Unix IO: Tried to fix strange behavior after POLLHUP or POLLERR.

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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.

	<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.

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

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

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


<chapt>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.


<sect>Graceful restart

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


<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. There are two
ways how to name symbols (like protocol names, filter names, constants etc.). You
can either use a simple string starting with a letter followed by any
combination of letters and numbers (e.g. "R123", "myfilter", "bgp5") or you can
enclose the name into apostrophes (<cf/'/) and than you can use any combination
of numbers, letters. hyphens, dots and colons (e.g. "'1:strange-name'",
"'-NAME-'", "'cool::name'").

<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. <m/Filename/ could also
	be a wildcard, in that case matching files are included in alphabetic
	order. The maximal depth is 8. Note that this statement could be used
	anywhere in the config file, not just as a top-level option.

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

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

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

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

	<tag>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 <m/name/ (or with a name like
	"bgp1" generated automatically if you don't specify any	<m/name/).
	Protocol templates can be used to group common options when many
	similarly configured protocol instances are to be defined. Protocol
	instances (and other templates) can use templates by using <cf/from/
	expression and the name of the template. At the moment templates (and
	<cf/from/ expression) are not implemented for OSPF protocol.

	<tag>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 with extended clauses.

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

	<tag>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 few 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 the <cf/iso short/ format for <cf/route/ and
	<cf/protocol/ times, and the <cf/iso long/ format for <cf/base/ and
	<cf/log/ times.

	In pre-1.4.0 versions, BIRD used 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 timeformats could be set by
	<cf/old short/ and <cf/old long/ compatibility shorthands.

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

<descrip>
	<tag>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 is a mask specified as a shell-like pattern. Interfaces are
	matched by their name.

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

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

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

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

	Default: none.

	Examples:

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

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

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

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

	<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 rip interfaces [<m/name/] ["<m/interface/"]</tag>
	Show detailed information about RIP interfaces.

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

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

	<tag>show bfd sessions [<m/name/]</tag>
	Show information about BFD sessions.

	<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|noexport) <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/, <cf/preexport/ and <cf/noexport/ switches ask for
	printing of routes that are exported to the specified protocol.
	With <cf/preexport/, the export filter of the protocol is skipped.
	With <cf/noexport/, routes rejected by the export filter are printed
	instead. Note that routes not exported to the protocol for other reasons
	(e.g. secondary routes or routes imported from that protocol) are not
	printed even with <cf/noexport/.

	<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 becomes inaccessible for an administrator. The
	config timeout expiration is equivalent to <cf/configure undo/
	command. The timeout duration could be specified, default is 300 s.

	<tag>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. It is an unsigned 32bit type; i.e., you
	can expect it to store values from 0 to 4294967295. Overflows are not
	checked. You can use <cf/0x1234/ syntax to write hexadecimal values.

	<tag/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"/. Additionally 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/pxlen/</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/len/+</cf>
	is a shorthand for <cf><m/address//<m/len/{<m/len/,<m/maxlen/}</cf>
	(where <cf><m/maxlen/</cf> is 32 for IPv4 and 128 for IPv6), that means
	network prefix <cf><m/address//<m/len/</cf> and all its	subnets.
	<cf><m/address//<m/len/-</cf> is a shorthand for
	<cf><m/address//<m/len/{0,<m/len/}</cf>, that means network prefix
	<cf><m/address//<m/len/</cf> and all its supernets (network prefixes
	that contain it).

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

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

	<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/.

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

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

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

	<cf>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/;
else <m/command2/;</cf> and you can use <cf>{ <m/command_1/; <m/command_2/;
<M>...</M> }</cf> instead of either command. The <cf>else</cf> clause may be
omitted. If the <cf><m>boolean expression</m></cf> is true, <m/command1/ is
executed, otherwise <m/command2/ 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>Babel

<sect1>Introduction

<p>The Babel protocol (RFC6126) is a loop-avoiding distance-vector routing
protocol that is robust and efficient both in ordinary wired networks and in
wireless mesh networks. Babel is conceptually very simple in its operation and
"just works" in its default configuration, though some configuration is possible
and in some cases desirable.

