xarpd

xarpd bridges ARP requests between separate networks.

xarpd listens for incoming ARP requests, and when a request matches a configured subnetwork, it asks other peers running xarpd to resolve the request. When a peer can resolve the ARP request, xarpd responds to the initial ARP request with the hardware address of the host it is running on, so that traffic can be forwarded through it.

Motivation

Scenario

• network1 is on 10.0.0.0/24 and has the hosts:
  • server1 with IP address 10.0.0.1
  • device1 with IP address 10.0.0.42
• network2 is on 10.0.0.0/24 and has the hosts:
  • server2 with IP address 10.0.0.129
  • device2 with IP address 10.0.0.170

network1 and network2 have the same address but are different networks.

server1 has a route to the subnetwork 10.0.0.128/25 of network2 via a VPN and can therefore reach server2 and device2.

It is assumed that server1 and server2 have IP forwarding enabled.

Goal

device1 needs to reach device2 without the ability to join VPNs or configure custom routes. Therefore, the traffic would need to flow: device1 → server1 → server2 → device2.

Solution

xarpd running on server1 intercepts ARP requests from device1 for 10.0.0.129 and replies with the hardware address of server1. Before replying, it asks server2 to check whether 10.0.0.129 exists on network2.

Usage

xarpd must be running on at least two networks to be useful.

On server1 with IP address 10.0.0.1/24 in network1:

$ xarpd 10.0.0.129/25
Forwarding ARP requests for 10.0.0.128/25 to 10.0.0.129
Listening for ARP on eth0
Listening for HTTP on 10.0.0.1:2707

On server2 with IP address 10.0.0.129/24 in network2:

$ xarpd 10.0.0.1/25
Forwarding ARP requests for 10.0.0.0/25 to 10.0.0.1
Listening for ARP on eth0
Listening for HTTP on 10.0.0.129:2707