Merge commit 'origin/master' into new

[thirdparty/bird.git] / doc / bird.sgml

<!doctype birddoc system>

<!--
        BIRD documentation

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.

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 100)

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

 -->

<book>

<title>BIRD User's Guide
<author>
Ondrej Filip <it/&lt;feela@network.cz&gt;/,
Pavel Machek <it/&lt;pavel@ucw.cz&gt;/,
Martin Mares <it/&lt;mj@ucw.cz&gt;/,
Ondrej Zajicek <it/&lt;santiago@crfreenet.org&gt;/
</author>

<abstract>
This document contains user documentation for the BIRD Internet Routing Daemon project.
</abstract>

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

<!-- Begin the document -->

<chapt>Introduction

<sect>What is BIRD

<p><label id="intro">
The name `BIRD' is actually an acronym standing for `BIRD Internet Routing Daemon'.
Let's take a closer look at the meaning of the name:

<p><em/BIRD/: Well, we think we have already explained that. It's an acronym standing
for `BIRD Internet Routing Daemon', you remember, don't you? :-)

<p><em/Internet Routing/: It's a program (well, a daemon, as you are going to discover in a moment)
which works as a dynamic router in an Internet type network (that is, in a network running either
the IPv4 or the IPv6 protocol). Routers are devices which forward packets between interconnected
networks in order to allow hosts not connected directly to the same local area network to
communicate with each other. They also communicate with the other routers in the Internet to discover
the topology of the network which allows them to find optimal (in terms of some metric) rules for
forwarding of packets (which are called routing tables) and to adapt themselves to the
changing conditions such as outages of network links, building of new connections and so on. Most of
these routers are costly dedicated devices running obscure firmware which is hard to configure and
not open to any changes (on the other hand, their special hardware design allows them to keep up with lots of high-speed network interfaces, better than general-purpose computer does). Fortunately, most operating systems of the UNIX family allow an ordinary 
computer to act as a router and forward packets belonging to the other hosts, but only according to
a statically configured table.

<p>A <em/Routing Daemon/ is in UNIX terminology a non-interactive program running on
background which does the dynamic part of Internet routing, that is it communicates
with the other routers, calculates routing tables and sends them to the OS kernel
which does the actual packet forwarding. There already exist other such routing
daemons: routed (RIP only), GateD (non-free), Zebra<HTMLURL URL="http://www.zebra.org">
and MRTD<HTMLURL URL="http://sourceforge.net/projects/mrt">, but their capabilities are
limited and they are relatively hard to configure and maintain.

<p>BIRD is an Internet Routing Daemon designed to avoid all of these shortcomings,
to support all the routing technology used in the today's Internet or planned to be
used in near future and to have a clean extensible architecture allowing new routing
protocols to be incorporated easily. Among other features, BIRD supports:

<itemize>
        <item>both IPv4 and IPv6 protocols
        <item>multiple routing tables
        <item>the Border Gateway Protocol (BGPv4)
        <item>the Routing Information Protocol (RIPv2)
        <item>the Open Shortest Path First protocol (OSPFv2, OSPFv3)
        <item>a virtual protocol for exchange of routes between different routing tables on a single host
        <item>a command-line interface allowing on-line control and inspection
                of status of the daemon
        <item>soft reconfiguration (no need to use complex online commands
                to change the configuration, just edit the configuration file
                and notify BIRD to re-read it and it will smoothly switch itself
                to the new configuration, not disturbing routing protocols
                unless they are affected by the configuration changes)
        <item>a powerful language for route filtering
</itemize>

<p>BIRD has been developed at the Faculty of Math and Physics, Charles University, Prague,
Czech Republic as a student project. It can be freely distributed under the terms of the GNU General
Public License.

<p>BIRD has been designed to work on all UNIX-like systems. It has been developed and
tested under Linux 2.0 to 2.4, and then ported to FreeBSD and NetBSD, porting to other
systems (even non-UNIX ones) should be relatively easy due to its highly modular architecture.

<sect>Installing BIRD

<p>On a recent UNIX system with GNU development tools (GCC, binutils, m4, make) and Perl, installing BIRD should be as easy as:

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

<p>You can use <tt>./configure --help</tt> to get a list of configure
options. The most important ones are:
<tt/--enable-ipv6/ which enables building of an IPv6 version of BIRD,
<tt/--with-protocols=/ to produce a slightly smaller BIRD executable by configuring out routing protocols you don't use, and
<tt/--prefix=/ to install BIRD to a place different from.
<file>/usr/local</file>.

<sect>Running BIRD

<p>You can pass several command-line options to bird:

<descrip>
        <tag>-c <m/config name/</tag>
        use given configuration file instead of <it/prefix/<file>/etc/bird.conf</file>.

        <tag>-d</tag>
        enable debug messages and run bird in foreground.

        <tag>-D <m/filename of debug log/</tag>
        log debugging information to given file instead of stderr.

        <tag>-p</tag>
        just parse the config file and exit. Return value is zero if the config file is valid,
        nonzero if there are some errors.

        <tag>-s <m/name of communication socket/</tag>
        use given filename for a  socket for communications with the client, default is <it/prefix/<file>/var/run/bird.ctl</file>.
</descrip>

<p>BIRD writes messages about its work to log files or syslog (according to config).

<chapt>About routing tables

<p>BIRD has one or more routing tables which may or may not be
synchronized with OS kernel and which may or may not be synchronized with
each other (see the Pipe protocol). Each routing table contains a list of
known routes. Each route consists of:

<itemize>
        <item>network prefix this route is for (network address and prefix length -- the number of bits forming the network part of the address; also known as a netmask)
        <item>preference of this route
        <item>IP address of router which told us about this route
        <item>IP address of router we should forward the packets to
        using this route
        <item>other attributes common to all routes
        <item>dynamic attributes defined by protocols which may or
        may not be present (typically protocol metrics)
</itemize>

Routing table maintains multiple entries
for a network, but at most one entry for one network and one
protocol. The entry with the highest preference is used for routing (we
will call such an entry the <it/selected route/). If
there are more entries with the same preference and they are from the same
protocol, the protocol decides (typically according to metrics). If they aren't,
an internal ordering is used to break the tie. You can
get the list of route attributes in the Route attributes section.

<p>Each protocol is connected to a routing table through two filters
which can accept, reject and modify the routes. An <it/export/
filter checks routes passed from the routing table to the protocol,
an <it/import/ filter checks routes in the opposite direction.
When the routing table gets a route from a protocol, it recalculates
the selected route and broadcasts it to all protocols connected to
the table. The protocols typically send the update to other routers
in the network.

<chapt>Configuration

<sect>Introduction

<p>BIRD is configured using a text configuration file. Upon startup, BIRD reads <it/prefix/<file>/etc/bird.conf</file> (unless the
<tt/-c/ command line option is given). Configuration may be changed at user's request: if you modify
the config file and then signal BIRD with <tt/SIGHUP/, it will adjust to the new
config. Then there's the client
which allows you to talk with BIRD in an extensive way.

<p>In the config, everything on a line after <cf/#/ or inside <cf>/*
*/</cf> is a comment, whitespace characters are treated as a single space. If there's a variable number of options, they are grouped using
the <cf/{ }/ brackets. Each option is terminated by a <cf/;/. Configuration
is case sensitive.

<p>Here is an example of a simple config file. It enables
synchronization of routing tables with OS kernel, scans for 
new network interfaces every 10 seconds and runs RIP on all network interfaces found.


<code>
protocol kernel {
        persist;                # Don't remove routes on BIRD shutdown
        scan time 20;           # Scan kernel routing table every 20 seconds
        export all;             # Default is export none
}

protocol device {
        scan time 10;           # Scan interfaces every 10 seconds
}

protocol rip {
        export all;
        import all;
        interface "*";
}
</code>


<sect>Global options

<p><descrip>
        <tag>log "<m/filename/"|syslog|stderr all|{ <m/list of classes/ }</tag> 
        Set logging of messages having the given class (either <cf/all/ or <cf/{
        error, trace }/ etc.) into selected destination. Classes are:
        <cf/info/, <cf/warning/, <cf/error/ and <cf/fatal/ for messages about local problems,
        <cf/debug/ for debugging messages, 
        <cf/trace/ when you want to know what happens in the network, 
        <cf/remote/ for messages about misbehavior of remote machines, 
        <cf/auth/ about authentication failures,
        <cf/bug/ for internal BIRD bugs. You may specify more than one <cf/log/ line to establish logging to multiple
        destinations. Default: log everything to the system log.

