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I have a computer (running Linux) with 2 interfaces eth0 and eth1.
eth0 with address 10.0.0.100/24
eth1 with address 192.168.1.100/24
They are connected to 2 different networks (10.0.0.0/24 and 192.168.1.0/24), these networks are connected with a router. Other computers on 10.0.0.0 can ping a computer on 192.168.1.0.

But if I, on this computer, try 

ping -I eth0 192.168.1.100

I don't get any reply. If I listen on eth1 I receive icmp request from 10.0.0.100 but it does not send any reply.

I've tried to set some static routes but did not make any difference. I also read up on kernel rp_filter and changed that to 2 (and 0, and 1) but did not make any difference either.

Anyone knows why and how to solve this?

The only reason I wanted to do this was to measure performace between the two networks using a single computer.

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Can you provide more information? Which computer is 192.168.1.100 (the computer on both networks, or some other one)? Also what is eth0? On the computer connected to both networks, what interface has which IP? – Patrick Feb 20 '12 at 3:04
Updated the post with interface and adresses. – jortan Feb 20 '12 at 10:14
Updated my answer below with an additional rule for the lo interface. This is because the reply to the ping is going across the loopback interface since youre pinging yourself. Creating source based routing rules cannot fix this since the rule that looks up local IPs is in the local table, and yanking rules out of that table gets messy (yes its technically possible, but not a good idea). – Patrick Feb 20 '12 at 16:51

3 Answers

up vote 5 down vote

The simplest solution is to disable reverse path filtering. By default linux filters out packets coming in on an interface which it thinks should have come in on a different interface (because the packet matches the other interface's subnet).

To do this

echo 'net.ipv4.conf.eth0.rp_filter = 0' >> /etc/sysctl.conf
echo 'net.ipv4.conf.eth1.rp_filter = 0' >> /etc/sysctl.conf
echo 'net.ipv4.conf.lo.rp_filter = 0' >> /etc/sysctl.conf
sysctl -p

This adds the setting to the sysctl config file and then reloads the config. You can also temporarily disable the setting by doing echo 0 > /proc/sys/net/ipv4/conf/eth0/rp_filter.

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Tried it with the same result. – jortan Feb 19 '12 at 20:58
up vote 0 down vote

You probably need to setup multiple default routes for both interfaces with iproute2 so that packets coming in from one interace are replied to on the same interface

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This seem to be the correct solution although I haven't got it to work yet. I found this blogg post: kindlund.wordpress.com/2007/11/19/… and will try some more on this. – jortan Feb 19 '12 at 21:00
That's the blog I've used to create this on my "multi-homed" server. If you set it up right pings will start working. – Jure1873 Feb 20 '12 at 8:13
up vote 0 down vote

You'll also need to set 'net.ipv4.ip_forward=1' in /etc/sysctl.conf.

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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. – HopelessN00b Jan 15 at 4:48

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