Routing based on the source ip

Question

I'm trying to configure a host in a way that the traffic is routed either via eth0 or vlan123@eth0, depending on the source address used. Both interfaces have access to the whole network and both have some ip assigned.

For example: eth0 has 1.2.3.1/24 and vlan123 has 4.5.6.1/24. The default gateway is set via eth0. How can I make sure that if:

• packet comes in to 1.2.3.1 (will arrive untagged), the response will be sent to eth0
• packet comes in to 4.5.6.1 (will arrive 802.1q tagged), the response will be sent to vlan123

Is it possible without additional routing protocols? (for example forcing via iptables)

Answer 1

Look for information on "policy routing". The particular use case you're looking at is covered in the Advanced Routing and Traffic Control HOWTO, as well as elsewhere.