Merge pull request #17859 from keszybz/hostnamed-export-hostname-origin-and-simplify-logic

Export hostname origin and simplify logic in hostamed

27 files changed, 598 insertions, 433 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>
diff --git a/man/hostnamectl.xml b/man/hostnamectl.xml
index 8c00867e73..224dab78a7 100644
--- a/man/hostnamectl.xml
+++ b/man/hostnamectl.xml
@@ -32,33 +32,23 @@
   <refsect1>
     <title>Description</title>
 
-    <para><command>hostnamectl</command> may be used to query and
-    change the system hostname and related settings.</para>
-
-    <para>This tool distinguishes three different hostnames: the
-    high-level "pretty" hostname which might include all kinds of
-    special characters (e.g. "Lennart's Laptop"), the static hostname
-    which is used to initialize the kernel hostname at boot (e.g.
-    "lennarts-laptop"), and the transient hostname which is a fallback
-    value received from network configuration. If a static hostname is
-    set, and is valid (something other than localhost), then the
-    transient hostname is not used.</para>
+    <para><command>hostnamectl</command> may be used to query and change the system hostname and related
+    settings.</para>
+
+    <para><citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    and this tool distinguish three different hostnames: the high-level "pretty" hostname which might include
+    all kinds of special characters (e.g. "Lennart's Laptop"), the "static" hostname which is the
+    user-configured hostname (e.g. "lennarts-laptop"), and the transient hostname which is a fallback value
+    received from network configuration (e.g. "node12345678"). If a static hostname is set to a valid value,
+    then the transient hostname is not used.</para>
 
     <para>Note that the pretty hostname has little restrictions on the characters and length used, while the static and
     transient hostnames are limited to the usually accepted characters of Internet domain names, and 64 characters at
     maximum (the latter being a Linux limitation).</para>
 
-    <para>The static hostname is stored in
-    <filename>/etc/hostname</filename>, see
-    <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-    for more information. The pretty hostname, chassis type, and icon
-    name are stored in <filename>/etc/machine-info</filename>, see
-    <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
-
     <para>Use
-    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
-    to initialize the system hostname for mounted (but not booted)
-    system images.</para>
+    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry> to
+    initialize the system hostname for mounted (but not booted) system images.</para>
   </refsect1>
 
   <refsect1>
diff --git a/man/org.freedesktop.hostname1.xml b/man/org.freedesktop.hostname1.xml
index f8e199ceaa..a17339ad07 100644
--- a/man/org.freedesktop.hostname1.xml
+++ b/man/org.freedesktop.hostname1.xml
@@ -62,6 +62,9 @@ node /org/freedesktop/hostname1 {
       readonly s Hostname = '...';
       readonly s StaticHostname = '...';
       readonly s PrettyHostname = '...';
+      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
+      readonly s FallbackHostname = '...';
+      readonly s HostnameSource = '...';
       readonly s IconName = '...';
       readonly s Chassis = '...';
       readonly s Deployment = '...';
@@ -113,6 +116,10 @@ node /org/freedesktop/hostname1 {
 
     <variablelist class="dbus-property" generated="True" extra-ref="PrettyHostname"/>
 
+    <variablelist class="dbus-property" generated="True" extra-ref="FallbackHostname"/>
+
+    <variablelist class="dbus-property" generated="True" extra-ref="HostnameSource"/>
+
     <variablelist class="dbus-property" generated="True" extra-ref="IconName"/>
 
     <variablelist class="dbus-property" generated="True" extra-ref="Chassis"/>
@@ -144,55 +151,61 @@ node /org/freedesktop/hostname1 {
   <refsect1>
     <title>Semantics</title>
 
-    <para>The <emphasis>static (configured) hostname</emphasis> is the one configured in
-    <filename>/etc/hostname</filename>. It is chosen by the local user. It is not always in sync with the
-    current hostname as returned by the
+    <para>The <varname>StaticHostname</varname> property exposes the "static" hostname configured in
+    <filename>/etc/hostname</filename>. It is not always in sync with the current hostname as returned by the
     <citerefentry project="man-pages"><refentrytitle>gethostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-    system call. If no hostname is configured this property will be the empty string. Setting this property
-    to the empty string will remove <filename>/etc/hostname</filename>. This property should be an
-    internet-style hostname, 7-bit lowercase ASCII, no special chars/spaces.</para>
+    system call. If no static hostname is configured this property will be the empty string.</para>
 
