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<book>

<title>BIRD 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;/,
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

<sect>What is BIRD

<p><label id="intro">
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), Zebra<HTMLURL URL="http://www.zebra.org">
and MRTD<HTMLURL URL="http://sourceforge.net/projects/mrt">, 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)
        <item>the Open Shortest Path First protocol (OSPFv2, OSPFv3)
        <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 supports either IPv4 or IPv6 protocol, but have to be compiled
separately for each one. Therefore, a dualstack router would run two
instances of BIRD (one for IPv4 and one for IPv6), with completely
separate setups (configuration files, tools ...).

<sect>Installing BIRD

<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/--enable-ipv6/ which enables building of an IPv6 version of BIRD,
<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

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

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

        <tag>-d</tag>
        enable debug messages and run bird in foreground.

        <tag>-D <m/filename of debug log/</tag>
        log debugging information to given file instead of stderr.

        <tag>-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>-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>-P <m/name of PID file/</tag>
        create a PID file with given filename</file>.

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

        <tag>-g <m/group/</tag>
        use that group ID, see the next section for details.

        <tag>-f</tag>
        run bird in foreground.
</descrip>

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

<sect>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>A nonprivileged 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>About routing tables

<p>BIRD has one or more routing tables which may or may not be
synchronized with OS kernel and which may or may not be synchronized with
each other (see the Pipe protocol). Each routing table contains a list of
known routes. Each route consists of:

<itemize>
        <item>network prefix this route is for (network address and prefix length -- the number of bits forming the network part of the address; also known as a netmask)
        <item>preference of this route
        <item>IP address of router which told us about this route
        <item>IP address of router we should forward the packets to
        using this route
        <item>other attributes common to all routes
        <item>dynamic attributes defined by protocols which may or
        may not be present (typically protocol metrics)
</itemize>

Routing table maintains multiple entries
for a network, but at most one entry for one network and one
protocol. The entry with the highest preference is used for routing (we
will call such an entry the <it/selected route/). If
there are more entries with the same preference and they are from the same
protocol, the protocol decides (typically according to metrics). If they aren't,
an internal ordering is used to break the tie. You can
get the list of route attributes in the Route attributes section.

<p>Each protocol is connected to a routing table through two filters
which can accept, reject and modify the routes. An <it/export/
filter checks routes passed from the routing table to the protocol,
an <it/import/ filter checks routes in the opposite direction.
When the routing table gets a route from a protocol, it recalculates
the selected route and broadcasts it to all protocols connected to
the table. The protocols typically send the update to other routers
in the network. Note that although most protocols are interested 
in receiving just selected routes, some protocols (e.g. the <cf/Pipe/
protocol) receive and process all entries in routing tables (accepted
by filters).

<p><label id="dsc-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.


<chapt>Configuration

<sect>Introduction

<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.

<p>Here is an example of a simple config file. It enables
synchronization of routing tables with OS kernel, scans for 
new network interfaces every 10 seconds and runs RIP on all network interfaces found.


<code>
protocol kernel {
        persist;                # Don't remove routes on BIRD shutdown
        scan time 20;           # Scan kernel routing table every 20 seconds
        export all;             # Default is export none
}

protocol device {
        scan time 10;           # Scan interfaces every 10 seconds
}

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


<sect>Global options

<p><descrip>
        <tag>include "<m/filename/"</tag> 
        This statement causes inclusion of a new file. The maximal depth is set to 5.

        <tag><label id="dsc-log">log "<m/filename/"|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 }/ etc.) into selected destination (a file specified as a filename string,
        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. You may specify more than one <cf/log/ line to establish logging to multiple
        destinations. Default: log everything to the system log.

        <tag>debug protocols all|off|{ states, routes, filters, interfaces, events, packets }</tag>
        Set global defaults of protocol debugging options. See <cf/debug/ in the following section. Default: off.

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

        <tag>mrtdump protocols all|off|{ states, messages }</tag>
        Set global defaults of MRTdump options. See <cf/mrtdump/ in the following section.
        Default: off.

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

        <tag>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>protocol rip|ospf|bgp|... [<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>template rip|bgp|... [<m/name/ [from <m/name2/]] { <m>protocol options</m> }</tag>
        Define a protocol template instance called <cf><m/name/</cf> (or with a name like "bgp1"
        generated automatically if you don't specify any <cf><m/name/</cf>). 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>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>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: in IPv4 version, the lowest IP address of a
        non-loopback interface. In IPv6 version, this option is
        mandatory.

        <tag>router id from [-] [ "<m/mask/" ] [ <m/prefix/ ] [, ...]</tag>
        Set BIRD's router ID based on an IP address of an interface
        specified by an interface pattern. The option is applicable
        for IPv4 version only. See <ref id="dsc-iface" name="interface">
        section for detailed description of interface patterns.

