Linux Packet Forwarding

Posted on 2008-11-06
Medium Priority
Last Modified: 2010-04-21
Can linux forward packets back through the same interface they came?  If so, how can this be accomplished with routes?  Thanks.
Question by:MainStaySolutions
  • 2

Accepted Solution

easyDK earned 500 total points
ID: 22901083
May be helpfull to get at least part of topology, but generally said, it is possible to route thru the same interface. If we do have two networks directly connected:
route add -net xx.xx.xx.0 netmask y.y.y.0 dev eth0
route add -net aa.aa.aa.0 netmask b.b.b.0 dev eth0

If there's gateway to one of the networks on the way, use
route add -net xx.xx.xx.0 netmask y.y.y.0 gw zz.zz.zz.zz

Don't forget to switch on packet forwarding
echo "1" > /proc/sys/net/ipv4/ip_forward
LVL 43

Assisted Solution

ravenpl earned 500 total points
ID: 22902917
Yes it can, use normal routing rules.
But mind, that if the connection gets SNATed or DNATed on the router as well - both actions are required SNAT and DNAT.
LVL 43

Expert Comment

ID: 22902924
Oh, one more thing, linux by default does not route such packets, but sends redirect to the sending device (so it knows there shorter path). To disable this
for f in /proc/sys/net/ipv4/conf/*/send_redirects; do echo 0 > $f; done

Author Closing Comment

ID: 31514145
Thank you for the quick response.

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