined
in <rfc id="4861">, router preferences and specific routes (<rfc id="4191">),
and DNS extensions (<rfc id="6106">).

<p>The RAdv protocols supports just IPv6 channel.

<sect1>Configuration
<label id="radv-config">

<p>There are several classes of definitions in RAdv configuration -- interface
definitions, prefix definitions and DNS definitions:

<descrip>
        <tag><label id="radv-iface">interface <m/pattern/ [, <m/.../] { <m/options/ }</tag>
        Interface definitions specify a set of interfaces on which the
        protocol is activated and contain interface specific options.
        See <ref id="proto-iface" name="interface"> common options for
        detailed description.

        <tag><label id="radv-prefix">prefix <m/prefix/ { <m/options/ }</tag>
        Prefix definitions allow to modify a list of advertised prefixes. By
        default, the advertised prefixes are the same as the network prefixes
        assigned to the interface. For each network prefix, the matching prefix
        definition is found and its options are used. If no matching prefix
        definition is found, the prefix is used with default options.

        Prefix definitions can be either global or interface-specific. The
        second ones are part of interface options. The prefix definition
        matching is done in the first-match style, when interface-specific
        definitions are processed before global definitions. As expected, the
        prefix definition is matching if the network prefix is a subnet of the
        prefix in prefix definition.

        <tag><label id="radv-rdnss">rdnss { <m/options/ }</tag>
        RDNSS definitions allow to specify a list of advertised recursive DNS
        servers together with their options. As options are seldom necessary,
        there is also a short variant <cf>rdnss <m/address/</cf> that just
        specifies one DNS server. Multiple definitions are cumulative. RDNSS
        definitions may also be interface-specific when used inside interface
        options. By default, interface uses both global and interface-specific
        options, but that can be changed by <cf/rdnss local/ option.

        <tag><label id="radv-dnssl">dnssl { <m/options/ }</tag>
        DNSSL definitions allow to specify a list of advertised DNS search
        domains together with their options. Like <cf/rdnss/ above, multiple
        definitions are cumulative, they can be used also as interface-specific
        options and there is a short variant <cf>dnssl <m/domain/</cf> that just
        specifies one DNS search domain.

        <tag><label id="radv-trigger">trigger <m/prefix/</tag>
        RAdv protocol could be configured to change its behavior based on
        availability of routes. When this option is used, the protocol waits in
        suppressed state until a <it/trigger route/ (for the specified network)
        is exported to the protocol, the protocol also returns to suppressed
        state if the <it/trigger route/ disappears. Note that route export
        depends on specified export filter, as usual. This option could be used,
        e.g., for handling failover in multihoming scenarios.

        During suppressed state, router advertisements are generated, but with
        some fields zeroed. Exact behavior depends on which fields are zeroed,
        this can be configured by <cf/sensitive/ option for appropriate
        fields. By default, just <cf/default lifetime/ (also called <cf/router
        lifetime/) is zeroed, which means hosts cannot use the router as a
        default router. <cf/preferred lifetime/ and <cf/valid lifetime/ could
        also be configured as <cf/sensitive/ for a prefix, which would cause
        autoconfigured IPs to be deprecated or even removed.

        <tag><label id="radv-propagate-routes">propagate routes <m/switch/</tag>
        This option controls propagation of more specific routes, as defined in
        <rfc id="4191">. If enabled, all routes exported to the RAdv protocol,
        with the exception of the trigger prefix, are added to advertisments as
        additional options. The lifetime and preference of advertised routes can
        be set individually by <cf/ra_lifetime/ and <cf/ra_preference/ route
        attributes, or per interface by <cf/route lifetime/ and
        <cf/route preference/ options. Default: disabled.

        Note that the RFC discourages from sending more than 17 routes and
        recommends the routes to be configured manually.
</descrip>

<p>Interface specific options:

<descrip>
        <tag><label id="radv-iface-max-ra-interval">max ra interval <m/expr/</tag>
        Unsolicited router advertisements are sent in irregular time intervals.
        This option specifies the maximum length of these intervals, in seconds.
        Valid values are 4-1800. Default: 600

        <tag><label id="radv-iface-min-ra-interval">min ra interval <m/expr/</tag>
        This option specifies the minimum length of that intervals, in seconds.
        Must be at least 3 and at most 3/4 * <cf/max ra interval/. Default:
        about 1/3 * <cf/max ra interval/.

        <tag><label id="radv-iface-min-delay">min delay <m/expr/</tag>
        The minimum delay between two consecutive router advertisements, in
        seconds. Default: 3

        <tag><label id="radv-solicited-ra-unicast">solicited ra unicast <m/switch/</tag>
        Solicited router advertisements are usually sent to all-nodes multicast
        group like unsolicited ones, but the router can be configured to send
        them as unicast directly to soliciting nodes instead. This is especially
        useful on wireless networks (see <rfc id="7772">). Default: no

        <tag><label id="radv-iface-managed">managed <m/switch/</tag>
        This option specifies whether hosts should use DHCPv6 for IP address
        configuration. Default: no

        <tag><label id="radv-iface-other-config">other config <m/switch/</tag>
        This option specifies whether hosts should use DHCPv6 to receive other
        configuration information. Default: no

        <tag><label id="radv-iface-link-mtu">link mtu <m/expr/</tag>
        This option specifies which value of MTU should be used by hosts. 0
        means unspecified. Default: 0

        <tag><label id="radv-iface-reachable-time">reachable time <m/expr/</tag>
        This option specifies the time (in milliseconds) how long hosts should
        assume a neighbor is reachable (from the last confirmation). Maximum is
        3600000, 0 means unspecified. Default 0.

