Hello my favorite mailing list! Recently I've been looking into TCP_USER_TIMEOUT and noticed some strange behaviour on fresh sockets in SYN-SENT state. Full writeup: https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/
Here's a reproducer. It does a simple thing: sets TCP_USER_TIMEOUT and does connect() to a blackholed IP: $ wget https://gist.githubusercontent.com/majek/b4ad53c5795b226d62fad1fa4a87151a/raw/cbb928cb99cd6c5aa9f73ba2d3bc0aef22fbc2bf/user-timeout-and-syn.py $ sudo python3 user-timeout-and-syn.py 00:00.000000 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] 00:01.007053 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] 00:03.023051 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] 00:05.007096 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] 00:05.015037 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] 00:05.023020 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] 00:05.034983 IP 192.1.1.1.52974 > 244.0.0.1.1234: Flags [S] The connect() times out with ETIMEDOUT after 5 seconds - as intended. But Linux (5.3.0-rc3) does something weird on the network - it sends remaining tcp_syn_retries packets aligned to the 5s mark. In other words: with TCP_USER_TIMEOUT we are sending spurious SYN packets on a timeout. For the record, the man page doesn't define what TCP_USER_TIMEOUT does on SYN-SENT state. Cheers, Marek