        <tag>debug protocols all|off|{ states, routes, filters, interfaces, events, packets }</tag>
        Set global defaults of protocol debugging options. See <cf/debug/ in the following section. Default: off.

        <tag>debug commands <m/number/</tag>
        Control logging of client connections (0 for no logging, 1 for
        logging of connects and disconnects, 2 and higher for logging of
        all client commands). Default: 0.

        <tag>mrtdump "<m/filename/"</tag>
        Set MRTdump file name. This option must be specified to allow MRTdump feature.
        Default: no dump file.

        <tag>mrtdump protocols all|off|{ states, messages }</tag>
        Set global defaults of MRTdump options. See <cf/mrtdump/ in the following section.
        Default: off.

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

        <tag>function <m/name/ (<m/parameters/) <m/local variables/ { <m/commands/ }</tag> Define a function. You can learn more
        about functions in the following chapter.
 
        <tag>protocol rip|ospf|bgp|... <m/[name]/ { <m>protocol options</m> }</tag> Define a protocol
        instance called <cf><m/name/</cf> (or with a name like "rip5" generated automatically if you don't specify any <cf><m/name/</cf>). You can learn more
        about configuring protocols in their own chapters. You can run more than one instance of
        most protocols (like RIP or BGP). By default, no instances are configured.

        <tag>define <m/constant/ = (<m/expression/)|<m/number/|<m/IP address/</tag> Define a constant. You can use it later in every place
        you could use a simple integer or an IP address.

        <tag>router id <m/IPv4 address/</tag> Set BIRD's router ID. It's a world-wide unique identification of your router, usually one of router's IPv4 addresses. Default: in IPv4 version, the lowest IP address of a non-loopback interface. In IPv6 version, this option is mandatory. 

        <tag>listen bgp [address <m/address/] [port <m/port/] [v6only]</tag>
        This option allows to specify address and port where BGP
        protocol should listen. It is global option as listening
        socket is common to all BGP instances. Default is to listen on
        all addresses (0.0.0.0) and port 179. In IPv6 mode, option
        <cf/v6only/ can be used to specify that BGP socket should
        listen to IPv6 connections only. This is needed if you want to
        run both bird and bird6 on the same port.

        <tag>table <m/name/</tag> Create a new routing table. The default
        routing table is created implicitly, other routing tables have
        to be added by this command.

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

<sect>Protocol options

<p>For each protocol instance, you can configure a bunch of options.
Some of them (those described in this section) are generic, some are
specific to the protocol (see sections talking about the protocols).

<p>Several options use a <cf><m/switch/</cf> 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 <cf><m/switch/</cf> is equivalent to <cf/on/
("silence means agreement").

<descrip>
        <tag>preference <m/expr/</tag> Sets the preference of routes generated by this protocol. Default: protocol dependent.

        <tag>disabled <m/switch/</tag> Disables the protocol. You can change the disable/enable status from the command
        line interface without needing to touch the configuration. Disabled protocols are not activated. Default: protocol is enabled.

        <tag>debug all|off|{ states, routes, filters, interfaces, events, packets }</tag>
        Set protocol debugging options. If asked, each protocol is capable of
        writing trace messages about its work to the log (with category
        <cf/trace/). You can either request printing of <cf/all/ trace messages
        or only of the types selected: <cf/states/ for protocol state changes
        (protocol going up, down, starting, stopping etc.),
        <cf/routes/ for routes exchanged with the routing table,
        <cf/filters/ for details on route filtering,
        <cf/interfaces/ for interface change events sent to the protocol,
        <cf/events/ for events internal to the protocol and
        <cf/packets/ for packets sent and received by the protocol. Default: off.

        <tag>mrtdump all|off|{ states, messages }</tag>

        Set protocol MRTdump flags. MRTdump is a standard binary
        format for logging information from routing protocols and
        daemons.  These flags control what kind of information is
        logged from the protocol to the MRTdump file (which must be
        specified by global <cf/mrtdump/ option, see the previous
        section). Although these flags are similar to flags of
        <cf/debug/ option, their meaning is different and
        protocol-specific. For BGP protocol, <cf/states/ logs BGP
        state changes and <cf/messages/ logs received BGP messages.
        Other protocols does not support MRTdump yet.

        <tag>router id <m/IPv4 address/</tag> This option can be used
        to override global router id for a given protocol. Default:
        uses global router id.

        <tag>import all | none | filter <m/name/ | filter { <m/filter commands/ } | where <m/filter expression/</tag> 
        Specify a filter to be used for filtering routes coming from the protocol to the routing table. <cf/all/ is shorthand for <cf/where true/ and <cf/none/ is shorthand for <cf/where false/. Default: <cf/all/.

        <tag>export <m/filter/</tag> This is similar to the <cf>import</cf> keyword, except that it
        works in the direction from the routing table to the protocol. Default: <cf/none/.

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

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

<p>There are several options that give sense only with certain protocols:

<descrip>
        <tag><label id="dsc-iface">interface [-] [ "<m/mask/" ] [ <m/prefix/ ] [, ...] [ { <m/option/ ; [...] } ]</tag>

        Specifies a set of interfaces on which the protocol is activated with
        given interface-specific options. A set of interfaces specified by one
        interface option is described using an interface pattern. The
        interface pattern consists of a sequence of clauses (separated by
        commas), each clause may contain a mask, a prefix, or both of them. An
        interface matches the clause if its name matches the mask (if
        specified) and its address matches the prefix (if specified). Mask is
        specified as shell-like pattern.

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

        An interface option can be used more times with different
        interfaces-specific options, in that case for given interface
        the first matching interface option is used.
        
        This option is allowed in Direct, OSPF and RIP protocols,
        but in OSPF protocol it is used in <cf/area/ subsection.

        Default: none.

        Examples:

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

        <cf>interface "eth1", "eth4", "eth5" { type pointopoint; };</cf> - start the protocol
        on enumerated interfaces with <cf>type pointopoint</cf> option.
        
        <cf>interface -192.168.1.0/24, 192.168.0.0/16;</cf> - start the protocol on all
        interfaces that have address from 192.168.0.0/16, but not
        from 192.168.1.0/24.

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

        <cf>interface "eth*" 192.168.1.0/24;</cf> - start the protocol on all
        ethernet interfaces that have address from 192.168.1.0/24.

        <tag><label id="dsc-pass">password "<m/password/" [ { id <m/num/; generate from <m/time/; generate to <m/time/; accept from <m/time/; accept to <m/time/; } ]</tag>
        Specifies a password that can be used by the protocol. Password option can
        be used more times to specify more passwords. If more passwords are
        specified, it is a protocol-dependent decision which one is really
        used. Specifying passwords does not mean that authentication is
        enabled, authentication can be enabled by separate, protocol-dependent
        <cf/authentication/ option.
        
        This option is allowed in OSPF and RIP protocols. BGP has also
        <cf/password/ option, but it is slightly different and described
        separately.

        Default: none.
</descrip>

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

<descrip>
        <tag>id <M>num</M></tag>
         ID of the password, (0-255). If it's not used, BIRD will choose
         ID based on an order of the password item in the interface. For
         example, second password item in one interface will have default
         ID 2. ID is used by some routing protocols to identify which
         password was used to authenticate protocol packets.

        <tag>generate from "<m/time/"</tag>
         The start time of the usage of the password for packet signing.
         The format of <cf><m/time/</cf> is <tt>dd-mm-yyyy HH:MM:SS</tt>.

        <tag>generate to "<m/time/"</tag>
         The last time of the usage of the password for packet signing.

        <tag>accept from "<m/time/"</tag>
         The start time of the usage of the password for packet verification.

        <tag>accept to "<m/time/"</tag>
         The last time of the usage of the password for packet verification.
</descrip>

<chapt>Remote control

<p>You can use the command-line client <file>birdc</file> to talk with
a running BIRD. Communication is done using a <file/bird.ctl/ UNIX domain
socket (unless changed with the <tt/-s/ option given to both the server and
the client). The commands can perform simple actions such as enabling/disabling
of protocols, telling BIRD to show various information, telling it to
show routing table filtered by filter, or asking BIRD to
reconfigure. Press <tt/?/ at any time to get online help. Option
<tt/-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).

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

<p>Here is a brief list of supported functions:

<descrip>
        <tag>dump resources|sockets|interfaces|neighbors|attributes|routes|protocols</tag>
        Dump contents of internal data structures to the debugging output.

        <tag>show status</tag>
        Show router status, that is BIRD version, uptime and time from last reconfiguration.

