path: root/doc/bird.sgml

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	BIRD documentation

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    Copyright 1999,2000 Pavel Machek <pavel@ucw.cz>, distribute under GPL version 2 or later.

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<article>

<title>BIRD
<author>
Pavel Machek <tt/pavel@ucw.cz/

<date>2000
<abstract>
This document contains documentation for BIRD Internet Routing Daemon
</abstract>

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<toc>

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<sect>Introduction

<sect1>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 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 will be called routes in the rest of this document) and to adapt 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.  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.

<p>There already exist some such routing daemons (routed, GateD <HTMLURL URL="http://www.gated.org/">
and Zebra <HTMLURL URL="http://www.zebra.org">), but their capabilities are very limited and
they are very 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 Interchange Protocol (RIPv2)
	<item>the Open Shortest Path First protocol (OSPFv2)
	<item>a virtual protocol for exchange of routes between internal routing tables
	<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
		to the new configuration, not disturbing routing protocols
		unless they are affected by the configuration changes)
	<item>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's 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 2.3, but porting to other systems (even non-UNIX ones) should
be relatively easy due to its highly modular architecture).

<sect1>About this documentation

<p>This documentation can have 4 forms: sgml (this is master copy), html, ASCII text (generated from
html) and dvi/postscript (generated from sgml using sgmltools). You should always edit master copy,
it is slightly modified linuxdoc dtd.  Anything in &lt;descrip&gt; tags is considered definition of
configuration primitives, &lt;cf&gt; is fragment of configuration within normal text, &lt;m&gt; is
"meta" information within fragment of configuration -- something in config which is not keyword.

<sect1>Installing BIRD

<p>On UNIX system, installing BIRD should be as easy as:

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

<sect1>About routing tables

<p>Bird has one or more routing tables. Each routing table contains
list of known routes. Each route has certain attributes, most
important is prefix of network this route is for. Routing table
maintains more than one entry for network, but at most one entry for
one network and one protocol. The entry with biggest preference is
used for routing. If there are more entries with same preference and
they are from same protocol, protocol decides (typically according to
metrics). You can get list of route attributes in "Route attributes"
section in filters.

<sect>Configuration

<sect1>Introduction

<p>BIRD is configured using text configuration file. At startup, BIRD reads <file/bird.conf/ (unless
-c command line parameter is given). Configuration may be changed on user request: if you modify
config file and then signal BIRD with SIGHUP, it will adjust to new config. There's BIRD client,
which allows you to talk with BIRD in more extensive way than just telling it to reconfigure. BIRD
writes messages about its work to log files or syslog (according to config).

<p>Bird is configured using text configuration file. At startup, bird
reads <file/bird.conf/ (unless -c command line parameter is
given). Everything on a line after <cf/#/ is a comment, whitespace is
ignored, C-style comments <cf>/* comment */</cf> are also
recognized. If there's variable number of options, it is grouped using
<cf/{ }/ brackets. Each option is terminated by <cf/;/.

<p>Really simple configuration file might look like this:

<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;
}
</code>

<sect1>Global options

<p><descrip>
	<tag>log "<m/filename/"|syslog|stderr all|{ <m/list of classes/ }</tag> 
	set logging of classes (either all or <cf/{
	error, trace }/ etc.) into selected destination. Classes are:
	<cf/info/, <cf/warning/, <cf/error/, <cf/fatal/ for messages about local problems
	<cf/debug/ for debugging messages, 
	<cf/trace/ when you want to know what happens on network, 
	<cf/remote/ for messages about misbehavior of remote side, 
	<cf/auth/ about authentication failures,  
	<cf/bug/ for internal bugs
	of BIRD. You may specify more than one <cf/log/ line to log to multiple
	destinations.
				  
	<tag>debug protocols all|off|{ states, routes, filters, interfaces, events, packets }</tag>
	sets global default of protocol debugging options.

	<tag>filter <m/name/{ <m/commands/ }</tag> define filter. You can learn more about filters
	in next chapter.
 
