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.
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:
/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)