udpproxy is a Server Type plugin for Caddy https://caddyserver.com, it is modified from https://github.com/pieterlouw/caddy-net by @pieterlouw.

It proxies UDP traffic to a specified destination, and listen the reply on multiple addresses (defined via the reply-addr-alias directive in Caddyfile).

It helps in the situation when a service replies via a different interface (and probably with a different source IP address) than where it receives the request from, mostly in a multi-homed environment like what is described here https://lists.zx2c4.com/pipermail/wireguard/2017-November/002016.html

SO_REUSEADDR and SO_REUSEPORT are used when creating connections. Multiple connections are created with all possible IP addresses defined in the Caddyfile which the service will use to reply. And these connections are all binded to the same local IP address and port opened when the request first gets forwarded to the destination. By doing this we can catch the reply from the service even it has been sent via a different interface (and a different source IP address).

Follow the guide here https://github.com/caddyserver/caddy/wiki/Plugging-in-Plugins-Yourself

The line to be added to run.go in the import section is

_ "github.com/def1oyd/udpproxy"

Suppose

• The service recives requests at 10.0.0.254:12345
• 10.0.0.254 is a VIP, floating between 10.0.0.252 and 10.0.0.253
• The service is not able to use the VIP to send reply since it lacks capability of interface binding
• The reply may come from 10.0.0.252 or 10.0.0.253

The Caddyfile will look like

proxy :12345 10.0.0.254:12345 {
    reply-addr-alias 10.0.0.253:12345 10.0.0.252:12345
}

Then start caddy with the following command

$ caddy -type=udpproxy

1. https://lists.zx2c4.com/pipermail/wireguard/2017-November/002016.html
2. https://utcc.utoronto.ca/~cks/space/blog/linux/WireGuardEarlyNotes
3. https://blog.kechako.com/entry/create-udp-conn-each-client