	<tag>protocol rip|ospf|bgp|... <m/[name]/ { <m>protocol options</m> }</tag> define protocol
	instance, called name (or called something like rip5 if you omit name). You can learn more
	about configuring protocols in their own chapters. You can run more than one instance of
	most protocols (like rip or bgp).

	<tag>define constant = expression</tag> define constant. You can use it later in every place
	you could use simple integer.

	<tag>router id <m/IPv4 address/</tag> set router id. Router id needs to be world-wide
	unique. It is usually one of router's IPv4 addresses.

	<tag>table <m/name/</tag> create new routing table.

	<tag>eval <m/expr/</tag> evaluates given filter expression. It is used for testing.
</descrip>

<sect1>Protocol options

<p>Several options are per-protocol, but all protocols support them. They are described here.

<descrip>
	<tag>preference <m/expr/</tag> sets preference of routes generated by this protocol.

	<tag>disabled</tag> disables given protocol. You can disable/enable protocol from command
	line interface without needing to touch config.

	<tag>debug <m/setting/</tag> this is similar to global debug setting, except that it only
	affects one protocol. Only messages in selected debugging categories will be written to
	logs.

	<tag>import <m/filter/</tag> filter can be either either <cf> { <m>filter commands</m>
	}</cf> or <cf>filter <m/name/</cf>. Import filter works in direction from protocol to main
	routing table.

	<tag>export <m/filter/</tag> This is similar to <cf>export</cf> keyword, except that it
	works in direction from main routing table to protocol.

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

<p>There are per-protocol options that give sense only with certain protocols.

<descrip>
	<tag>passwords { password "<m/password/" from <m/time/ to <m/time/ passive <m/time/ id
	<m/num/ [...] }</tag> specifies passwords to be used with this protocol. Passive time is
	time from which password is not announced but is allowed. id is password id, as needed by
	certain protocols.

	<tag>interface "<m/mask/"|<m/prefix/ [ { <m/option/ ; [ ... ] } ]</tag> specifies, which
	interfaces this protocol is active at, and allows you to set options on
	interface-by-interface basis. Mask is specified in shell-like patters, thus <cf>interface
	"*" { mode broadcast; };</cf> will start given protocol on all interfaces, with <cf>mode
	broadcast;</cf> option.
						  
</descrip>

<sect>Filters

<sect1>Introduction

<p>BIRD contains rather simple programming language. (No, it can not yet read mail :-). There are
two objects in this language: filters and functions. Filters are called by BIRD core when route is
being passed between protocol and main routing table, and filters may call functions. Functions may
call other functions, but recursion is not allowed. Filter language contains control structures such
as if's and switches, but it allows no loops. Filters are
interpreted. Filter using many features can be found in <file>filter/test.conf</file>. 

<p>You can find sources of filters language in <file>filter/</file>
directory. <file>filter/config.Y</file> contains filter grammar, and basically translates source from
user into tree of <cf>f_inst</cf> structures. These trees are later interpreted using code in
<file>filter/filter.c</file>. Filters internally work with values/variables in <tt>struct
f_val</tt>, which contains type of value and value.

<p>Filter basically gets the route, looks at its attributes and
modifies some of them if it wishes. At the end, it decides, whether to
pass change route through (using <cf/accept/), or whether to <cf/reject/ given route. It 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, filter has a header, list of local variables, and body. Header consists of
<cf/filter/ keyword, followed by (unique) name of filter. List of local variables consists of
pairs <cf><M>type name</M>;</cf>, where each pair defines one local variable. Body consists of
<cf> { <M>statements</M> }</cf>. Statements are terminated by <cf/;/. You can group
several statements into one by <cf>{ <M>statements</M> }</cf> construction, that is useful if
you want to make bigger block of code conditional.

<p>There are two special filters, <cf/all/ (which accepts all routes) and <cf/none/ (which rejects
all routes).


<p>Bird supports functions, so that you don't have to repeat same blocks of code over and
over. Functions can have zero or more parameters, and can have local variables. Function basically
looks like this:

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

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

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

<p>Filters are declared in similar way to functions, except they can not have explicit
parameters. They get route table entry as implicit parameter. Route table entry is passed implicitly
to any functions being called. Filter must terminate with either
accept or reject statement. If there's runtime error in filter, route
is rejected. 