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

        <tag>show ospf interface [<m/name/] ["<m/interface/"]</tag>
        Show detailed information about OSPF interfaces.

        <tag>show ospf neighbors [<m/name/] ["<m/interface/"]</tag>
        Show a list of OSPF neighbors and a state of adjacency to them.

        <tag>show ospf state [<m/name/]</tag>
        Show detailed information about OSPF areas based on a content of link-state database.
        It shows network topology,  aggregated networks and routers from other areas and external routes.

        <tag>show ospf topology [<m/name/]</tag>
        Show a topology of OSPF areas based on a content of link-state database.
        It is just a stripped-down version of 'show ospf state'.

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

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

        <tag>show symbols</tag>
        Show the list of symbols defined in the configuration (names of protocols, routing tables etc.).

        <tag>show route [[for] <m/prefix/|<m/IP/] [table <m/sym/] [filter <m/f/|where <m/c/] [(export|preexport) <m/p/] [protocol <m/p/] [<m/options/]</tag>
        Show contents of a routing table (by default of the main one),
        that is routes, their metrics and (in case the <cf/all/ switch is given)
        all their attributes.

        <p>You can specify a <m/prefix/ if you want to print routes for a
        specific network. If you use <cf>for <m/prefix or IP/</cf>, you'll get
        the entry which will be used for forwarding of packets to the given
        destination. By default, all routes for each network are printed with
        the selected one at the top, unless <cf/primary/ is given in which case
        only the selected route is shown.

        <p>You can also ask for printing only routes processed and accepted by
        a given filter (<cf>filter <m/name/</cf> or <cf>filter { <m/filter/ }
        </cf> or matching a given condition (<cf>where <m/condition/</cf>).
        The <cf/export/ and <cf/preexport/ switches ask for printing of entries
        that are exported to the specified protocol. With <cf/preexport/, the
        export filter of the protocol is skipped.

        <p>You can also select just routes added by a specific protocol.
        <cf>protocol <m/p/</cf>.

        <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>configure [soft] ["<m/config file/"]</tag>
        Reload configuration from a given file. BIRD will smoothly
        switch itself to the new configuration, protocols are
        reconfigured if possible, restarted otherwise. Changes in
        filters usually lead to restart of affected protocols. If
        <cf/soft/ option is used, changes in filters does not cause
        BIRD to restart affected protocols, therefore already accepted
        routes (according to old filters) would be still propagated,
        but new routes would be processed according to the new
        filters.

        <tag>enable|disable|restart <m/name/|"<m/pattern/"|all</tag>
        Enable, disable or restart a given protocol instance, instances matching the <cf><m/pattern/</cf> or <cf/all/ instances.

        <tag>reload [in|out] <m/name/|"<m/pattern/"|all</tag>
        
        Reload a given protocol instance, that means re-import routes
        from the protocol instance and re-export preferred routes to
        the instance. If <cf/in/ or <cf/out/ options are used, the
        command is restricted to one direction (re-import or
        re-export).

        This command is useful if appropriate filters have changed but
        the protocol instance was not restarted (or reloaded),
        therefore it still propagates the old set of routes. For example
        when <cf/configure soft/ command was used to change filters.

        Re-export always succeeds, but re-import is protocol-dependent
        and might fail (for example, if BGP neighbor does not support
        route-refresh extension). In that case, re-export is also
        skipped. Note that for the pipe protocol, both directions are
        always reloaded together (<cf/in/ or <cf/out/ options are
        ignored in that case).

        <tag/down/
        Shut BIRD down.

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

<chapt>Filters

<sect>Introduction

<p>BIRD contains a simple programming language. (No, it can't yet read mail :-). There are
two objects in this language: filters and functions. Filters are interpreted by BIRD core when a route is
being passed between protocols and routing tables. The filter language contains control structures such
as if's and switches, but it allows no loops. An example of a filter using many features can be found in <file>filter/test.conf</file>. 

<p>Filter gets the route, looks at its attributes and
modifies some of them if it wishes. At the end, it decides whether to
pass the changed route through (using <cf/accept/) or whether to <cf/reject/ it. A simple filter looks
like this:

<code>
filter not_too_far
int var;
{
        if defined( rip_metric ) then
                var = rip_metric;
        else {
                var = 1;
                rip_metric = 1;
        }
        if rip_metric &gt; 10 then
                reject "RIP metric is too big";
        else
                accept "ok";
}
</code>

<p>As you can see, a filter has a header, a list of local variables, and a body. The header consists of
the <cf/filter/ keyword followed by a (unique) name of filter. The list of local variables consists of
<cf><M>type name</M>;</cf> pairs where each pair defines one local variable. The body consists of
<cf> { <M>statements</M> }</cf>. Each <m/statement/ is terminated by a <cf/;/. You can group
several statements to a single compound statement by using braces (<cf>{ <M>statements</M> }</cf>) which is useful if
you want to make a bigger block of code conditional.

<p>BIRD supports functions, so that you don't have to repeat the same blocks of code over and
over. Functions can have zero or more parameters and they can have local variables. Recursion is not allowed. Function definitions
look like this:

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

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

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

<p>Filters are declared in a way similar to functions except they can't have explicit
parameters. They get a route table entry as an implicit parameter, it is also passed automatically 
to any functions called. The filter must terminate with either
<cf/accept/ or <cf/reject/ statement. If there's a runtime error in filter, the route
is rejected. 

<p>A nice trick to debug filters is to use <cf>show route filter
<m/name/</cf> from the command line client. An example session might look
like:

<code>
pavel@bug:~/bird$ ./birdc -s bird.ctl
BIRD 0.0.0 ready.
bird> show route
10.0.0.0/8         dev eth0 [direct1 23:21] (240)
195.113.30.2/32    dev tunl1 [direct1 23:21] (240)
127.0.0.0/8        dev lo [direct1 23:21] (240)
bird> show route ?
show route [<prefix>] [table <t>] [filter <f>] [all] [primary]...
bird> show route filter { if 127.0.0.5 &tilde; net then accept; }
127.0.0.0/8        dev lo [direct1 23:21] (240)
bird>
</code>

<sect>Data types

<p>Each variable and each value has certain type. Booleans, integers and enums are
incompatible with each other (that is to prevent you from shooting in the foot).

<descrip>
        <tag/bool/ This is a boolean type, it can have only two values, <cf/true/ and
          <cf/false/. Boolean is the only type you can use in <cf/if/
          statements.

        <tag/int/ This is a general integer type, you can expect it to store signed values from -2000000000
          to +2000000000. Overflows are not checked. You can use <cf/0x1234/ syntax to write hexadecimal values.

        <tag/pair/ This is a pair of two short integers. Each component can have values from 0 to
          65535. Literals of this type are written as <cf/(1234,5678)/. The same syntax can also be
          used to construct a pair from two arbitrary integer expressions (for example <cf/(1+2,a)/).

        <tag/string/ This is a string of characters. There are no ways to modify strings in
          filters. You can pass them between functions, assign them to variables of type <cf/string/, print
          such variables, but you can't concatenate two strings. String literals
          are written as <cf/"This is a string constant"/.

        <tag/ip/ This type can hold a single IP address. Depending on the compile-time configuration of BIRD you are using, it
          is either an IPv4 or IPv6 address. IP addresses are written in the standard notation (<cf/10.20.30.40/ or <cf/fec0:3:4::1/). You can apply special operator <cf>.mask(<M>num</M>)</cf>
          on values of type ip. It masks out all but first <cf><M>num</M></cf> bits from the IP
          address. So <cf/1.2.3.4.mask(8) = 1.0.0.0/ is true.

        <tag/prefix/ This type can hold a network prefix consisting of IP address and prefix length. Prefix literals are written as
          <cf><M>ipaddress</M>/<M>pxlen</M></cf>, or
          <cf><m>ipaddress</m>/<m>netmask</m></cf>. There are two special
          operators on prefixes:
          <cf/.ip/ which extracts the IP address from the pair, and <cf/.len/, which separates prefix
          length from the pair. So <cf>1.2.0.0/16.pxlen = 16</cf> is true.

        <tag/int|ip|prefix|pair|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>set int</cf> look 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: 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> iff 
          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 iff it matches any prefix pattern in the
          prefix set literal.

          There are also two shorthands for prefix patterns: <cf><m>address</m>/<m/len/+</cf> is a shorthand for
          <cf><m>address</m>/<m/len/{<m/len/,<m/maxlen/}</cf> (where <cf><m>maxlen</m></cf> is 32 for IPv4 and 128 for IPv6), 
          that means network prefix <cf><m>address</m>/<m/len/</cf> and all its subnets. <cf><m>address</m>/<m/len/-</cf>
          is a shorthand for <cf><m>address</m>/<m/len/{0,<m/len/}</cf>, that means network prefix <cf><m>address</m>/<m/len/</cf>
          and all its supernets (network prefixes that contain it).