        <tag>listen bgp [address <m/address/] [port <m/port/] [dual]</tag>
        This option allows to specify address and port where BGP
        protocol should listen. It is global option as listening
        socket is common to all BGP instances. Default is to listen on
        all addresses (0.0.0.0) and port 179. In IPv6 mode, option
        <cf/dual/ can be used to specify that BGP socket should accept
        both IPv4 and IPv6 connections (but even in that case, BIRD
        would accept IPv6 routes only). Such behavior was default in
        older versions of BIRD.

        <tag>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). <m/limit> and
        "<m/format2/" allow to specify the second format string for
        times in past deeper than <m/limit/ seconds. There are two
        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"/. <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"/.

        By default, BIRD uses an short, ad-hoc format for <cf/route/
        and <cf/protocol/ times, and a <cf/iso long/ similar format
        (DD-MM-YYYY hh:mm:ss) for <cf/base/ and <cf/log/. These
        defaults are here for a compatibility with older versions
        and might change in the future.

        <tag>table <m/name/ [sorted]</tag>
        Create a new routing table. The default routing table is
        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-sorted" name="sorted table">
        description for details.

        <tag>roa table <m/name/ [ { roa table options ... } ]</tag>
        Create a new ROA (Route Origin Authorization) table. ROA
        tables can be used to validate route origination of BGP
        routes. A ROA table contains ROA entries, each consist of a
        network prefix, a max prefix length and an AS number. A ROA
        entry specifies prefixes which could be originated by that AS
        number. ROA tables could be filled with data from RPKI (RFC
        6480) or from public databases like Whois. ROA tables are 
        examined by <cf/roa_check()/ operator in filters.

        Currently, there is just one option,
        <cf>roa <m/prefix/ max <m/num/ as <m/num/</cf>, which
        can be used to populate the ROA table with static ROA
        entries. The option may be used multiple times. Other entries
        can be added dynamically by <cf/add roa/ command.

        <tag>eval <m/expr/</tag>
        Evaluates given filter expression. It is used by us for testing of filters.
</descrip>

<sect>Protocol options

<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 <cf><m/switch/</cf> 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 <cf><m/switch/</cf> is equivalent to <cf/on/
("silence means agreement").

<descrip>
        <tag>preference <m/expr/</tag> Sets the preference of routes generated by this protocol. Default: protocol dependent.

        <tag>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>debug all|off|{ states, routes, filters, interfaces, events, packets }</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>mrtdump all|off|{ states, messages }</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>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>import all | none | filter <m/name/ | filter { <m/filter commands/ } | where <m/filter expression/</tag> 
        Specify a filter to be used for filtering routes coming from
        the protocol to the routing table. <cf/all/ is shorthand for
        <cf/where true/ and <cf/none/ is shorthand for
        <cf/where false/. Default: <cf/all/.

        <tag>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/.

        <tag>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="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>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>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/.

        <tag>description "<m/text/"</tag>
        This is an optional description of the protocol. It is
        displayed as a part of the output of 'show route all' command.

        <tag>table <m/name/</tag>
        Connect this protocol to a non-default routing table.
</descrip>

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

<descrip>
        <tag><label id="dsc-iface">interface [-] [ "<m/mask/" ] [ <m/prefix/ ] [, ...] [ { <m/option/ ; [...] } ]</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 may contain a mask, a prefix, or both of them. An
        interface matches the clause if its name matches the mask (if
        specified) and its address matches the prefix (if specified). Mask is
        specified as shell-like pattern. For IPv6, the prefix part of a clause
        is generally ignored and interfaces are matched just 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 parsed left-to-right, thus
        <cf/interface "eth0", -"eth*", "*";/ means eth0 and all
        non-ethernets.

        An interface option can be used more times with different
        interfaces-specific options, in that case for given interface
        the first matching interface option is used.
        
        This option is allowed in Direct, OSPF, RIP and RAdv protocols,
        but in OSPF protocol it is used in <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 -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="dsc-prio">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>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="dsc-pass">password "<m/password/" [ { id <m/num/; generate from <m/time/; generate to <m/time/; accept from <m/time/; accept to <m/time/; } ]</tag>
        Specifies a password that can be used by the protocol. 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 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>id <M>num</M></tag>
         ID of the password, (0-255). If it's 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>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>generate to "<m/time/"</tag>
         The last time of the usage of the password for packet signing.

        <tag>accept from "<m/time/"</tag>
         The start time of the usage of the password for packet verification.

        <tag>accept to "<m/time/"</tag>
         The last time of the usage of the password for packet verification.
</descrip>

<chapt>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>show status</tag>
        Show router status, that is BIRD version, uptime and time from last reconfiguration.

        <tag>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.

        <tag>show ospf interface [<m/name/] ["<m/interface/"]</tag>
        Show detailed information about OSPF interfaces.

