NAME

       sysctl.d - Configure kernel parameters at boot

SYNOPSIS

       /etc/sysctl.d/*.conf

       /run/sysctl.d/*.conf

       /usr/lib/sysctl.d/*.conf

DESCRIPTION

       At boot, systemd-sysctl.service(8) reads configuration files from the above directories to
       configure sysctl(8) kernel parameters.

CONFIGURATION FORMAT

       The configuration files contain a list of variable assignments, separated by newlines.
       Empty lines and lines whose first non-whitespace character is "#" or ";" are ignored.

       Note that either "/" or "."  may be used as separators within sysctl variable names. If
       the first separator is a slash, remaining slashes and dots are left intact. If the first
       separator is a dot, dots and slashes are interchanged.  "kernel.domainname=foo" and
       "kernel/domainname=foo" are equivalent and will cause "foo" to be written to
       /proc/sys/kernel/domainname. Either "net.ipv4.conf.enp3s0/200.forwarding" or
       "net/ipv4/conf/enp3s0.200/forwarding" may be used to refer to
       /proc/sys/net/ipv4/conf/enp3s0.200/forwarding.

       The settings configured with sysctl.d files will be applied early on boot. The network
       interface-specific options will also be applied individually for each network interface as
       it shows up in the system. (More specifically, net.ipv4.conf.*, net.ipv6.conf.*,
       net.ipv4.neigh.*  and net.ipv6.neigh.*).

       Many sysctl parameters only become available when certain kernel modules are loaded.
       Modules are usually loaded on demand, e.g. when certain hardware is plugged in or network
       brought up. This means that systemd-sysctl.service(8) which runs during early boot will
       not configure such parameters if they become available after it has run. To set such
       parameters, it is recommended to add an udev(7) rule to set those parameters when they
       become available. Alternatively, a slightly simpler and less efficient option is to add
       the module to modules-load.d(5), causing it to be loaded statically before sysctl settings
       are applied (see example below).

CONFIGURATION DIRECTORIES AND PRECEDENCE

       Configuration files are read from directories in /etc/, /run/, and /lib/, in order of
       precedence. Each configuration file in these configuration directories shall be named in
       the style of filename.conf. Files in /etc/ override files with the same name in /run/ and
       /lib/. Files in /run/ override files with the same name in /lib/.

       Packages should install their configuration files in /lib/. Files in /etc/ are reserved
       for the local administrator, who may use this logic to override the configuration files
       installed by vendor packages. All configuration files are sorted by their filename in
       lexicographic order, regardless of which of the directories they reside in. If multiple
       files specify the same option, the entry in the file with the lexicographically latest
       name will take precedence. It is recommended to prefix all filenames with a two-digit
       number and a dash, to simplify the ordering of the files.

       If the administrator wants to disable a configuration file supplied by the vendor, the
       recommended way is to place a symlink to /dev/null in the configuration directory in
       /etc/, with the same filename as the vendor configuration file. If the vendor
       configuration file is included in the initrd image, the image has to be regenerated.

EXAMPLES

       Example 1. Set kernel YP domain name

       /etc/sysctl.d/domain-name.conf:

           kernel.domainname=example.com

       Example 2. Apply settings available only when a certain module is loaded (method one)

       /etc/udev/rules.d/99-bridge.rules:

           ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", \
                 RUN+="/lib/systemd/systemd-sysctl --prefix=/net/bridge"

       /etc/sysctl.d/bridge.conf:

           net.bridge.bridge-nf-call-ip6tables = 0
           net.bridge.bridge-nf-call-iptables = 0
           net.bridge.bridge-nf-call-arptables = 0

       This method applies settings when the module is loaded. Please note that, unless the
       br_netfilter module is loaded, bridged packets will not be filtered by Netfilter (starting
       with kernel 3.18), so simply not loading the module is sufficient to avoid filtering.

       Example 3. Apply settings available only when a certain module is loaded (method two)

       /etc/modules-load.d/bridge.conf:

           br_netfilter

       /etc/sysctl.d/bridge.conf:

           net.bridge.bridge-nf-call-ip6tables = 0
           net.bridge.bridge-nf-call-iptables = 0
           net.bridge.bridge-nf-call-arptables = 0

       This method forces the module to be always loaded. Please note that, unless the
       br_netfilter module is loaded, bridged packets will not be filtered with Netfilter
       (starting with kernel 3.18), so simply not loading the module is sufficient to avoid
       filtering.

SEE ALSO

       systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8), sysctl.conf(5),
       modprobe(8)