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.
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 /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.
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:
/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)