        <tag><label id="radv-iface-retrans-timer">retrans timer <m/expr/</tag>
        This option specifies the time (in milliseconds) how long hosts should
        wait before retransmitting Neighbor Solicitation messages. 0 means
        unspecified. Default 0.

        <tag><label id="radv-iface-current-hop-limit">current hop limit <m/expr/</tag>
        This option specifies which value of Hop Limit should be used by
        hosts. Valid values are 0-255, 0 means unspecified. Default: 64

        <tag><label id="radv-iface-default-lifetime">default lifetime <m/expr/ [sensitive <m/switch/]</tag>
        This option specifies the time (in seconds) how long (since the receipt
        of RA) hosts may use the router as a default router. 0 means do not use
        as a default router. For <cf/sensitive/ option, see <ref id="radv-trigger" name="trigger">.
        Default: 3 * <cf/max ra interval/, <cf/sensitive/ yes.

        <tag><label id="radv-iface-default-preference">default preference low|medium|high</tag>
        This option specifies the Default Router Preference value to advertise
        to hosts. Default: medium.

        <tag><label id="radv-iface-route-lifetime">route lifetime <m/expr/ [sensitive <m/switch/]</tag>
        This option specifies the default value of advertised lifetime for
        specific routes; i.e., the time (in seconds) for how long (since the
        receipt of RA) hosts should consider these routes valid. A special value
        0xffffffff represents infinity. The lifetime can be overriden on a per
        route basis by the <ref id="rta-ra-lifetime" name="ra_lifetime"> route
        attribute. Default: 3 * <cf/max ra interval/, <cf/sensitive/ no.

        For the <cf/sensitive/ option, see <ref id="radv-trigger" name="trigger">.
        If <cf/sensitive/ is enabled, even the routes with the <cf/ra_lifetime/
        attribute become sensitive to the trigger.

        <tag><label id="radv-iface-route-preference">route preference low|medium|high</tag>
        This option specifies the default value of advertised route preference
        for specific routes. The value can be overriden on a per route basis by
        the <ref id="rta-ra-preference" name="ra_preference"> route attribute.
        Default: medium.

        <tag><label id="radv-prefix-linger-time">prefix linger time <m/expr/</tag>
        When a prefix or a route disappears, it is advertised for some time with
        zero lifetime, to inform clients it is no longer valid. This option
        specifies the time (in seconds) for how long prefixes are advertised
        that way. Default: 3 * <cf/max ra interval/.

        <tag><label id="radv-route-linger-time">route linger time <m/expr/</tag>
        When a prefix or a route disappears, it is advertised for some time with
        zero lifetime, to inform clients it is no longer valid. This option
        specifies the time (in seconds) for how long routes are advertised
        that way. Default: 3 * <cf/max ra interval/.

        <tag><label id="radv-iface-rdnss-local">rdnss local <m/switch/</tag>
        Use only local (interface-specific) RDNSS definitions for this
        interface. Otherwise, both global and local definitions are used. Could
        also be used to disable RDNSS for given interface if no local definitons
        are specified. Default: no.

        <tag><label id="radv-iface-dnssl-local">dnssl local <m/switch/</tag>
        Use only local DNSSL definitions for this interface. See <cf/rdnss local/
        option above. Default: no.
</descrip>

<p>Prefix specific options

<descrip>
        <tag><label id="radv-prefix-skip">skip <m/switch/</tag>
        This option allows to specify that given prefix should not be
        advertised. This is useful for making exceptions from a default policy
        of advertising all prefixes. Note that for withdrawing an already
        advertised prefix it is more useful to advertise it with zero valid
        lifetime. Default: no

        <tag><label id="radv-prefix-onlink">onlink <m/switch/</tag>
        This option specifies whether hosts may use the advertised prefix for
        onlink determination. Default: yes

        <tag><label id="radv-prefix-autonomous">autonomous <m/switch/</tag>
        This option specifies whether hosts may use the advertised prefix for
        stateless autoconfiguration. Default: yes

        <tag><label id="radv-prefix-valid-lifetime">valid lifetime <m/expr/ [sensitive <m/switch/]</tag>
        This option specifies the time (in seconds) how long (after the
        receipt of RA) the prefix information is valid, i.e., autoconfigured
        IP addresses can be assigned and hosts with that IP addresses are
        considered directly reachable. 0 means the prefix is no longer
        valid. For <cf/sensitive/ option, see <ref id="radv-trigger" name="trigger">.
        Default: 86400 (1 day), <cf/sensitive/ no.

        <tag><label id="radv-prefix-preferred-lifetime">preferred lifetime <m/expr/ [sensitive <m/switch/]</tag>
        This option specifies the time (in seconds) how long (after the
        receipt of RA) IP addresses generated from the prefix using stateless
        autoconfiguration remain preferred. For <cf/sensitive/ option,
        see <ref id="radv-trigger" name="trigger">. Default: 14400 (4 hours),
        <cf/sensitive/ no.
</descrip>

<p>RDNSS specific options:

<descrip>
        <tag><label id="radv-rdnss-ns">ns <m/address/</tag>
        This option specifies one recursive DNS server. Can be used multiple
        times for multiple servers. It is mandatory to have at least one
        <cf/ns/ option in <cf/rdnss/ definition.