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

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

        <tag/enum/
          Enumeration types are fixed sets of possibilities. You can't define your own
          variables of such type, but some route attributes are of enumeration
          type. Enumeration types are incompatible with each other.

        <tag/bgppath/
          BGP path is a list of autonomous system numbers. You can't write literals of this type.
          There are several special operators on bgppaths:

          <cf><m/P/.first</cf> returns the first ASN (the neighbor ASN) in path <m/P/.

          <cf><m/P/.last</cf> returns the last ASN (the source ASN) in path <m/P/.

          Both <cf/first/ and <cf/last/ return zero if there is no appropriate ASN,
          for example if the path contains an AS set element as the first (or the last) part.

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

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

        <tag/bgpmask/
          BGP masks are patterns used for BGP path matching
          (using <cf>path &tilde; [= 2 3 5 * =]</cf> syntax). The masks
          resemble wildcard patterns as used by UNIX shells. Autonomous
          system numbers match themselves, <cf/*/ matches any (even empty)
          sequence of arbitrary AS numbers and <cf/?/ matches one arbitrary AS number.
          For example, if <cf>bgp_path</cf> is 4 3 2 1, then:
          <tt>bgp_path &tilde; [= * 4 3 * =]</tt> is true, but 
          <tt>bgp_path &tilde; [= * 4 5 * =]</tt> is false.
          BGP mask expressions can also contain integer expressions enclosed in parenthesis
          and integer variables, for example <tt>[= * 4 (1+2) a =]</tt>.
          There is also old syntax that uses / .. / instead of [= .. =] and ? instead of *.

        <tag/clist/ 
          Community list is similar to set of pairs,
          except that unlike other sets, it can be modified.
          There exist no literals of this type.
          There are two special operators on clists:

          <cf>add(<m/C/,<m/P/)</cf> adds pair <m/P/ to clist <m/C/ and returns the result.

          <cf>delete(<m/C/,<m/P/)</cf> deletes pair <m/P/ from clist <m/C/ and returns the result.

          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/.

</descrip>

<sect>Operators

<p>The filter language supports common integer operators <cf>(+,-,*,/)</cf>, parentheses <cf/(a*(b+c))/, comparison
<cf/(a=b, a!=b, a&lt;b, a&gt;=b)/. Logical operations include unary not (<cf/!/), and (<cf/&amp;&amp;/) and or (<cf/&verbar;&verbar;/). 
Special operators include <cf/&tilde;/ for "is element of a set" operation - it can be
used on element and set of elements of the same type (returning true if element is contained in the given set), or
on two strings (returning true if first string matches a shell-like pattern stored in second string) or on IP and prefix (returning true if IP is within the range defined by that prefix), or on
prefix and prefix (returning true if first prefix is more specific than second one) or on bgppath and bgpmask (returning true if the path matches the mask) or on pair and clist (returning true if the community is element of the community list).


<sect>Control structures

<p>Filters support two control structures: conditions and case switches. 

<p>Syntax of a condition is: <cf>if
<M>boolean expression</M> then <M>command1</M>; else <M>command2</M>;</cf> and you can use <cf>{
<M>command_1</M>; <M>command_2</M>; <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, <cf><m>command1</m></cf> is executed, otherwise <cf><m>command2</m></cf> is executed.

<p>The <cf>case</cf> is similar to case from Pascal. Syntax is <cf>case <m/expr/ { else |
<m/num_or_prefix [ .. num_or_prefix]/: <m/statement/ ; [ ... ] }</cf>. The expression after
<cf>case</cf> can be of any type which can be on the left side of the &tilde; operator and anything that could
be a member of a set is allowed before <cf/:/. Multiple commands are allowed without <cf/{}/ grouping.
If <cf><m/expr/</cf> matches one of the <cf/:/ clauses, statements between it and next <cf/:/ statement are executed. If <cf><m/expr/</cf> matches neither of the <cf/:/ clauses, the statements after <cf/else:/ are executed.

<p>Here is example that uses <cf/if/ and <cf/case/ structures:

<code>
case arg1 {
        2: print "two"; print "I can do more commands without {}";
        3 .. 5: print "three to five";
        else: print "something else";
}

if 1234 = i then printn "."; else { 
  print "not 1234"; 
  print "You need {} around multiple commands"; 
}
</code>

<sect>Route attributes

<p>A filter is implicitly passed a route, and it can access its
attributes just like it accesses variables. Attempts to access undefined
attribute result in a runtime error; you can check if an attribute is
defined by using the <cf>defined( <m>attribute</m> )</cf> operator.

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

        <tag><m/enum/ scope</tag>
        Address scope of the network (<cf/SCOPE_HOST/ for addresses local to this host, <cf/SCOPE_LINK/ for those specific for a physical link, <cf/SCOPE_SITE/ and <cf/SCOPE_ORGANIZATION/ for private addresses, <cf/SCOPE_UNIVERSE/ for globally visible addresses).

        <tag><m/int/ preference</tag>
        Preference of the route. Valid values are 0-65535. (See the chapter about routing tables.)

        <tag><m/ip/ from</tag>
        The router which the route has originated from. Read-only.
        
        <tag><m/ip/ gw</tag>
        Next hop packets routed using this route should be forwarded to.

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

        <tag><m/enum/ source</tag>
        what protocol has told me about this route. Possible values: <cf/RTS_DUMMY/, <cf/RTS_STATIC/, <cf/RTS_INHERIT/, <cf/RTS_DEVICE/, <cf/RTS_STATIC_DEVICE/, <cf/RTS_REDIRECT/, <cf/RTS_RIP/, <cf/RTS_OSPF/, <cf/RTS_OSPF_IA/, <cf/RTS_OSPF_EXT/, <cf/RTS_BGP/, <cf/RTS_PIPE/.

        <tag><m/enum/ cast</tag>
        Route type (<cf/RTC_UNICAST/ for normal routes, <cf/RTC_BROADCAST/, <cf/RTC_MULTICAST/, <cf/RTC_ANYCAST/ for broadcast, multicast and anycast routes). Read-only.

        <tag><m/enum/ dest</tag>
        Type of destination the packets should be sent to (<cf/RTD_ROUTER/ for forwarding to a neighboring router, <cf/RTD_NETWORK/ for routing to a directly-connected network, <cf/RTD_BLACKHOLE/ for packets to be silently discarded, <cf/RTD_UNREACHABLE/, <cf/RTD_PROHIBIT/ for packets that should be returned with ICMP host unreachable / ICMP administratively prohibited messages). Read-only.
</descrip>

<p>There also exist some protocol-specific attributes which are described in the corresponding protocol sections.

<sect>Other statements

<p>The following statements are available:

<descrip>
        <tag><m/variable/ = <m/expr/</tag> Set variable to a given value.

        <tag>accept|reject [ <m/expr/ ]</tag> Accept or reject the route, possibly printing <cf><m>expr</m></cf>.

        <tag>return <m/expr/</tag> Return <cf><m>expr</m></cf> from the current function, the function ends at this point.

        <tag>print|printn <m/expr/ [<m/, expr.../]</tag>
        Prints given expressions; useful mainly while debugging
        filters. The <cf/printn/ variant does not terminate the line.

        <tag>quitbird</tag>
        Terminates BIRD. Useful when debugging the filter interpreter.
</descrip>

<chapt>Protocols

<sect>BGP

<p>The Border Gateway Protocol is the routing protocol used for backbone
level routing in the today's Internet. Contrary to the 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. It is identified by a unique 16-bit number.
Routers within each AS usually communicate with each other 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 the particular
route) in order to avoid routing loops.

<p>BIRD supports all requirements of the BGP4 standard as defined in
RFC 4271<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4271.txt">
It also supports the community attributes
(RFC 1997<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt">),
capability negotiation
(RFC 3392<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc3392.txt">),
MD5 password authentication
(RFC 2385<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2385.txt">),
route reflectors 
(RFC 4456<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4456.txt">),
multiprotocol extensions
(RFC 4760<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt">),
and 4B AS numbers 
(RFC 4893<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4893.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">.

<sect1>Route selection rules

<p>BGP doesn't have any simple metric, so the rules for selection of an optimal
route among multiple BGP routes with the same preference are a bit more complex
and they are implemented according to the following algorithm. It starts the first
rule, if there are more "best" routes, then it uses the second rule to choose
among them and so on.

