+ same filename.</para>
+
+ <para>The settings configured with
+ <filename>sysctl.d</filename> 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,
+ <filename>net.ipv4.conf.*</filename>,
+ <filename>net.ipv6.conf.*</filename>,
+ <filename>net.ipv4.neigh.*</filename> and <filename>net.ipv6.neigh.*</filename>).</para>
+
+ <para>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
+ <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> 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 <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> rule to set those parameters when they become
+ available. Alternatively, a slightly simpler and
+ less efficient option is to add the module to
+ <citerefentry><refentrytitle>modules-load.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, causing it to be loaded statically
+ before sysctl settings are applied (see
+ example below).</para>