        <tag><label id="radv-rdnss-lifetime">lifetime [mult] <m/expr/</tag>
        This option specifies the time how long the RDNSS information may be
        used by clients after the receipt of RA. It is expressed either in
        seconds or (when <cf/mult/ is used) in multiples of <cf/max ra
        interval/. Note that RDNSS information is also invalidated when
        <cf/default lifetime/ expires. 0 means these addresses are no longer
        valid DNS servers. Default: 3 * <cf/max ra interval/.
</descrip>

<p>DNSSL specific options:

<descrip>
        <tag><label id="radv-dnssl-domain">domain <m/address/</tag>
        This option specifies one DNS search domain. Can be used multiple times
        for multiple domains. It is mandatory to have at least one <cf/domain/
        option in <cf/dnssl/ definition.

        <tag><label id="radv-dnssl-lifetime">lifetime [mult] <m/expr/</tag>
        This option specifies the time how long the DNSSL information may be
        used by clients after the receipt of RA. Details are the same as for
        RDNSS <cf/lifetime/ option above. Default: 3 * <cf/max ra interval/.
</descrip>

<sect1>Attributes
<label id="radv-attr">

<p>RAdv defines two route attributes:

<descrip>
        <tag><label id="rta-ra-preference">enum ra_preference</tag>
        The preference of the route. The value can be <it/RA_PREF_LOW/,
        <it/RA_PREF_MEDIUM/ or <it/RA_PREF_HIGH/. If the attribute is not set,
        the <ref id="radv-iface-route-preference" name="route preference">
        option is used.

        <tag><label id="rta-ra-lifetime">int ra_lifetime</tag>
        The advertised lifetime of the route, in seconds. The special value of
        0xffffffff represents infinity. If the attribute is not set, the
        <ref id="radv-iface-route-lifetime" name="route lifetime">
        option is used.
</descrip>

<sect1>Example
<label id="radv-exam">

<p><code>
ipv6 table radv_routes;                 # Manually configured routes go here

protocol static {
        ipv6 { table radv_routes; };

        route 2001:0DB8:4000::/48 unreachable;
        route 2001:0DB8:4010::/48 unreachable;

        route 2001:0DB8:4020::/48 unreachable {
                ra_preference = RA_PREF_HIGH;
                ra_lifetime = 3600;
        };
}

protocol radv {
        propagate routes yes;           # Propagate the routes from the radv_routes table
        ipv6 { table radv_routes; export all; };

        interface "eth2" {
                max ra interval 5;      # Fast failover with more routers
                managed yes;            # Using DHCPv6 on eth2
                prefix ::/0 {
                        autonomous off; # So do not autoconfigure any IP
                };
        };

        interface "eth*";               # No need for any other options

        prefix 2001:0DB8:1234::/48 {
                preferred lifetime 0;   # Deprecated address range
        };

        prefix 2001:0DB8:2000::/48 {
                autonomous off;         # Do not autoconfigure
        };

        rdnss 2001:0DB8:1234::10;       # Short form of RDNSS

        rdnss {
                lifetime mult 10;
                ns 2001:0DB8:1234::11;
                ns 2001:0DB8:1234::12;
        };

        dnssl {
                lifetime 3600;
                domain "abc.com";
                domain "xyz.com";
        };
}
</code>


<sect>RIP
<label id="rip">

<sect1>Introduction
<label id="rip-intro">

<p>The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol,
where each router broadcasts (to all its neighbors) distances to all networks it
can reach. When a router hears distance to another network, it increments it and
broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some
network goes unreachable, routers keep telling each other that its distance is
the original distance plus 1 (actually, plus interface metric, which is usually
one). After some time, the distance reaches infinity (that's 15 in RIP) and all
routers know that network is unreachable. RIP tries to minimize situations where
counting to infinity is necessary, because it is slow. Due to infinity being 16,
you can't use RIP on networks where maximal distance is higher than 15
hosts.

<p>BIRD supports RIPv1 (<rfc id="1058">), RIPv2 (<rfc id="2453">), RIPng (<rfc
id="2080">), and RIP cryptographic authentication (<rfc id="4822">).

<p>RIP is a very simple protocol, and it has a lot of shortcomings. Slow
convergence, big network load and inability to handle larger networks makes it
pretty much obsolete. It is still usable on very small networks.

<sect1>Configuration
<label id="rip-config">

<p>RIP configuration consists mainly of common protocol options and interface
definitions, most RIP options are interface specific. RIPng (RIP for IPv6)
protocol instance can be configured by using <cf/rip ng/ instead of just
<cf/rip/ as a protocol type.

<p>RIP needs one IPv4 channel. RIPng needs one IPv6 channel. If no channel is
configured, appropriate channel is defined with default parameters.

<code>
protocol rip [ng] [&lt;name&gt;] {
        infinity &lt;number&gt;;
        ecmp &lt;switch&gt; [limit &lt;number&gt;];
        interface &lt;interface pattern&gt; {
                metric &lt;number&gt;;
                mode multicast|broadcast;
                passive &lt;switch&gt;;
                address &lt;ip&gt;;
                port &lt;number&gt;;
                version 1|2;
                split horizon &lt;switch&gt;;
                poison reverse &lt;switch&gt;;
                check zero &lt;switch&gt;;
                update time &lt;number&gt;;
                timeout time &lt;number&gt;;
                garbage time &lt;number&gt;;
                ecmp weight &lt;number&gt;;
                ttl security &lt;switch&gt;; | tx only;
                tx class|dscp &lt;number&gt;;
                tx priority &lt;number&gt;;
                rx buffer &lt;number&gt;;
                tx length &lt;number&gt;;
                check link &lt;switch&gt;;
                authentication none|plaintext|cryptographic;
                password "&lt;text&gt;";
                password "&lt;text&gt;" {
                        id &lt;num&gt;;
                        generate from "&lt;date&gt;";
                        generate to "&lt;date&gt;";
                        accept from "&lt;date&gt;";
                        accept to "&lt;date&gt;";
                        from "&lt;date&gt;";
                        to "&lt;date&gt;";
                        algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
                };
        };
}
</code>

<descrip>
        <tag><label id="rip-infinity">infinity <M>number</M></tag>
        Selects the distance of infinity. Bigger values will make
        protocol convergence even slower. The default value is 16.

