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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
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 /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 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 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)
systemd 249 SYSCTL.D(5)
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