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<!doctype birddoc system>
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	BIRD documentation

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This documentation can have 4 forms: sgml (this is master copy), html, ASCII
text and dvi/postscript (generated from sgml using sgmltools). You should always
edit master copy.

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This is a slightly modified linuxdoc dtd. Anything in <descrip> tags is
considered definition of configuration primitives, <cf> is fragment of
configuration within normal text, <m> is "meta" information within fragment of
configuration - something in config which is not keyword.

    (set-fill-column 80)
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    Copyright 1999,2000 Pavel Machek <>, distribute under GPL version 2 or later.



<title>BIRD User's Guide
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Ondrej Filip <it/&lt;;/,
Pavel Machek <it/&lt;;/,
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Martin Mares <it/&lt;;/,
Ondrej Zajicek <it/&lt;;/
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This document contains user documentation for the BIRD Internet Routing Daemon project.
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<!-- Table of contents -->

<!-- Begin the document -->



<sect>What is BIRD

<p><label id="intro">
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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=""> and
but their capabilities are limited and they are relatively hard to configure
and maintain.
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<p>BIRD is an Internet Routing Daemon designed to avoid all of these shortcomings,
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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
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	<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
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	<item>a virtual protocol for exchange of routes between different
		routing tables on a single host
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	<item>a command-line interface allowing on-line control and inspection
		of status of the daemon
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	<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
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<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.

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<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 ...).
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<sect>Installing BIRD

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<p>On a recent UNIX system with GNU development tools (GCC, binutils, m4, make)
and Perl, installing BIRD should be as easy as:
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	make install
	vi /usr/local/etc/bird.conf
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<p>You can use <tt>./configure --help</tt> to get a list of configure
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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
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<p>You can pass several command-line options to bird:

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	<tag>-c <m/config name/</tag>
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	use given configuration file instead of <it/prefix/<file>/etc/bird.conf</file>.
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	enable debug messages and run bird in foreground.
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	<tag>-D <m/filename of debug log/</tag>
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	log debugging information to given file instead of stderr.

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	just parse the config file and exit. Return value is zero if the config
	file is valid, nonzero if there are some errors.
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	<tag>-s <m/name of communication socket/</tag>
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	use given filename for a socket for communications with the client,
	default is <it/prefix/<file>/var/run/bird.ctl</file>.
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	<tag>-P <m/name of PID file/</tag>
	create a PID file with given filename.
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	<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.
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	run bird in foreground.

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

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	apply graceful restart recovery after start.
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<p>BIRD writes messages about its work to log files or syslog (according to config).


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

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

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


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<chapt>About routing tables

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<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:
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	<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)
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	<item>preference of this route
	<item>IP address of router which told us about this route
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	<item>IP address of router we should forward the packets to using this
	<item>other attributes common to all routes
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	<item>dynamic attributes defined by protocols which may or may not be
		present (typically protocol metrics)
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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.

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<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/.


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<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, constats 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.

protocol kernel {
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	persist;		# Don't remove routes on BIRD shutdown
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	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;
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	interface "*";


<sect>Global options
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	<tag>include "<m/filename/"</tag>
	This statement causes inclusion of a new file. <m/Filename/ could also
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	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>
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	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,
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	<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,
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	<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>
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	Set global defaults of protocol debugging options. See <cf/debug/ in the
	following section. Default: off.
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	<tag>debug commands <m/number/</tag>
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	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.

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	<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>
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	Set MRTdump file name. This option must be specified to allow MRTdump
	feature. Default: no dump file.
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	<tag>mrtdump protocols all|off|{ states, messages }</tag>
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	Set global defaults of MRTdump options. See <cf/mrtdump/ in the
	following section. Default: off.

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

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	<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>
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	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.
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	<tag>template rip|bgp|... [<m/name/ [from <m/name2/]] { <m>protocol options</m> }</tag>
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	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.

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	<tag>define <m/constant/ = <m/expression/</tag>
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	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>
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 	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.
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	<tag>router id from [-] [ "<m/mask/" ] [ <m/prefix/ ] [, ...]</tag>
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	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>
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	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 ( 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.
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	<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>
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	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"/.

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	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.

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	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.

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	<tag>table <m/name/ [sorted]</tag>
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	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.
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	<tag>roa table <m/name/ [ { roa table options ... } ]</tag>
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	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.

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

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<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).

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<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

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	<tag>preference <m/expr/</tag>
	Sets the preference of routes generated by this protocol. Default:
	protocol dependent.
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	<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.
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	<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
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	(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.
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	<tag>mrtdump all|off|{ states, messages }</tag>
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	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.

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	<tag>router id <m/IPv4 address/</tag>
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	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>
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	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>
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	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/.
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	<tag>import keep filtered <m/switch/</tag>
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	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>
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	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>
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	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>
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	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/.

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

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	<tag>table <m/name/</tag>
	Connect this protocol to a non-default routing table.
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<p>There are several options that give sense only with certain protocols:
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	<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
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	interface option is described using an interface pattern. The interface
	pattern consists of a sequence of clauses (separated by commas), each
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	clause is a mask specified as a shell-like pattern. Interfaces are
	matched by their name.
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	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*", "*";/
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	means eth0 and all non-ethernets.

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	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.

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	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.

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	This option is allowed in BFD, Direct, OSPF, RAdv and RIP protocols, but
	in OSPF protocol it is used in the <cf/area/ subsection.
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	Default: none.


