reqsock:peek for UDP Does it work?

[cgambrel]

I have been trying to use client_sock.peek to pull some data from UDP to determine my packet routing. When I try peek with UDP it appears empty (nil). I have seen documentation around peek with TCP and conversations debating whether or not UDP would work the same. Can you please confirm if peek with UDP should function or not?

[linsite]

I'm not an expert.
There is no peek implementation in ngx_stream_lua_socket_udp.c, so sadly it doesn't work.
But, what I saw in Linux's recv manual page, it supports both UDP and TCP. You can try to peek data with FFI on the socket fd with a MSG_PEEK flag.

The  recv(),  recvfrom(), and recvmsg() calls are used to receive messages from a socket.  They may be used to receive data on both connectionless and connection-oriented sock‐
       ets.  This page first describes common features of all three system calls, and then describes the differences between the calls.

[aaurizon]

Hello,

I’ve opened a PR that addresses this: #383

If you’d like to test it right away, I’ve also published a Docker image that already includes this change. You can find it here: anna/openresty (based on the latest OpenResty, with UDP peek support added in preread)

This should make it easier to experiment and validate before the PR is merged.