        <tag><label id="rip-ecmp">ecmp <M>switch</M> [limit <M>number</M>]</tag>
        This option specifies whether RIP is allowed to generate ECMP
        (equal-cost multipath) routes. Such routes are used when there are
        several directions to the destination, each with the same (computed)
        cost. This option also allows to specify a limit on maximum number of
        nexthops in one route. By default, ECMP is enabled if supported by
        Kernel. Default value of the limit is 16.

        <tag><label id="rip-iface">interface <m/pattern/ [, <m/.../] { <m/options/ }</tag>
        Interface definitions specify a set of interfaces on which the
        protocol is activated and contain interface specific options.
        See <ref id="proto-iface" name="interface"> common options for
        detailed description.
</descrip>

<p>Interface specific options:

<descrip>
        <tag><label id="rip-iface-metric">metric <m/num/</tag>
        This option specifies the metric of the interface. When a route is
        received from the interface, its metric is increased by this value
        before further processing. Valid values are 1-255, but values higher
        than infinity has no further meaning. Default: 1.

        <tag><label id="rip-iface-mode">mode multicast|broadcast</tag>
        This option selects the mode for RIP to use on the interface. The
        default is multicast mode for RIPv2 and broadcast mode for RIPv1.
        RIPng always uses the multicast mode.

        <tag><label id="rip-iface-passive">passive <m/switch/</tag>
        Passive interfaces receive routing updates but do not transmit any
        messages. Default: no.

        <tag><label id="rip-iface-address">address <m/ip/</tag>
        This option specifies a destination address used for multicast or
        broadcast messages, the default is the official RIP (224.0.0.9) or RIPng
        (ff02::9) multicast address, or an appropriate broadcast address in the
        broadcast mode.

        <tag><label id="rip-iface-port">port <m/number/</tag>
        This option selects an UDP port to operate on, the default is the
        official RIP (520) or RIPng (521) port.

        <tag><label id="rip-iface-version">version 1|2</tag>
        This option selects the version of RIP used on the interface. For RIPv1,
        automatic subnet aggregation is not implemented, only classful network
        routes and host routes are propagated. Note that BIRD allows RIPv1 to be
        configured with features that are defined for RIPv2 only, like
        authentication or using multicast sockets. The default is RIPv2 for IPv4
        RIP, the option is not supported for RIPng, as no further versions are
        defined.

        <tag><label id="rip-iface-version-only">version only <m/switch/</tag>
        Regardless of RIP version configured for the interface, BIRD accepts
        incoming packets of any RIP version. This option restrict accepted
        packets to the configured version. Default: no.

        <tag><label id="rip-iface-split-horizon">split horizon <m/switch/</tag>
        Split horizon is a scheme for preventing routing loops. When split
        horizon is active, routes are not regularly propagated back to the
        interface from which they were received. They are either not propagated
        back at all (plain split horizon) or propagated back with an infinity
        metric (split horizon with poisoned reverse). Therefore, other routers
        on the interface will not consider the router as a part of an
        independent path to the destination of the route. Default: yes.

        <tag><label id="rip-iface-poison-reverse">poison reverse <m/switch/</tag>
        When split horizon is active, this option specifies whether the poisoned
        reverse variant (propagating routes back with an infinity metric) is
        used. The poisoned reverse has some advantages in faster convergence,
        but uses more network traffic. Default: yes.

        <tag><label id="rip-iface-check-zero">check zero <m/switch/</tag>
        Received RIPv1 packets with non-zero values in reserved fields should
        be discarded. This option specifies whether the check is performed or
        such packets are just processed as usual. Default: yes.

        <tag><label id="rip-iface-update-time">update time <m/number/</tag>
        Specifies the number of seconds between periodic updates. A lower number
        will mean faster convergence but bigger network load. Default: 30.

        <tag><label id="rip-iface-timeout-time">timeout time <m/number/</tag>
        Specifies the time interval (in seconds) between the last received route
        announcement and the route expiration. After that, the network is
        considered unreachable, but still is propagated with infinity distance.
        Default: 180.

        <tag><label id="rip-iface-garbage-time">garbage time <m/number/</tag>
        Specifies the time interval (in seconds) between the route expiration
        and the removal of the unreachable network entry. The garbage interval,
        when a route with infinity metric is propagated, is used for both
        internal (after expiration) and external (after withdrawal) routes.
        Default: 120.

        <tag><label id="rip-iface-ecmp-weight">ecmp weight <m/number/</tag>
        When ECMP (multipath) routes are allowed, this value specifies a
        relative weight used for nexthops going through the iface. Valid
        values are 1-256. Default value is 1.

        <tag><label id="rip-iface-auth">authentication none|plaintext|cryptographic</tag>
        Selects authentication method to be used. <cf/none/ means that packets
        are not authenticated at all, <cf/plaintext/ means that a plaintext
        password is embedded into each packet, and <cf/cryptographic/ means that
        packets are authenticated using some cryptographic hash function
        selected by option <cf/algorithm/ for each key. The default
        cryptographic algorithm for RIP keys is Keyed-MD5. If you set
        authentication to not-none, it is a good idea to add <cf>password</cf>
        section. Default: none.

