sysctl changes persist after removing conf file and reboot

Question

I have a package which installs a sysctl config file to /etc/sysctl.d in order to "tune" some default settings. For example, some are

net.ipv4.tcp_congestion_control = agilesd
net.ipv4.tcp_fin_timeout = 20
net.ipv4.tcp_sack = 0

The defaults for these are cubic, 60 and 1 (respectively).

I have a need where I must temporarily "disable" them. That's simply accomplished by setting them to the default. However, my experimentation lead me to moving the file /etc/sysctl.d/99-my_ip_changes.conf from that directory to /root and rebooting. On one of the CentOS 7.6 systems on which this package is installed, after a reboot, things do in fact revert to the default. However, on two others, they do not. After a reboot, the TCP Congestion Algorithm is still agilesd for example.

I have verified, to the best that I can, that there is no other sysctl conf file setting these things to the same values set in this config file. Can anyone help me in understanding why this is happening?

Answer 1

First of all, I think you have a mistake in the sysctl fields names as they should start with 'net' and not with 'sys', like so:

net.ipv4.tcp_congestion_control = agilesd
net.ipv4.tcp_fin_timeout = 20
net.ipv4.tcp_sack = 0

Besides that, from what I can tell the file '/etc/sysctl.d/99-sysctl.conf' is actually a symbolic link to '/etc/sysctl.conf':

$ll /etc/sysctl.d/99-sysctl.conf
lrwxrwxrwx. 1 root root 14 Dec 31 17:03 /etc/sysctl.d/99-sysctl.conf -> ../sysctl.conf

So my guess is that on the systems that didn't revert the change the original file was changed and even after moving your file ('/etc/sysctl.d/99-my_ip_changes.conf') to '/root' the original file '/etc/sysctl.conf' was still holding the "tuned" values.

If you still having trouble with this issue please check this out. If not, maybe this would help someone else in the future.

Hope this helps.