Introduction

What is bird

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.

About this documentation

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.

Configuration

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.

Filters

Introduction

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.

Data types

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).

bool

this is boolean type, it can have only two values, TRUE and FALSE. Boolean is not compatible with integer and is the only type you can use in if statments.

int

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

pair

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

string

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

ip

this type can hold single ip address. Depending on version of bird you are using, it can be ipv4 or ipv6 address. Ipv4 addresses addresses are written (as you would expect) as 1.2.3.4. You can apply special operator .mask(num) on values of type ip. It masks out all but first num bits from ip address. So 1.2.3.4.mask(8) = 1.0.0.0 is true.

prefix

this type can hold ip address, prefix len pair. Prefixes are written as ip address/px len. There are two special operators on prefix: .ip, which separates ip address from the pair, and .len, which separates prefix len from the pair.

set int|ip|prefix|pair

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

enum

enumerational types are halfway-internal in the bird. You can not define your own variable of enumerational type, but some pre-defined variables are of enumerational type. Enumerational types are incompatible with each other, again, its for your protection.

Operations

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.

Functions

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.

Control structures

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";
        }

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; };
}