<sect1>Data types

<p>Each variable and each value has certain type. Unlike C, filters distinguish between integers and
booleans (that is to prevent you from shooting in the foot).

<descrip>
	<tag/bool/ this is boolean type, it can have only two values, <cf/TRUE/ and
	  <cf/FALSE/. Boolean is not compatible with integer and is the only type you can use in if
	  statements.

	<tag/int/ this is common integer, you can expect it to store signed values from -2000000000
	  to +2000000000.

	<tag/pair/ this is pair of two short integers. Each component can have values from 0 to
	  65535. Constant of this type is written as <cf/(1234,5678)/.

	<tag/string/ this is string of characters. There are no ways to modify strings in
	  filters. You can pass them between functions, assign to variable of type string, print
	  such variables, but you can not concatenate two strings (for example). String constants
	  are written as <cf/"This is a string constant"/.

	<tag/ip/ this type can hold single ip address. Depending on version of BIRD you are using, it
	  can be IPv4 or IPv6 address. IPv4 addresses are written (as you would expect) as
	  <cf/1.2.3.4/. 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 ip
	  address. So <cf/1.2.3.4.mask(8) = 1.0.0.0/ is true.

	<tag/prefix/ this type can hold ip address, prefix len pair. Prefixes are written as
	  <cf><M>ipaddress</M>/<M>pxlen</M></cf>, or
	  <cf><m>ipaddress</m>/<m>netmask</m></cf> There are two special
	  operators on prefix:
	  <cf/.ip/, which separates ip address from the pair, and <cf/.len/, which separates prefix
	  len from the pair.

	<tag/int|ip|prefix|pair set/
	  filters know four types of sets. Sets are similar to strings: you can pass them around
	  but you can not modify them. Constant of type <cf>set int</cf> looks like <cf>
	  [ 1, 2, 5..7 ]</cf>. As you can see, both simple values and ranges are permitted in
	  sets. Sets of prefixes are special: you can specify which prefixes should match them by
	  using <cf>[ 1.0.0.0/8+, 2.0.0.0/8-, 3.0.0.0/8{5,6} ]</cf>. 3.0.0.0/8{5,6} matches
	  prefixes 3.X.X.X, whose prefix length is 5 to 6. 3.0.0.0/8+ is shorthand for 3.0.0.0/{0,8},
	  3.0.0.0/8- is shorthand for 3.0.0.0/{0,7}.

	<tag/enum/
	  enumeration types are halfway-internal in the BIRD. You can not define your own
	  variable of enumeration type, but some predefined variables are of enumeration
	  type. Enumeration types are incompatible with each other, again, for your
	  protection.

	<tag/bgppath/
	  bgp path is list of autonomous systems.

	<tag/bgpmask/ 
	  bgp mask is mask used for matching bgp paths
	  (using <cf>path ~ / 2 3 5 ? / syntax </cf>). <cf/?/ is
	  really serving in "any number of autonomous systems", but we
	  did not want to use * because then it becomes too easy to
	  write <cf>/*</cf> which is start of comment.

	<tag/clist/ 
	  community list. This is similar to set of pairs,
	  except that unlike other sets, it can be modified.

	  
</descrip>

<sect1>Operations

<p>Filter language supports common integer operations <cf>(+,-,*,/)</cf>, parentheses <cf/(a*(b+c))/, comparison
<cf/(a=b, a!=b, a&lt;b, a&gt;=b)/. Special operators include <cf/&tilde;/ for "in" operation. In operation can be
used on element and set of that elements, or on ip and prefix, or on
prefix and prefix or on bgppath and bgpmask or on pair and clist. Its result
is true if element is in given set or if ip address is inside given prefix. Operator <cf/=/ is used to assign value
to variable.

<sect1>Control structures

<p>Filters support two control structures: if/then/else and case. Syntax of if/then/else is <cf>if
<M>expression</M> then <M>command</M>; else <M>command</M>;</cf> and you can use <cf>{
<M>command_1</M>; <M>command_2</M>; <M>...</M> }</cf> instead of one or both commands. <cf>else</cf>
clause may be omitted.