-    <para>The <emphasis>transient (dynamic) hostname</emphasis> is the one configured via the kernel's
+    <para>When <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> or
+    <citerefentry><refentrytitle>systemd-hostnamed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    set the hostname, this static hostname <emphasis>has the highest priority</emphasis>.</para>
+
+    <para>The <varname>Hostname</varname> property exposes the actual hostname configured in the kernel via
     <citerefentry project="man-pages"><refentrytitle>sethostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
-    It can be different from the static hostname if DHCP or mDNS have been configured to change the name
-    based on network information. <!-- FIXME: it's not DHCP that configures this... -->
-    This property is never empty. If no hostname is set this will default to
-    <literal>&FALLBACK_HOSTNAME;</literal> (configurable at compilation time). Setting this property to the
-    empty string will reset the dynamic hostname to the static hostname. If no static hostname is
-    configured the dynamic hostname will be reset to <literal>&FALLBACK_HOSTNAME;</literal>. This property
-    should be an internet-style hostname, 7-bit lowercase ASCII, no special chars/spaces.</para>
-
-    <para>The <emphasis>pretty hostname</emphasis> is a free-form UTF-8 hostname for presentation to the
-    user. User interfaces should ensure that the pretty hostname and the static hostname stay in sync.
-    I.e. when the former is <literal>Lennart’s Computer</literal> the latter should be
-    <literal>lennarts-computer</literal>. If no pretty hostname is set this setting will be the empty
-    string. Applications should then find a suitable fallback, such as the dynamic hostname.</para>
-
-    <para>The <emphasis>icon name</emphasis> is a name following the XDG icon naming spec. If not set,
-    information such as the chassis type (see below) is used to find a suitable fallback icon name
-    (i.e. <literal>computer-laptop</literal> vs. <literal>computer-desktop</literal> is picked based on the
-    chassis information). If no such data is available, the empty string is returned. In that case an application
-    should fall back to a replacement icon, for example <literal>computer</literal>. If this property is set
-    to the empty string, the automatic fallback name selection is enabled again.</para>
-
-    <para>The <emphasis>chassis type</emphasis> should be one of the currently defined chassis types:
-    <literal>desktop</literal>, <literal>laptop</literal>, <literal>server</literal>,
-    <literal>tablet</literal>, <literal>handset</literal>, as well as the special chassis types
-    <literal>vm</literal> and <literal>container</literal> for virtualized systems. Note that in most cases
-    the chassis type will be determined automatically from DMI/SMBIOS/ACPI firmware information. Writing to
-    this setting is hence useful only to override misdetected chassis types, or to configure the chassis type if
-    it could not be auto-detected. Set this property to the empty string to reenable the automatic detection of
-    the chassis type from firmware information.</para>
+    It can be different from the static hostname. This property is never empty.</para>
+
+    <para>The <varname>PrettyHostname</varname> property exposes the <emphasis>pretty hostname</emphasis>
+    which is a free-form UTF-8 hostname for presentation to the user. User interfaces should ensure that the
+    pretty hostname and the static hostname stay in sync. E.g. when the former is <literal>Lennart’s
+    Computer</literal> the latter should be <literal>lennarts-computer</literal>. If no pretty hostname is
+    set this setting will be the empty string. Applications should then find a suitable fallback, such as the
+    dynamic hostname.</para>
+
+    <para>The <varname>FallbackHostname</varname> property exposes the fallback hostname (configured at
+    compilation time).</para>
+
+    <para>The <varname>HostnameSource</varname> property exposes the origin of the currently configured
+    hostname. One of <literal>static</literal> (set from <filename>/etc/hostname</filename>),
+    <literal>transient</literal> (a non-permanent hostname from an external source),
+    <literal>fallback</literal> (the compiled-in fallback value).</para>
+
+    <para>The <varname>IconName</varname> property exposes the <emphasis>icon name</emphasis> following the
+    XDG icon naming spec. If not set, information such as the chassis type (see below) is used to find a
+    suitable fallback icon name (i.e. <literal>computer-laptop</literal>
+    vs. <literal>computer-desktop</literal> is picked based on the chassis information). If no such data is
+    available, the empty string is returned. In that case an application should fall back to a replacement
+    icon, for example <literal>computer</literal>. If this property is set to the empty string, the automatic
+    fallback name selection is enabled again.</para>
+
+    <para>The <varname>Chassis</varname> property exposes a <emphasis>chassis type</emphasis>, one of the
+    currently defined chassis types: <literal>desktop</literal>, <literal>laptop</literal>,
+    <literal>server</literal>, <literal>tablet</literal>, <literal>handset</literal>, as well as the special
+    chassis types <literal>vm</literal> and <literal>container</literal> for virtualized systems. Note that
+    in most cases the chassis type will be determined automatically from DMI/SMBIOS/ACPI firmware
+    information. Writing to this setting is hence useful only to override misdetected chassis types, or to
+    configure the chassis type if it could not be auto-detected. Set this property to the empty string to
+    reenable the automatic detection of the chassis type from firmware information.</para>
 
     <para>Note that <filename>systemd-hostnamed</filename> starts only on request and terminates after a
     short idle period. This effectively means that <function>PropertyChanged</function> messages are not sent
     out for changes made directly on the files (as in: administrator edits the files with vi). This is
     the intended behavior: manual configuration changes should require manual reloading.</para>
 
-    <para>The transient (dynamic) hostname maps directly to the kernel hostname. This hostname should be
-    assumed to be highly dynamic, and hence should be watched directly, without depending on
-    <function>PropertyChanged</function> messages from <filename>systemd-hostnamed</filename>. To accomplish
-    this, open <filename>/proc/sys/kernel/hostname</filename> and
+    <para>The transient (dynamic) hostname exposed by the <varname>Hostname</varname> property maps directly
+    to the kernel hostname. This hostname should be assumed to be highly dynamic, and hence should be watched
+    directly, without depending on <function>PropertyChanged</function> messages from
+    <filename>systemd-hostnamed</filename>. To accomplish this, open
+    <filename>/proc/sys/kernel/hostname</filename> and
     <citerefentry project="man-pages"><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     for <constant>SIGHUP</constant> which is triggered by the kernel every time the hostname changes. Again:
     this is special for the transient (dynamic) hostname, and does not apply to the configured (fixed)
@@ -206,15 +219,17 @@ node /org/freedesktop/hostname1 {
     for that. For more information on these files and syscalls see the respective man pages.</para>
 
