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NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION FORMAT | CONFIGURATION DIRECTORIES AND PRECEDENCE | EXAMPLES | SEE ALSO | COLOPHON |
SYSCTL.D(5) sysctl.d SYSCTL.D(5)
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. 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. If the vendor configuration file is
included in the initrd image, the image has to be regenerated.
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+="/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.
systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8),
sysctl.conf(5), modprobe(8)
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2017-07-05. If you dis‐
cover any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
systemd 234 SYSCTL.D(5)
Pages that refer to this page: systemd.exec(5), file-hierarchy(7), systemd.directives(7), systemd.index(7), systemd-coredump(8), systemd-sysctl.service(8)