<p><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>. Expression after
<cf>case</cf> can be of any type that can be on the left side of &tilde; operator, and anything that could
be member of set is allowed before :. Multiple commands are allowed without {} grouping. If argument
matches neither of : clauses, else: clause is used. (Case is actually implemented as set matching,
internally.)

<p>Here is example that uses if and 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 "*** FAIL: if 1 else"; }
</code>

<sect1>Route attributes

<p>Filter is implicitly passed route, and it can access its attributes, just like it accesses variables.

<descrip>
	<tag>defined( <m>attribute</m> )</tag>
	returns TRUE if given attribute is defined. Access to undefined attribute results in runtime error.

	<tag/<m/prefix/ network/
	network this route is talking about.

	<tag/<m/ip/ from/
	who told me about this route.
	
	<tag/<m/ip/ gw/
	what is next hop packets routed using this route should be forwarded to.

	<tag/<m/enum/ source/
	what protocol told me about this route. This can have values such as <cf/RTS_RIP/ or <cf/RTS_OSPF_EXT/.
</descrip>

<p>Plus, there are protocol-specific attributes, which are described in protocol sections.

<sect1>Utility functions

<p>There are few functions you might find convenient to use:

<descrip>
	<tag>print|printn <m/expr/ [ <m/, expr .../ ]</tag>
	prints given expressions, useful mainly while debugging
	filters. Printn variant does not go to new line.

	<tag>quitbird</tag>
	terminates bird. Useful while debugging filter interpreter.
</descrip>

<sect>Protocols

<sect1>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
doesn't rely on all routers following the same rules for route selection,
making it possible to implement any routing policy at any router in the
network, the only restriction being that if a router advertises a route,
it must accept and forward packets according to it.

<p>BGP works in terms of autonomous systems (often abbreviated as AS). Each
AS is a part of the network with common management and common routing policy.
Routers within each AS usually communicate using either a interior routing
protocol (such as OSPF or RIP) or an interior variant of BGP (called iBGP).
Boundary routers at the border of the AS communicate with their peers
in the neighboring AS'es via exterior BGP (eBGP).

<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 that particular
route) in order to avoid routing loops.

<p>BIRD supports all requirements of the BGP4 standard as defined in
RFC 1771<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1771.txt">
including several enhancements from the
latest draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-09.txt">.
It also supports the community attributes as per
RFC 1997<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt">,
capability negotiation draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-cap-neg-06.txt">.
For IPv6, it uses the standard multiprotocol extensions defined in
RFC 2283<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt">
including changes described in the
latest draft <htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt">
and applied to IPv6 according to
RFC 2545<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt">.

<sect2>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 are implemented according to the following algorithm. First it uses 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 over incomplete.
	<item>Prefer the lowest value of the Multiple Exit Discriminator.
	<item>Prefer internal routes over external routes.
	<item>Prefer route with the lowest value of router ID of the
	advertising router.
</itemize>

