systemd/man/systemd.dnssd.xml

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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="systemd.dnssd"
          xmlns:xi="http://www.w3.org/2001/XInclude"
          conditional='ENABLE_RESOLVE'>

  <refentryinfo>
    <title>systemd.dnssd</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.dnssd</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.dnssd</refname>
    <refpurpose>DNS-SD configuration</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename><replaceable>network_service</replaceable>.dnssd</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>DNS-SD setup is performed by
    <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
    </para>

    <para>The main network service file must have the extension <filename>.dnssd</filename>; other
    extensions are ignored.</para>

    <para>The <filename>.dnssd</filename> files are read from the files located in the system network
    directories <filename>/usr/lib/systemd/dnssd</filename> and
    <filename>/usr/local/lib/systemd/dnssd</filename>, the volatile runtime network directory
    <filename>/run/systemd/dnssd</filename> and the local administration network directory
    <filename>/etc/systemd/dnssd</filename>. All configuration 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 configuration file with a
    local file if needed.</para>

    <para>Along with the network service file <filename>foo.dnssd</filename>, a "drop-in" directory
    <filename>foo.dnssd.d/</filename> may exist. All files with the suffix
    <literal>.conf</literal> from this directory will be parsed after the file itself is
    parsed. This is useful to alter or add configuration settings, without having to modify the main
    configuration file. Each drop-in file must have appropriate section headers.</para>

    <para>In addition to <filename>/etc/systemd/dnssd</filename>, drop-in <literal>.d</literal> directories
    can be placed in <filename>/usr/lib/systemd/dnssd</filename> or <filename>/run/systemd/dnssd</filename>
    directories. Drop-in files in <filename>/etc/</filename> take precedence over those in
    <filename>/run/</filename> which in turn take precedence over those in <filename>/usr/lib/</filename> or
    <filename>/usr/local/lib</filename>. Drop-in files under any of these directories take precedence over
    the main network service file wherever located.</para>
  </refsect1>

  <refsect1>
    <title>[Service] Section Options</title>

      <para>The network service file contains a [Service]
      section, which specifies a discoverable network service announced in a
      local network with Multicast DNS broadcasts.</para>

      <variablelist class='network-directives'>
        <varlistentry>
          <term><varname>Name=</varname></term>
          <listitem>
            <para>An instance name of the network service as defined in the section 4.1.1 of <ulink
            url="https://tools.ietf.org/html/rfc6763">RFC 6763</ulink>, e.g. <literal>webserver</literal>.</para>
            <para>The option supports simple specifier expansion. The following expansions are understood:</para>
            <table class='specifiers'>
              <title>Specifiers available</title>
              <tgroup cols='3' align='left' colsep='1' rowsep='1'>
                <colspec colname="spec" />
                <colspec colname="mean" />
                <colspec colname="detail" />
                <thead>
                  <row>
                    <entry>Specifier</entry>
                    <entry>Meaning</entry>
                    <entry>Details</entry>
                  </row>
                </thead>
                <tbody>
                  <xi:include href="standard-specifiers.xml" xpointer="a"/>
                  <xi:include href="standard-specifiers.xml" xpointer="A"/>
                  <xi:include href="standard-specifiers.xml" xpointer="b"/>
                  <xi:include href="standard-specifiers.xml" xpointer="B"/>
                  <xi:include href="standard-specifiers.xml" xpointer="H"/>
                  <xi:include href="standard-specifiers.xml" xpointer="m"/>
                  <xi:include href="standard-specifiers.xml" xpointer="M"/>
                  <xi:include href="standard-specifiers.xml" xpointer="o"/>
                  <xi:include href="standard-specifiers.xml" xpointer="v"/>
                  <xi:include href="standard-specifiers.xml" xpointer="w"/>
                  <xi:include href="standard-specifiers.xml" xpointer="W"/>
                  <xi:include href="standard-specifiers.xml" xpointer="percent"/>
                </tbody>
              </tgroup>
            </table>

