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 2020-12-18.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2020-12-18.)  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 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 247                                                  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)