docs/docs/routing/bird/bird.md

Configuring Bird2
=================

We now need to configure the routing daemon for your router which will allow you to
exchange routes with other routers over the tunnels you will setup later. This is at
the core of what makes CRXN an inter-network. 

The software we use for this procedure is known as BIRD or _BIRD Internet Routing Daemon_,
of which there are two versions:

1. Bird 1.6
2. Bird 2

You can use Bird 1.6 but you are on your own then in terms of configuration, the syntax
differs slightly but we recommend (and for the rest of this document we will be) using
Bird 2 as it comes with many bug fixes and improvements and most distributions (including Debian)
now have support for it.

## Installation

In order to install the BIRD daemon on your machine you should look for a package named `bird2` or something
similar (some repositories name it slightly differently - such as _just_ `bird`). On a Debian-based system you
can use:

```bash
sudo apt install bird2 -y
```

You can confirm that the version of BIRD you installed is version 2 with the command `bird -v`. If that shows the correct version number then continue to the next step:

```bash
sudo systemctl enable --now bird
```

This will ensure that the routing daemon starts on boot.

## Configuration

TODO: Re-do this sections

The configuration template is constructed out of the following files:

1. `filters.conf`
	* Filter functions and the filter itself
2. `networks.conf`
	* Advertisement of ULA
3. `tables.conf`
	* The table definitions
4. `router.conf`
	* This contains the needed protocol definition for discovering
	your interface's prefixes and generating routes form them
	* It also contains the needed protocol definitions to sync bird
	routes into the Linux kernel's routing table (so you cna forward
	packets based on the routes from Bird)
5. `protocols.conf`
	* Depending on what protocol you want to use this will contains
	configurations for each

All of these will be included in a file saved at `/etc/bird/bird.conf` like so:

```
router id <ipv4>;

include "/etc/bird/crxn/tables.conf";
include "/etc/bird/crxn/filters.conf";
include "/etc/bird/crxn/router.conf";
include "/etc/bird/crxn/networks.conf";
```

Additionally, add the files for the route distribution protocol which we configure in the next steps.
```
include "/etc/bird/crxn/babel.conf"; # For babel routing
include "/etc/bird/crxn/ospfv3.conf"; # For OSPFv3 routing
```

Remember to set a unique router ID in `<ipv4>`, make it anything - it doesn't have to even be an address you own.

#### `filters.conf`

This file holds all the required functions for subnet matching and also
filters that match to the specific prefix aggregates (regional subnets)
that CRXN uses.

```
filter crxnFilter
{
    if (net ~ fd00::/8) then accept;
    reject;
}
```

#### `tables.conf`

This file holds all table definitions. There are only two actually.
The table `crxn` is the one we actually use, `master` is optional
and is only present because if one uses `bird-lg-go` (the looking glass
we use) then it, by default, only shows routes in the `master` table.
It is meant to have the same routes as the `crxn` table.

```
# CRXN table
ipv6 table crxn;
```

#### `router.conf`

This contains an instance of the `direct` protocol which reads the address
and prefix assigned to your AF_INET6 interfaces and generates routes from
those that represent routes to directly atrtached networks those interfaces
are on. The reason for this is that the `kernel` protocol never learns routes
in the Linux kernel's routing table that have the `kernel` protocol which
is what you get when you assign interfaces addresses and prefixes. This
doesn't even need those, it gets them from the interface.

```
# The kernel protocol doesn't grab kernel routes that are added by you when you assign an
# address and prefix. So instead of reading this from all routes with `proto kernel` this just
# yeets the routes off of the interface structure itself (even if you didn't have a route for your
# directly attached networks - i.e. nexthop = 0.0.0.0)
protocol direct crxnDirect
{
    ipv6
    {
        table crxn;
        import filter crxnFilter;
    };
    # Interfaces to find neighbours on
    interface "eth*";
}

protocol device {
}
```

The second part is for syncing routes from Bird to the Linux kernel's routing
table such that you can forward traffic based on the routes in Bird.

TODO: Check, defualt `learn` should learn non `kernel` and non-`bird` routes

```
# CRXN Kernel protocol
# We import any routes from the kernel table other than `proto bird` and `proto kernel`,
# could be `proto static` for example. By default it will learn these.
# Of course we also then export all routes from our Bird tables into the kernel so you can actually forward packets
protocol kernel crxnKernel
{
    ipv6 {
        # bird's crxn table -> kernel
        table crxn;
        export filter crxnFilter;
    };
}
```

#### `networks.conf`

This is just something we normally add. Usually you would assign a `/64` within your ULA `/48` but you also want to claim the whole `/48` by advertising a blackhole for it. Here our `/48`/ULA is `fd40:ec65:5b4c::/48`.

```
protocol static crxnStatic
{
        # Advertise your /48 with a blackhole
        route fd40:ec65:5b4c::/48 blackhole;

        ipv6 {
                import filter crxn6;
                table crxn;
        }
}
```