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NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION FORMAT | CONFIGURATION DIRECTORIES AND PRECEDENCE | EXAMPLES | SEE ALSO | NOTES | COLOPHON |
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SYSCTL.D(5) sysctl.d SYSCTL.D(5)
sysctl.d - Configure kernel parameters at boot
/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
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. 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 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.
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 do not 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)
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.
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 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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
[email protected]
systemd 258~rc2 SYSCTL.D(5)
Pages that refer to this page: sysctl.conf(5), systemd.exec(5), file-hierarchy(7), systemd.directives(7), systemd.index(7), systemd.system-credentials(7), systemd-coredump(8), systemd-sysctl.service(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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