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rdrmntn
Asked: 2012-02-20 06:15:18 +0800 CST

Ping one interface from the other

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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.

linux routing networking ping

3 Answers

  1. 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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  2. 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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  3. Patrick's answer

    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

    did not work for me on Ubuntu 16.04.

    This worked:

    sysctl -w net.ipv4.conf.all.rp_filter=0
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