This repo contains code for reproducing port binding problems in Go and for understanding how socket stuff works in general.

Sometimes your TCP server written in Go fails with error bind: address already in use

It could happen in such cases as:

1. Other process already listens on this port on the same address. (non interesting case)
2. Server code explicitly disables SO_REUSEADDR (enabled by default in *nix systems) and tries to bind on same port and address which have unfinished connection in TIME_WAIT state.
3. Client uses random port for itself and make a lot of reconnects so it exhausted all ephemeral ports (left all potential available port in TIME_WAIT state)
4. Client uses the same port to itself

Or sometimes your TCP client fails bind to ephemeral port.

TODO: describe

1. Server listens on 1215 port, close connection and exit after 2 seconds: go run server/main.go -l 127.0.0.1:1215 -s 2.
2. Client connects to 1215 port, exit without closing connection after 4 seconds: go run client/main.go -e 127.0.0.1:1215 -s 5 -noclose. Lefts server side in TIME_WAIT state for 30s (depends on system settings).
3. Check connections statuses: (netstat -a -n | head -n 2) ; (netstat -a -n | grep 1215).
4. Try to bind on port w/o SO_REUSEADDR option: go run server/main.go -l 127.0.0.1:1215 -s 2 -no-reuse-addr (should fail if you are fast enough)
5. Try to bind on port with default mode: go run server/main.go -l 127.0.0.1:1215 -s 2 (should works if no other process listens on the same ip:port pair)

• SO_REUSEPORT, SO_REUSEADDR related discussion: golang/go#9661

• SO_REUSEPORT/ADDR

  • How different about the condition of binding
  • How packets forwarded to multiple sockets

• «why we do not use SO_REUSEADDR on windows»: https://github.com/golang/go/commit/c3733b29d494995859bb6d6241797f67ece4c53d

• This is strictly a violation of the TCP specification

• TCP: About FIN_WAIT_2, TIME_WAIT and CLOSE_WAIT