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NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION FORMAT | CONFIGURATION DIRECTORIES AND PRECEDENCE | EXAMPLES | SEE ALSO | COLOPHON |
SYSCTL.D(5) sysctl.d SYSCTL.D(5)
sysctl.d - Configure kernel parameters at boot
/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
At boot, systemd-sysctl.service(8) reads configuration files from the
above directories to configure sysctl(8) kernel parameters.
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. Moreover, if a variable assignment is
prefixed with a single "-" character, failure to set the variable for
other reasons will be logged at debug level and will not cause the
service to fail. In other cases, errors when setting variables are
logged with higher priority and cause the service to return failure
at the end (after processing other variables).
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 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.
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.
systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8),
sysctl.conf(5), modprobe(8)
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 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) 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 246 SYSCTL.D(5)
Pages that refer to this page: systemd.exec(5) , 30-systemd-environment-d-generator(7) , file-hierarchy(7) , systemd.directives(7) , systemd.index(7) , systemd-coredump(8) , systemd-coredump.service(8) , systemd-coredump@.service(8) , systemd-coredump.socket(8) , systemd-sysctl(8) , systemd-sysctl.service(8)