man/hostnamectl,hostaned,hostname1: adjust the docs to match reality

The semantics were significantly changed in c779a44222161155c039a7fd2fd304c006590ac7
("hostnamed: Fix the way that static and transient host names interact", Feb. 2014),
but when the dbus api documentation was imported much later, it wasn't properly
adjusted to describe those new semantics.

34293dfafd2a81d80727938199769906dab321bd which added systemd.hostname= also
added new behaviour.

Let's ove various bits and pieces around so that they are in more appropriate
places. Drop recommendations to set the hostname for DHCP or mDNS purposes.
Nowadays we expect tools that want to expose some different hostname to the
outside to manage that internally without affecting visible state. Also drop
mentions of DHCP or mDNS directly setting the hostname, since nowadays network
management software is expected to (and does) go through hostnamed.

Also, add a high-level description of semantics. It glosses over the details of
handling of localhost-style names. Later commits will remove this special handling
anyway.

1 files changed, 61 insertions, 16 deletions

diff --git a/man/hostname.xml b/man/hostname.xml
index edbeef8f4a..d050703792 100644
--- a/man/hostname.xml
+++ b/man/hostname.xml
@@ -1,6 +1,9 @@
 <?xml version='1.0'?> <!--*-nxml-*-->
 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
-  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
+<!ENTITY % entities SYSTEM "custom-entities.ent" >
+%entities;
+]>
 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
 
 <refentry id="hostname">
@@ -26,23 +29,65 @@
   <refsect1>
     <title>Description</title>
 
-    <para>The <filename>/etc/hostname</filename> file configures the
-    name of the local system that is set during boot using the
-    <citerefentry><refentrytitle>sethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
-    system call. It should contain a single newline-terminated
-    hostname string.  Comments (lines starting with a `#') are ignored.
-    The hostname may be a free-form string up to 64 characters in length;
-    however, it is recommended that it consists only of 7-bit ASCII lower-case
-    characters and no spaces or dots, and limits itself to the format allowed
-    for DNS domain name labels, even though this is not a strict
-    requirement.</para>
+    <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 recommended that this name
+    contains only a single label, i.e. without any dots. 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>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>
+    <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 commandline 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>