     <refsect2>
-      <title>Methods and Properties</title>
+      <title>Methods</title>
 
-      <para><function>SetHostname()</function> sets the transient (dynamic) hostname which is exposed by the
-      <varname>Hostname</varname> property. If empty, the transient hostname is set to the static hostname.
-      </para>
+      <para><function>SetHostname()</function> sets the transient (dynamic) hostname, which is used if no
+      static hostname is set. This value must be an internet-style hostname, 7-bit lowercase ASCII, no
+      special chars/spaces. An empty string will unset the transient hostname.</para>
 
       <para><function>SetStaticHostname()</function> sets the static hostname which is exposed by the
-      <varname>StaticHostname</varname> property. If empty, the built-in default of
-      <literal>&FALLBACK_HOSTNAME;</literal> is used.</para>
+      <varname>StaticHostname</varname> property. When called with an empty argument, the static
+      configuration in <filename>/etc/hostname</filename> is removed. Since the static hostname has the
+      highest priority, calling this function usually affects also the <varname>Hostname</varname> property
+      and the effective hostname configured in the kernel.</para>
 
       <para><function>SetPrettyHostname()</function> sets the pretty hostname which is exposed by the
       <varname>PrettyHostname</varname> property.</para>
@@ -287,10 +302,6 @@ node /org/freedesktop/hostname1 {
     with <citerefentry><refentrytitle>nss-myhostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
     </para>
 
-    <para>A client that wants to change the local hostname for DHCP/mDNS should invoke
-    <code>SetHostname("newname", false)</code> as soon as the name is available and afterwards reset it via
-    <code>SetHostname("")</code>.</para>
-
     <para>Here are some recommendations to follow when generating a static (internet) hostname from a pretty
     name:
     <itemizedlist>
diff --git a/man/systemd-hostnamed.service.xml b/man/systemd-hostnamed.service.xml
index c0c46b6609..0e42f671c2 100644
--- a/man/systemd-hostnamed.service.xml
+++ b/man/systemd-hostnamed.service.xml
@@ -51,9 +51,15 @@
       </listitem>
     </itemizedlist></para>
 
+    <para>The static hostname is stored in <filename>/etc/hostname</filename>, see
+    <citerefentry><refentrytitle>hostname</refentrytitle><manvolnum>5</manvolnum></citerefentry> for more
+    information. The pretty hostname, chassis type, and icon name are stored in
+    <filename>/etc/machine-info</filename>, see
+    <citerefentry><refentrytitle>machine-info</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
     <para>The tool
-    <citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
-    is a command line client to this service.</para>
+    <citerefentry><refentrytitle>hostnamectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> is a
+    command line client to this service.</para>
 
     <para>See
     <citerefentry><refentrytitle>org.freedesktop.hostname1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
diff --git a/src/basic/hostname-util.c b/src/basic/hostname-util.c
index e1e3d1b061..d7aba2c263 100644
--- a/src/basic/hostname-util.c
+++ b/src/basic/hostname-util.c
@@ -7,28 +7,10 @@
 #include <unistd.h>
 
 #include "alloc-util.h"
-#include "fd-util.h"
-#include "fileio.h"
 #include "hostname-util.h"
-#include "macro.h"
 #include "string-util.h"
 #include "strv.h"
 
-bool hostname_is_set(void) {
-        struct utsname u;
-
-        assert_se(uname(&u) >= 0);
-
-        if (isempty(u.nodename))
-                return false;
-
-        /* This is the built-in kernel default hostname */
-        if (streq(u.nodename, "(none)"))
-                return false;
-
-        return true;
-}
-
 char* gethostname_malloc(void) {
         struct utsname u;
         const char *s;
@@ -208,105 +190,3 @@ bool is_localhost(const char *hostname) {
                 endswith_no_case(hostname, ".localhost.localdomain") ||
                 endswith_no_case(hostname, ".localhost.localdomain.");
 }
-
-int sethostname_idempotent(const char *s) {
-        char buf[HOST_NAME_MAX + 1] = {};
-
-        assert(s);
-
-        if (gethostname(buf, sizeof(buf)) < 0)
-                return -errno;
-
-        if (streq(buf, s))
-                return 0;
-
-        if (sethostname(s, strlen(s)) < 0)
-                return -errno;
-
-        return 1;
-}
-
-int shorten_overlong(const char *s, char **ret) {
-        char *h, *p;
-
-        /* Shorten an overlong name to HOST_NAME_MAX or to the first dot,
-         * whatever comes earlier. */
-
-        assert(s);
-
-        h = strdup(s);
-        if (!h)
-                return -ENOMEM;
-
-        if (hostname_is_valid(h, 0)) {
-                *ret = h;
-                return 0;
-        }
-
-        p = strchr(h, '.');
-        if (p)
-                *p = 0;
-
-        strshorten(h, HOST_NAME_MAX);
-
-        if (!hostname_is_valid(h, 0)) {
-                free(h);
-                return -EDOM;
-        }
-
-        *ret = h;
-        return 1;
-}
-
-int read_etc_hostname_stream(FILE *f, char **ret) {
-        int r;
-
-        assert(f);
-        assert(ret);
-
-        for (;;) {
-                _cleanup_free_ char *line = NULL;
-                char *p;
-
-                r = read_line(f, LONG_LINE_MAX, &line);
-                if (r < 0)
-                        return r;
-                if (r == 0) /* EOF without any hostname? the file is empty, let's treat that exactly like no file at all: ENOENT */
-                        return -ENOENT;
-
-                p = strstrip(line);
-
-                /* File may have empty lines or comments, ignore them */
-                if (!IN_SET(*p, '\0', '#')) {
-                        char *copy;
-
-                        hostname_cleanup(p); /* normalize the hostname */
-
-                        if (!hostname_is_valid(p, VALID_HOSTNAME_TRAILING_DOT)) /* check that the hostname we return is valid */
-                                return -EBADMSG;
-
-                        copy = strdup(p);
-                        if (!copy)
-                                return -ENOMEM;
-
-                        *ret = copy;
-                        return 0;
-                }
-        }
-}
-
-int read_etc_hostname(const char *path, char **ret) {
-        _cleanup_fclose_ FILE *f = NULL;
-
-        assert(ret);
-
-        if (!path)
-                path = "/etc/hostname";
-
-        f = fopen(path, "re");
-        if (!f)
-                return -errno;
-
-        return read_etc_hostname_stream(f, ret);
-
-}
diff --git a/src/basic/hostname-util.h b/src/basic/hostname-util.h
index fe417297ee..6cff9c1d4c 100644
--- a/src/basic/hostname-util.h
+++ b/src/basic/hostname-util.h
@@ -7,8 +7,6 @@
 #include "macro.h"
 #include "strv.h"
 