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

        <tag>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>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>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>show static [<m/name/]</tag>
        Show detailed information about static routes.

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

        <tag>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>show route [[for] <m/prefix/|<m/IP/] [table <m/sym/] [filter <m/f/|where <m/c/] [(export|preexport) <m/p/] [protocol <m/p/] [<m/options/]</tag>
        Show contents of a routing table (by default of the main one or
        the table attached to a respective protocol),
        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>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/ and <cf/preexport/ switches ask for printing of entries
        that are exported to the specified protocol. With <cf/preexport/, the
        export filter of the protocol is skipped.

        <p>You can also select just routes added by a specific protocol.
        <cf>protocol <m/p/</cf>.

        <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>show roa [<m/prefix/ | in <m/prefix/ | for <m/prefix/] [as <m/num/] [table <m/t/>]</tag>
        Show contents of a ROA table (by default of the first one).
        You can specify a <m/prefix/ to print ROA entries for a
        specific network. If you use <cf>for <m/prefix/</cf>, you'll
        get all entries relevant for route validation of the network
        prefix; i.e., ROA entries whose prefixes cover the network
        prefix. Or you can use <cf>in <m/prefix/</cf> to get ROA entries
        covered by the network prefix. You could also use <cf/as/ option
        to show just entries for given AS.

        <tag>add roa <m/prefix/ max <m/num/] as <m/num/ [table <m/t/>]</tag>
        Add a new ROA entry to a ROA table. Such entry is called
        <it/dynamic/ compared to <it/static/ entries specified in the
        config file. These dynamic entries survive reconfiguration.

        <tag>delete roa <m/prefix/ max <m/num/] as <m/num/ [table <m/t/>]</tag>
        Delete the specified ROA entry from a ROA table. Only dynamic
        ROA entries (i.e., the ones added by <cf/add roa/ command) can
        be deleted.

        <tag>flush roa [table <m/t/>]</tag>
        Remove all dynamic ROA entries from a ROA table.

        <tag>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 becames
        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>configure confirm</tag>
        Deactivate the config undo timer and therefore confirm the current
        configuration.

        <tag>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>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>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>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/down/
        Shut BIRD down.

        <tag>debug <m/protocol/|<m/pattern/|all all|off|{ states | routes | filters | events | packets }</tag>
        Control protocol debugging.

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

        <tag>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="dsc-log" name="log option"> for a list of log classes.

        <tag>eval <m/expr/</tag>
        Evaluate given expression.

</descrip>

<chapt>Filters

<sect>Introduction

<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 defines 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 declared 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

<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 in the foot).

<descrip>
        <tag/bool/ 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/int/ This is a general integer type, you can expect it to store
          signed values from -2000000000 to +2000000000. Overflows are not
          checked. You can use <cf/0x1234/ syntax to write hexadecimal values.

        <tag/pair/ 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/quad/ 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/string/ 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"/. Additionaly matching
          <cf/&tilde;/ operator could be used to match a string value against a
          shell pattern (represented also as a string).

        <tag/ip/ This type can hold a single IP address. Depending on the
          compile-time configuration of BIRD you are using, it is either an IPv4
          or IPv6 address. 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/prefix/ This type can hold a network prefix consisting of IP
          address and prefix length. Prefix literals are written
          as <cf><M>ipaddress</M>/<M>pxlen</M></cf>, or
          <cf><m>ipaddress</m>/<m>netmask</m></cf>. There are two special
          operators on prefixes: <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.pxlen = 16</cf> is true.

        <tag/ec/ 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/int|pair|quad|ip|prefix|ec|enum set/ 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).

          You can also use expressions for int, pair and EC 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>/<m/len/+</cf> is a shorthand for
          <cf><m>address</m>/<m/len/{<m/len/,<m/maxlen/}</cf> (where <cf><m>maxlen</m></cf> is 32 for IPv4 and 128 for IPv6), 
          that means network prefix <cf><m>address</m>/<m/len/</cf> and all its subnets. <cf><m>address</m>/<m/len/-</cf>
          is a shorthand for <cf><m>address</m>/<m/len/{0,<m/len/}</cf>, that means network prefix <cf><m>address</m>/<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>.

        <tag/enum/
          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/bgppath/
          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/.

          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.

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

          <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/bgpmask/
          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>.
          There is also old syntax that uses / .. / instead of [= .. =] and ? instead of *.

        <tag/clist/
          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>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/eclist/
          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;/ membership operator) can be
          used to modify or test eclists, with ECs instead of pairs as
          arguments.
</descrip>

<sect>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;/ for "is 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 6483 route
origin validation for a given network prefix. The basic usage
is <cf>roa_check(<m/table/)</cf>, which checks 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

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

<p>Syntax of a condition is: <cf>if
<M>boolean expression</M> then <M>command1</M>; else <M>command2</M>;</cf> and you can use <cf>{
<M>command_1</M>; <M>command_2</M>; <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, <cf><m>command1</m></cf> is executed, otherwise <cf><m>command2</m></cf> 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

<p>A filter is implicitly passed a route, and it can access its
attributes just like it accesses variables. 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 clist type, where
undefined value is regarded as empty clist for most purposes.

