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298 lines
13 KiB
HTML
298 lines
13 KiB
HTML
<HTML><HEAD><TITLE>Bird</TITLE></HEAD><BODY>
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<!-- This is bird documentation system. It looks like html, but it is _not_ html: nonstandard
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extensions are in use in order to auto-generate nice tex source. Use TT tag to markup short
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texts that should be rendered in fixed-space font, and further specify what kind of text this
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is. Currently TT file and TT conf are being used. For multi-line texts, use PRE section, again
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with option saying what kind of section this is. Use DL conf for definition of configuration
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keywords.
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(set-fill-column 100)
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Copyright 1999 Pavel Machek <pavel@ucw.cz>, distribute under GPL version 2 or later.
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-->
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<TEX t="Insert nice, hand-generated title page here">
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<h1>Introduction</h1>
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<h2>What is bird</h2>
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<p>You may wonder what 'bird' means. It is acronym of 'Basic Internet Routing Daemon', and we think
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that's cool name. Its task is similar to what firmware of Cisco routers does, or what <A
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HREF="fixme">gated</A> does. However, you can not run Cisco's firmware on "normal" computer and
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gated is really hard to configure and comes under wrong license. Bird is being developed on Charles
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University, Prague, and can be freely distributed under terms of GNU General Public License. Bird is
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designed to run on unix and unix-like systems, it is primarily developed on Linux.
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<h2>About this documentation</h2>
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<p>This documentation can have 4 forms: extended html (this is master copy), html with stripped
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extensions, ascii text (generated from html) and dvi/postscript (generated from html using
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html2latex and latex). You should always edit master copy; if you do so be sure to read comment at
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beggining of file. If you want to view documentation, you can either launch your www browser at
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master copy (and hope that browser does not have incompatible extensions from our), or you can
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generate nice printed copy.
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<h1>Configuration</h1>
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<p>Bird is configured using text configuration file. At startup, bird reads <TT file>bird.conf</TT>
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(unless -c command line parameter is given). Really simple configuration file might look like this:
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<PRE conf>
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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
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export all; # Default is export none
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}
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protocol device {
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scan time 10; # Scan interfaces every 10 seconds
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}
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protocol rip {
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export all;
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import all;
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}
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</PRE>
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<p>You can find example of more complicated configuration file in <TT file>doc/bird.conf.example</TT>.
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<h1>Filters</h1>
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<h2>Introduction</h2>
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<p>Bird contains rather simple programming language. (No, it can not yet read mail :-). There are
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two objects in this language: filters and functions. Filters are called by bird core when route is
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being passed between protocol and main routing table, and filters may call functions. Functions may
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call other functions but recursion is not allowed. Filter language contains control structures such
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as if's and switches, but it allows no loops. Filters are
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interpretted. Filter using many features can be found in <TT file>filter/test.conf</TT>.
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<p>There's one strange thing with filter language: it does not permit you to create loops. There's
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no equivalent of while() or for() command, and recursive functions are not permitted.
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<p pgm>You can find sources of filters language in <TT file>filter/</TT> directory. <TT
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file>filter/config.Y</TT> contains filter gramar, and basically translates source from user into
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tree of <TT c>f_inst</TT> structures. These trees are later interpreted using code in <TT
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file>filter/filter.c</TT>. Filters internally work with values/variables in <TT c>struct f_val</TT>,
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which contains type of value and value.
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<p>Filter basically looks like this:
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<PRE filt>
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filter not_too_far
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int var;
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{
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if defined( rip_metric ) then
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var = rip_metric;
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else {
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var = 1;
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rip_metric = 1;
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}
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if rip_metric > 10 then
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reject "RIP metric is too big";
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else
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accept "ok";
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}
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</PRE>
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<p>As you can see, filter has a header, list of local variables, and body. Header consists of <TT
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filt>filter</TT> keyword, followed by (unique) name of filter. List of local variables consists of
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pairs <TT filt><I>type name</I>;</TT>, where each pair defines one local variable. Body consists of
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<TT filt> { <I>statments</I> }</TT>. Statements are terminated by <TT filt>;</TT>. You can group
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several statments into one by <TT filt>{ <I>statments</I> }</TT> construction, that is usefull if
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you want to make bigger block of code conditional.