-bool hostname_is_set(void);
-
 char* gethostname_malloc(void);
 char* gethostname_short_malloc(void);
 int gethostname_strict(char **ret);
@@ -29,10 +27,3 @@ static inline bool is_gateway_hostname(const char *hostname) {
         /* This tries to identify the valid syntaxes for the our synthetic "gateway" host. */
         return STRCASE_IN_SET(hostname, "_gateway", "_gateway.");
 }
-
-int sethostname_idempotent(const char *s);
-
-int shorten_overlong(const char *s, char **ret);
-
-int read_etc_hostname_stream(FILE *f, char **ret);
-int read_etc_hostname(const char *path, char **ret);
diff --git a/src/basic/string-table.h b/src/basic/string-table.h
index 4911a455c5..ddc9b9a559 100644
--- a/src/basic/string-table.h
+++ b/src/basic/string-table.h
@@ -78,6 +78,7 @@ ssize_t string_table_lookup(const char * const *table, size_t len, const char *k
         _DEFINE_STRING_TABLE_LOOKUP_FROM_STRING_WITH_BOOLEAN(name,type,yes,scope)
 
 #define DEFINE_STRING_TABLE_LOOKUP(name,type) _DEFINE_STRING_TABLE_LOOKUP(name,type,)
+#define DEFINE_STRING_TABLE_LOOKUP_TO_STRING(name,type) _DEFINE_STRING_TABLE_LOOKUP_TO_STRING(name,type,)
 #define DEFINE_PRIVATE_STRING_TABLE_LOOKUP(name,type) _DEFINE_STRING_TABLE_LOOKUP(name,type,static)
 #define DEFINE_PRIVATE_STRING_TABLE_LOOKUP_TO_STRING(name,type) _DEFINE_STRING_TABLE_LOOKUP_TO_STRING(name,type,static)
 #define DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING(name,type) _DEFINE_STRING_TABLE_LOOKUP_FROM_STRING(name,type,static)
diff --git a/src/core/hostname-setup.c b/src/core/hostname-setup.c
deleted file mode 100644
index 6ccaa479de..0000000000
--- a/src/core/hostname-setup.c
+++ /dev/null
@@ -1,62 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-
-#include <errno.h>
-#include <stdio.h>
-#include <stdlib.h>
-
-#include "alloc-util.h"
-#include "fileio.h"
-#include "hostname-setup.h"
-#include "hostname-util.h"
-#include "log.h"
-#include "macro.h"
-#include "proc-cmdline.h"
-#include "string-util.h"
-#include "util.h"
-
-int hostname_setup(void) {
-        _cleanup_free_ char *b = NULL;
-        const char *hn = NULL;
-        bool enoent = false;
-        int r;
-
-        r = proc_cmdline_get_key("systemd.hostname", 0, &b);
-        if (r < 0)
-                log_warning_errno(r, "Failed to retrieve system hostname from kernel command line, ignoring: %m");
-        else if (r > 0) {
-                if (hostname_is_valid(b, VALID_HOSTNAME_TRAILING_DOT))
-                        hn = b;
-                else  {
-                        log_warning("Hostname specified on kernel command line is invalid, ignoring: %s", b);
-                        b = mfree(b);
-                }
-        }
-
-        if (!hn) {
-                r = read_etc_hostname(NULL, &b);
-                if (r == -ENOENT)
-                        enoent = true;
-                else if (r < 0)
-                        log_warning_errno(r, "Failed to read configured hostname, ignoring: %m");
-                else
-                        hn = b;
-        }
-
-        if (isempty(hn)) {
-                /* Don't override the hostname if it is already set and not explicitly configured */
-                if (hostname_is_set())
-                        return 0;
-
-                if (enoent)
-                        log_info("No hostname configured.");
-
-                hn = FALLBACK_HOSTNAME;
-        }
-
-        r = sethostname_idempotent(hn);
-        if (r < 0)
-                return log_warning_errno(r, "Failed to set hostname to <%s>: %m", hn);
-
-        log_info("Set hostname to <%s>.", hn);
-        return 0;
-}
diff --git a/src/core/hostname-setup.h b/src/core/hostname-setup.h
deleted file mode 100644
index 7fd0a02747..0000000000
--- a/src/core/hostname-setup.h
+++ /dev/null
@@ -1,4 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-#pragma once
-
-int hostname_setup(void);
diff --git a/src/core/main.c b/src/core/main.c
index ef4d03750f..eaa56aca2a 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -2064,7 +2064,7 @@ static int initialize_runtime(
                         }
 
