Im running Ubuntu 14.0.4 LTS and im trying to change some default values to optimize.

I run sysctl -a to see all values.

I have /etc/sysctl.conf and /etc/sysctl.d/ which has a few 10-*.conf files in it.

I want to change net.ipv4.tcp_max_syn_backlog value but it is not present in any of the configuration files above.

a) Shall I just enter the new value in any of those files? Where is the default value stored?

b) After changing the value shall I run service procps start or sysctl -p?

I have read that the configuration files can be in the following places:

  1. /run/sysctl.d/*.conf (not present on my system)
  2. /usr/local/lib/sysctl.d/*.conf (not present on my system)
  3. /usr/lib/sysctl.d/*.conf (not present on my system)
  4. /lib/sysctl.d/*.conf (not present on my system)
  5. /etc/sysctl.d/*.conf
  6. /etc/sysctl.conf

Thanks in advance


The default value for kernel runtime parameters is selected/set at compilation time of the kernel (module). sysctl is used to read those parameters and allows you to modify/override those parameters at both system boot and at runtime.

Personally I have never used any other file than /etc/sysctl.conf and I would use sysctl -p to verify the syntax of the new parameters I set.
(Except when I don't use sysctl at all and manipulate kernel paramaters directly e.g. with the traditional echo 1 > /proc/sys/net/ipv4/ip_forward )

  • 1
    I prefer making a new file under sysctl.d. That way possible OS updates on the sysctl.conf file won't overwrite my changes. – Tero Kilkanen Oct 29 '16 at 10:05
  • Thanks for your reply. I see, so if the default is 256 and I set it to 5000, then in order to "go back" to default I will have to set it to the value it originally was, removing the new value wont restore the default value. However isn't directly writing to "/proc/sys/..." temporary? Or will it survive reboot? Thanks in advance – Kevin Oct 29 '16 at 13:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.