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I have a CentOS 7 box where there is tun0 interface. I want to forward traffic to a specific IP, originating from within the box, to go over the tun0.

I tried doing this

route add -host 12.12.12.12 dev tun0

also tried adding

iptables -t nat -I POSTROUTING 1 -o tun0 -j MASQUERADE

Nothing seems to t work

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What you're wanting is called policy routing (or rule-based routing), where the routing process looks at something other than the destination (in addition to the destination) for its decision-making. This can be done with ip using the rule command:

# echo '1 tunroute' >> /etc/iproute2/rt_tables
# ip rule add from 12.12.12.12/24 table tunroute
# ip route add default via {tun0-address} dev tun0 table tunroute

You only need to run the first command once, as it establishes an alias; technically, you don't need it if you only want to use table numbers.

  • Thanks for the suggestion. I was not aware of this. However, it is still not working or the traffic is not reaching the destination. When I do the tcpdump on the tun0 interface, I see the message being send to the interface, but no response. Not sure what is the issue – Sajan Jun 28 '18 at 21:30
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    Does the other side have its routes set up correctly? As well, are there any firewall rules preventing input traffic? – ErikF Jun 28 '18 at 21:34
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    Note that {tun0-address} is the "remote address" for tun0, and not its own address. – Ralph Rönnquist Jun 29 '18 at 6:46

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