<p>While the Babel protocol is dual stack (i.e., can carry both IPv4 and IPv6
routes over the same IPv6 transport), BIRD presently implements only the IPv6
subset of the protocol. No Babel extensions are implemented, but the BIRD
implementation can coexist with implementations using the extensions (and will
just ignore extension messages).

<p>The Babel protocol implementation in BIRD is currently in alpha stage.

<sect1>Configuration

<p>Babel supports no global configuration options apart from those common to all
other protocols, but supports the following per-interface configuration options:

<code>
protocol babel [<name>] {
	interface <interface pattern> {
		type <wired|wireless>;
		rxcost <number>;
		hello interval <number>;
		update interval <number>;
		port <number>;
		tx class|dscp <number>;
		tx priority <number>;
		rx buffer <number>;
		tx length <number>;
		check link <switch>;
	};
}
</code>

<descrip>
      <tag>type wired|wireless </tag>
      This option specifies the interface type: Wired or wireless. Wired
      interfaces are considered more reliable, and so the default hello
      interval is higher, and a neighbour is considered unreachable after only
      a small number of "hello" packets are lost. On wireless interfaces,
      hello packets are sent more often, and the ETX link quality estimation
      technique is used to compute the metrics of routes discovered over this
      interface. This technique will gradually degrade the metric of routes
      when packets are lost rather than the more binary up/down mechanism of
      wired type links. Default: <cf/wired/.

      <tag>rxcost <m/num/</tag>
      This specifies the RX cost of the interface. The route metrics will be
      computed from this value with a mechanism determined by the interface
      <cf/type/. Default: 96 for wired interfaces, 256 for wireless.

      <tag>hello interval <m/num/</tag>
      Interval at which periodic "hello" messages are sent on this interface,
      in seconds. Default: 4 seconds.

      <tag>update interval <m/num/</tag>
      Interval at which periodic (full) updates are sent. Default: 4 times the
      hello interval.

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

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

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

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

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


<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.

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

<sect1>Configuration

<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 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 endpoints try to establish the session by
	sending control packets to the other side. This option allows to enable
	passive mode, which means that the router does not send BFD packets
	until it has received one from the other side. Default: disabled.
</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 other protocols, its convergence
does not rely on all routers following the same rules for route selection,
making it possible to implement any routing policy at any router in the network,
the only restriction being that if a router advertises a route, it must accept
and forward packets according to it.

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

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

<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 5492<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc5492.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">),
graceful restart
(RFC 4724<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4724.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 4760<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4760.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/] [port <m/number/] [as <m/number/]</tag>
	Define neighboring router this instance will be talking to and what AS
	it is located in. In case the neighbor is in the same AS as we are, we
	automatically switch to iBGP. Optionally, the remote port may also be
	specified. The parameter may be used multiple times with different
	sub-options (e.g., both <cf/neighbor 10.0.0.1 as 65000;/ and
	<cf/neighbor 10.0.0.1; neighbor as 65000;/ are valid). This parameter is
	mandatory.

	<tag>interface <m/string/</tag>
	Define interface we should use for link-local BGP IPv6 sessions.
	Interface can also be specified as a part of <cf/neighbor address/
	(e.g., <cf/neighbor fe80::1234%eth0 as 65000;/). It is an error to use
	this parameter for non link-local sessions.

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

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

	<tag>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/. Note that there is just one level of indirection in
	recursive mode - the route obtained by the lookup must not be recursive
	itself, to prevent mutually recursive routes.

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

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

	<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 (RFC 2385).
	When used on BSD systems, see also <cf/setkey/ option below. Default:
	no authentication.

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

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

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

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

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

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

	<tag>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>enable extended messages <m/switch/</tag>
	The BGP protocol uses maximum message length of 4096 bytes. This option
	provides an extension to allow extended messages with length up
	to 65535 bytes. Default: off.