<itemize>
        <item>Prefer route with the highest Local Preference attribute.
        <item>Prefer route with the shortest AS path.
        <item>Prefer IGP origin over EGP and EGP over incomplete.
        <item>Prefer the lowest value of the Multiple Exit Discriminator.
        <item>Prefer internal routes over external ones.
        <item>Prefer the route with the lowest value of router ID of the
        advertising router.
</itemize>

<sect1>Configuration

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

<descrip>
        <tag>local 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. In case the neighbor is in the same AS
        as we are, we automatically switch to iBGP. 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>missing lladdr self|drop|ignore</tag>Next Hop attribute
        in BGP-IPv6 sometimes contains just the global IPv6 address,
        but sometimes it has to contain both global and link-local
        IPv6 addresses. This option specifies what to do if BIRD have
        to send both addresses but does not know link-local address.
        This situation might happen when routes from other protocols
        are exported to BGP, or when improper updates are received
        from BGP peers.  <cf/self/ means that BIRD advertises its own
        local address instead. <cf/drop/ means that BIRD skips that
        prefixes and logs error. <cf/ignore/ means that BIRD ignores
        the problem and sends just the global address (and therefore
        forms improper BGP update). Default: <cf/self/, unless BIRD
        is configured as a route server (option <cf/rs client/), in
        that case default is <cf/drop/, because route servers usually
        does not forward packets ifselves.
        
        <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>password <m/string/</tag> Use this password for MD5 authentication
        of BGP sessions. Default: no authentication. Password has to be set by
        external utility (e.g. setkey(8)) on BSD systems.

        <tag>passive <m/switch/</tag> Standard BGP behavior is both
        initiating outgoing connections and accepting incoming
        connections. In passive mode, outgoing connections are not
        initiated. Default: off.

        <tag>rr client</tag> Be a route reflector and treat the neighbor as
        a route reflection client. Default: disabled.

        <tag>rr cluster id <m/IPv4 address/</tag> Route reflectors use cluster id
        to avoid route reflection loops. When there is one route reflector in a cluster
        it usually uses its router id as a cluster id, but when there are more route
        reflectors in a cluster, these need to be configured (using this option) to
        use a common cluster id. Clients in a cluster need not know their cluster
        id and this option is not allowed for them. Default: the same as router id.

        <tag>rs client</tag> Be a route server and treat the neighbor
        as a route server client. A route server is used as a
        replacement for full mesh EBGP routing in Internet exchange
        points in a similar way to route reflectors used in IBGP routing.
        BIRD does not implement obsoleted RFC 1863, but uses ad-hoc implementation,
        which behaves like plain EBGP but reduces modifications to advertised route
        attributes to be transparent (for example does not prepend its AS number to
        AS PATH attribute and keep MED attribute). Default: disabled.

        <tag>enable route refresh <m/switch/</tag> When BGP speaker
        changes its import filter, it has to re-examine all routes
        received from its neighbor against the new filter. As these
        routes might not be available, there is a BGP protocol
        extension Route Refresh (specified in RFC 2918) that allows
        BGP speaker to request re-advertisement of all routes from its
        neighbor. This option specifies whether BIRD advertises this
        capability and accepts such requests. Even when disabled, BIRD
        can send route refresh requests. Default: on.

        <tag>interpret communities <m/switch/</tag> RFC 1997 demands
        that BGP speaker should process well-known communities like
        no-export (65535, 65281) or no-advertise (65535, 65282). For
        example, received route carrying a no-adverise community
        should not be advertised to any of its neighbors. If this
        option is enabled (which is by default), BIRD has such
        behavior automatically (it is evaluated when a route is
        exported to the protocol just before the export filter).
        Otherwise, this integrated processing of well-known
        communities is disabled. In that case, similar behavior can be
        implemented in the export filter.  Default: on.

        <tag>enable as4 <m/switch/</tag> BGP protocol was designed to use 2B AS numbers
        and was extended later to allow 4B AS number. BIRD supports 4B AS extension,
        but by disabling this option it can be persuaded not to advertise it and
        to maintain old-style sessions with its neighbors. This might be useful for
        circumventing bugs in neighbor's implementation of 4B AS extension.
        Even when disabled (off), BIRD behaves internally as AS4-aware BGP router.
        Default: on.

        <tag>capabilities <m/switch/</tag> Use capability advertisement
        to advertise optional capabilities. This is standard behavior
        for newer BGP implementations, but there might be some older
        BGP implementations that reject such connection attempts.
        When disabled (off), features that request it (4B AS support)
        are also disabled. Default: on, with automatic fallback to
        off when received capability-related error.

        <tag>advertise ipv4 <m/switch/</tag> Advertise IPv4 multiprotocol capability.
        This is not a correct behavior according to the strict interpretation
        of RFC 4760, but it is widespread and required by some BGP
        implementations (Cisco and Quagga). This option is relevant
        to IPv4 mode with enabled capability advertisement only. Default: on.

        <tag>route limit <m/number/</tag> The maximal number of routes
        that may be imported from the protocol. If the route limit is
        exceeded, the connection is closed with error. Default: no limit.

        <tag>disable after error <m/switch/</tag> When an error is encountered (either
        locally or by the other side), disable the instance automatically
        and wait for an administrator to fix the problem manually. Default: off.

        <tag>hold time <m/number/</tag> Time in seconds to wait for a Keepalive
        message from the other side before considering the connection stale.
        Default: depends on agreement with the neighboring router, we prefer
        240 seconds if the other side is willing to accept it.

        <tag>startup hold time <m/number/</tag> Value of the hold timer used
        before the routers have a chance to exchange open messages and agree
        on the real value. Default: 240 seconds.

        <tag>keepalive time <m/number/</tag> Delay in seconds between sending
        of two consecutive Keepalive messages. Default: One third of the hold time.

        <tag>connect retry time <m/number/</tag> Time in seconds to wait before
        retrying a failed attempt to connect. Default: 120 seconds.

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

        <tag>error wait time <m/number/,<m/number/</tag> Minimum and maximum delay in seconds between a protocol
        failure (either local or reported by the peer) and automatic restart.
        Doesn't apply when <cf/disable after error/ is configured. If consecutive
        errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.

        <tag>error forget time <m/number/</tag> Maximum time in seconds between two protocol
        failures to treat them as a error sequence which makes 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>prefer older <m/switch/</tag> Standard route selection algorithm
        breaks ties by comparing router IDs. This changes the behavior
        to prefer older routes (when both are external and from different
        peer). For details, see RFC 5004. Default: off.

        <tag>default bgp_med <m/number/</tag> Value of the Multiple Exit
        Discriminator to be used during route selection when the MED attribute
        is missing. Default: 0.

        <tag>default bgp_local_pref <m/number/</tag> A default value
        for the Local Preference attribute. It is used when a new
        Local Preference attribute is attached to a route by the BGP
        protocol itself (for example, if a route is received through
        eBGP and therefore does not have such attribute). Default: 100
        (0 in pre-1.2.0 versions of BIRD).
</descrip>

<sect1>Attributes

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

<descrip>
        <tag>bgppath <cf/bgp_path/</tag> Sequence of AS numbers describing the AS path
        the packet will travel through when forwarded according to the particular route. In case of 
        internal BGP it doesn't contain the number of the local AS.

        <tag>int <cf/bgp_local_pref/ [I]</tag> Local preference value used for
        selection among multiple BGP routes (see the selection rules above). It's
        used as an additional metric which is propagated through the whole local AS.

        <tag>int <cf/bgp_med/ [O]</tag> The Multiple Exit Discriminator of the route
        is an optional attribute which is used on on external (inter-AS) links to
        convey to an adjacent AS the optimal entry point into the local AS.
        The received attribute may be also propagated over internal BGP links
        (and this is default behavior). The attribute value is zeroed when a route
        is exported from a routing table to a BGP instance to ensure that the attribute
        received from a neighboring AS is not propagated to other neighboring ASes.
        A new value might be set in the export filter of a BGP instance.
        See RFC 4451<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt">
        for further discussion of BGP MED attribute.

        <tag>enum <cf/bgp_origin/</tag> Origin of the route: either <cf/ORIGIN_IGP/
        if the route has originated in an interior routing protocol or
        <cf/ORIGIN_EGP/ if it's been imported from the <tt>EGP</tt> protocol
        (nowadays it seems to be obsolete) or <cf/ORIGIN_INCOMPLETE/ if the origin
        is unknown.

