NAME

       sysctl.d - Configure kernel parameters at boot

SYNOPSIS

           /etc/sysctl.d/*.conf
           /run/sysctl.d/*.conf
           /usr/local/lib/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) [1]. 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. It is recommended to use the range 10-40 for configuration files in /usr/
       and the range 60-90 for configuration files in /etc/ and /run/, to make sure that local
       and transient configuration files will always take priority over configuration files
       shipped by the OS vendor.

       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)

NOTES

        1. 💣💥🧨💥💥💣 Please note that those configuration files must be available at all times.
           If /usr/local/ is a separate partition, it may not be available during early boot, and
           must not be used for configuration.