sysctl.d

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



NAME
       sysctl.d - Configure kernel parameters at boot

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

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



systemd 247                                                          SYSCTL.D(5)