        <tag><label id="rip-iface-pass">password "<m/text/"</tag>
        Specifies a password used for authentication. See <ref id="proto-pass"
        name="password"> common option for detailed description.

        <tag><label id="rip-iface-ttl-security">ttl security [<m/switch/ | tx only]</tag>
        TTL security is a feature that protects routing protocols from remote
        spoofed packets by using TTL 255 instead of TTL 1 for protocol packets
        destined to neighbors. Because TTL is decremented when packets are
        forwarded, it is non-trivial to spoof packets with TTL 255 from remote
        locations.

        If this option is enabled, the router will send RIP packets with TTL 255
        and drop received packets with TTL less than 255. If this option si set
        to <cf/tx only/, TTL 255 is used for sent packets, but is not checked
        for received packets. Such setting does not offer protection, but offers
        compatibility with neighbors regardless of whether they use ttl
        security.

        For RIPng, TTL security is a standard behavior (required by <rfc
        id="2080">) and therefore default value is yes. For IPv4 RIP, default
        value is no.

        <tag><label id="rip-iface-tx-class">tx class|dscp|priority <m/number/</tag>
        These options specify the ToS/DiffServ/Traffic class/Priority of the
        outgoing RIP packets. See <ref id="proto-tx-class" name="tx class"> common
        option for detailed description.

        <tag><label id="rip-iface-rx-buffer">rx buffer <m/number/</tag>
        This option specifies the size of buffers used for packet processing.
        The buffer size should be bigger than maximal size of received packets.
        The default value is 532 for IPv4 RIP and interface MTU value for RIPng.

        <tag><label id="rip-iface-tx-length">tx length <m/number/</tag>
        This option specifies the maximum length of generated RIP packets. To
        avoid IP fragmentation, it should not exceed the interface MTU value.
        The default value is 532 for IPv4 RIP and interface MTU value for RIPng.

        <tag><label id="rip-iface-check-link">check link <m/switch/</tag>
        If set, the hardware link state (as reported by OS) is taken into
        consideration. When the link disappears (e.g. an ethernet cable is
        unplugged), neighbors are immediately considered unreachable and all
        routes received from them are withdrawn. It is possible that some
        hardware drivers or platforms do not implement this feature.
        Default: yes.
</descrip>

<sect1>Attributes
<label id="rip-attr">

<p>RIP defines two route attributes:

<descrip>
        <tag><label id="rta-rip-metric">int rip_metric</tag>
        RIP metric of the route (ranging from 0 to <cf/infinity/). When routes
        from different RIP instances are available and all of them have the same
        preference, BIRD prefers the route with lowest <cf/rip_metric/. When a
        non-RIP route is exported to RIP, the default metric is 1.

        <tag><label id="rta-rip-tag">int rip_tag</tag>
        RIP route tag: a 16-bit number which can be used to carry additional
        information with the route (for example, an originating AS number in
        case of external routes). When a non-RIP route is exported to RIP, the
        default tag is 0.
</descrip>

<sect1>Example
<label id="rip-exam">

<p><code>
protocol rip {
        ipv4 {
                import all;
                export all;
        };
        interface "eth*" {
                metric 2;
                port 1520;
                mode multicast;
                update time 12;
                timeout time 60;
                authentication cryptographic;
                password "secret" { algorithm hmac sha256; };
        };
}
</code>


<sect>RPKI
<label id="rpki">

<sect1>Introduction

<p>The Resource Public Key Infrastructure (RPKI) is mechanism for origin
validation of BGP routes (RFC 6480). BIRD supports only so-called RPKI-based
origin validation. There is implemented RPKI to Router (RPKI-RTR) protocol (RFC
6810). It uses some of the RPKI data to allow a router to verify that the
autonomous system announcing an IP address prefix is in fact authorized to do
so. This is not crypto checked so can be violated. But it should prevent the
vast majority of accidental hijackings on the Internet today, e.g. the famous
Pakastani accidental announcement of YouTube's address space.

<p>The RPKI-RTR protocol receives and maintains a set of ROAs from a cache
server (also called validator). You can validate routes (RFC 6483) using
function <cf/roa_check()/ in filter and set it as import filter at the BGP
protocol. BIRD should re-validate all of affected routes after RPKI update by
RFC 6811, but we don't support it yet! You can use a BIRD's client command
<cf>reload in <m/bgp_protocol_name/</cf> for manual call of revalidation of all
routes.

<sect1>Supported transports
<p>
<itemize>
        <item>Unprotected transport over TCP uses a port 323. The cache server
        and BIRD router should be on the same trusted and controlled network
        for security reasons.
        <item>SSHv2 encrypted transport connection uses the normal SSH port
        22.
</itemize>

<sect1>Configuration

<p>We currently support just one cache server per protocol. However you can
define more RPKI protocols generally.

<code>
protocol rpki [&lt;name&gt;] {
        roa4 { table &lt;tab&gt;; };
        roa6 { table &lt;tab&gt;; };
        remote &lt;ip&gt; | "&lt;domain&gt;" [port &lt;num&gt;];
        port &lt;num&gt;;
        refresh [keep] &lt;num&gt;;
        retry [keep] &lt;num&gt;;
        expire [keep] &lt;num&gt;;
        transport tcp;
        transport ssh {
                bird private key "&lt;/path/to/id_rsa&gt;";
                remote public key "&lt;/path/to/known_host&gt;";
                user "&lt;name&gt;";
        };
}
</code>

<p>Alse note that you have to specify the ROA channel. If you want to import
only IPv4 prefixes you have to specify only roa4 channel. Similarly with IPv6
prefixes only. If you want to fetch both IPv4 and even IPv6 ROAs you have to
specify both channels.

