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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.

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2 Answers 2

37

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

ip rule add from <source>/<mask> table <name>
ip route add 1.2.3.4/24 via <router> dev eth4 table <name>

<name> 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 <name>. 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.

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  • 4
    one correction (but not sure if your one will also work. for the second command: ip route add default dev eth4 table <name> (table directive should go at the end I think, just like in the first command... also default can be replaced by a subnet, just like in your example)
    – Alex
    Jan 12, 2012 at 2:23
  • I had to issue these commands in reverse order: 1st sudo ip route add default via <router-addr> dev <device-name> table <table-id>, 2nd sudo ip rule add from <source-addr>/<mask> lookup <table-id>. I think the 1st one creates the table and the 2nd adds a line to it, so the reverse order shouldn't work.
    – kol
    Aug 11 at 9:38
10

http://wiki.wlug.org.nz/SourceBasedRouting

This site has a nice example of source based routing.

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  • Please quote the essential parts of the answer from the reference link(s), as the answer can become invalid if the linked page(s) change.
    – DavidPostill
    Nov 1, 2020 at 8:23

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