                         status_welcome();
-                        hostname_setup();
+                        (void) hostname_setup(true);
                         /* Force transient machine-id on first boot. */
                         machine_id_setup(NULL, first_boot, arg_machine_id, NULL);
                         (void) loopback_setup();
diff --git a/src/core/meson.build b/src/core/meson.build
index 77767eb603..662e6376f1 100644
--- a/src/core/meson.build
+++ b/src/core/meson.build
@@ -76,8 +76,6 @@ libcore_sources = '''
         execute.h
         generator-setup.c
         generator-setup.h
-        hostname-setup.c
-        hostname-setup.h
         ima-setup.c
         ima-setup.h
         ip-address-access.c
diff --git a/src/fuzz/fuzz-hostname-util.c b/src/fuzz/fuzz-hostname-setup.c
index 0a81e74424..b8d36da54a 100644
--- a/src/fuzz/fuzz-hostname-util.c
+++ b/src/fuzz/fuzz-hostname-setup.c
@@ -4,7 +4,7 @@
 #include "fd-util.h"
 #include "fileio.h"
 #include "fuzz.h"
-#include "hostname-util.h"
+#include "hostname-setup.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;
diff --git a/src/fuzz/meson.build b/src/fuzz/meson.build
index 80fa0f6fcc..83527a68fb 100644
--- a/src/fuzz/meson.build
+++ b/src/fuzz/meson.build
@@ -123,7 +123,7 @@ fuzzers += [
          [libshared],
          []],
 
-        [['src/fuzz/fuzz-hostname-util.c'],
+        [['src/fuzz/fuzz-hostname-setup.c'],
          [libshared],
          []],
 
diff --git a/src/hostname/hostnamed.c b/src/hostname/hostnamed.c
index a0dc25c834..f0bf29028f 100644
--- a/src/hostname/hostnamed.c
+++ b/src/hostname/hostnamed.c
@@ -8,6 +8,7 @@
 
 #include "alloc-util.h"
 #include "bus-common-errors.h"
+#include "bus-get-properties.h"
 #include "bus-log-control-api.h"
 #include "bus-polkit.h"
 #include "def.h"
@@ -16,6 +17,7 @@
 #include "env-util.h"
 #include "fileio-label.h"
 #include "fileio.h"
+#include "hostname-setup.h"
 #include "hostname-util.h"
 #include "id128-util.h"
 #include "main-func.h"
@@ -29,6 +31,7 @@
 #include "service-util.h"
 #include "signal-util.h"
 #include "stat-util.h"
+#include "string-table.h"
 #include "strv.h"
 #include "user-util.h"
 #include "util.h"
@@ -58,6 +61,8 @@ enum {
 typedef struct Context {
         char *data[_PROP_MAX];
 
+        HostnameSource hostname_source;
+
         struct stat etc_hostname_stat;
         struct stat etc_os_release_stat;
         struct stat etc_machine_info_stat;
@@ -66,11 +71,9 @@ typedef struct Context {
 } Context;
 
 static void context_reset(Context *c, uint64_t mask) {
-        int p;
-
         assert(c);
 
-        for (p = 0; p < _PROP_MAX; p++) {
+        for (int p = 0; p < _PROP_MAX; p++) {
                 if (!FLAGS_SET(mask, UINT64_C(1) << p))
                         continue;
 
@@ -312,63 +315,63 @@ static char* context_fallback_icon_name(Context *c) {
         return strdup("computer");
 }
 
-static bool hostname_is_useful(const char *hn) {
-        return !isempty(hn) && !is_localhost(hn);
-}
-
 static int context_update_kernel_hostname(
                 Context *c,
                 const char *transient_hn) {
 
-        const char *static_hn, *hn;
-        struct utsname u;
+        const char *hn;
+        HostnameSource hns;
+        int r;
 
         assert(c);
 
-        if (!transient_hn) {
-                /* If no transient hostname is passed in, then let's check what is currently set. */
-                assert_se(uname(&u) >= 0);
-                transient_hn =
-                        isempty(u.nodename) || streq(u.nodename, "(none)") ? NULL : u.nodename;
-        }
-
-        static_hn = c->data[PROP_STATIC_HOSTNAME];
-
-        /* /etc/hostname with something other than "localhost"
-         * has the highest preference ... */
-        if (hostname_is_useful(static_hn))
-                hn = static_hn;
+        /* /etc/hostname has the highest preference ... */
+        if (c->data[PROP_STATIC_HOSTNAME]) {
+                hn = c->data[PROP_STATIC_HOSTNAME];
+                hns = HOSTNAME_STATIC;
 
         /* ... the transient hostname, (ie: DHCP) comes next ... */
-        else if (!isempty(transient_hn))
+        } else if (transient_hn) {
                 hn = transient_hn;
-
-        /* ... fallback to static "localhost.*" ignored above ... */
-        else if (!isempty(static_hn))
-                hn = static_hn;
+                hns = HOSTNAME_TRANSIENT;
 