<sect2>RPKI protocol options
<p>
<descrip>
        <tag>remote <m/ip/ | "<m/hostname/" [port <m/num/]</tag> Specifies
        a destination address of the cache server.  Can be specified by an IP
        address or by full domain name string.  Only one cache can be specified
        per protocol. This option is required.

        <tag>port <m/num/</tag> Specifies the port number. The default port
        number is 323 for transport without any encryption and 22 for transport
        with SSH encryption.

        <tag>refresh [keep] <m/num/</tag> Time period in seconds. Tells how
        long to wait before next attempting to poll the cache using a Serial
        Query or a Reset Query packet. Must be lower than 86400 seconds (one
        day). Too low value can caused a false positive detection of
        network connection problems.  A keyword <cf/keep/ suppresses updating
        this value by a cache server.
        Default: 3600 seconds

        <tag>retry [keep] <m/num/</tag> Time period in seconds between a failed
        Serial/Reset Query and a next attempt.  Maximum allowed value is 7200
        seconds (two hours). Too low value can caused a false positive
        detection of network connection problems.  A keyword <cf/keep/
        suppresses updating this value by a cache server.
        Default: 600 seconds

        <tag>expire [keep] <m/num/</tag> Time period in seconds. Received
        records are deleted if the client was unable to successfully refresh
        data for this time period.  Must be in range from 600 seconds (ten
        minutes) to 172800 seconds (two days).  A keyword <cf/keep/
        suppresses updating this value by a cache server.
        Default: 7200 seconds

        <tag>transport tcp</tag> Unprotected transport over TCP. It's a default
        transport. Should be used only on secure private networks.
        Default: tcp

        <tag>transport ssh { <m/SSH transport options.../ }</tag> It enables a
        SSHv2 transport encryption. Cannot be combined with a TCP transport.
        Default: off
</descrip>

<sect3>SSH transport options
<p>
<descrip>
        <tag>bird private key "<m>/path/to/id_rsa</m>"</tag>
        A path to the BIRD's private SSH key for authentication.
        It can be a <cf><m>id_rsa</m></cf> file.

        <tag>remote public key "<m>/path/to/known_host</m>"</tag>
        A path to the cache's public SSH key for verification identity
        of the cache server. It could be a path to <cf><m>known_host</m></cf> file.

        <tag>user "<m/name/"</tag>
        A SSH user name for authentication. This option is a required.
</descrip>

<sect1>Examples
<sect2>BGP origin validation
<p>Policy: Don't import <cf/ROA_INVALID/ routes.
<code>
roa4 table r4;
roa6 table r6;

protocol rpki {
        debug all;

        roa4 { table r4; };
        roa6 { table r6; };

        # Please, do not use rpki-validator.realmv6.org in production
        remote "rpki-validator.realmv6.org" port 8282;

        retry keep 5;
        refresh keep 30;
        expire 600;
}

filter peer_in_v4 {
        if (roa_check(r4, net, bgp_path.last) = ROA_INVALID) then
        {
                print "Ignore RPKI invalid ", net, " for ASN ", bgp_path.last;
                reject;
        }
        accept;
}

protocol bgp {
        debug all;
        local as 65000;
        neighbor 192.168.2.1 as 65001;
        ipv4 {
                import filter peer_in_v4;
                export none;
        };
}
</code>

<sect2>SSHv2 transport encryption
<p>
<code>
roa4 table r4;
roa6 table r6;

protocol rpki {
        debug all;

        roa4 { table r4; };
        roa6 { table r6; };

        remote 127.0.0.1 port 2345;
        transport ssh {
                bird private key "/home/birdgeek/.ssh/id_rsa";
                remote public key "/home/birdgeek/.ssh/known_hosts";
                user "birdgeek";
        };

        # Default interval values
}
</code>


<sect>Static
<label id="static">

<p>The Static protocol doesn't communicate with other routers in the network,
but instead it allows you to define routes manually. This is often used for
specifying how to forward packets to parts of the network which don't use
dynamic routing at all and also for defining sink routes (i.e., those telling to
return packets as undeliverable if they are in your IP block, you don't have any
specific destination for them and you don't want to send them out through the
default route to prevent routing loops).

<p>There are three classes of definitions in Static protocol configuration --
global options, static route definitions, and per-route options. Usually, the
definition of the protocol contains mainly a list of static routes.
Static routes have no specific attributes.

<p>Global options:

<descrip>
        <tag><label id="static-check-link">check link <m/switch/</tag>
        If set, hardware link states of network interfaces are taken into
        consideration.  When link disappears (e.g. ethernet cable is unplugged),
        static routes directing to that interface are removed. It is possible
        that some hardware drivers or platforms do not implement this feature.
        Default: off.

        <tag><label id="static-igp-table">igp table <m/name/</tag>
        Specifies a table that is used for route table lookups of recursive
        routes. Default: the same table as the protocol is connected to.
</descrip>

<p>Route definitions (each may also contain a block of per-route options):

<sect1>Regular routes; MPLS switching rules

<p>There exist several types of routes; keep in mind that <m/prefix/ syntax is
<ref id="type-prefix" name="dependent on network type">.

