+ <term><option>%%</option></term>
+ <listitem>
+ <para>The <literal>%</literal> character itself.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>$$</option></term>
+ <listitem>
+ <para>The <literal>$</literal> character itself.</para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </refsect1>
+
+ <refsect1><title>Hardware Database Files</title>
+ <para>The hwdb files are read from the files located in the
+ system hwdb directory <filename>/usr/lib/udev/hwdb.d</filename>,
+ the volatile runtime directory <filename>/run/udev/hwdb.d</filename>
+ and the local administration directory <filename>/etc/udev/hwdb.d</filename>.
+ All hwdb files are collectively sorted and processed in lexical order,
+ regardless of the directories in which they live. However, files with
+ identical filenames replace each other. Files in <filename>/etc</filename>
+ have the highest priority, files in <filename>/run</filename> take precedence
+ over files with the same name in <filename>/usr/lib</filename>. This can be
+ used to override a system-supplied hwdb file with a local file if needed;
+ a symlink in <filename>/etc</filename> with the same name as a hwdb file in
+ <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
+ disables the hwdb file entirely. hwdb files must have the extension
+ <filename>.hwdb</filename>; other extensions are ignored.</para>
+
+ <para>The hwdb file contains data records consisting of matches and
+ associated key-value pairs. Every record in the hwdb starts with one or
+ more match string, specifying a shell glob to compare the database
+ lookup string against. Multiple match lines are specified in additional
+ consecutive lines. Every match line is compared indivdually, they are
+ combined by OR. Every match line must start at the first character of
+ the line.</para>
+
+ <para>The match lines are followed by one or more key-value pair lines, which
+ are recognized by a leading space character. The key name and value are separated
+ by <literal>=</literal>. An empty line signifies the end
+ of a record. Lines beginning with <literal>#</literal> are ignored.</para>
+
+ <para>The content of all hwdb files is read by
+ <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>.
+ During runtime only the binary database is used.</para>
+ </refsect1>
+
+ <refsect1><title>Network Link Configuration</title>
+ <para>Network link configuration is performed by the <literal>net_setup_link</literal>
+ udev builtin.</para>
+
+ <para>The link files are read from the files located in the
+ system network directory <filename>/usr/lib/systemd/network</filename>,
+ the volatile runtime network directory <filename>/run/systemd/network</filename>
+ and the local administration network directory <filename>/etc/systemd/network</filename>.
+ Link files must have the extension <filename>.link</filename>; other extensions are ignored.
+ All link files are collectively sorted and processed in lexical order,
+ regardless of the directories in which they live. However, files with
+ identical filenames replace each other. Files in <filename>/etc</filename>
+ have the highest priority, files in <filename>/run</filename> take precedence
+ over files with the same name in <filename>/usr/lib</filename>. This can be
+ used to override a system-supplied link file with a local file if needed;
+ a symlink in <filename>/etc</filename> with the same name as a link file in
+ <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
+ disables the link file entirely.</para>
+
+ <para>The link file contains a <literal>[Match]</literal> section, which
+ determines if a given link file may be applied to a given device; and a
+ <literal>[Link]</literal> section specifying how the device should be
+ configured. The first (in lexical order) of the link files that matches
+ a given device is applied.</para>
+
+ <para>A link file is said to match a device if each of the entries in the
+ <literal>[Match]</literal> section matches, or if the section is empty.
+ The following keys are accepted:</para>
+
+ <variablelist class='network-directives'>
+ <varlistentry>
+ <term><varname>MACAddress</varname></term>
+ <listitem>
+ <para>The hardware address.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><varname>Path</varname></term>
+ <listitem>
+ <para>The persistent path, as exposed by the udev property <literal>ID_PATH</literal>.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><varname>Driver</varname></term>
+ <listitem>
+ <para>The driver currently bound to the device, as exposed by the udev property <literal>DRIVER</literal>.</para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term><varname>Type</varname></term>