<descrip>
        <tag><m/prefix/ net</tag>
        Network the route is talking about. Read-only. (See the chapter about routing tables.)

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

        <tag><m/ip/ from</tag>
        The router which the route has originated from.
        
        <tag><m/ip/ gw</tag>
        Next hop packets routed using this route should be forwarded to.

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

        <tag><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/.

        <tag><m/enum/ cast</tag>
        Route type (Currently <cf/RTC_UNICAST/ for normal routes,
        <cf/RTC_BROADCAST/, <cf/RTC_MULTICAST/, <cf/RTC_ANYCAST/ will
        be used in the future for broadcast, multicast and anycast
        routes). Read-only.

        <tag><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><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. Read-only.

        <tag><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><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). It is also used when the route is exported
        to OSPF as a default value for OSPF type 1 metric.
</descrip>

<p>There also exist some protocol-specific attributes which are described in the corresponding protocol sections.

<sect>Other statements

<p>The following statements are available:

<descrip>
        <tag><m/variable/ = <m/expr/</tag> Set variable to a given value.

        <tag>accept|reject [ <m/expr/ ]</tag> Accept or reject the route, possibly printing <cf><m>expr</m></cf>.

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

        <tag>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>quitbird</tag>
        Terminates BIRD. Useful when debugging the filter interpreter.
</descrip>

<chapt>Protocols

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

<sect1>Introduction

<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 5880<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc5880.txt">
(some advanced features like the echo mode or authentication are not implemented),
IP transport for BFD as defined in
RFC 5881<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc5881.txt"> and
RFC 5883<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc5883.txt">
and interaction with client protocols as defined in
RFC 5882<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc5882.txt">.

<p>Note that BFD implementation in BIRD is currently a new feature in
development, expect some rough edges and possible UI and configuration changes
in the future. Also note that we currently support at most one protocol instance.

<sect1>Configuration

<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>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;;
        };
        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>interface <m/pattern [, ...]/  { <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="dsc-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>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>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 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>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>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>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>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>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>passive <m/switch/</tag>
        Generally, both BFD session endpoinds 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.
</descrip>

<sect1>Example

<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

<p>The Border Gateway Protocol is the routing protocol used for backbone
level routing in the today's Internet. Contrary to the other protocols, its convergence
doesn't 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.

<p>BIRD supports all requirements of the BGP4 standard as defined in
RFC 4271<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4271.txt">
It also supports the community attributes
(RFC 1997<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt">),
capability negotiation
(RFC 3392<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc3392.txt">),
MD5 password authentication
(RFC 2385<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2385.txt">),
extended communities
(RFC 4360<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4360.txt">),
route reflectors 
(RFC 4456<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4456.txt">),
multiprotocol extensions
(RFC 4760<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt">),
4B AS numbers 
(RFC 4893<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4893.txt">),
and 4B AS numbers in extended communities
(RFC 5668<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc5668.txt">).


For IPv6, it uses the standard multiprotocol extensions defined in
RFC 2283<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt">
including changes described in the
latest draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt">
and applied to IPv6 according to
RFC 2545<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt">.

<sect1>Route selection 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

<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>Configuration

<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>local [<m/ip/] 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).
        This parameter is mandatory.

        <tag>neighbor <m/ip/ as <m/number/</tag> Define neighboring router
        this instance will be talking to and what AS it's located in. Unless
        you use the <cf/multihop/ clause, it must be directly connected to one
        of your router's interfaces. In case the neighbor is in the same AS
        as we are, we automatically switch to iBGP. This parameter is mandatory.

        <tag>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. For multihop BGP it is recommended to
        explicitly configure <cf/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: switched off.

        <tag>source address <m/ip/</tag> Define local address we
        should use for next hop calculation and as a source address
        for the BGP session. Default: the address of the local
        end of the interface our neighbor is connected to.

        <tag>next hop self</tag> Avoid calculation of the Next Hop
        attribute and always advertise our own source address as a
        next hop.  This needs to be used only occasionally to
        circumvent misconfigurations of other routers.  Default:
        disabled.

        <tag>next hop keep</tag> Forward the received Next Hop
        attribute even in situations where the local address should be
        used instead, like when the route is sent to an interface with
        a different subnet. Default: disabled.

        <tag>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>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 if it is directly reachable,
        otherwise the neighbor IP address is used. Recursive mode
        means that the gateway is computed by an IGP routing table
        lookup for the IP address from <cf/bgp_next_hop/. 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-sorted" name="sorted
        tables">. Default: <cf/direct/ for singlehop eBGP,
        <cf/recursive/ otherwise.

