如果你遇過 linux server 上有著大量 TIME_WAIT 你一定在 google 上看過把 net.ipv4.tcp_tw_recycle 跟 net.ipv4.tcp_tw_reuse 設定為 1 的優化方法，想要介此來減少 TIME_WAIT 或是增加 server 可以處理的連線數量

但是這些做法可能是無效的，甚至是「有風險在」的

在 tcp(7) 的 man page 有清楚的說明

tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11) Enable fast recycling of TIME_WAIT sockets. Enabling this option is not recommended as the remote IP may not use monotonically increasing timestamps (devices behind NAT, devices with per-connection timestamp offsets). See RFC 1323 (PAWS) and RFC 6191.

tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6) Allow to reuse TIME_WAIT sockets for new connections when it is safe from protocol viewpoint. It should not be changed without advice/request of technical experts.

在 Coping with the TCP TIME-WAIT state on busy Linux servers 這篇文章中，有完整的 TCP 運作狀態說明

tcp_tw_recycle

在 NAT 設備後面的 server 如果啟用 tcp_tw_recycle 會造成一些難以追查的連線中斷問題

在 kernel 4.12 版本開始，tcp_tw_recycle 因為無法正常運作已經被移除，詳細的原因可以參考 4396e46187 這個 commit

tcp: remove tcp_tw_recycle The tcp_tw_recycle was already broken for connections behind NAT, since the per-destination timestamp is not monotonically increasing for multiple machines behind a single destination address.

After the randomization of TCP timestamp offsets in commit 8a5bd45f6616 (tcp: randomize tcp timestamp offsets for each connection), the tcp_tw_recycle is broken for all types of connections for the same reason: the timestamps received from a single machine is not monotonically increasing, anymore.

Remove tcp_tw_recycle, since it is not functional. Also, remove the PAWSPassive SNMP counter since it is only used for tcp_tw_recycle, and simplify tcp_v4_route_req and tcp_v6_route_req since the strict argument is only set when tcp_tw_recycle is enabled.

所以在 4.12 以後的版本，已經沒有 tcp_tw_recycle 這個設定可以使用，而在 4.12 之前的版本則不建議啟用這個設定。

tcp_tw_reuse

在 server 端啟用 tcp_tw_reuse 對於要進來的連線沒有幫助

在 client 端啟用 tcp_tw_reuse 可以允許 TIME_WAIT 連線被使用在新建立要出去的連線，這個設定基本上是安全的

Summary

大部份的情況下 TIME-WAIT 是無害的，並不會佔用太多資源

比修改 tcp_tw_recycle 跟 tcp_tw_reuse，有很多更適合的方法可以幫助解決問題 ex:

  • 增加可用 port 數量
  • 增加可用 IP 數量

Reference

linux - Why is tcp_tw_reuse turned off by default? - Stack Overflow Coping with the TCP TIME-WAIT state on busy Linux servers linux - tcp_tw_reuse vs tcp_tw_recycle : Which to use (or both)? - Stack Overflow