sysctl.d

systemd
Developer Lennart Poettering lennart@poettering.net
sysctl.d 5
sysctl.d Configure kernel parameters at boot

`/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).

## Examples

Set kernel YP domain name

`/etc/sysctl.d/domain-name.conf`:

    kernel.domainname=example.com

Disable packet filter on bridged packets (method one)

`/etc/udev/rules.d/99-bridge.rules`:

    ACTION=="add", SUBSYSTEM=="module", KERNEL=="bridge", 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

Disable packet filter on bridged packets (method two)

`/etc/modules-load.d/bridge.conf`:

    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

## See Also

systemd 1, systemd-sysctl.service 8, systemd-delta 1, sysctl 8, sysctl.conf 5, modprobe 8