labs-bird/doc/bird.sgml

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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 Basic Internet Routing Daemon
</abstract>

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

<!-- Begin the document -->

<sect>Introduction

<sect1>What is bird

<p><label id="intro">
You may wonder what 'bird' means. It is acronym of 'Basic Internet Routing Daemon', and we think
that's cool name. Its task is similar to what firmware of Cisco routers does, or what gated
(<HTMLURL URL="fixme">) does. However, you can not run Cisco's firmware on "normal" computer
and gated is really hard to configure and comes under wrong license. Bird is being developed on
Charles University, Prague, and can be freely distributed under terms of GNU General Public
License. Bird is designed to run on unix and unix-like systems, it is primarily developed on Linux.

<sect1>About this documentation

<p>This documentation can have 4 forms: extended html (this is master copy), html with stripped
extensions, ascii text (generated from html) and dvi/postscript (generated from html using
html2latex and latex). You should always edit master copy; if you do so be sure to read comment at
beggining of file. If you want to view documentation, you can either launch your www browser at
master copy (and hope that browser does not have incompatible extensions from our), or you can
generate nice printed copy.

<sect1>Configuration

<p>Bird is configured using text configuration file. At startup, bird reads <file/bird.conf/
(unless -c command line parameter is given). Really simple configuration file might look like this:

<cf>

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;
}
</cf>

<p>You can find example of more complicated configuration file in <file>doc/bird.conf.example</file>.

<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
interpretted. Filter using many features can be found in <file>filter/test.conf</file>. 

<p>There's one strange thing with filter language: it does not permit you to create loops. There's
no equivalent of while() or for() command, and recursive functions are not permitted.

<p pgm>You can find sources of filters language in
<file>filter/</file> directory. <file>filter/config.Y</file> contains
filter gramar, 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 c>struct f_val</TT>, which contains type of
value and value.

<p>Filter basically looks like this:

<cf>
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";
}
</cf>

<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><I>type name</I>;</cf>, where each pair defines one local variable. Body consists of
<cf> { <I>statments</I> }</cf>. Statements are terminated by <cf/;/. You can group
several statments into one by <cf>{ <I>statments</I> }</cf> construction, that is usefull if
you want to make bigger block of code conditional.

<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 statments.

	<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 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 addresses are written (as you would expect) as
	  <cf/1.2.3.4/. You can apply special operator <cf>.mask(<I>num</I>)</cf>
	  on values of type ip. It masks out all but first <cf><I>num</I></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><I>ip address</I>/<I>px
	  len</I></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/set int|ip|prefix|pair/
	  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 prefixlength 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/
	  enumerational types are halfway-internal in the bird. You can not define your own
	  variable of enumerational type, but some pre-defined variables are of enumerational
	  type. Enumerational types are incompatible with each other, again, its for your
	  protection.
</descrip>

<sect1>Operations

<p>Filter language supports common integer operations (+,-,*,/), parenthesis (a*(b+c)), comparation
(a=b, a!=b, a&lt;b, a&gt;=b). Special operators include ~ 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. Its result
is true if element is in given set or if ip adress is inside given prefix.

<sect1>Functions

<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:

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

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

<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>.

<p>Filters are declared in similar way to functions, except they can not have explicit
parameters. They get route table entry as implicit parameter.

<sect1>Control structures

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

<cf>
        case <I>argument</I> {
        2: print "dva"; print "jeste jednou dva";
        3 .. 5: print "tri az pet";
        else: print "neco jineho";
        }
</cf>

where <I>argument</I> is any argument that can be on the left side of ~ 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.)

<sect>Protocols

<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 neccessary, 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 <A
HREF="fixme">rfc1234</A>.

<sect2>Configuration

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

<descrip>
	<tag/authentication none|password|md5/
	  selects authenticaion 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 cryptographics hash. See <A
	  HREF="fixme">rfc1234</A>. If you set authentication to non-none, it is good idea to add
	  <cf>passwords { }</cf> section.
</descrip>

<p>There are two options that can be specified per-interface. First is <cf>metric</cf>, with
default one.  Second is <cf>mode broadcast|quiet|nolisten|version1</cf>, it selects mode for
rip to work in. If nothing is specified, rip runs in multicasts 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 behaviour 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 (mis-)configured bird. I warned you.

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

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

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

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

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

<p>In addition, rip defines two filter variables, both of type it. <cf>rip_metric</cf> is rip
metric of current route, <cf>rip_tag</cf> is tag of current route.

<cf>

protocol rip MyRIP_test {
        debug all;
        port 1520;
        period 7;
        garbagetime 60;
        interface "*";
        honour neighbour;
        passwords { password "ahoj" from 0 to 10;
                password "nazdar" from 10;
        }
        authentication none;
        import filter { print "importing"; accept; };
        export filter { print "exporting"; accept; };
}
</cf>

</article>