<descrip>
        <tag>route <m/prefix/ via <m/ip/|<m/"interface"/ [mpls <m/num/[/<m/num/[/<m/num/[...]]]]</tag>
        Next hop routes may bear one or more <ref id="route-next-hop" name="next hops">.
        Every next hop is preceded by <cf/via/ and configured as shown.

        <tag>route <m/prefix/ recursive <m/ip/ [mpls <m/num/[/<m/num/[/<m/num/[...]]]]</tag>
        Recursive nexthop resolves the given IP in the configured IGP table and
        uses that route's next hop. The MPLS stacks are concatenated; on top is
        the IGP's nexthop stack and on bottom is this route's stack.

        <tag>route <m/prefix/ blackhole|unreachable|prohibit</tag>
        Special routes specifying to silently drop the packet, return it as
        unreachable or return it as administratively prohibited. First two
        targets are also known as <cf/drop/ and <cf/reject/.
</descrip>

<p>When the particular destination is not available (the interface is down or
the next hop of the route is not a neighbor at the moment), Static just
uninstalls the route from the table it is connected to and adds it again as soon
as the destination becomes adjacent again.

<sect1>Route Origin Authorization

<p>The ROA config is just <cf>route <m/prefix/ max <m/int/ as <m/int/</cf> with no nexthop.

<sect1>Flowspec
<label id="flowspec-network-type">

<p>The flow specification are rules for routers and firewalls for filtering
purpose. It is described by <rfc id="5575">. There are 3 types of arguments:
<m/inet4/ or <m/inet6/ prefixes, bitmasks matching expressions and numbers
matching expressions.

Bitmasks matching is written using <m/value/<cf>/</cf><m/mask/ or
<cf/!/<m/value/<cf>/</cf><m/mask/ pairs. It means that <cf/(/<m/data/ <cf/&/
<m/mask/<cf/)/ is or is not equal to <m/value/.

Numbers matching is a matching sequence of numbers and ranges separeted by a
commas (<cf/,/) (e.g. <cf/10,20,30/). Ranges can be written using double dots
<cf/../ notation (e.g. <cf/80..90,120..124/). An alternative notation are
sequence of one or more pairs of relational operators and values separated by
logical operators <cf/&&/ or <cf/||/. Allowed relational operators are <cf/=/,
<cf/!=/, <cf/</, <cf/<=/, <cf/>/, <cf/>=/, <cf/true/ and <cf/false/.

<sect2>IPv4 Flowspec

<p><descrip>
        <tag><label id="flow-dst">dst <m/inet4/</tag>
        Set a matching destination prefix (e.g. <cf>dst 192.168.0.0/16</cf>).
        Only this option is mandatory in IPv4 Flowspec.

        <tag><label id="flow-src">src <m/inet4/</tag>
        Set a matching source prefix (e.g. <cf>src 10.0.0.0/8</cf>).

        <tag><label id="flow-proto">proto <m/numbers-match/</tag>
        Set a matching IP protocol numbers (e.g.  <cf/proto 6/).

        <tag><label id="flow-port">port <m/numbers-match/</tag>
        Set a matching source or destination TCP/UDP port numbers (e.g.
        <cf>port 1..1023,1194,3306</cf>).

        <tag><label id="flow-dport">dport <m/numbers-match/</tag>
        Set a mating destination port numbers (e.g. <cf>dport 49151</cf>).

        <tag><label id="flow-sport">sport <m/numbers-match/</tag>
        Set a matching source port numbers (e.g. <cf>sport = 0</cf>).

        <tag><label id="flow-icmp-type">icmp type <m/numbers-match/</tag>
        Set a matching type field number of an ICMP packet (e.g. <cf>icmp type
        3</cf>)

        <tag><label id="flow-icmp-code">icmp code <m/numbers-match/</tag>
        Set a matching code field number of an ICMP packet (e.g. <cf>icmp code
        1</cf>)

        <tag><label id="flow-tcp-flags">tcp flags <m/bitmask-match/</tag>
        Set a matching bitmask for TCP header flags (aka control bits) (e.g.
        <cf>tcp flags 0x03/0x0f;</cf>). The maximum length of mask is 12 bits
        (0xfff).

        <tag><label id="flow-length">length <m/numbers-match/</tag>
        Set a matching packet length (e.g. <cf>length > 1500;</cf>)

        <tag><label id="flow-dscp">dscp <m/numbers-match/</tag>
        Set a matching DiffServ Code Point number (e.g. <cf>length > 1500;</cf>).

        <tag><label id="flow-fragment">fragment <m/fragmentation-type/</tag>
        Set a matching type of packet fragmentation. Allowed fragmentation
        types are <cf/dont_fragment/, <cf/is_fragment/, <cf/first_fragment/,
        <cf/last_fragment/ (e.g. <cf>fragment is_fragment &&
        !dont_fragment</cf>).
</descrip>

<p><code>
protocol static {
        flow4;

        route flow4 {
                dst 10.0.0.0/8;
                port > 24 && < 30 || 40..50,60..70,80 && >= 90;
                tcp flags 0x03/0x0f;
                length > 1024;
                dscp = 63;
                fragment dont_fragment, is_fragment || !first_fragment;
        };
}
</code>

<sect2>Differences for IPv6 Flowspec

<p>Flowspec IPv6 are same as Flowspec IPv4 with a few exceptions.
<itemize>
        <item>Prefixes <m/inet6/ can be specified not only with prefix length,
        but with prefix <cf/offset/ <m/num/ too (e.g.
        <cf>::1234:5678:9800:0000/101 offset 64</cf>). Offset means to don't
        care of <m/num/ first bits.
        <item>IPv6 Flowspec hasn't mandatory any flowspec component.
        <item>In IPv6 packets, there is a matching the last next header value
        for a matching IP protocol number (e.g. <cf>next header 6</cf>).
        <item>It is not possible to set <cf>dont_fragment</cf> as a type of
        packet fragmentation.
</itemize>

<p><descrip>
        <tag><label id="flow6-dst">dst <m/inet6/ [offset <m/num/]</tag>
        Set a matching destination IPv6 prefix (e.g. <cf>dst
        ::1c77:3769:27ad:a11a/128 offset 64</cf>).

