I ran into an interesting situation the other day which I expected would have more documentation online.

Situation: You have a multi-homed router and you would like to route traffic based on client IP addresses, or the source address. In my case I wanted a /24 (Net 1) to be directed (forwarded) through interface A, and another /24 (Net 2) to be forwarded through interface B. In my case I also NATed traffic forwarded to interface A.

This is called source address routing or policy-based routing.

Linux / IP

Create two routing tables:

ip route flush 11 #interface A
ip route add table 11 to dev eth1
ip route add table 11 to default via dev eth1
ip route flush 10 #interface B
ip route add table 10 to dev eth2
ip route add table 10 to default via dev eth2

Here we have two interfaces, eth1 and eth2, which are assigned different networks, let’s assume that we also have an eth0 which includes clients with both and addresses.

Then create two rules to send traffic to each table based on the traffic’s source IP address:

ip rule add from table 11 priority 11 #net 1
ip rule add from table 10 priority 10 #net 2

Now in this example, a default route was assigned to each table. You can add more routing entries, remove the default route, and do whatever. In my scenario both interfaces had a default route and were connected to the Internet. Issuing these commands does not make the routing persistant. You’ll need to add this somewhere to your start up scripts.

OpenBSD / PF

To accomplish this on a BSD system, use PF. You can install PF on FreeBSD, and it ships with OpenBSD. Add the following to the pf.conf file. Then reload pf using pf -F all -f /etc/pf.conf. For more information on pf check the guide.

ext\_if="sis0" #interface A
ext\_gw = ""
int\_net = "" #net 1

With one line we can choose to route from an internal network/interface pair out an external interface/gateway pair.

pass in on $int\_if from $int\_net route-to {($ext\_if $ext\_gw)}

We can do the same for net 2 and interface B, without using variables.

pass in on sis1 from route-to {(sis2}

These configurations to not NAT route, they just allow a more granular approach to routing/forwarding. Hope this helps.