NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION FORMAT | CONFIGURATION DIRECTORIES AND PRECEDENCE | EXAMPLES | SEE ALSO | COLOPHON

SYSCTL.D(5)                       sysctl.d                       SYSCTL.D(5)

NAME         top

       sysctl.d - Configure kernel parameters at boot

SYNOPSIS         top

       /etc/sysctl.d/*.conf

       /run/sysctl.d/*.conf

       /usr/lib/sysctl.d/*.conf

DESCRIPTION         top

       At boot, systemd-sysctl.service(8) reads configuration files from the
       above directories to configure sysctl(8) kernel parameters.

CONFIGURATION FORMAT         top

       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         top

       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.

EXAMPLES         top

       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.

SEE ALSO         top

       systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8),
       sysctl.conf(5), modprobe(8)

COLOPHON         top

       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 2016-07-16.  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 230                                                      SYSCTL.D(5)