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

	<tag>connect retry time <m/number/</tag>
	Time in seconds to wait before retrying a failed attempt to connect.
	Default: 120 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 <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>There are just few configuration options for the Direct protocol:

<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 or addresses (e.g. if you're using multiple routing tables
	for policy routing and some of the policy domains don't contain all
	interfaces), just use this clause. See <ref id="dsc-iface" name="interface">
	common option for detailed description. The Direct protocol uses
	extended interface clauses.

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

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

<p>Example config might look like this:

<p><code>
protocol direct {
	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.

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

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

<sect1>Attributes

<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. Has to be one of the IP addresses of the router.

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

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

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

<sect1>Example

<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;;
	instance id &lt;num&gt;;
	stub router &lt;switch&gt;;
	tick &lt;num&gt;;
	ecmp &lt;switch&gt; [limit &lt;num&gt;];
	merge external &lt;switch&gt;;
	area &lt;id&gt; {
		stub;
		nssa;
		summary &lt;switch&gt;;
		default nssa &lt;switch&gt;;
		default cost &lt;num&gt;;
		default cost2 &lt;num&gt;;
		translator &lt;switch&gt;;
		translator stability &lt;num&gt;;

                networks {
			&lt;prefix&gt;;
			&lt;prefix&gt; hidden;
		}
                external {
			&lt;prefix&gt;;
			&lt;prefix&gt; hidden;
			&lt;prefix&gt; tag &lt;num&gt;;
		}
		stubnet &lt;prefix&gt;;
		stubnet &lt;prefix&gt; {
			hidden &lt;switch&gt;;
			summary &lt;switch&gt;;
			cost &lt;num&gt;;
		}
		interface &lt;interface pattern&gt; [instance &lt;num&gt;] {
			cost &lt;num&gt;;
			stub &lt;switch&gt;;
			hello &lt;num&gt;;
			poll &lt;num&gt;;
			retransmit &lt;num&gt;;
			priority &lt;num&gt;;
			wait &lt;num&gt;;
			dead count &lt;num&gt;;
			dead &lt;num&gt;;
			secondary &lt;switch&gt;;
			rx buffer [normal|large|&lt;num&gt;];
			tx length &lt;num&gt;;
			type [broadcast|bcast|pointopoint|ptp|
				nonbroadcast|nbma|pointomultipoint|ptmp];
			link lsa suppression &lt;switch&gt;;
			strict nonbroadcast &lt;switch&gt;;
			real broadcast &lt;switch&gt;;
			ptp netmask &lt;switch&gt;;
			check link &lt;switch&gt;;
			bfd &lt;switch&gt;;
			ecmp weight &lt;num&gt;;
			ttl security [&lt;switch&gt;; | tx only]
			tx class|dscp &lt;num&gt;;
			tx priority &lt;num&gt;;
			authentication [none|simple|cryptographic];
			password "&lt;text&gt;";
			password "&lt;text&gt;" {
				id &lt;num&gt;;
				generate from "&lt;date&gt;";
				generate to "&lt;date&gt;";
				accept from "&lt;date&gt;";
				accept to "&lt;date&gt;";
			};
			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>instance id <m/num/</tag>
	When multiple OSPF protocol instances are active on the same links, they
	should use different instance IDs to distinguish their packets. Although
	it could be done on per-interface basis, it is often preferred to set
	one instance ID to whole OSPF domain/topology (e.g., when multiple
	instances are used to represent separate logical topologies on the same
	physical network). This option specifies the default instance ID for all
	interfaces of the OSPF instance. Note that this option, if used, must
	precede interface definitions. Default value is 0.

	<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. In OSPFv3, the stub router behavior is announced by
	clearing the R-bit in the router LSA. See RFC 6987
	<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc6987.txt"> for
	details. 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 maximum number of
	nexthops in one route. By default, ECMP is disabled. If enabled,
	default	value of the limit is 16.

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

	<tag>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 OSPFv2, extended interface clauses are used, because
	each network prefix is handled as a separate virtual interface.