        <tag>ip <cf/bgp_next_hop/</tag> Next hop to be used for forwarding of packets
        to this destination. On internal BGP connections, it's an address of the
        originating router if it's inside the local AS or a boundary router the
        packet will leave the AS through if it's an exterior route, so each BGP
        speaker within the AS has a chance to use the shortest interior path
        possible to this point.

        <tag>void <cf/bgp_atomic_aggr/ [O]</tag> This is an optional attribute
        which carries no value, but the sole presence of which indicates that the route
        has been aggregated from multiple routes by some router on the path from
        the originator.

<!-- we don't handle aggregators right since they are of a very obscure type
        <tag>bgp_aggregator</tag>
-->
        <tag>clist <cf/bgp_community/ [O]</tag> List of community values associated
        with the route. Each such value is a pair (represented as a <cf/pair/ data
        type inside the filters) of 16-bit integers, the first of them containing the number of the AS which defines
        the community and the second one being a per-AS identifier. There are lots
        of uses of the community mechanism, but generally they are used to carry
        policy information like "don't export to USA peers". As each AS can define
        its own routing policy, it also has a complete freedom about which community
        attributes it defines and what will their semantics be.
</descrip>

<sect1>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
                        # Assign our community
                        bgp_community.add((65000,5678));
                        # Artificially increase path length
                        # by advertising local AS number twice
                        if bgp_path ~ [= 65000 =] then    
                                bgp_path.prepend(65000);  
                        accept;
                }
                reject;
        };
        import all;
        source address 62.168.0.1;      # Use a non-standard source address
}
</code>

<sect>Device

<p>The Device protocol is not a real routing protocol.  It doesn't generate
any routes and it only serves as a module for getting information about network
interfaces from the kernel.

<p>Except for very unusual circumstances, you probably should include
this protocol in the configuration since almost all other protocols
require network interfaces to be defined for them to work with.

<sect1>Configuration

<p><descrip>
        <tag>scan time <m/number/</tag> Time in seconds between two scans
        of the network interface list. On systems where we are notified about
        interface status changes asynchronously (such as newer versions of
        Linux), we need to scan the list only in order to avoid confusion by lost
        notification messages, so the default time is set to a large value.

        <tag>primary  [ "<m/mask/" ] <m/prefix/</tag>
        If a network interface has more than one network address,
        BIRD has to choose one of them as a primary one, because some
        routing protocols (for example OSPFv2) suppose there is only
        one network address per interface. By default, BIRD chooses
        the lexicographically smallest address as the primary one.

        This option allows to specify which network address should be
        chosen as a primary one. Network addresses that match
        <m/prefix/ are preferred to non-matching addresses. If more
        <cf/primary/ options are used, the first one has the highest
        preference. If "<m/mask/" is specified, then such
        <cf/primary/ option is relevant only to matching network
        interfaces.

        In all cases, an address marked by operating system as
        secondary cannot be chosen as the primary one. 
</descrip>

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

<p><code>
protocol device {
        scan time 10;           # Scan the interfaces often
        primary "eth0" 192.168.1.1;
        primary 192.168.0.0/16;
}
</code>

<sect>Direct

<p>The Direct protocol is a simple generator of device routes for all the
directly connected networks according to the list of interfaces provided
by the kernel via the Device protocol.

<p>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 itself and only participates in
distribution of routing information.

<p>The 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>

<sect>Kernel

<p>The Kernel protocol is not a real routing protocol. Instead of communicating
the with other routers in the network, it performs synchronization of BIRD's routing
tables with the OS kernel. Basically, it sends all routing table updates to the kernel
and from time to time it scans the kernel tables to see whether some routes have
disappeared (for example due to unnoticed up/down transition of an interface)
or whether an `alien' route has been added by someone else (depending on the
<cf/learn/ switch, such routes are either deleted or accepted to our
table).

<p>If your OS supports only a single routing table, you can configure only one
instance of the Kernel protocol. If it supports multiple tables (in order to
allow policy routing; such an OS is for example Linux 2.2), 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.

<sect1>Configuration

<p><descrip>
        <tag>persist <m/switch/</tag> Tell BIRD to leave all its routes in the
        routing tables when it exits (instead of cleaning them up).
        <tag>scan time <m/number/</tag> Time in seconds between two consecutive scans of the
        kernel routing table.
        <tag>learn <m/switch/</tag> Enable learning of routes added to the kernel
        routing tables by other routing daemons or by the system administrator.
        This is possible only on systems which support identification of route
        authorship.
        <tag>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>The Kernel protocol doesn't define any route attributes.
<p>A 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>

<sect>OSPF

<sect1>Introduction

<p>Open Shortest Path First (OSPF) is a quite complex interior gateway
protocol. The current IPv4 version (OSPFv2) is defined in RFC
2328<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt"> and
the current IPv6 version (OSPFv3) is defined in RFC 5340<htmlurl
url="ftp://ftp.rfc-editor.org/in-notes/rfc5340.txt">  It's a link state
(a.k.a. shortest path first) protocol -- each router maintains a
database describing the autonomous system's topology. Each participating
router has an identical copy of the database and all routers run the
same algorithm calculating a shortest path tree with themselves as a
root. OSPF chooses the least cost path as the best path.

<p>In OSPF, the autonomous system can be split to several areas in order
to reduce the amount of resources consumed for exchanging the routing
information and to protect the other areas from incorrect routing data.
Topology of the area is hidden to the rest of the autonomous system.

<p>Another very important feature of OSPF is that
it can keep routing information from other protocols (like Static or BGP)
in its link state database as external routes. Each external route can
be tagged by the advertising router, making it possible to pass additional
information between routers on the boundary of the autonomous system.

<p>OSPF quickly detects topological changes in the autonomous system (such
as router interface failures) and calculates new loop-free routes after a short
period of convergence. Only a minimal amount of 
routing traffic is involved.

<p>Each router participating in OSPF routing periodically sends Hello messages
to all its interfaces. This allows neighbors to be discovered dynamically.
Then the neighbors exchange theirs parts of the link state database and keep it
identical by flooding updates. The flooding process is reliable and ensures
that each router detects all changes.

<sect1>Configuration

<p>In the main part of configuration, there can be multiple definitions of
OSPF area witch different id included. These definitions includes many other
switches and multiple definitions of interfaces. Definition of interface
may contain many switches and constant definitions and list of neighbors
on nonbroadcast networks.

<code>
protocol ospf &lt;name&gt; {
        rfc1583compat &lt;switch&gt;;
        tick &lt;num&gt;;
        area &lt;id&gt; {
                stub cost &lt;num&gt;;
                networks {
                        &lt;prefix&gt;;
                        &lt;prefix&gt; hidden;
                }
                stubnet &lt;prefix&gt;;
                stubnet &lt;prefix&gt; {
                        hidden &lt;switch&gt;;
                        summary &lt;switch&gt;;
                        cost &lt;num&gt;;
                }
                interface &lt;interface pattern&gt; {
                        cost &lt;num&gt;;
                        stub &lt;switch&gt;;
                        hello &lt;num&gt;;
                        poll &lt;num&gt;;
                        retransmit &lt;num&gt;;
                        priority &lt;num&gt;;
                        wait &lt;num&gt;;
                        dead count &lt;num&gt;;
                        dead &lt;num&gt;;
                        rx buffer [normal|large|&lt;num&gt;];
                        type [broadcast|nonbroadcast|pointopoint];
                        strict nonbroadcast &lt;switch&gt;;
                        authentication [none|simple|cryptographic];
                        password "&lt;text&gt;";
                        password "&lt;text&gt;" {
                                id &lt;num&gt;;
                                generate from "&lt;date&gt;";
                                generate to "&lt;date&gt;";
                                accept from "&lt;date&gt;";
                                accept to "&lt;date&gt;";
                        };
                        neighbors {
                                &lt;ip&gt;;
                                &lt;ip&gt; eligible;
                        };
                };
                virtual link &lt;id&gt; {
                        hello &lt;num&gt;;
                        retransmit &lt;num&gt;;
                        wait &lt;num&gt;;
                        dead count &lt;num&gt;;
                        dead &lt;num&gt;;
                        authentication [none|simple|cryptographic];
                        password "&lt;text&gt;";
                };
        };
}
</code>

<descrip>
        <tag>rfc1583compat <M>switch</M></tag>
         This option controls compatibility of routing table
         calculation with RFC 1583<htmlurl
         url="ftp://ftp.rfc-editor.org/in-notes/rfc1583.txt">. Default
         value is no.
        