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<h2>Data types</h2>
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<p>Each variable and each value has certain type. Unlike C, filters distinguish between integers and
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booleans (that is to prevent you from shooting in the foot).
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<DL filt>
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<DT>bool
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<DD>this is boolean type, it can have only two values, <TT filt>TRUE</TT> and <TT
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filt>FALSE</TT>. Boolean is not compatible with integer and is the only type you can use
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in if statments.
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<DT>int
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<DD>this is common integer, you can expect it to store signed values from -2000000000 to
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+2000000000.
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<DT>pair
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<DD>this is pair of two short integers. Each component can have values from 0 to
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65535. Constant of this type is written as <TT filt>(1234,5678)</TT>.
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<DT>string
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<DD>this is string of characters. There are no ways to modify strings in filters. You can
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pass them between functions, assign to variable of type string, print such variables, but
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you can not concatenate two strings (for example). String constants are written as <TT
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filt>"This is string constant"</TT>.
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<DT>ip
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<DD>this type can hold single ip address. Depending on version of bird you are using, it
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can be ipv4 or ipv6 address. Ipv4 addresses addresses are written (as you would expect) as
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<TT filt>1.2.3.4</TT>. You can apply special operator <TT filt>.mask(<I>num</I>)</TT>
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on values of type ip. It masks out all but first <TT filt><I>num</I></TT> bits from ip
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address. So <TT filt>1.2.3.4.mask(8) = 1.0.0.0</TT> is true.
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<DT>prefix
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<DD>this type can hold ip address, prefix len pair. Prefixes are written as <TT filt><I>ip
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address</I>/<I>px len</I></TT>. There are two special operators on prefix: <TT
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filt>.ip</TT>, which separates ip address from the pair, and <TT filt>.len</TT>, which
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separates prefix len from the pair.
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<DT>set int|ip|prefix|pair
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<DD>filters know four types of sets. Sets are similar to strings: you can pass them around
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but you can not modify them. Constant of type <TT filt>set int</TT> looks like <TT filt>
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[ 1, 2, 5..7 ]</TT>. As you can see, both simple values and ranges are permitted in
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sets. Sets of prefixes are special: you can specify which prefixes should match them by
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using <TT filt>[ 1.0.0.0/8+, 2.0.0.0/8-, 3.0.0.0/8{5,6} ]</TT>. 3.0.0.0/8{5,6} matches
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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},
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3.0.0.0/8- is shorthand for 3.0.0.0/{0,7}.
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<DT>enum
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<DD>enumerational types are halfway-internal in the bird. You can not define your own
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variable of enumerational type, but some pre-defined variables are of enumerational
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type. Enumerational types are incompatible with each other, again, its for your
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protection.
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</DL>
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<h2>Operations</h2>
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<p>Filter language supports common integer operations (+,-,*,/), parenthesis (a*(b+c)), comparation
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(a=b, a!=b, a<b, a>=b). Special operators include ~ for "in" operation. In operation can be
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used on element and set of that elements, or on ip and prefix, or on prefix and prefix. Its result
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is true if element is in given set or if ip adress is inside given prefix.
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<h2>Functions</h2>
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<p>Bird supports functions, so that you don't have to repeat same blocks of code over and
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over. Functions can have zero or more parameters, and can have local variables. Function basically
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looks like this:
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<PRE filt>
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function name ()
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int local_variable;
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{
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local_variable = 5;
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}
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function with_parameters (int parameter)
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{
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print parameter;
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}
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</PRE>
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<p>Unlike C, variables are declared after function line but before first {. You can not declare
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variables in nested blocks. Functions are called like in C: <TT filt>name(); with_parameters(5);</TT>.
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<p>Filters are declared in similar way to functions, except they can not have explicit
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parameters. They get route table entry as implicit parameter.
