I found the solution for this myself: Routing for multiple uplinks/providers
A common configuration is the following, in which there are two providers that connect a local network (or even a single machine) to the big Internet.
| | |
+-------------+ Provider 1 +-------
__ | | | /
___/ \_ +------+-------+ +------------+ |
_/ \__ | if1 | /
/ \ | | |
| Local network -----+ Linux router | | Internet
\_ __/ | | |
\__ __/ | if2 | \
\___/ +------+-------+ +------------+ |
| | | \
+-------------+ Provider 2 +-------
| | |
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