sysctl.d — Configure kernel parameters at boot
/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
At boot, systemd-sysctl.service(8) reads configuration files from the above directories to configure sysctl(8) kernel parameters.
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 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/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.
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