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/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 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.
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.
systemd 257.9 SYSCTL.D(5)
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