SYSCTL.D(5)                                           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

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          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 /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 /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

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          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+="/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

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          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)

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