	You can specify alternative instance ID for the interface definition,
	therefore it is possible to have several instances of that interface
	with different options or even in different areas. For OSPFv2,
	instance ID support is an extension (RFC 6549
	<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc6549.txt">) and is
	supposed to be set per-protocol. For OSPFv3, it is an integral feature.

	<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 a transport area. This item cannot be in the backbone. Like with
	<cf/interface/ option, you could also use several virtual links to one
	destination with different instance IDs.

	<tag>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) Designated Router
	and Backup Designated router are elected. These routers have some special
	functions in the flooding process. Higher priority increases preferences
	in this election. Routers with priority 0 are not eligible. Default
	value is 1.

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

	<tag>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
	<cf/dead count/ and <cf/dead/ are used, <cf/dead/ has precedence.

	<tag>secondary <M>switch</M></tag>
	On BSD systems, older versions of BIRD supported OSPFv2 only for the
	primary IP address of an interface, other IP ranges on the interface
	were handled as stub networks. Since v1.4.1, regular operation on
	secondary IP addresses is supported, but disabled by default for
	compatibility. This option allows to enable it. The option is a
	transitional measure, will be removed in the next major release as the
	behavior will be changed. On Linux systems, the option is irrelevant, as
	operation on non-primary addresses is already the regular behavior.

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

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

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

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

	<tag>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/.

<p>Metric is ranging from 1 to infinity (65535). External routes use
<cf/metric type 1/ or <cf/metric type 2/. A <cf/metric of type 1/ is comparable
with internal <cf/metric/, a <cf/metric of type 2/ is always longer than any
<cf/metric of type 1/ or any <cf/internal metric/. <cf/Internal metric/ or
<cf/metric of type 1/ is stored in attribute <cf/ospf_metric1/, <cf/metric type
2/ is stored in attribute <cf/ospf_metric2/. If you specify both metrics only
metric1 is used.

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

<sect1>Example

<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>default preference low|medium|high</tag>
	This option specifies the Default Router Preference value to advertise
	to hosts. Default: medium.

	<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.

<p>BIRD supports RIPv1
(RFC 1058<htmlurl url="http://www.rfc-editor.org/rfc/rfc1058.txt">),
RIPv2 (RFC 2453<htmlurl url="http://www.rfc-editor.org/rfc/rfc2453.txt">),
RIPng (RFC 2080<htmlurl url="http://www.rfc-editor.org/rfc/rfc2080.txt">),
and RIP cryptographic authentication (SHA-1 not implemented)
(RFC 4822<htmlurl url="http://www.rfc-editor.org/rfc/rfc4822.txt">).

<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>RIP configuration consists mainly of common protocol options and interface
definitions, most RIP options are interface specific.

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

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

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

	<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.
</descrip>

<p>Interface specific options:

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

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

	<tag>passive <m/switch/</tag>
	Passive interfaces receive routing updates but do not transmit any
	messages. Default: no.

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

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

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

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

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

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

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

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

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

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

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

	<tag>authentication none|plaintext|cryptographic</tag>
	Selects authentication method to be used. <cf/none/ means that packets
	are not authenticated at all, <cf/plaintext/ means that a plaintext
	password is embedded into each packet, and <cf/cryptographic/ means that
	packets are authenticated using 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>password "<m/text/"</tag>
	Specifies a password used for authentication. See <ref id="dsc-pass"
	name="password"> common option for detailed description.

	<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.

	For RIPng, TTL security is a standard behavior (required by RFC 2080)
	and therefore default value is yes. For IPv4 RIP, default value is no.

	<tag>tx class|dscp|priority <m/number/</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.

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

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

	<tag>check link <m/switch/</tag>
	If set, the hardware link state (as reported by OS) is taken into
	consideration. When the link disappears (e.g. an ethernet cable is
	unplugged), neighbors are immediately considered unreachable and all
	routes received from them are withdrawn. It is possible that some
	hardware drivers or platforms do not implement this feature. Default:
	no.
</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 a
	non-RIP route is exported to RIP, the default metric is 1.