        <tag>area <M>id</M></tag>
         This defines an OSPF area with given area ID (an integer or an IPv4
         address, similarly to a router ID).
         The most important area is
         the backbone (ID 0) to which every other area must be connected.

        <tag>stub cost <M>num</M></tag>
         No external (except default) routes are flooded into stub areas.
         Setting this value marks area stub with defined cost of default route.
         Default value is no. (Area is not stub.)

        <tag>tick <M>num</M></tag>
         The routing table calculation and clean-up of areas' databases
         is not performed when a single link state
         change arrives. To lower the CPU utilization, it's processed later
         at periodical intervals of <m/num/ seconds. The default value is 1.

        <tag>networks { <m/set/ }</tag>
         Definition of area IP ranges. This is used in summary LSA origination.
         Hidden networks are not propagated into other areas.

        <tag>stubnet <m/prefix/ { <m/options/ }</tag>
         Stub networks are networks that are not transit networks
         between OSPF routers. They are also propagated through an
         OSPF area as a part of a link state database. By default,
         BIRD generates a stub network record for each primary network
         address on each OSPF interface that does not have any OSPF
         neighbors, and also for each non-primary network address on
         each OSPF interface. This option allows to alter a set of
         stub networks propagated by this router. 

         Each instance of this option adds a stub network with given
         network prefix to the set of propagated stub network, unless
         option <cf/hidden/ is used. It also suppresses default stub
         networks for given network prefix. When option
         <cf/summary/ is used, also default stub networks that are
         subnetworks of given stub network are suppressed. This might
         be used, for example, to aggregate generated stub networks.
         
        <tag>interface <M>pattern</M></tag>
         Defines that the specified interfaces belong to the area being defined.
         See <ref id="dsc-iface" name="interface"> common option for detailed description.

        <tag>virtual link <M>id</M></tag>
         Virtual link to router with the router id. Virtual link acts as a
         point-to-point interface belonging to backbone. The actual area is
         used as transport area. This item cannot be in the backbone.

        <tag>cost <M>num</M></tag>
         Specifies output cost (metric) of an interface. Default value is 10.

        <tag>stub <M>switch</M></tag>
         If set to interface it does not listen to any packet and does not send
         any hello. Default value is no.

        <tag>hello <M>num</M></tag>
         Specifies interval in seconds between sending of Hello messages. Beware, all
         routers on the same network need to have the same hello interval.
         Default value is 10.

        <tag>poll <M>num</M></tag>
         Specifies interval in seconds between sending of Hello messages for
         some neighbors on NBMA network. Default value is 20.

        <tag>retransmit <M>num</M></tag>
         Specifies interval in seconds between retransmissions of unacknowledged updates.
         Default value is 5.

        <tag>priority <M>num</M></tag>
         On every multiple access network (e.g., the Ethernet) Designed Router
         and Backup Designed router are elected. These routers have some
         special functions in the flooding process. Higher priority increases
         preferences in this election. Routers with priority 0 are not
         eligible. Default value is 1.

        <tag>wait <M>num</M></tag>
         After start, router waits for the specified number of seconds between starting
         election and building adjacency. Default value is 40.
         
        <tag>dead count <M>num</M></tag>
         When the router does not receive any messages from a neighbor in
         <m/dead count/*<m/hello/ seconds, it will consider the neighbor down.

        <tag>dead <M>num</M></tag>
         When the router does not receive any messages from a neighbor in
         <m/dead/ seconds, it will consider the neighbor down. If both directives
         <m/dead count/ and <m/dead/ are used, <m/dead/ has precendence.

        <tag>rx buffer <M>num</M></tag>
         This sets the size of buffer used for receiving packets. The buffer should
         be bigger than maximal size of any packets. Value NORMAL (default)
         means 2*MTU, value LARGE means maximal allowed packet - 65536.

        <tag>type broadcast</tag>
         BIRD detects a type of a connected network automatically, but sometimes it's
         convenient to force use of a different type manually.
         On broadcast networks, flooding and Hello messages are sent using multicasts
         (a single packet for all the neighbors).

        <tag>type pointopoint</tag>
         Point-to-point networks connect just 2 routers together. No election
         is performed there which reduces the number of messages sent.

        <tag>type nonbroadcast</tag>
         On nonbroadcast networks, the packets are sent to each neighbor
         separately because of lack of multicast capabilities.

        <tag>strict nonbroadcast <M>switch</M></tag>
         If set, don't send hello to any undefined neighbor. This switch
         is ignored on any non-NBMA network. Default is No.

        <tag>authentication none</tag>
         No passwords are sent in OSPF packets. This is the default value.

        <tag>authentication simple</tag>
         Every packet carries 8 bytes of password. Received packets
         lacking this password are ignored. This authentication mechanism is
         very weak.

        <tag>authentication cryptographic</tag>
         16-byte long MD5 digest is appended to every packet. For the digest
         generation 16-byte long passwords are used. Those passwords are 
         not sent via network, so this mechanism is quite secure.
         Packets can still be read by an attacker.

        <tag>password "<M>text</M>"</tag>
         An 8-byte or 16-byte password used for authentication.
         See <ref id="dsc-pass" name="password"> common option for detailed description.

        <tag>neighbors { <m/set/ } </tag>
         A set of neighbors to which Hello messages on nonbroadcast networks
         are to be sent. Some of them could be marked as eligible.

</descrip>

<sect1>Attributes

<p>OSPF defines three route attributes. Each internal route has a <cf/metric/
Metric is ranging from 1 to infinity (65535).
External routes use <cf/metric type 1/ or <cf/metric type 2/.
A <cf/metric of type 1/ is comparable with internal <cf/metric/, a
<cf/metric of type 2/ is always longer
than any <cf/metric of type 1/ or any <cf/internal metric/.
If you specify both metrics only metric1 is used.
Each external route can also carry a <cf/tag/ which is a 32-bit
integer which is used when exporting routes to other protocols;
otherwise, it doesn't affect routing inside the OSPF domain at all.
Default is <cf/metric of type 2 = 10000/ and <cf/tag = 0/.

<sect1>Example

<p>

<code>
protocol ospf MyOSPF {
        rfc1583compat yes;
        tick 2;
        export filter {
                if source = RTS_BGP then {
                        ospf_metric1 = 100;
                        accept;
                }
                reject;
        };
        area 0.0.0.0 {
                interface "eth*" {
                        cost 11;
                        hello 15;
                        priority 100;
                        retransmit 7;
                        authentication simple;
                        password "aaa";
                };
                interface "ppp*" {
                        cost 100;
                        authentication cryptographic;
                        password "abc" {
                                id 1;
                                generate to "22-04-2003 11:00:06";
                                accept from "17-01-2001 12:01:05";
                        };
                        password "def" {
                                id 2;
                                generate to "22-07-2005 17:03:21";
                                accept from "22-02-2001 11:34:06";
                        };
                };
                interface "arc0" {
                        cost 10;
                        stub yes;
                };
                interface "arc1";
        };
        area 120 {
                stub yes;
                networks {
                        172.16.1.0/24;
                        172.16.2.0/24 hidden;
                }
                interface "-arc0" , "arc*" {
                        type nonbroadcast;
                        authentication none;
                        strict nonbroadcast yes;
                        wait 120;
                        poll 40;
                        dead count 8;
                        neighbors {
                                192.168.120.1 eligible;
                                192.168.120.2;
                                192.168.120.10;
                        };
                };
        };
}
</code>

<sect>Pipe

<sect1>Introduction

<p>The Pipe protocol serves as a link between two routing tables, allowing routes to be
passed from a table declared as primary (i.e., the one the pipe is connected to using the
<cf/table/ configuration keyword) to the secondary one (declared using <cf/peer table/)
and vice versa, depending on what's allowed by the filters. Export filters control export
of routes from the primary table to the secondary one, import filters control the opposite
direction.

<p>The Pipe protocol may work in the opaque mode or in the transparent
mode. In the opaque mode, the Pipe protocol retransmits optimal route
from one table to the other table in a similar way like other
protocols send and receive routes. Retransmitted route will have the
source set to the Pipe protocol, which may limit access to protocol
specific route attributes. The opaque mode is a default mode.

<p>In transparent mode, the Pipe protocol retransmits all routes from
one table to the other table, retaining their original source and
attributes.  If import and export filters are set to accept, then both
tables would have the same content. The mode can be set by
<tt/mode/ option.