        <tag>igp table <m/name/</tag> Specifies a table that is used
        as an IGP routing table. Default: the same as the table BGP is
        connected to.

        <tag>bfd <M>switch</M></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. Note that BFD protocol also has to be configured, see
        <ref id="sect-bfd" name="BFD"> section for details. Default: disabled.

        <tag>ttl security <m/switch/</tag> Use GTSM (RFC 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 5082 support is provided by Linux
        only. Default: disabled.
        
        <tag>password <m/string/</tag> Use this password for MD5 authentication
        of BGP sessions. Default: no authentication. Password has to be set by
        external utility (e.g. setkey(8)) on BSD systems.

        <tag>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>rr client</tag> Be a route reflector and treat the neighbor as
        a route reflection client. Default: disabled.

        <tag>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>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 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>secondary <m/switch/</tag> Usually, if an import 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-sorted" name="sorted">. Default: off.

        <tag>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>enable route refresh <m/switch/</tag> When BGP speaker
        changes its import filter, it has to re-examine all routes
        received from its neighbor against the new filter. As these
        routes might not be available, there is a BGP protocol
        extension Route Refresh (specified in RFC 2918) that allows
        BGP speaker to request re-advertisement of all routes from its
        neighbor. This option specifies whether BIRD advertises this
        capability and accepts such requests. Even when disabled, BIRD
        can send route refresh requests. Default: on.

        <tag>interpret communities <m/switch/</tag> RFC 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>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>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>advertise ipv4 <m/switch/</tag> Advertise IPv4 multiprotocol capability.
        This is not a correct behavior according to the strict interpretation
        of RFC 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>route limit <m/number/</tag> The maximal number of routes
        that may be imported from the protocol. If the route limit is
        exceeded, the connection is closed with an error. Limit is currently implemented as
        <cf/import limit <m/number/ action restart/. This option is obsolete and it is
        replaced by <ref id="import-limit" name="import limit option">.  Default: no limit.

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

        <tag>connect retry time <m/number/</tag> Time in seconds to wait before
        retrying a failed attempt to connect. Default: 120 seconds.

        <tag>start delay time <m/number/</tag> Delay in seconds between protocol
        startup and the first attempt to connect. Default: 5 seconds.

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

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

        <tag>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>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 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 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-sorted" name="sorted tables">.
        Default: off.

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

        <tag>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 5004. Default: off.

        <tag>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>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>Attributes

<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>bgppath <cf/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>int <cf/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>int <cf/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 4451<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt">
        for further discussion of BGP MED attribute.

        <tag>enum <cf/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>ip <cf/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>void <cf/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.

<!-- we don't handle aggregators right since they are of a very obscure type
        <tag>bgp_aggregator</tag>
-->
        <tag>clist <cf/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>eclist <cf/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>quad <cf/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>clist <cf/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.
</descrip>

<sect1>Example

<p><code>
protocol bgp {
        local as 65000;                      # Use a private AS number
        neighbor 198.51.100.130 as 64496;    # Our neighbor ...
        multihop;                            # ... which is connected indirectly
        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;
        source address 198.51.100.14;   # Use a non-standard source address
}
</code>

<sect>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.

<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

<p><descrip>
        <tag>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>primary  [ "<m/mask/" ] <m/prefix/</tag>
        If a network interface has more than one network address, BIRD
        has to choose one of them as a primary one. By default, BIRD
        chooses the lexicographically smallest address as the primary
        one.

        This option allows to specify which network address should be
        chosen as a primary one. Network addresses that match
        <m/prefix/ are preferred to non-matching addresses. If more
        <cf/primary/ options are used, the first one has the highest
        preference. If "<m/mask/" is specified, then such
        <cf/primary/ option is relevant only to matching network
        interfaces.

        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
        primary "eth0" 192.168.1.1;
        primary 192.168.0.0/16;
}
</code>

<sect>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.

<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. Although there are some
use cases that use the direct protocol (like abusing eBGP as an IGP
routing protocol), in most cases it is not needed to have these device
routes in BIRD routing table and to use the direct protocol.

<p>There is one notable case when you definitely want to use the
direct protocol -- running BIRD on BSD systems. Having high priority
device routes for directly connected networks from the direct protocol
protects kernel device routes from being overwritten or removed by IGP
routes during some transient network conditions, because a lower
priority IGP route for the same network is not exported to the kernel
routing table. This is an issue on BSD systems only, as on Linux
systems BIRD cannot change non-BIRD route in the kernel routing table.