              <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><varname>Type=</varname></term>
          <listitem>
            <para>A type of the network service as defined in the section 4.1.2 of <ulink
            url="https://tools.ietf.org/html/rfc6763">RFC 6763</ulink>, e.g. <literal>_http._tcp</literal>.
            </para>

          <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><varname>Port=</varname></term>
          <listitem>
            <para>An IP port number of the network service.</para>

          <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><varname>Priority=</varname></term>
          <listitem>
            <para>A priority number set in <constant class='dns'>SRV</constant> resource records corresponding
            to the network service.</para>

          <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><varname>Weight=</varname></term>
          <listitem>
            <para>A weight number set in <constant class='dns'>SRV</constant> resource records corresponding
            to the network service.</para>

          <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><varname>TxtText=</varname></term>
          <listitem>
            <para>A whitespace-separated list of arbitrary key/value pairs
            conveying additional information about the named service in the corresponding TXT resource record,
            e.g. <literal>path=/portal/index.html</literal>. Keys and values can contain C-style escape
            sequences which get translated upon reading configuration files.
            </para>
            <para>This option together with <varname>TxtData=</varname> may be specified more than once, in which
            case multiple TXT resource records will be created for the service. If the empty string is assigned to
            this option, the list is reset and all prior assignments will have no effect.
            </para>

            <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><varname>TxtData=</varname></term>
          <listitem>
            <para>A whitespace-separated list of arbitrary key/value pairs
            conveying additional information about the named service in the corresponding TXT resource record
            where values are base64-encoded string representing any binary data,
            e.g. <literal>data=YW55IGJpbmFyeSBkYXRhCg==</literal>. Keys can contain C-style escape
            sequences which get translated upon reading configuration files.
            </para>
            <para>This option together with <varname>TxtText=</varname> may be specified more than once, in which
            case multiple TXT resource records will be created for the service. If the empty string is assigned to
            this option, the list is reset and all prior assignments will have no effect.
            </para>

            <xi:include href="version-info.xml" xpointer="v236"/>
          </listitem>
        </varlistentry>
      </variablelist>

  </refsect1>

  <refsect1>
    <title>Examples</title>
    <example>
      <title>HTTP service</title>

      <programlisting># /etc/systemd/dnssd/http.dnssd
[Service]
Name=%H
Type=_http._tcp
Port=80
TxtText=path=/stats/index.html t=temperature_sensor</programlisting>

      <para>This makes the http server running on the host discoverable in the local network
      given MulticastDNS is enabled on the network interface.</para>

      <para>Now the utility <literal>resolvectl</literal> should be able to resolve the
      service to the host's name:</para>

      <programlisting>$ resolvectl service meteo._http._tcp.local
meteo._http._tcp.local: meteo.local:80 [priority=0, weight=0]
                        169.254.208.106%senp0s21f0u2u4
                        fe80::213:3bff:fe49:8aa%senp0s21f0u2u4
                        path=/stats/index.html
                        t=temperature_sensor
                        (meteo/_http._tcp/local)

-- Information acquired via protocol mDNS/IPv6 in 4.0ms.
-- Data is authenticated: yes</programlisting>

      <para><literal>Avahi</literal> running on a different host in the same local network should see the service as well:</para>

      <programlisting>$ avahi-browse -a -r
+ enp3s0 IPv6 meteo                                         Web Site             local
+ enp3s0 IPv4 meteo                                         Web Site             local
= enp3s0 IPv6 meteo                                         Web Site             local
   hostname = [meteo.local]
   address = [fe80::213:3bff:fe49:8aa]
   port = [80]
   txt = ["path=/stats/index.html" "t=temperature_sensor"]
= enp3s0 IPv4 meteo                                         Web Site             local
   hostname = [meteo.local]
   address = [169.254.208.106]
   port = [80]
   txt = ["path=/stats/index.html" "t=temperature_sensor"]</programlisting>

    </example>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>