         /* ... and the ultimate fallback */
-        else
+        } else {
                 hn = FALLBACK_HOSTNAME;
+                hns = HOSTNAME_FALLBACK;
+        }
 
-        if (sethostname_idempotent(hn) < 0)
-                return -errno;
+        r = sethostname_idempotent(hn);
+        if (r < 0)
+                return log_error_errno(r, "Failed to set hostname: %m");
+
+        if (c->hostname_source != hns) {
+                c->hostname_source = hns;
+                r = 1;
+        }
 
         (void) nscd_flush_cache(STRV_MAKE("hosts"));
 
-        return 0;
+        if (r == 0)
+                log_debug("Hostname was already set to <%s>.", hn);
+        else {
+                log_info("Hostname set to <%s> (%s)", hn, hostname_source_to_string(hns));
+
+                hostname_update_source_hint(hn, hns);
+        }
+
+        return r; /* 0 if no change, 1 if something was done  */
 }
 
 static int context_write_data_static_hostname(Context *c) {
         assert(c);
 
         if (isempty(c->data[PROP_STATIC_HOSTNAME])) {
-
                 if (unlink("/etc/hostname") < 0)
                         return errno == ENOENT ? 0 : -errno;
-
                 return 0;
         }
+
         return write_string_file_atomic_label("/etc/hostname", c->data[PROP_STATIC_HOSTNAME]);
 }
 
@@ -383,7 +386,7 @@ static int context_write_data_machine_info(Context *c) {
         };
 
         _cleanup_strv_free_ char **l = NULL;
-        int r, p;
+        int r;
 
         assert(c);
 
@@ -391,7 +394,7 @@ static int context_write_data_machine_info(Context *c) {
         if (r < 0 && r != -ENOENT)
                 return r;
 
-        for (p = PROP_PRETTY_HOSTNAME; p <= PROP_LOCATION; p++) {
+        for (int p = PROP_PRETTY_HOSTNAME; p <= PROP_LOCATION; p++) {
                 _cleanup_free_ char *t = NULL;
                 char **u;
 
@@ -461,6 +464,52 @@ static int property_get_static_hostname(
         return sd_bus_message_append(reply, "s", c->data[PROP_STATIC_HOSTNAME]);
 }
 
+static BUS_DEFINE_PROPERTY_GET_GLOBAL(property_get_fallback_hostname, "s", FALLBACK_HOSTNAME);
+
+static int property_get_hostname_source(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *reply,
+                void *userdata,
+                sd_bus_error *error) {
+
+        Context *c = userdata;
+        int r;
+        assert(c);
+
+        context_read_etc_hostname(c);
+
+        if (c->hostname_source < 0) {
+                char hostname[HOST_NAME_MAX + 1] = {};
+                _cleanup_free_ char *fallback = NULL;
+
+                (void) get_hostname_filtered(hostname);
+
+                if (streq_ptr(hostname, c->data[PROP_STATIC_HOSTNAME]))
+                        c->hostname_source = HOSTNAME_STATIC;
+
+                else {
+                        /* If the hostname was not set by us, try to figure out where it came from. If we set
+                         * it to the fallback hostname, the file will tell us. We compare the string because
+                         * it is possible that the hostname was set by an older version that had a different
+                         * fallback, in the initramfs or before we reexecuted. */
+
+                        r = read_one_line_file("/run/systemd/fallback-hostname", &fallback);
+                        if (r < 0 && r != -ENOENT)
+                                log_warning_errno(r, "Failed to read /run/systemd/fallback-hostname, ignoring: %m");
+
+                        if (streq_ptr(fallback, hostname))
+                                c->hostname_source = HOSTNAME_FALLBACK;
+                        else
+                                c->hostname_source = HOSTNAME_TRANSIENT;
+                }
+        }
+
+        return sd_bus_message_append(reply, "s", hostname_source_to_string(c->hostname_source));
+}
+
 static int property_get_machine_info_field(
                 sd_bus *bus,
                 const char *path,
@@ -579,7 +628,6 @@ static int method_set_hostname(sd_bus_message *m, void *userdata, sd_bus_error *
         Context *c = userdata;
         const char *name;
         int interactive, r;
-        struct utsname u;
 
         assert(m);
         assert(c);
@@ -588,20 +636,15 @@ static int method_set_hostname(sd_bus_message *m, void *userdata, sd_bus_error *
         if (r < 0)
                 return r;
 
-        context_read_etc_hostname(c);
-
-        if (isempty(name))
-                name = c->data[PROP_STATIC_HOSTNAME];
+        name = empty_to_null(name);
 
-        if (isempty(name))
-                name = FALLBACK_HOSTNAME;
+        /* We always go through with the procedure below without comparing to the current hostname, because
+         * we might want to adjust hostname source information even if the actual hostname is unchanged. */
 
         if (!hostname_is_valid(name, 0))
                 return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid hostname '%s'", name);
 