<p>The only configurable thing about direct is what interfaces it watches:

<p><descrip>
        <tag>interface <m/pattern [, ...]/</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
        (for example 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.
</descrip>

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

<p>Example config might look like this:

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

<sect>Kernel

<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>Unfortunately, there is one thing that makes the routing table
synchronization a bit more complicated. In the kernel routing table
there are also device routes for directly connected networks. These
routes are usually managed by OS itself (as a part of IP address
configuration) and we don't want to touch that.  They are completely
ignored during the scan of the kernel tables and also the export of
device routes from BIRD tables to kernel routing tables is restricted
to prevent accidental interference. This restriction can be disabled using
<cf/device routes/ switch.

<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.

<sect1>Configuration

<p><descrip>
        <tag>persist <m/switch/</tag> Tell BIRD to leave all its routes in the
        routing tables when it exits (instead of cleaning them up).
        <tag>scan time <m/number/</tag> Time in seconds between two consecutive scans of the
        kernel routing table.
        <tag>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>device routes <m/switch/</tag> Enable export of device
        routes to the kernel routing table. By default, such routes
        are rejected (with the exception of explicitly configured
        device routes from the static protocol) regardless of the
        export filter to protect device routes in kernel routing table
        (managed by OS itself) from accidental overwriting or erasing.

        <tag>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.
</descrip>

<sect1>Attributes

<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>int <cf/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>int <cf/krt_metric/</tag> The kernel metric of
        the route.  When multiple same routes are in a kernel routing
        table, the Linux kernel chooses one with lower metric.

        <tag>ip <cf/krt_prefsrc/</tag> (Linux) The preferred source address.
        Used in source address selection for outgoing packets. Have to
        be one of IP addresses of the router.

        <tag>int <cf/krt_realm/</tag> (Linux) The realm of the route. Can be
        used for traffic classification.
</descrip>

<sect1>Example

<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
        import all;
        export all;
}

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

<sect>OSPF

<sect1>Introduction

<p>Open Shortest Path First (OSPF) is a quite complex interior gateway
protocol. The current IPv4 version (OSPFv2) is defined in RFC
2328<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt"> and
the current IPv6 version (OSPFv3) is defined in RFC 5340<htmlurl
url="ftp://ftp.rfc-editor.org/in-notes/rfc5340.txt">  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

<p>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.

<code>
protocol ospf &lt;name&gt; {
        rfc1583compat &lt;switch&gt;;
        stub router &lt;switch&gt;;
        tick &lt;num&gt;;
        ecmp &lt;switch&gt; [limit &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;;
                        rx buffer [normal|large|&lt;num&gt;];
                        type [broadcast|bcast|pointopoint|ptp|
                                nonbroadcast|nbma|pointomultipoint|ptmp];
                        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;";
                        };
                        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;";
                };
        };
}
</code>

<descrip>
        <tag>rfc1583compat <M>switch</M></tag>
         This option controls compatibility of routing table
         calculation with RFC 1583<htmlurl
         url="ftp://ftp.rfc-editor.org/in-notes/rfc1583.txt">. Default
         value is no.

        <tag>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, as suggested
         by RFC 3137<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc3137.txt">.
         In OSPFv3, the stub router behavior is announced by clearing
         the R-bit in the router LSA. Default value is no.

        <tag>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>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 maximal number of nexthops in one route. By
         default, ECMP is disabled.  If enabled, default value of the
         limit is 16.

        <tag>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>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>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>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>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>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>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>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>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>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>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>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>interface <M>pattern</M> [instance <m/num/]</tag>
         Defines that the specified interfaces belong to the area being defined.
         See <ref id="dsc-iface" name="interface"> common option for detailed description.
         In OSPFv3, you can specify instance ID for that interface
         description, so it is possible to have several instances of
         that interface with different options or even in different areas.

        <tag>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 transport area. This item cannot be in
         the backbone. In OSPFv3, you could also use several virtual
         links to one destination with different instance IDs.

        <tag>cost <M>num</M></tag>
         Specifies output cost (metric) of an interface. Default value is 10.

        <tag>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>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>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>retransmit <M>num</M></tag>
         Specifies interval in seconds between retransmissions of unacknowledged updates.
         Default value is 5.

        <tag>priority <M>num</M></tag>
         On every multiple access network (e.g., the Ethernet) Designed Router
         and Backup Designed 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>wait <M>num</M></tag>
         After start, router waits for the specified number of seconds between starting
         election and building adjacency. Default value is 40.
         
        <tag>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>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
         <m/dead count/ and <m/dead/ are used, <m/dead/ has precendence.

        <tag>rx buffer <M>num</M></tag>
         This sets the size of buffer used for receiving packets. The buffer should
         be bigger than maximal size of any packets. Value NORMAL (default)
         means 2*MTU, value LARGE means maximal allowed packet - 65535.

        <tag>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>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>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>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>strict nonbroadcast <M>switch</M></tag>
         If set, don't send hello to any undefined neighbor. This switch
         is ignored on other than NBMA or PtMP networks. Default value is no.

