systemd sysctl.d 5 sysctl.d Configure kernel parameters at boot /etc/sysctl.d/*.conf /run/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf At boot, systemd-sysctl.service8 reads configuration files from the above directories to configure sysctl8 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. Any access permission errors and attempts to write variables not defined on the local system are logged, but do not cause the service to fail. Moreover, if a variable assignment is prefixed with a single - character, failure to set the variable will be logged, but will not cause the service to fail. All other errors when setting variables cause the service to return failure at the end (other variables are still processed). 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.service8 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 udev7 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.d5, causing it to be loaded statically before sysctl settings are applied (see example below). /etc/sysctl.d/domain-name.conf: kernel.domainname=example.com /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. /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. systemd1, systemd-sysctl.service8, systemd-delta1, sysctl8, sysctl.conf5, modprobe8