<p>The primary use of multiple routing tables and the Pipe protocol is for policy routing,
where handling of a single packet doesn't depend only on its destination address, but also
on its source address, source interface, protocol type and other similar parameters.
In many systems (Linux being a good example), the kernel allows to enforce routing policies
by defining routing rules which choose one of several routing tables to be used for a packet
according to its parameters. Setting of these rules is outside the scope of BIRD's work
(on Linux, you can use the <tt/ip/ command), but you can create several routing tables in BIRD,
connect them to the kernel ones, use filters to control which routes appear in which tables
and also you can employ the Pipe protocol for exporting a selected subset of one table to
another one.

<sect1>Configuration

<p><descrip>
        <tag>peer table <m/table/</tag> Defines secondary routing table to connect to. The
        primary one is selected by the <cf/table/ keyword.

        <tag>mode opaque|transparent</tag> Specifies the mode for the pipe to work in. Default is opaque.
</descrip>

<sect1>Attributes

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

<sect1>Example

<p>Let's consider a router which serves as a boundary router of two different autonomous
systems, each of them connected to a subset of interfaces of the router, having its own
exterior connectivity and wishing to use the other AS as a backup connectivity in case
of outage of its own exterior line.

<p>Probably the simplest solution to this situation is to use two routing tables (we'll
call them <cf/as1/ and <cf/as2/) and set up kernel routing rules, so that packets having
arrived from interfaces belonging to the first AS will be routed according to <cf/as1/
and similarly for the second AS. Thus we have split our router to two logical routers,
each one acting on its own routing table, having its own routing protocols on its own
interfaces. In order to use the other AS's routes for backup purposes, we can pass
the routes between the tables through a Pipe protocol while decreasing their preferences
and correcting their BGP paths to reflect the AS boundary crossing.

<code>
table as1;                              # Define the tables
table as2;

protocol kernel kern1 {                 # Synchronize them with the kernel
        table as1;
        kernel table 1;
}

protocol kernel kern2 {
        table as2;
        kernel table 2;
}

protocol bgp bgp1 {                     # The outside connections
        table as1;
        local as 1;
        neighbor 192.168.0.1 as 1001;
        export all;
        import all;
}

protocol bgp bgp2 {
        table as2;
        local as 2;
        neighbor 10.0.0.1 as 1002;
        export all;
        import all;
}

protocol pipe {                         # The Pipe
        table as1;
        peer table as2;
        export filter {
                if net ~ [ 1.0.0.0/8+] then {   # Only AS1 networks
                        if preference>10 then preference = preference-10;
                        if source=RTS_BGP then bgp_path.prepend(1);
                        accept;
                }
                reject;
        };
        import filter {
                if net ~ [ 2.0.0.0/8+] then {   # Only AS2 networks
                        if preference>10 then preference = preference-10;
                        if source=RTS_BGP then bgp_path.prepend(2);
                        accept;
                }
                reject;
        };
}
</code>

<sect>RIP

<sect1>Introduction

<p>The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol, where each router broadcasts (to all its neighbors)
distances to all networks it can reach. When a router hears distance to another network, it increments
it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes
unreachable, routers keep telling each other that its distance is the original distance plus 1 (actually, plus
interface metric, which is usually one). After some time, the distance reaches infinity (that's 15 in
RIP) and all routers know that network is unreachable. RIP tries to minimize situations where
counting to infinity is necessary, because it is slow. Due to infinity being 16, you can't use
RIP on networks where maximal distance is higher than 15 hosts. You can read more about RIP at <HTMLURL
URL="http://www.ietf.org/html.charters/rip-charter.html" name="http://www.ietf.org/html.charters/rip-charter.html">. Both IPv4  
(RFC 1723<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1723.txt">)
and IPv6 (RFC 2080<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2080.txt">) versions of RIP are supported by BIRD, historical RIPv1 (RFC 1058<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1058.txt">)is
not currently supported. RIPv4 MD5 authentication (RFC 2082<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2082.txt">) is supported.

<p>RIP is a very simple protocol, and it has a lot of shortcomings. Slow
convergence, big network load and inability to handle larger networks
makes it pretty much obsolete. (It is still usable on very small networks.)

<sect1>Configuration

<p>In addition to options common for all to other protocols, RIP supports the following ones:

<descrip>
        <tag/authentication none|plaintext|md5/ selects authentication method to be used. <cf/none/ means that
          packets are not authenticated at all, <cf/plaintext/ means that a plaintext password is embedded
          into each packet, and <cf/md5/ means that packets are authenticated using a MD5 cryptographic
          hash. If you set authentication to not-none, it is a good idea to add <cf>password</cf>
          section. Default: none.

        <tag>honor always|neighbor|never </tag>specifies when should requests for dumping routing table
          be honored. (Always, when sent from a  host on a directly connected
          network or never.) Routing table updates are honored only from
          neighbors, that is not configurable. Default: 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 to a broadcast address even
through multicast mode is possible. <cf>quiet</cf> option means that RIP will not transmit
any periodic messages to this interface and <cf>nolisten</cf> means that RIP will send to this
interface but not listen to it.

<p>The following options generally override behavior specified in RFC. If you use any of these
options, BIRD will no longer be RFC-compliant, which means it will not be able to talk to anything
other than equally configured BIRD. I have 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 an address &gt;1024, you will not need to run bird with UID==0).

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

        <tag>period <M>number</M>
          </tag>specifies the number of seconds between periodic updates. Default is 30 seconds. A lower
          number will mean faster convergence but bigger network
          load. Do not use values lower than 10.

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

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

<sect1>Attributes

<p>RIP defines two route attributes:

<descrip>
        <tag>int <cf/rip_metric/</tag> RIP metric of the route (ranging from 0 to <cf/infinity/).
        When routes from different RIP instances are available and all of them have the same
        preference, BIRD prefers the route with lowest <cf/rip_metric/.
        When importing a non-RIP route, the metric defaults to 5.

        <tag>int <cf/rip_tag/</tag> RIP route tag: a 16-bit number which can be used
        to carry additional information with the route (for example, an originating AS number
        in case of external routes). When importing a non-RIP route, the tag defaults to 0.
</descrip>

<sect1>Example

<p><code>
protocol rip MyRIP_test {
        debug all;
        port 1520;
        period 10;
        garbage time 60;
        interface "eth0" { metric 3; mode multicast; };
        interface "eth*" { metric 2; mode broadcast; };
        honor neighbor;
        authentication none;
        import filter { print "importing"; accept; };
        export filter { print "exporting"; accept; };
}
</code>

<sect>Static

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

<p>There are 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 it is connected to and adds it again as soon
as the destination becomes adjacent again.

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

<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 a 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>

<chapt>Conclusions

<sect>Future work

<p>Although BIRD supports all the commonly used routing protocols,
there are still some features which would surely deserve to be
implemented in future versions of BIRD:

<itemize>
<item>OSPF NSSA areas and opaque LSA's
<item>Route aggregation and flap dampening
<item>Generation of IPv6 router advertisements
<item>Multipath routes
<item>Multicast routing protocols
<item>Ports to other systems
</itemize>

<sect>Getting more help

<p>If you use BIRD, you're welcome to join the bird-users mailing list
(<HTMLURL URL="mailto:bird-users@bird.network.cz" name="bird-users@bird.network.cz">)
where you can share your experiences with the other users and consult
your problems with the authors. To subscribe to the list, just send a
<tt/subscribe bird-users/ command in a body of a mail to
(<HTMLURL URL="mailto:majordomo@bird.network.cz" name="majordomo@bird.network.cz">).
The home page of BIRD can be found at <HTMLURL URL="http://bird.network.cz/" name="http://bird.network.cz/">.

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

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

<p><it/Good luck!/

</book>

<!--
LocalWords:  GPL IPv GateD BGPv RIPv OSPFv Linux sgml html dvi sgmltools Pavel
LocalWords:  linuxdoc dtd descrip config conf syslog stderr auth ospf bgp Mbps
LocalWords:  router's eval expr num birdc ctl UNIX if's enums bool int ip GCC
LocalWords:  len ipaddress pxlen netmask enum bgppath bgpmask clist gw md eth
LocalWords:  RTS printn quitbird iBGP AS'es eBGP RFC multiprotocol IGP Machek
LocalWords:  EGP misconfigurations keepalive pref aggr aggregator BIRD's RTC
LocalWords:  OS'es AS's multicast nolisten misconfigured UID blackhole MRTD MTU
LocalWords:  uninstalls ethernets IP binutils ANYCAST anycast dest RTD ICMP rfc
LocalWords:  compat multicasts nonbroadcast pointopoint loopback sym stats
LocalWords:  Perl SIGHUP dd mm yy HH MM SS EXT IA UNICAST multihop Discriminator txt
LocalWords:  proto wildcard
-->