port: builtin: UDP reply packet is dropped

[AkihiroSuda]

Terminal 1:

$ rootlesskit --net=slirp4netns --port-driver=builtin -p 0.0.0.0:1234:1234/udp  -- sh -c "nc -u -l -p 1234 < /etc/issue"

Terminal 2:

$ echo foo | nc -u 127.0.0.1 1234

foo is printed on Terminal 1, but /etc/issue is not printed on Terminal 2 when --port-driver is set to builtin.

--port-driver=socat works as expected.

version: e6152ff

podman-container-tools/podman#4586 (comment)

[AkihiroSuda]

seems we need to port over this: https://github.com/docker/libnetwork/blob/master/cmd/proxy/udp_proxy.go (moby/moby@fac0d87)

[AkihiroSuda]

Curious, why doesn't VXLAN (Flannel) seem to hit this?