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.
If a variable assignment is prefixed with a single "-" character, any attempts to
set it that fail will be ignored (though are logged). Moreover, any access permission errors, and
attempts to write variables not defined on the local system are ignored (and logged) too.
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. If the vendor configuration file is included in
the initrd image, the image has to be regenerated.
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+="/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
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.