        <tag><label id="flow6-src">src <m/inet6/ [offset <m/num/]</tag>
        Set a matching source IPv6 prefix (e.g. <cf>src fe80::/64</cf>).

        <tag><label id="flow6-next-header">next header <m/numbers-match/</tag>
        Set a matching IP protocol numbers (e.g. <cf>next header != 6</cf>).

        <tag><label id="flow6-label">label <m/bitmask-match/</tag>
        Set a 20-bit bitmask for matching Flow Label field in IPv6 packets
        (e.g. <cf>label 0x8e5/0x8e5</cf>).
</descrip>

<p><code>
protocol static {
        flow6 { table myflow6; };

        route flow6 {
                dst fec0:1122:3344:5566:7788:99aa:bbcc:ddee/128;
                src 0000:0000:0000:0001:1234:5678:9800:0000/101 offset 63;
                next header = 23;
                sport > 24 && < 30 || = 40 || 50,60,70..80;
                dport = 50;
                tcp flags 0x03/0x0f, !0/0xff || 0x33/0x33;
                fragment !is_fragment || !first_fragment;
                label 0xaaaa/0xaaaa && 0x33/0x33;
        };
}
</code>

<sect1>Per-route options
<p>
<descrip>
        <tag><label id="static-route-bfd">bfd <m/switch/</tag>
        The Static protocol could use BFD protocol for next hop liveness
        detection. If enabled, a BFD session to the route next hop is created
        and the static route is BFD-controlled -- the static route is announced
        only if the next hop liveness is confirmed by BFD. If the BFD session
        fails, the static route is removed. Note that this is a bit different
        compared to other protocols, which may use BFD as an advisory mechanism
        for fast failure detection but ignores it if a BFD session is not even
        established.

        This option can be used for static routes with a direct next hop, or
        also for for individual next hops in a static multipath route (see
        above). Note that BFD protocol also has to be configured, see
        <ref id="bfd" name="BFD"> section for details. Default value is no.

        <tag><label id="static-route-filter"><m/filter expression/</tag>
        This is a special option that allows filter expressions to be configured
        on per-route basis. Can be used multiple times. These expressions are
        evaluated when the route is originated, similarly to the import filter
        of the static protocol. This is especially useful for configuring route
        attributes, e.g., <cf/ospf_metric1 = 100;/ for a route that will be
        exported to the OSPF protocol.
</descrip>

<sect1>Example static config

<p><code>
protocol static {
        ipv4 { table testable; };       # Connect to a non-default routing table
        check link;                     # Advertise routes only if link is up
        route 0.0.0.0/0 via 198.51.100.130; # Default route
        route 10.0.0.0/8                # Multipath route
                via 198.51.100.10 weight 2
                via 198.51.100.20 bfd   # BFD-controlled next hop
                via 192.0.2.1;
        route 203.0.113.0/24 unreachable; # Sink route
        route 10.2.0.0/24 via "arc0";   # Secondary network
        route 192.168.10.0/24 via 198.51.100.100 {
                ospf_metric1 = 20;      # Set extended attribute
        }
        route 192.168.10.0/24 via 198.51.100.100 {
                ospf_metric2 = 100;     # Set extended attribute
                ospf_tag = 2;           # Set extended attribute
                bfd;                    # BFD-controlled route
        }
}
</code>


<chapt>Conclusions
<label id="conclusion">

<sect>Future work
<label id="future-work">

<p>Although BIRD supports all the commonly used routing protocols, there are
still some features which would surely deserve to be implemented in future
versions of BIRD:

<itemize>
<item>Opaque LSA's
<item>Route aggregation and flap dampening
<item>Multicast routing protocols
<item>Ports to other systems
</itemize>


<sect>Getting more help
<label id="help">

<p>If you use BIRD, you're welcome to join the bird-users mailing list
(<HTMLURL URL="mailto:bird-users@network.cz" name="bird-users@network.cz">)
where you can share your experiences with the other users and consult
your problems with the authors. To subscribe to the list, visit
<HTMLURL URL="http://bird.network.cz/?m_list" name="http://bird.network.cz/?m_list">.
The home page of BIRD can be found at <HTMLURL URL="http://bird.network.cz/" name="http://bird.network.cz/">.

<p>BIRD is a relatively young system and it probably contains some bugs. You can
report any problems to the bird-users list and the authors will be glad to solve
them, but before you do so, please make sure you have read the available
documentation and that you are running the latest version (available at
<HTMLURL URL="ftp://bird.network.cz/pub/bird" name="bird.network.cz:/pub/bird">).
(Of course, a patch which fixes the bug is always welcome as an attachment.)

<p>If you want to understand what is going inside, Internet standards are a good
and interesting reading. You can get them from
<HTMLURL URL="ftp://ftp.rfc-editor.org/" name="ftp.rfc-editor.org"> (or a
nicely sorted version from <HTMLURL URL="ftp://atrey.karlin.mff.cuni.cz/pub/rfc"
name="atrey.karlin.mff.cuni.cz:/pub/rfc">).

<p><it/Good luck!/

</book>

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