Provided by: systemd_229-4ubuntu21.31_amd64 
NAME
sysctl.d - Configure kernel parameters at boot
SYNOPSIS
/etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf
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.
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/, and /lib/, in order of precedence. Each configuration file in these configuration directories shall be named in the style of filename.conf. Files in /etc/ override files with the same name in /run/ and /lib/. Files in /run/ override files with the same name in /lib/. Packages should install their configuration files in /lib/. Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. 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. 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.
SEE ALSO
systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8), sysctl.conf(5), modprobe(8)