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

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	<cf>interface -,;</cf> - start the protocol
	on all interfaces that have address from, but not from
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	<cf>interface -,;</cf> - start the protocol
	on all interfaces that have address from, but not from
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	<cf>interface "eth*";</cf> - start the protocol on all
	ethernet interfaces that have address from

	<tag><label id="dsc-prio">tx class|dscp <m/num/</tag>
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	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).
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	<tag>tx priority <m/num/</tag>
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	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).

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	<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>
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	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
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	specified, it is a protocol-dependent decision which one is really
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	used. Specifying passwords does not mean that authentication is enabled,
	authentication can be enabled by separate, protocol-dependent
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	<cf/authentication/ option.

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	This option is allowed in OSPF and RIP protocols. BGP has also
	<cf/password/ option, but it is slightly different and described

	Default: none.

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

	<tag>id <M>num</M></tag>
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	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.
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	<tag>generate from "<m/time/"</tag>
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	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>.
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	<tag>generate to "<m/time/"</tag>
	The last time of the usage of the password for packet signing.
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	<tag>accept from "<m/time/"</tag>
	The start time of the usage of the password for packet verification.
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	<tag>accept to "<m/time/"</tag>
	The last time of the usage of the password for packet verification.

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

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<p>Here is a brief list of supported functions:
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	<tag>show status</tag>
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	Show router status, that is BIRD version, uptime and time from last
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	<tag>show interfaces [summary]</tag>
	Show the list of interfaces. For each interface, print its type, state,
	MTU and addresses assigned.

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	<tag>show protocols [all]</tag>
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	Show list of protocol instances along with tables they are connected to
	and protocol status, possibly giving verbose information, if <cf/all/ is

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	<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>
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	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
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	<tag>show ospf topology [all] [<m/name/]</tag>
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	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>
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	Show contents of an OSPF LSA database. Options could be used to filter

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	<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.

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

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

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	<tag>show symbols [table|filter|function|protocol|template|roa|<m/symbol/]</tag>
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	Show the list of symbols defined in the configuration (names of
	protocols, routing tables etc.).
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	<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>
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	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.
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	<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>).
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	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/.
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	<p>You can also select just routes added by a specific protocol.
	<cf>protocol <m/p/</cf>.

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	<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.

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	<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>
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	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
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	to show just entries for given AS.

	<tag>add roa <m/prefix/ max <m/num/] as <m/num/ [table <m/t/>]</tag>
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	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.
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	<tag>delete roa <m/prefix/ max <m/num/] as <m/num/ [table <m/t/>]</tag>
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	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.
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	<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>
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	Reload configuration from a given file. BIRD will smoothly switch itself
	to the new configuration, protocols are reconfigured if possible,
	restarted otherwise. Changes in filters usually lead to restart of
	affected protocols.

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

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

	<tag>configure undo</tag>
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	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.
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	<tag>configure check ["<m/config file/"]</tag>
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	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>
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	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>
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	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).

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	Shut BIRD down.

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

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<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

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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";
		accept "ok";

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<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.

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<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:
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function name ()
int local_variable;
	local_variable = 5;

function with_parameters (int parameter)
	print parameter;

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<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).

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<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.

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

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<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
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	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.

	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.

	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)/).

	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.

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

	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/ 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/ = is true.

	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> = 16</cf> is true.

	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).

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	<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.

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

	 odds = [ one, 2+1, 6-one, 2*2*2-1, 9, 11 ];
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	 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, *) ];

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	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

	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>[,,,{16,24}
	]</cf> matches prefix <cf></cf>, all subprefixes of
	<cf></cf>, all superprefixes of <cf></cf> and prefixes
	<cf/4.X.X.X/ whose prefix length is 16 to 24. <cf>[{20,24} ]</cf>
	matches all prefixes (regardless of IP address) whose prefix length is
	20 to 24, <cf>[ ]</cf> matches any prefix that contains IP
	address <cf></cf>. <cf> &tilde; [{15,17} ]</cf>
	is true, but <cf> &tilde; [ ]</cf> is false.

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

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

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	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
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	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/.

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

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	<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/.

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	<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/.

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	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/.

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	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>
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 	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 *.

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	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/.
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	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.
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<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

<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
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<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.

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<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:
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case arg1 {
	2: print "two"; print "I can do more commands without {}";
	3 .. 5: print "three to five";
	else: print "something else";
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if 1234 = i then printn "."; else {
  print "not 1234";
  print "You need {} around multiple commands";
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<sect>Route attributes

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<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.

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

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	<tag><m/int/ preference</tag>
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	Preference of the route. Valid values are 0-65535. (See the chapter
	about routing tables.)
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	<tag><m/ip/ from</tag>
	The router which the route has originated from.

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	<tag><m/ip/ gw</tag>
	Next hop packets routed using this route should be forwarded to.

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

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	<tag><m/enum/ source</tag>
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	what protocol has told me about this route. Possible values:
	<cf/RTS_OSPF_IA/, <cf/RTS_OSPF_EXT1/, <cf/RTS_OSPF_EXT2/, <cf/RTS_BGP/,
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	<tag><m/enum/ cast</tag>
	Route type (Currently <cf/RTC_UNICAST/ for normal routes,
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	<cf/RTC_BROADCAST/, <cf/RTC_MULTICAST/, <cf/RTC_ANYCAST/ will be used in
	the future for broadcast, multicast and anycast routes). Read-only.
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	<tag><m/enum/ dest</tag>
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	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,
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	<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/,
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	<tag><m/string/ ifname</tag>
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	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.
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	<tag><m/int/ ifindex</tag>
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	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
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	interfaces. Read-only.

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	<tag><m/int/ igp_metric</tag>
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	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.
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<p>There also exist some protocol-specific attributes which are described in the
corresponding protocol sections.