So back in the day I was trying to bridge two layer-2 networks over a wireless relay and I was using a TP-Link Archer C7V5 for the two routers. I initially tried out relayd however I found that it wasn’t doing a good job at managing the ARP/route table entries as they were getting out of sync and not being updated and refreshed properly. I tried modding the framework but eventually gave up and wrote my own solution in C because these router units had very limited RAM and CPU available. The original framework was called ARP-Relay-Bridge (arprb) and it did a lot of work to manage the ARP table, PING the clients, listen for ARP Requests, send Proxied Replies, manage the routing table and I also even had DHCP relay capability as well.

Now, I have replaced all my TP-Link units with the Linksys E8450 which have a bit more power to them so I tried to re-write a simpler solution to this layer-2 bridging problem in Python. This solution simply just sends out the current list of active ARP table entries to another router and when it receives the entries of another router, it adds an entry to the route table to properly redirect the traffic. This simple solution solves the LAN-to-WAN issue and with the help of OpenWRTs Linux based proxy_arp functionality, the LAN-to-LAN issue can also be solved as an extension since the routers know where to route the packets. Basic instructions in the readme!

Source code: https://github.com/stoops/arpsync