Troubleshooting sysctl: cannot stat /proc/sys/net/ipv4/tcp_tw_recycle: No such file or directory

Troubleshooting setting net.ipv4.tcp_tw_recycle = 0 in /etc/sysctl.conf caused error: sysctl: cannot stat /proc/sys/net/ipv4/tcp_tw_recycle: No such file or directory

Last Update: March 08, 2022

What is `net.ipv4.tcp_tw_recycle`

Per Linux kernel document networking/ip-sysctl.txt:

tcp_tw_recycle - BOOLEAN
Enable fast recycling TIME-WAIT sockets. Default value is 0. It should not be changed without advice/request of technical experts.

This documentation is not clear what is mechanism and what is the side effect.

Symptom - sysctl: cannot stat /proc/sys/net/ipv4/tcp_tw_recycle: No such file or directory

When I tried to put following line into /etc/sysctl.conf:

net.ipv4.tcp_tw_recycle = 0

Then use sysctl -p to reload it, I got following error:

$ sudo sysctl -p
sysctl: cannot stat /proc/sys/net/ipv4/tcp_tw_recycle: No such file or directory

Analysis

The net.ipv4.tcp_tw_recycle has been removed from Linux 4.12 on 2017.

Check kernel version:

$ uname -a
Linux pi3 4.19.97-v7+ #1294 SMP

My kernel version is 4.19 which is newer than 4.12, net.ipv4.tcp_tw_recycle is not available anymore. This error can be safely ignored.

Should not set this option anymore.

Reason to remove

Per kernel git commit history, tcp_tw_recycle is not functional and cause problem.

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.
Linux kernel commit 4396e4618: tcp: remove tcp_tw_recycle .

Suggestion

Consider to use tcp_tw_reuse by set to 1 to improve network server performance:

net.ipv4.tcp_tw_reuse = 1

tcp_tw_reuse - INTEGER
Enable reuse of TIME-WAIT sockets for new connections when it is safe from protocol viewpoint.
0 - disable
1 - global enable
2 - enable for loopback traffic only
It should not be changed without advice/request of technical experts.
Default: 2

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