	<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 a non-RIP route is exported to RIP, the
	default tag is 0.
</descrip>

<sect1>Example

<p><code>
protocol rip {
        debug all;
        port 1520;
        period 12;
        garbage time 60;
        interface "eth0" { metric 3; mode multicast; };
	interface "eth*" { metric 2; mode broadcast; };
        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>There are three classes of definitions in Static protocol configuration --
global options, static route definitions, and per-route options. Usually, the
definition of the protocol contains mainly a list of static routes.

<p>Global options:

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

<descrip>
	<tag>route <m/prefix/ via <m/ip/</tag>
	Static route through a neighboring router. For link-local next hops,
	interface can be specified as a part of the address (e.g.,
	<cf/via fe80::1234%eth0/).

	<tag>route <m/prefix/ multipath via <m/ip/ [weight <m/num/] [bfd <m/switch/] [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/.
</descrip>

<p>Per-route options:

<descrip>
	<tag>bfd <m/switch/</tag>
	The Static protocol could use BFD protocol for next hop liveness
	detection. If enabled, a BFD session to the route next hop is created
	and the static route is BFD-controlled -- the static route is announced
	only if the next hop liveness is confirmed by BFD. If the BFD session
	fails, the static route is removed. Note that this is a bit different
	compared to other protocols, which may use BFD as an advisory mechanism
	for fast failure detection but ignores it if a BFD session is not even
	established.

	This option can be used for static routes with a direct next hop, or
	also for for individual next hops in a static multipath route (see
	above). Note that BFD protocol also has to be configured, see
	<ref id="sect-bfd" name="BFD"> section for details. Default value is no.

	<tag><m/filter expression/</tag>
	This is a special option that allows filter expressions to be configured
	on per-route basis. Can be used multiple times. These expressions are
	evaluated when the route is originated, similarly to the import filter
	of the static protocol. This is especially useful for configuring route
	attributes, e.g., <cf/ospf_metric1 = 100;/ for a route that will be
	exported to the OSPF protocol.
</descrip>

<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
	check link;			# Advertise routes only if link is up
	route 0.0.0.0/0 via 198.51.100.130; # Default route
	route 10.0.0.0/8 multipath	# Multipath route
		via 198.51.100.10 weight 2
		via 198.51.100.20 bfd	# BFD-controlled next hop
		via 192.0.2.1;
	route 203.0.113.0/24 unreachable; # Sink route
	route 10.2.0.0/24 via "arc0";	# Secondary network
	route 192.168.10.0/24 via 198.51.100.100 {
		ospf_metric1 = 20;	# Set extended attribute
	}
	route 192.168.10.0/24 via 198.51.100.100 {
		ospf_metric2 = 100;	# Set extended attribute
		ospf_tag = 2;		# Set extended attribute
		bfd;			# BFD-controlled route
	}
}
</code>


<chapt>Conclusions

<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@network.cz" name="bird-users@network.cz">)
where you can share your experiences with the other users and consult
your problems with the authors. To subscribe to the list, visit
<HTMLURL URL="http://bird.network.cz/?m_list" name="http://bird.network.cz/?m_list">.
The home page of BIRD can be found at <HTMLURL URL="http://bird.network.cz/" name="http://bird.network.cz/">.

<p>BIRD is a relatively young system and it probably contains some bugs. You can
report any problems to the bird-users list and the authors will be glad to solve
them, but before you do so, please make sure you have read the available
documentation and that you are running the latest version (available at
<HTMLURL URL="ftp://bird.network.cz/pub/bird" name="bird.network.cz:/pub/bird">).
(Of course, a patch which fixes the bug is always welcome as an attachment.)

<p>If you want to understand what is going inside, Internet standards are a good
and interesting reading. You can get them from
<HTMLURL URL="ftp://ftp.rfc-editor.org/" name="ftp.rfc-editor.org"> (or a
nicely sorted version from <HTMLURL URL="ftp://atrey.karlin.mff.cuni.cz/pub/rfc"
name="atrey.karlin.mff.cuni.cz:/pub/rfc">).

<p><it/Good luck!/

</book>

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