Linux answers to ARP who-is on the wrong network interface

Assume a server has two network interfaces as follows:

  • eth0 : 192.168.0.1/24
  • eth1 : 192.168.10.1/24

Let’s assume these interfaces reside on the different VLANs. Lets assume they were connected incorrectly, in such a way that eth0 is connected to VLAN 10, which servers 192.168.10.0/24 and eth1 is connected to VLAN 2, which serves 192.168.0.0/24.

You would expect that queries by other hosts on VLAN 2 (which is connected to eth1, but serves 192.168.0.0/24!) would not get responses from the server. You are wrong.

Linux will answer who-is queries on VLAN 2, replying with eth1’s MAC address to queries for 192.168.0.1 IP address.

This example is a simple example, but it can get ugly if your eth0 mimics a different network, and you want the server to be disconnected. I have had to “forge” a network setup on a different VLAN, mimicking the original network and subnet. However – a “backdoor” I have opened (on an additional NIC) between the mimicking server and the original server on a different IP class (a private one) resulted in the mimicking server answering to ARP queries, causing the clients to attempt connecting to the mimicking server instead of the production server. The clients could not complete the TCP handcheck because the mimicking server attempted to contact them via eth0, which was on the fake network, and did not actually reach anywhere.

This was a more complex example, however – the result is the same – the response on the “wrong” network interface to ARP who-is queries might hijack data which should be delivered elsewhere.

There is a solution! You need to setup the sysctl parameter arp_ignore  to either of the following values. The parameter is hidden in /proc/sys/net/ipv4/conf/<NIC>/arp_ignore

The parameters documentation is as follows:

arp_ignore – INTEGER
Define different modes for sending replies in response to received ARP requests that resolve local target IP addresses:
0 – (default): reply for any local target IP address, configured on any interface
1 – reply only if the target IP address is local address configured on the incoming interface
2 – reply only if the target IP address is local address configured on the incoming interface and both with the sender’s IP address are part from same subnet on this interface
3 – do not reply for local addresses configured with scope host, only resolutions for global and link addresses are replied

The value “1” or “2” would do the trick in such cases.

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