<sect2>Configuration

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

<descrip>
	<tag>local 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.)
	This parameter is mandatory.
	<tag>neighbor <m/ip/ as <m/number/</tag> Define neighboring router
	this instance will be talking to and what AS it's located in. Unless
	you use the <cf/multihop/ clause, it must be directly connected to one
	of your router's interfaces. This parameter is mandatory.
	<tag>multihop <m/number/ via <m/ip/</tag> Configure multihop BGP to a
	neighbor which is connected at most <m/number/ hops far and to which
	we should route via our direct neighbor with address <m/ip/.
	Default: switched off.
	<tag>next hop self</tag> Avoid calculation of the Next Hop attribute
	and always advertise our own source address (see below) as a next hop.
	This needs to be used only
	occasionally to circumvent misconfigurations of other routers.
	Default: disabled.
	<tag>source address <m/ip/</tag> Define local address we should use
	for next hop calculation. Default: the address of the local end
	of the interface our neighbor is connected to.
	<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 solve the problem manually. Default: off.
	<tag>hold time <m/number/</tag> Time in seconds to wait for a keepalive
	message from the other side before considering the connection stale.
	Default: depends on agreement with the neighboring router, we prefer
	240 seconds if the other side is willing to accept it.
	<tag>startup hold time <m/number/</tag> Value of the hold timer used
	before the routers have a chance to exchange OPEN messages and agree
	on the real value. Default: 240 seconds.
	<tag>keepalive time <m/number/</tag> Delay in seconds between sending
	of two consecutive keepalive messages. Default: One third of the hold time.
	<tag>connect retry time <m/number/</tag> Time in seconds to wait before
	retrying a failed connect attempt. Default: 120 seconds.
	<tag>start delay time <m/number/</tag> Delay in seconds between protocol
	startup and first attempt to connect. Default: 5 seconds.
	<tag>error wait time <m/number/, <m/number/</tag> Minimum and maximum delay in seconds between protocol
	failure (either local or reported by the peer) and automatic startup.
	Doesn't apply when <cf/disable after error/ is configured. If consecutive
	errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.
	<tag>error forget time <m/number/</tag> Maximum time in seconds between two protocol
	failures to treat them as a error sequence which makes the <cf/error wait time/
	increase exponentially. Default: 300 seconds.
	<tag>path metric <m/switch/</tag> Enable comparison of path lengths
	when deciding which BGP route is the best one. Default: on.
	<tag>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: infinite.
	<tag>default bgp_local_pref <m/number/</tag> Value of the Local Preference
	to be used during route selection when the Local Preference attribute
	is missing. Default: 0.
</descrip>

<sect2>Attributes

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

<descrip>
	<tag>path <cf/bgp_path/</tag> Sequence of AS numbers describing the AS path
	the packet will travel through when forwarded according to this route. On
	internal BGP connections 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/ [IO]</tag> The Multiple Exit Discriminator of the route
	which is an optional attribute which is often used within the local AS to
	reflect interior distances to various boundary routers. See the route selection
	rules above for exact semantics.
	<tag>enum <cf/bgp_origin/</tag> Origin of the route: either <cf/ORIGIN_IGP/
	if the route has originated in interior routing protocol of an AS 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 which sole presence indicates that the route
	has been aggregated from multiple routes by some AS 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 a number of the AS which defines
	the community and the second one is 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's also has a complete freedom about which community
	attributes it defines and what their semantics will be.
</descrip>

<sect2>Example

<p><code>
protocol bgp {
	local as 65000;				# Use a private AS number
	neighbor 62.168.0.130 as 5588;		# Our neighbor
	multihop 20 via 62.168.0.13;		# Which is connected indirectly
	export filter {				# We use non-trivial export rules
		if source = RTS_STATIC then {	# Export only static routes
			bgp_community.add((65000,5678));  # Assign our community
			if bgp_path ~ / 65000 / then	  # Artificially increase path length
				bgp_path.prepend(65000);  # by prepending local AS number twice
			accept;
		}
		reject;
	};
	import all;
	source address 62.168.0.1;		# Use non-standard source address
}
</code>

<sect1>Device

<p>The Device protocol is not a real routing protocol as it doesn't generate
any routes and 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 protocol
require network interfaces to be defined in order to work.

<p>The only configurable thing is interface scan time:

<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 to avoid confusion by lost
	notifications, so the default time is set to a large value.
</descrip>

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

<p><code>
protocol device {
	scan time 10;		# Scan the interfaces often
}
</code>

<sect1>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>It's highly recommended to include this protocol in your configuration
unless you want to use BIRD as a route server or a route reflector, that is
on a machine which doesn't forward packets and only participates in
distribution of routing information.

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

<p><descrip>
	<tag>interface <m/pattern [, ...]/</tag> By default, the Direct
	protocol will generate device routes for all the interfaces
	available. If you want to restrict it to some subset of interfaces
	(for example if you're using multiple routing tables for policy
	routing and some of the policy domains don't contain all interfaces),
	just use this clause.
</descrip>

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

<p>Example config might look like this:

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

<sect1>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 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.

<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), 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.