        <tag>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>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>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 no.

        <tag>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="sect-bfd" name="BFD"> section for details. Default value is no.

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

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

        <tag>authentication simple</tag>
         Every packet carries 8 bytes of password. Received packets
         lacking this password are ignored. This authentication mechanism is
         very weak.

        <tag>authentication cryptographic</tag>
         16-byte long MD5 digest is appended to every packet. For the digest
         generation 16-byte long passwords are used. Those passwords are 
         not sent via network, so this mechanism is quite secure.
         Packets can still be read by an attacker.

        <tag>password "<M>text</M>"</tag>
         An 8-byte or 16-byte password used for authentication.
         See <ref id="dsc-pass" name="password"> common option for detailed description.

        <tag>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

<p>OSPF defines four route attributes. Each internal route has a <cf/metric/.
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/.
If you specify both metrics only metric1 is used.

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

<p>

<code>
protocol ospf MyOSPF {
        rfc1583compat yes;
        tick 2;
        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";
                        };
                        password "def" {
                                id 2;
                                generate to "22-07-2005 17:03:21";
                                accept from "22-02-2001 11:34:06";
                        };
                };
                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>Pipe

<sect1>Introduction

<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.

<p>The Pipe protocol may work in the transparent mode mode or in the opaque mode.
In the transparent mode, 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. The transparent mode is the default mode.

<p>In the opaque mode, the Pipe protocol retransmits optimal route
from one table to the other table in a similar way like other
protocols send and receive routes. Retransmitted route will have the
source set to the Pipe protocol, which may limit access to protocol
specific route attributes. This mode is mainly for compatibility, it
is not suggested for new configs. The mode can be changed by
<tt/mode/ option.

<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

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

        <tag>mode opaque|transparent</tag> Specifies the mode for the pipe to work in. Default is transparent.
</descrip>

<sect1>Attributes

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

<sect1>Example

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

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

protocol kernel kern2 {
        table as2;
        kernel table 2;
}

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

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

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

<sect1>Introduction

<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 4861<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4861.txt">
and also the DNS extensions from
RFC 6106<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc6106.txt">.

<sect1>Configuration

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

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

        <tag>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>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>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.

        <label id="dsc-trigger"> <tag>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 returnsd 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.
</descrip>

<p>Interface specific options:

<descrip>
        <tag>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>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>min delay <m/expr/</tag>
        The minimum delay between two consecutive router advertisements,
        in seconds. Default: 3

        <tag>managed <m/switch/</tag>
        This option specifies whether hosts should use DHCPv6 for
        IP address configuration. Default: no

        <tag>other config <m/switch/</tag>
        This option specifies whether hosts should use DHCPv6 to
        receive other configuration information. Default: no

        <tag>link mtu <m/expr/</tag>
        This option specifies which value of MTU should be used by
        hosts. 0 means unspecified. Default: 0

        <tag>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>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>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>default lifetime <m/expr/ [sensitive <m/switch/]</tag>
        This option specifies the time (in seconds) how long (after
        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="dsc-trigger" name="trigger">.
        Default: 3 * <cf/max ra interval/, <cf/sensitive/ yes.

        <tag>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>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>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>onlink <m/switch/</tag>
        This option specifies whether hosts may use the advertised
        prefix for onlink determination. Default: yes

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

        <tag>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="dsc-trigger" name="trigger">. Default: 86400 (1 day), <cf/sensitive/ no.

        <tag>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="dsc-trigger" name="trigger">.
        Default: 14400 (4 hours), <cf/sensitive/ no.
</descrip>


<p>RDNSS specific options:

<descrip>
        <tag>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>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>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>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>Example

<p><code>
protocol radv {
        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

<sect1>Introduction

<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. You can read more about RIP at <HTMLURL
URL="http://www.ietf.org/html.charters/rip-charter.html" name="http://www.ietf.org/html.charters/rip-charter.html">. Both IPv4  
(RFC 1723<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1723.txt">)
and IPv6 (RFC 2080<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2080.txt">) versions of RIP are supported by BIRD, historical RIPv1 (RFC 1058<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1058.txt">)is
not currently supported. RIPv4 MD5 authentication (RFC 2082<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2082.txt">) is supported.

<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

<p>In addition to options common for all to other protocols, RIP supports the following ones:

<descrip>
        <tag/authentication none|plaintext|md5/ 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/md5/ means that packets are authenticated using a MD5 cryptographic
          hash. If you set authentication to not-none, it is a good idea to add <cf>password</cf>
          section. Default: none.

