You may wonder what 'bird' means. It is acronym of 'Basic Internet Routing Daemon', and we think -that's cool name. Its task is similar to what firmware of Cisco routers does, or what gated does. However, you can not run Cisco's firmware on "normal" computer and -gated is really hard to configure and comes under wrong license. Bird is being developed on Charles -University, Prague, and can be freely distributed under terms of GNU General Public License. Bird is -designed to run on unix and unix-like systems, it is primarily developed on Linux. - -
This documentation can have 4 forms: extended html (this is master copy), html with stripped -extensions, ascii text (generated from html) and dvi/postscript (generated from html using -html2latex and latex). You should always edit master copy; if you do so be sure to read comment at -beggining of file. If you want to view documentation, you can either launch your www browser at -master copy (and hope that browser does not have incompatible extensions from our), or you can -generate nice printed copy. - -
Bird is configured using text configuration file. At startup, bird reads bird.conf -(unless -c command line parameter is given). Really simple configuration file might look like this: - -
-
-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;
-}
-
-
-You can find example of more complicated configuration file in doc/bird.conf.example. - -
Bird contains rather simple programming language. (No, it can not yet read mail :-). There are -two objects in this language: filters and functions. Filters are called by bird core when route is -being passed between protocol and main routing table, and filters may call functions. Functions may -call other functions but recursion is not allowed. Filter language contains control structures such -as if's and switches, but it allows no loops. Filters are -interpretted. Filter using many features can be found in filter/test.conf. - -
There's one strange thing with filter language: it does not permit you to create loops. There's -no equivalent of while() or for() command, and recursive functions are not permitted. - -
You can find sources of filters language in filter/ directory. filter/config.Y contains filter gramar, and basically translates source from user into -tree of f_inst structures. These trees are later interpreted using code in filter/filter.c. Filters internally work with values/variables in struct f_val, -which contains type of value and value. - -
Filter basically looks like this: - -
-filter not_too_far
-int var;
-{
- if defined( rip_metric ) then
- var = rip_metric;
- else {
- var = 1;
- rip_metric = 1;
- }
- if rip_metric > 10 then
- reject "RIP metric is too big";
- else
- accept "ok";
-}
-
-
-As you can see, filter has a header, list of local variables, and body. Header consists of filter keyword, followed by (unique) name of filter. List of local variables consists of -pairs type name;, where each pair defines one local variable. Body consists of - { statments }. Statements are terminated by ;. You can group -several statments into one by { statments } construction, that is usefull if -you want to make bigger block of code conditional. - -
Each variable and each value has certain type. Unlike C, filters distinguish between integers and -booleans (that is to prevent you from shooting in the foot). - -
Filter language supports common integer operations (+,-,*,/), parenthesis (a*(b+c)), comparation -(a=b, a!=b, a<b, a>=b). Special operators include ~ for "in" operation. In operation can be -used on element and set of that elements, or on ip and prefix, or on prefix and prefix. Its result -is true if element is in given set or if ip adress is inside given prefix. - -
Bird supports functions, so that you don't have to repeat same blocks of code over and -over. Functions can have zero or more parameters, and can have local variables. Function basically -looks like this: - -
-function name ()
-int local_variable;
-{
- local_variable = 5;
-}
-
-function with_parameters (int parameter)
-{
- print parameter;
-}
-
-
-Unlike C, variables are declared after function line but before first {. You can not declare -variables in nested blocks. Functions are called like in C: name(); with_parameters(5);. - -
Filters are declared in similar way to functions, except they can not have explicit -parameters. They get route table entry as implicit parameter. - -
Filters support two control structures: if/then/else and case. Syntax of if/then/else is if expression then command; else command; and you can use {
-command_1; command_2; ... } instead of one or both commands. else clause may be ommited. Case is used like this:
-
-
- case argument {
- 2: print "dva"; print "jeste jednou dva";
- 3 .. 5: print "tri az pet";
- else: print "neco jineho";
- }
-
-
-where argument is any argument that can be on the left side of ~ operator, and anything that
-could be member of set is allowed before :. Multiple commands are allowed without {} grouping. If
-argument matches neither of : clauses, else: clause is used. (Case is actually implemented as set
-matching, internally.)
-
-Protocols
-
-Rip
-
-Introduction
-
-
Rip protocol (sometimes called Rest In Pieces) is simple protocol, where each router broadcasts -distances to all networks he can reach. When router hears distance to other network, it increments -it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes -unreachable, routers keep telling each other that distance is old distance plus 1 (actually, plus -interface metric, which is usually one). After some time, distance reaches infinity (that's 15 in -rip) and all routers know that network is unreachable. Rip tries to minimize situations where -counting to infinity is neccessary, because it is slow. Due to infinity being 16, you can not use -rip on networks where maximal distance is bigger than 15 hosts. You can read more about rip at rfc1234. - -
In addition to options generic to other protocols, rip supports following options: - -
There are two options that can be specified per-interface. First is metric, with -default one. Second is mode broadcast|quiet|nolisten|version1, it selects mode for -rip to work in. If nothing is specified, rip runs in multicasts mode. version1 is -currently equivalent to broadcast, and it makes rip talk at broadcast address even -through multicast mode is possible. quiet option means that rip will not transmit -periodic messages onto this interface and nolisten means that rip will talk to this -interface but not listen on it. - -
Following options generally override specified behaviour from rfc. If you use any of these -options, bird will no longer be rfc-compatible, which means it will not be able to talk to anything -other than equally (mis-)configured bird. I warned you. - -
In addition, rip defines two filter variables, both of type it. rip_metric is rip -metric of current route, rip_tag is tag of current route. - -
-
-protocol rip MyRIP_test {
- debug all;
- port 1520;
- period 7;
- garbagetime 60;
- interface "*";
- honour neighbour;
- passwords { password "ahoj" from 0 to 10;
- password "nazdar" from 10;
- }
- authentication none;
- import filter { print "importing"; accept; };
- export filter { print "exporting"; accept; };
-}
-
-
-