sysctl.d(5) — Linux manual page

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

       key.name.under.proc.sys = some value
       key/name/under/proc/sys = some value
       key/middle.part.with.dots/foo = 123
       key.middle/part/with/dots.foo = 123
       -key.that.will.not.fail = value
       key.pattern.*.with.glob = whatever
       -key.pattern.excluded.with.glob
       key.pattern.overridden.with.glob = custom

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. A glob glob(7) pattern
       may be used to write the same value to all matching keys. Keys for
       which an explicit pattern exists will be excluded from any glob
       matching. In addition, a key may be explicitly excluded from being
       set by any matching glob patterns by specifying the key name prefixed
       with a "-" character and not followed by "=", see SYNOPSIS.

       Any access permission errors and attempts to write variables not
       present on the local system are logged at debug level and do not
       cause the service to fail. Other types of errors when setting
       variables are logged with higher priority and cause the service to
       return failure at the end (after processing other variables). As an
       exception, if a variable assignment is prefixed with a single "-"
       character, failure to set the variable for any reason will be logged
       at debug level and will not cause the service to fail.

       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/,
       /usr/local/lib/, and /usr/lib/, in order of precedence, as listed in
       the SYNOPSIS section above. Files must have the ".conf" extension.
       Files in /etc/ override files with the same name in /run/,
       /usr/local/lib/, and /usr/lib/. Files in /run/ override files with
       the same name under /usr/.

       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. Thus, the
       configuration in a certain file may either be replaced completely (by
       placing a file with the same name in a directory with higher
       priority), or individual settings might be changed (by specifying
       additional settings in a file with a different name that is ordered
       later).

       Packages should install their configuration files in /usr/lib/
       (distribution packages) or /usr/local/lib/ (local installs). Files in
       /etc/ are reserved for the local administrator, who may use this
       logic to override the configuration files installed by vendor
       packages. 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.

       Example 4. Set network routing properties for all interfaces

       /etc/sysctl.d/20-rp_filter.conf:

           net.ipv4.conf.default.rp_filter = 2
           net.ipv4.conf.*.rp_filter = 2
           -net.ipv4.conf.all.rp_filter
           net.ipv4.conf.hub0.rp_filter = 1

       The rp_filter key will be set to "2" for all interfaces, except
       "hub0". We set net.ipv4.conf.default.rp_filter first, so any
       interfaces which are added later will get this value (this also
       covers any interfaces detected while we're running). The glob matches
       any interfaces which were detected earlier. The glob will also match
       net.ipv4.conf.all.rp_filter, which we don't want to set at all, so it
       is explicitly excluded. And "hub0" is excluded from the glob because
       it has an explicit setting.

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 2020-11-01.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-11-01.)  If you discover 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 im‐
       provements to the information in this COLOPHON (which is not part of
       the original manual page), send a mail to man-pages@man7.org

systemd 247                                                      SYSCTL.D(5)

Pages that refer to this page: systemd.exec(5)30-systemd-environment-d-generator(7)file-hierarchy(7)systemd.directives(7)systemd.index(7)systemd-coredump(8)systemd-coredump.service(8)systemd-coredump@.service(8)systemd-coredump.socket(8)systemd-sysctl(8)systemd-sysctl.service(8)