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sysctl

The sysctl tool is used to manage kernel runtime parameters at runtime (without rebooting).

It works with parameters located under /proc/sys, and changes made via sysctl are immediate but not persistent unless written to a config file.

Basic Usage

  • View all kernel runtime parameters as they're currently set: 

    sysctl -a

  • Filter for a specific kernel setting, e.g., ipv4: 

    sysctl -a | grep -i 'ipv4'

  • Filter for settings that start with net.ipv4: 

    sysctl net.ipv4
    Note that this will not do the same as grep, it will only show settings that start with the given argument.

  • Filter for a specific setting by name: 

    sysctl net.ipv4.ip_forward
    This will look in /proc/sys/ for the current state of that setting.
    This will follow the path: /proc/sys/net/ipv4/ip_forward

  • Set a runtime kernel parameter (non-persistent): 

    sysctl net.ipv4.ip_forward=0
    This change will not persist throughout reboots.
    You'll need to add a rule to a config file to persist the change.

  • Reload kernel runtime parameters without rebooting: 

    sysctl --system

  • Filter kernel parameters by using a regular expression (ERE):

    sysctl -a -r '^net\.ipv[46]\>'
    This will show all kernel parameter that match the pattern.

    • start with the word net, followed by a ., then match either ipv4 or ipv6.

Config Files

To configure kernel runtime parameters to persist throughout reboots, you need to add them to a config file in /etc/sysctl.d/.

Custom settings should go in /etc/sysctl.d/, but there are other locations where settings are stored:

  • /usr/lib/sysctl.d/: Vendor settings go in here.
  • /etc/sysctl.d/: This is where you should put your settings.
  • /run/sysctl.d/

sysctl Priority Order

When kernel paremeters are loaded in, either on boot or with sysctl --system, it looks for files in this order:

  • /etc/sysctl.d/*.conf
  • /run/sysctl.d/*.conf
  • /usr/local/lib/sysctl.d/*.conf
  • /usr/lib/sysctl.d/*.conf
  • /lib/sysctl.d/*.conf
  • /etc/sysctl.conf

Once a file has been loaded, any other files with the same name will be ignored. Since files in /etc/sysctl.d are loaded first, this is where we put our settings.

Resources

  • man 8 sysctl
  • man 5 sysctl.conf