-        assert_se(uname(&u) >= 0);
-        if (streq_ptr(name, u.nodename))
-                return sd_bus_reply_method_return(m, NULL);
+        context_read_etc_hostname(c);
 
         r = bus_verify_polkit_async(
                         m,
@@ -618,14 +661,12 @@ static int method_set_hostname(sd_bus_message *m, void *userdata, sd_bus_error *
                 return 1; /* No authorization for now, but the async polkit stuff will call us again when it has it */
 
         r = context_update_kernel_hostname(c, name);
-        if (r < 0) {
-                log_error_errno(r, "Failed to set hostname: %m");
+        if (r < 0)
                 return sd_bus_error_set_errnof(error, r, "Failed to set hostname: %m");
-        }
-
-        log_info("Changed hostname to '%s'", name);
-
-        (void) sd_bus_emit_properties_changed(sd_bus_message_get_bus(m), "/org/freedesktop/hostname1", "org.freedesktop.hostname1", "Hostname", NULL);
+        else if (r > 0)
+                (void) sd_bus_emit_properties_changed(sd_bus_message_get_bus(m),
+                                                      "/org/freedesktop/hostname1", "org.freedesktop.hostname1",
+                                                      "Hostname", "HostnameSource", NULL);
 
         return sd_bus_reply_method_return(m, NULL);
 }
@@ -650,7 +691,7 @@ static int method_set_static_hostname(sd_bus_message *m, void *userdata, sd_bus_
         if (streq_ptr(name, c->data[PROP_STATIC_HOSTNAME]))
                 return sd_bus_reply_method_return(m, NULL);
 
-        if (!isempty(name) && !hostname_is_valid(name, 0))
+        if (name && !hostname_is_valid(name, 0))
                 return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid static hostname '%s'", name);
 
         r = bus_verify_polkit_async(
@@ -671,21 +712,21 @@ static int method_set_static_hostname(sd_bus_message *m, void *userdata, sd_bus_
         if (r < 0)
                 return r;
 
-        r = context_update_kernel_hostname(c, NULL);
-        if (r < 0) {
-                log_error_errno(r, "Failed to set hostname: %m");
-                return sd_bus_error_set_errnof(error, r, "Failed to set hostname: %m");
-        }
-
         r = context_write_data_static_hostname(c);
         if (r < 0) {
                 log_error_errno(r, "Failed to write static hostname: %m");
                 return sd_bus_error_set_errnof(error, r, "Failed to set static hostname: %m");
         }
 
-        log_info("Changed static hostname to '%s'", strna(c->data[PROP_STATIC_HOSTNAME]));
+        r = context_update_kernel_hostname(c, NULL);
+        if (r < 0) {
+                log_error_errno(r, "Failed to set hostname: %m");
+                return sd_bus_error_set_errnof(error, r, "Failed to set hostname: %m");
+        }
 
-        (void) sd_bus_emit_properties_changed(sd_bus_message_get_bus(m), "/org/freedesktop/hostname1", "org.freedesktop.hostname1", "StaticHostname", NULL);
+        (void) sd_bus_emit_properties_changed(sd_bus_message_get_bus(m),
+                                              "/org/freedesktop/hostname1", "org.freedesktop.hostname1",
+                                              "StaticHostname", "Hostname", "HostnameSource", NULL);
 
         return sd_bus_reply_method_return(m, NULL);
 }
@@ -850,6 +891,8 @@ static const sd_bus_vtable hostname_vtable[] = {
         SD_BUS_PROPERTY("Hostname", "s", property_get_hostname, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("StaticHostname", "s", property_get_static_hostname, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("PrettyHostname", "s", property_get_machine_info_field, offsetof(Context, data) + sizeof(char*) * PROP_PRETTY_HOSTNAME, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
+        SD_BUS_PROPERTY("FallbackHostname", "s", property_get_fallback_hostname, 0, SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("HostnameSource", "s", property_get_hostname_source, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("IconName", "s", property_get_icon_name, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("Chassis", "s", property_get_chassis, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("Deployment", "s", property_get_machine_info_field, offsetof(Context, data) + sizeof(char*) * PROP_DEPLOYMENT, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
@@ -960,7 +1003,9 @@ static int connect_bus(Context *c, sd_event *event, sd_bus **ret) {
 }
 