<sect2>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 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>kernel table <m/number/</tag> Select which kernel table should
	this particular instance of the Kernel protocol work with. Available
	only on systems supporting multiple routing tables.
</descrip>

<p>A default simple configuration can look this way:

<p><code>
protocol kernel {
	import all;
	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>

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

<sect1>OSPF

<sect1>Pipe

<sect1>Rip

<sect2>Introduction

<p>Rip protocol (sometimes called Rest In Pieces) is simple protocol, where each router broadcasts
distances to all networks he can reach. When router hears distance to other 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 distance is old distance plus 1 (actually, plus
interface metric, which is usually one). After some time, 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 not use
rip on networks where maximal distance is bigger than 15 hosts. You can read more about rip at <HTMLURL
URL="http://www.ietf.org/html.charters/rip-charter.html">. Both IPv4 and IPv6 versions of rip are supported by BIRD.

<p>Rip is very simple protocol, and it is not too good. Slow
convergence, big network load and inability to handle bigger networks
makes it pretty much obsolete in IPv4 world. (It is still usable on
very small networks, through.) It is widely used in IPv6 world,
because they are no good implementations of OSPFv3.

<sect2>Configuration

<p>In addition to options generic to other protocols, rip supports following options:

<descrip>
	<tag/authentication none|password|md5/ selects authentication method to use. None means that
	  packets are not authenticated at all, password means that plaintext password is embedded
	  into each packet, and md5 means that packets are authenticated using md5 cryptographic
	  hash. If you set authentication to non-none, it is good idea to add <cf>passwords { }</cf>
	  section.

	<tag>honor always|neighbor|never </tag>specifies, when should be routing table updates
	  honored. (Always, when sent from host on directly connected network, or never.)
</descrip>

<p>There are two options that can be specified per-interface. First is <cf>metric</cf>, with
default one.  Second is <cf>mode multicast|broadcast|quiet|nolisten|version1</cf>, it selects mode for
rip to work in. If nothing is specified, rip runs in multicast mode. <cf>version1</cf> is
currently equivalent to <cf>broadcast</cf>, and it makes rip talk at broadcast address even
through multicast mode is possible. <cf>quiet</cf> option means that rip will not transmit
periodic messages onto this interface and <cf>nolisten</cf> means that rip will talk to this
interface but not listen on it.

<p>Following options generally override specified behavior from RFC. If you use any of these
options, BIRD will no longer be RFC-compatible, which means it will not be able to talk to anything
other than equally misconfigured BIRD. I warned you.

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

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

	<tag>period <M>number</M>
	  </tag>specifies number of seconds between periodic updates. Default is 30 seconds. Lower
	  number will mean faster convergence but bigger network load.

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

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

<sect2>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/.

	<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).
</descrip>

<sect2>Example

<p><code>
protocol rip MyRIP_test {
        debug all;
        port 1520;
        period 7;
        garbagetime 60;
        interface "eth0" { metric 3; mode multicast; } "eth1" { metric 2; mode broadcast; };
        honor neighbour;
        passwords { password "ahoj" from 0 to 10;
                password "nazdar" from 10;
        }
        authentication none;
        import filter { print "importing"; accept; };
        export filter { print "exporting"; accept; };
}
</code>

<sect1>Static

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

<p>There are three types of static routes: `classical' routes telling to
forward packets to a neighboring router, device routes specifying forwarding
to hosts on a directly connected network 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 its connected to and adds it again as soon
as the destinations becomes adjacent again.

<p>The Static protocol has no configuration options. Instead, the
definition of the protocol contains a list of static routes which
can contain:

<descrip>
	<tag>route <m/prefix/ via <m/ip/</tag> Static route through
	a neighboring router.
	<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/ drop|reject|prohibit</tag> Special routes
	specifying to drop the packet, return it as unreachable or return
	it as administratively prohibited.
</descrip>

<p>Static routes have no specific attributes.

<p>Example static config might look like this:

<p><code>
protocol static {
	table testable;				# Connect to non-default routing table
	route 0.0.0.0/0 via 62.168.0.13;	# Default route
	route 62.168.0.0/25 reject;		# Sink route
	route 10.2.0.0/24 via "arc0";		# Secondary network
}
</code>

<sect>Getting more help

<p>This is really last section of this file, should give pointers to
programmers documentation, web pages mailing lists and similar stuff.


</article>


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