The first is how to route answers to packets coming in over a
particular provider, say Provider 1, back out again over that same provider.
Let us first set some symbolical names. Let $IF1 be the name of the
first interface (if1 in the picture above) and $IF2 the name of the
second interface. Then let $IP1 be the IP address associated with
$IF1 and $IP2 the IP address associated with
$IF2. Next, let $P1 be the IP address of the gateway at
Provider 1, and $P2 the IP address of the gateway at provider 2.
Finally, let $P1_NET be the IP network $P1 is in,
and $P2_NET the IP network $P2 is in.
One creates two additional routing tables, say T1 and T2.
These are added in /etc/iproute2/rt_tables. Then you set up routing in
these tables as follows:
ip route add $P1_NET dev $IF1 src $IP1 table T1
ip route add default via $P1 table T1
ip route add $P2_NET dev $IF2 src $IP2 table T2
ip route add default via $P2 table T2
|
Nothing spectacular, just build a route to the gateway and build a
default route via that gateway, as you would do in the case of a single
upstream provider, but put the routes in a separate table per provider.
Note that the network route suffices, as it tells you how to find any host
in that network, which includes the gateway, as specified above.
Next you set up the main routing table. It is a good idea to route
things to the direct neighbour through the interface connected to that
neighbour. Note the `src' arguments, they make sure the right outgoing IP
address is chosen.
ip route add $P1_NET dev $IF1 src $IP1
ip route add $P2_NET dev $IF2 src $IP2
|
Then, your preference for default route:
ip route add default via $P1
|
Next, you set up the routing rules. These actually choose what routing table
to route with. You want to make sure that you route out a given
interface if you already have the corresponding source address:
ip rule add from $IP1 table T1
ip rule add from $IP2 table T2
|
This set of commands makes sure all answers to traffic coming in on a
particular interface get answered from that interface.
Now, this is just the very basic setup. It will work for all processes
running on the router itself, and for the local network, if it is
masqueraded. If it is not, then you either have IP space from both providers
or you are going to want to masquerade to one of the two providers. In both
cases you will want to add rules selecting which provider to route out from
based on the IP address of the machine in the local network.
