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SYSCTL.D(5) sysctl.d SYSCTL.D(5)
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 /usr/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
/usr/lib/. Files in /run/ override files with the same name in /usr/lib/.
Packages should install their configuration files in /usr/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.
EXAMPLES
Example 1. Set kernel YP domain name
/etc/sysctl.d/domain-name.conf:
kernel.domainname=example.com
Example 2. Disable packet filter on bridged packets (method one)
/etc/udev/rules.d/99-bridge.rules:
ACTION=="add", SUBSYSTEM=="module", KERNEL=="bridge", RUN+="/usr/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
Example 3. Disable packet filter on bridged packets (method two)
/etc/modules-load.d/bridge.conf:
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
SEE ALSO
systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8), sysctl.conf(5),
modprobe(8)
systemd 219 SYSCTL.D(5)
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