systemd/man/hostname.xml

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<refentry id="hostname" xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>hostname</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>hostname</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>hostname</refname>
    <refpurpose>Local hostname configuration file</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>/etc/hostname</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>The <filename>/etc/hostname</filename> file configures the name of the local system. Unless
    overridden as described in the next section,
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will set this
    hostname during boot using the
    <citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry> system
    call.</para>

    <para>The file should contain a single newline-terminated hostname string. Comments (lines starting with
    a <literal>#</literal>) are ignored. The hostname should be composed of up to 64 7-bit ASCII lower-case
    alphanumeric characters or hyphens forming a valid DNS domain name. It is strongly recommended that this
    name contains only a single DNS label, i.e does not contain any dots. This recommendation reflects both
    usual expectations of applications, and the fact that the Linux kernel limits the length of the system
    hostname to 64 (i.e. close to the maximum DNS label length of 63) rather than 255 (the maximum DNS domain
    name length). When applied, invalid characters will be filtered out in an attempt to make the name valid,
    but obviously it is recommended to use a valid name and not rely on this filtering.</para>

    <para id="question-mark-hostname-pattern">If the question mark character <literal>?</literal> appears in
    the hostname, it is automatically substituted by a hexadecimal character derived from the
    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry> when
    applied, securely and deterministically by cryptographic hashing. Example:
    <literal>foobar-????-????</literal> will automatically expand to <literal>foobar-92a9-061c</literal> or
    similar, depending on the local machine ID.</para>

    <para id="word-hostname-pattern">In addition, the token <literal>$</literal> is substituted by a word picked
    deterministically from a word list, again derived from the
    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry> by
    cryptographic hashing. Each <literal>$</literal> is positional: the first <literal>$</literal> uses the word
    list file named <filename>1</filename>, the second <filename>2</filename>, and so on. This allows
    human-friendly names, for example <literal>$-$-$-????</literal> might expand to
    <literal>wildly-happy-octopus-92a9</literal>. The word lists are searched for in
    <filename>/etc/systemd/hostname-wordlist/</filename>, <filename>/run/systemd/hostname-wordlist/</filename>,
    <filename>/usr/local/lib/systemd/hostname-wordlist/</filename> and
    <filename>/usr/lib/systemd/hostname-wordlist/</filename> (the first directory providing a given list wins,
    lists are not merged); one word per line, with empty lines and lines starting with <literal>#</literal>
    ignored.</para>

    <para>The word for each token is derived deterministically from the machine ID and recomputed on every
    boot (the lists are not loaded wholesale: a word is chosen by hashing to a byte offset into the file).
    Consequently the word lists must be kept stable: changing a list (adding, removing, or reordering words)
    may change the name a machine already has, so installations that rely on persistent hostnames must not
    modify the lists after deployment. If a referenced list is missing the
    name is treated as invalid and the built-in fallback hostname is used. The combined name space is the
    product of the list sizes, so collisions follow the birthday bound; append a few <literal>?</literal>
    characters for extra entropy when uniqueness across a large fleet matters.</para>

    <para>Note that hostname can be at most 64 characters long. The word lists and the pattern should be
    chosen so that the longest possible expansion (the longest words from each list plus any literal and
    <literal>?</literal> characters and separators) stays within this limit. An expanded name that exceeds
    the limit is considered invalid and the built-in fallback hostname is used.</para>

    <para>Because the name only stays stable as long as the pattern and word lists are unchanged, the
    preferred way to obtain a stable, generated hostname is to provide the pattern through the
    <varname>firstboot.hostname</varname> credential (see
    <citerefentry><refentrytitle>systemd.system-credentials</refentrytitle><manvolnum>7</manvolnum></citerefentry>)
    rather than placing it directly in this file. Concretely, write the pattern to
    <filename>/etc/credstore/firstboot.hostname</filename> (see the description of
    <varname>LoadCredential=</varname> in
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for the
    credential store): when firstboot runs on the live system the credential is resolved once and the
    concrete result is persisted here, so the word lists may be updated afterwards without changing the
    hostname. A pattern placed directly in <filename>/etc/hostname</filename> (or provisioned into an offline
    image) is instead re-evaluated on every boot, which only really makes sense for systems that keep the
    wordlist and the pattern stable.</para>

    <xi:include href="version-info.xml" xpointer="v262"/>

    <para>You may use
    <citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to change
    the value of this file during runtime from the command line. Use
    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> to
    initialize it on mounted (but not booted) system images.</para>
  </refsect1>

  <refsect1>
    <title>Hostname semantics</title>

    <para><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> and the
    associated tools will obtain the hostname in the following ways:</para>
    <itemizedlist>
      <listitem><para>If the kernel command line parameter <varname>systemd.hostname=</varname> specifies a
      valid hostname,
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will use it
      to set the hostname during early boot, see
      <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      </para></listitem>

      <listitem><para>Otherwise, the "static" hostname specified by <filename>/etc/hostname</filename> as
      described above will be used.</para></listitem>

      <listitem><para>Otherwise, a transient hostname may be set during runtime, for example based on
      information in a DHCP lease, see
      <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
      Both <ulink url="https://developer.gnome.org/NetworkManager/stable/">NetworkManager</ulink> and
      <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      allow this. Note that
      <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      gives higher priority to the static hostname, so the transient hostname will only be used if the static
      hostname is not configured.</para></listitem>

      <listitem><para>Otherwise, a fallback hostname configured at compilation time will be used
      (<literal>&FALLBACK_HOSTNAME;</literal>).</para></listitem>

      <!-- what about the "linux" fallback fallback? -->
    </itemizedlist>

    <para>Effectively, the static hostname has higher priority than a transient hostname, which has higher
    priority than the fallback hostname. Transient hostnames are equivalent, so setting a new transient
    hostname causes the previous transient hostname to be forgotten. The hostname specified on the kernel
    command line is like a transient hostname, with the exception that it has higher priority when the
    machine boots. Also note that those are the semantics implemented by systemd tools, but other programs
    may also set the hostname.</para>
  </refsect1>

  <refsect1>
    <title>History</title>

    <para>The simple configuration file format of
    <filename>/etc/hostname</filename> originates from Debian
    GNU/Linux.</para>
  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para><simplelist type="inline">
        <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      </simplelist></para>
  </refsect1>

</refentry>