        <tag>honor always|neighbor|never </tag>specifies when should requests for dumping routing table
          be honored. (Always, when sent from a  host on a directly connected
          network or never.) Routing table updates are honored only from
          neighbors, that is not configurable. Default: never.
</descrip>

<p>There are some options that can be specified per-interface:

<descrip>
        <tag>metric <m/num/</tag>
          This option specifies the metric of the interface. Valid

        <tag>mode multicast|broadcast|quiet|nolisten|version1</tag>
          This option selects the mode for RIP to work in. If nothing is
          specified, RIP runs in multicast mode. <cf/version1/ is
          currently equivalent to <cf/broadcast/, and it makes RIP talk
          to a broadcast address even through multicast mode is
          possible. <cf/quiet/ option means that RIP will not transmit
          any periodic messages to this interface and <cf/nolisten/
          means that RIP will send to this interface butnot listen to it.

        <tag>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.

         Note that for RIPng, TTL security is a standard behavior
         (required by RFC 2080), but BIRD uses <cf/tx only/ by
         default, for compatibility with older versions. For IPv4 RIP,
         default value is no.

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

<p>The following options generally override behavior specified in RFC. If you use any of these
options, BIRD will no longer be RFC-compliant, which means it will not be able to talk to anything
other than equally configured BIRD. I have warned you.

<descrip>
        <tag>port <M>number</M></tag>
          selects IP port to operate on, default 520. (This is useful when testing BIRD, if you
          set this to an address &gt;1024, you will not need to run bird with UID==0).

        <tag>infinity <M>number</M></tag>
          selects the value of infinity, default is 16. Bigger values will make protocol convergence
          even slower.

        <tag>period <M>number</M>
          </tag>specifies the number of seconds between periodic updates. Default is 30 seconds. A lower
          number will mean faster convergence but bigger network
          load. Do not use values lower than 12.

        <tag>timeout time <M>number</M>
          </tag>specifies how old route has to be to be considered unreachable. Default is 4*<cf/period/.

        <tag>garbage time <M>number</M>
          </tag>specifies how old route has to be to be discarded. Default is 10*<cf/period/.
</descrip>

<sect1>Attributes

<p>RIP defines two route attributes:

<descrip>
        <tag>int <cf/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 importing a non-RIP route, the metric defaults to 5.

        <tag>int <cf/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 importing a non-RIP route, the tag defaults to 0.
</descrip>

<sect1>Example

<p><code>
protocol rip MyRIP_test {
        debug all;
        port 1520;
        period 12;
        garbage time 60;
        interface "eth0" { metric 3; mode multicast; };
        interface "eth*" { metric 2; mode broadcast; };
        honor neighbor;
        authentication none;
        import filter { print "importing"; accept; };
        export filter { print "exporting"; accept; };
}
</code>

<sect>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 five types of static routes: `classical' routes telling
to forward packets to a neighboring router, multipath routes
specifying several (possibly weighted) neighboring routers, device
routes specifying forwarding to hosts on a directly connected network,
recursive routes computing their nexthops by doing route table lookups
for a given IP and special routes (sink, blackhole etc.) which specify
a special action to be done instead of forwarding the packet.

<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.

<p>The Static protocol does not have many configuration options. The
definition of the protocol contains mainly a list of static routes:

<descrip>
        <tag>route <m/prefix/ via <m/ip/</tag> Static route through
        a neighboring router.
        <tag>route <m/prefix/ multipath via <m/ip/ [weight <m/num/] [via ...]</tag>
        Static multipath route. Contains several nexthops (gateways), possibly
        with their weights.
        <tag>route <m/prefix/ via <m/"interface"/</tag> Static device
        route through an interface to hosts on a directly connected network.
        <tag>route <m/prefix/ recursive <m/ip/</tag> Static recursive route,
        its nexthop depends on a route table lookup for given IP address.
        <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/.

        <tag>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>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>Static routes have no specific attributes.

<p>Example static config might look like this:

<p><code>
protocol static {
        table testable;                  # Connect to a non-default routing table
        route 0.0.0.0/0 via 198.51.100.130; # Default route
        route 10.0.0.0/8 multipath       # Multipath route
                via 198.51.100.10 weight 2
                via 198.51.100.20
                via 192.0.2.1;
        route 203.0.113.0/24 unreachable; # Sink route
        route 10.2.0.0/24 via "arc0";    # Secondary network
}
</code>

<chapt>Conclusions

<sect>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>Multipath routes
<item>Multicast routing protocols
<item>Ports to other systems
</itemize>

<sect>Getting more help

<p>If you use BIRD, you're welcome to join the bird-users mailing list
(<HTMLURL URL="mailto:bird-users@bird.network.cz" name="bird-users@bird.network.cz">)
where you can share your experiences with the other users and consult
your problems with the authors. To subscribe to the list, just send a
<tt/subscribe bird-users/ command in a body of a mail to
(<HTMLURL URL="mailto:majordomo@bird.network.cz" name="majordomo@bird.network.cz">).
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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