forwarp

Linux only. This is a PoC.

$ make
$ sudo setcap cap_net_admin,cap_net_raw=ep ./forwarp
$ ./forwarp eth0 eth1

This will forward ARP requests from eth0 to eth1 and update the ARP table of eth0 by capturing all the ARP responses received by eth1.

,---------------------------------------.
| ,-----------------------------------. |
| |              forwarp              | |
| |             ,-----------.         | |
'-|-------------|-----------|---------|-'
  |             |           |         |
,-'------.      v           |  ,------|-.
|        |,-------------.   |  |      '-|--ARP-REQ->
|  eth0  ==  ARP cache  |   |  |  eth1  |
|        |'-------------'   '--|--------|--ARP-REP--
'--------'                     '--------'

Notes

• I wasn't able to capture local ARP requests with ETH_P_ARP. I had to use ETH_P_ALL with BPF to do it without impacting perf too much.

• I wasn't able to use the same socket for eth0 and eth1. It's sad but it makes sense...