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<h2>Control structures</h2>
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<p>Filters support two control structures: if/then/else and case. Syntax of if/then/else is <TT
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filt>if <I>expression</I> then <I>command</I>; else <I>command</I>;<TT> and you can use <TT filt>{
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<I>command_1</I>; <I>command_2</I>; <I>...</I> }</TT> instead of one or both commands. <TT
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filt>else</TT> clause may be ommited. Case is used like this:
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<PRE filt>
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case <I>argument</I> {
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2: print "dva"; print "jeste jednou dva";
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3 .. 5: print "tri az pet";
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else: print "neco jineho";
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}
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</PRE>
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where <I>argument</I> is any argument that can be on the left side of ~ operator, and anything that
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could be member of set is allowed before :. Multiple commands are allowed without {} grouping. If
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argument matches neither of : clauses, else: clause is used. (Case is actually implemented as set
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matching, internally.)
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<h1>Protocols</h1>
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<h2>Rip</h2>
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<h3>Introduction</h3>
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<p>Rip protocol (sometimes called Rest In Pieces) is simple protocol, where each router broadcasts
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distances to all networks he can reach. When router hears distance to other network, it increments
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it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes
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unreachable, routers keep telling each other that distance is old distance plus 1 (actually, plus
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interface metric, which is usually one). After some time, distance reaches infinity (that's 15 in
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rip) and all routers know that network is unreachable. Rip tries to minimize situations where
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counting to infinity is neccessary, because it is slow. Due to infinity being 16, you can not use
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rip on networks where maximal distance is bigger than 15 hosts. You can read more about rip at <A
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HREF="fixme">rfc1234</A>.
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<h3>Configuration</h3>
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<p>In addition to options generic to other protocols, rip supports following options:
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<DL conf>
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<DT>authentication none|password|md5
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<DD>selects authenticaion method to use. None means that packets are not authenticated at
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all, password means that plaintext password is embedded into each packet, and md5 means
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that packets are authenticated using md5 cryptographics hash. See <A
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HREF="fixme">rfc1234</A>. If you set authentication to non-none, it is good idea to add
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<TT conf>passwords { }</TT> section.
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</DL>
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<p>There are two options that can be specified per-interface. First is <TT conf>metric</TT>, with
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default one. Second is <TT conf>mode broadcast|quiet|nolisten|version1</TT>, it selects mode for
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rip to work in. If nothing is specified, rip runs in multicasts mode. <TT conf>version1</TT> is
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currently equivalent to <TT conf>broadcast</TT>, and it makes rip talk at broadcast address even
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through multicast mode is possible. <TT conf>quiet</TT> option means that rip will not transmit
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periodic messages onto this interface and <TT conf>nolisten</TT> means that rip will talk to this
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interface but not listen on it.
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<p>Following options generally override specified behaviour from rfc. If you use any of these
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options, bird will no longer be rfc-compatible, which means it will not be able to talk to anything
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other than equally (mis-)configured bird. I warned you.
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<DL conf>
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<DT>port <I>number</I>
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<DD>selects IP port to operate on, default 520. (This is usefull when testing bird, if you
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set this to address >1024, you will not need to run bird with uid==0).
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<DT>infinity <I>number</I>
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<DD>select value of infinity, default 16. Bigger values will make protocol convergence
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even slower.
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<DT>period <I>number</I>
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<DD>specifies number of seconds between periodic updates. Default is 30 seconds. Lower
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number will mean faster convergence but bigger network load.
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<DT>timeouttime <I>number</I>
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<DD>specifies how old route has to be to be considered unreachable. Default is 4*period.
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<DT>garbagetime <I>number</I>
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<DD>specifies how old route has to be to be discarded. Default is 10*period.
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</DL>
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<p>In addition, rip defines two filter variables, both of type it. <TT filt>rip_metric</TT> is rip
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metric of current route, <TT filt>rip_tag</TT> is tag of current route.
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<pre conf>
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protocol rip MyRIP_test {
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debug all;
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port 1520;
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period 7;
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garbagetime 60;
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interface "*";
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honour neighbour;
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passwords { password "ahoj" from 0 to 10;
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password "nazdar" from 10;
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}
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authentication none;
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import filter { print "importing"; accept; };
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export filter { print "exporting"; accept; };
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}
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</pre>
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</BODY></HTML>
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