linux - How to route only specific subnet (source ip) to a particular interface?

How to route only specific subnet (source ip) to a particular interface?
OS: Linux

I know I can do easily by destination IP by using something like

route add 1.2.3.4/24 dev eth4

but I do not see how can route based on source IP.

Answer

You need to use policy based routing. Something kind of like

ip rule add from / table 
ip route add 1.2.3.4/24 via  dev eth4 table 

is either table name specified in /etc/iproute2/rt_tables or you can use numeric id ...

This pretty much says, that all traffic from 1.2.3.4/24 will be routed using routing table . IIRC it doesen't use the default table after going through this, so if you need other routes (ie. default gateway), you need to add them to the table as well.