huschi.net

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
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.
       If a variable assignment is prefixed with a single "-" character, any
       attempts to set it that fail will be ignored (though are logged).
       Moreover, any access permission errors, and attempts to write variables
       not defined on the local system are ignored (and logged) too.
       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/, 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
       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
       systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8),
       sysctl.conf(5), modprobe(8)
systemd 239                                                        SYSCTL.D(5)