 static int run(int argc, char *argv[]) {
-        _cleanup_(context_destroy) Context context = {};
+        _cleanup_(context_destroy) Context context = {
+                .hostname_source = _HOSTNAME_INVALID, /* appropriate value will be set later */
+        };
         _cleanup_(sd_event_unrefp) sd_event *event = NULL;
         _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
         int r;
diff --git a/src/network/networkd-dhcp4.c b/src/network/networkd-dhcp4.c
index 14e7a28774..080dcedab5 100644
--- a/src/network/networkd-dhcp4.c
+++ b/src/network/networkd-dhcp4.c
@@ -8,6 +8,7 @@
 #include "escape.h"
 #include "alloc-util.h"
 #include "dhcp-client-internal.h"
+#include "hostname-setup.h"
 #include "hostname-util.h"
 #include "parse-util.h"
 #include "network-internal.h"
diff --git a/src/network/test-network.c b/src/network/test-network.c
index 03c94409fa..45cd3fa973 100644
--- a/src/network/test-network.c
+++ b/src/network/test-network.c
@@ -8,7 +8,7 @@
 #include "alloc-util.h"
 #include "dhcp-lease-internal.h"
 #include "ether-addr-util.h"
-#include "hostname-util.h"
+#include "hostname-setup.h"
 #include "network-internal.h"
 #include "networkd-manager.h"
 #include "string-util.h"
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index cfbc8f11bf..0f6411d193 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -46,6 +46,7 @@
 #include "fs-util.h"
 #include "gpt.h"
 #include "hexdecoct.h"
+#include "hostname-setup.h"
 #include "hostname-util.h"
 #include "id128-util.h"
 #include "io-util.h"
diff --git a/src/shared/dissect-image.c b/src/shared/dissect-image.c
index 113538bb97..b1d7d689ea 100644
--- a/src/shared/dissect-image.c
+++ b/src/shared/dissect-image.c
@@ -32,7 +32,7 @@
 #include "fsck-util.h"
 #include "gpt.h"
 #include "hexdecoct.h"
-#include "hostname-util.h"
+#include "hostname-setup.h"
 #include "id128-util.h"
 #include "mkdir.h"
 #include "mount-util.h"
diff --git a/src/shared/hostname-setup.c b/src/shared/hostname-setup.c
new file mode 100644
index 0000000000..c0465d3dcd
--- /dev/null
+++ b/src/shared/hostname-setup.c
@@ -0,0 +1,236 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/utsname.h>
+#include <unistd.h>
+
+#include "alloc-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "fs-util.h"
+#include "hostname-setup.h"
+#include "hostname-util.h"
+#include "log.h"
+#include "macro.h"
+#include "proc-cmdline.h"
+#include "string-table.h"
+#include "string-util.h"
+#include "util.h"
+
+static int sethostname_idempotent_full(const char *s, bool really) {
+        char buf[HOST_NAME_MAX + 1] = {};
+
+        assert(s);
+
+        if (gethostname(buf, sizeof(buf) - 1) < 0)
+                return -errno;
+
+        if (streq(buf, s))
+                return 0;
+
+        if (really &&
+            sethostname(s, strlen(s)) < 0)
+                return -errno;
+
+        return 1;
+}
+
+int sethostname_idempotent(const char *s) {
+        return sethostname_idempotent_full(s, true);
+}
+
+bool get_hostname_filtered(char ret[static HOST_NAME_MAX + 1]) {
+        char buf[HOST_NAME_MAX + 1] = {};
+
+        /* Returns true if we got a good hostname, false otherwise. */
+
+        if (gethostname(buf, sizeof(buf) - 1) < 0)
+                return false;  /* This can realistically only fail with ENAMETOOLONG.
+                                * Let's treat that case the same as an invalid hostname. */
+
+        if (isempty(buf))
+                return false;
+
+        /* This is the built-in kernel default hostname */
+        if (streq(buf, "(none)"))
+                return false;
+
+        memcpy(ret, buf, sizeof buf);
+        return true;
+}
+
+int shorten_overlong(const char *s, char **ret) {
+        char *h, *p;
+
+        /* Shorten an overlong name to HOST_NAME_MAX or to the first dot,
+         * whatever comes earlier. */
+
+        assert(s);
+
+        h = strdup(s);
+        if (!h)
+                return -ENOMEM;
+
+        if (hostname_is_valid(h, 0)) {
+                *ret = h;
+                return 0;
+        }
+
+        p = strchr(h, '.');
+        if (p)
+                *p = 0;
+
+        strshorten(h, HOST_NAME_MAX);
+
+        if (!hostname_is_valid(h, 0)) {
+                free(h);
+                return -EDOM;
+        }
+
+        *ret = h;
+        return 1;
+}
+
+int read_etc_hostname_stream(FILE *f, char **ret) {
+        int r;
+
+        assert(f);
+        assert(ret);
+
+        for (;;) {
+                _cleanup_free_ char *line = NULL;
+                char *p;
+
+                r = read_line(f, LONG_LINE_MAX, &line);
+                if (r < 0)
+                        return r;
+                if (r == 0) /* EOF without any hostname? the file is empty, let's treat that exactly like no file at all: ENOENT */
+                        return -ENOENT;
+
+                p = strstrip(line);
+
+                /* File may have empty lines or comments, ignore them */
+                if (!IN_SET(*p, '\0', '#')) {
+                        char *copy;
+
+                        hostname_cleanup(p); /* normalize the hostname */
+
+                        if (!hostname_is_valid(p, VALID_HOSTNAME_TRAILING_DOT)) /* check that the hostname we return is valid */
+                                return -EBADMSG;
+
+                        copy = strdup(p);
+                        if (!copy)
+                                return -ENOMEM;
+
+                        *ret = copy;
+                        return 0;
+                }
+        }
+}
+
+int read_etc_hostname(const char *path, char **ret) {
+        _cleanup_fclose_ FILE *f = NULL;
+
+        assert(ret);
+
+        if (!path)
+                path = "/etc/hostname";
+
+        f = fopen(path, "re");
+        if (!f)
+                return -errno;
+
+        return read_etc_hostname_stream(f, ret);
+}
+
+void hostname_update_source_hint(const char *hostname, HostnameSource source) {
+        int r;
+
+        /* Why save the value and not just create a flag file? This way we will
+         * notice if somebody sets the hostname directly (not going through hostnamed).
+         */
+
+        if (source == HOSTNAME_FALLBACK) {
+                r = write_string_file("/run/systemd/fallback-hostname", hostname,
+                                      WRITE_STRING_FILE_CREATE | WRITE_STRING_FILE_ATOMIC);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to create \"/run/systemd/fallback-hostname\": %m");
+        } else
+                unlink_or_warn("/run/systemd/fallback-hostname");
+}
+
+int hostname_setup(bool really) {
+        _cleanup_free_ char *b = NULL;
+        const char *hn = NULL;
+        HostnameSource source;
+        bool enoent = false;
+        int r;
+
+        r = proc_cmdline_get_key("systemd.hostname", 0, &b);
+        if (r < 0)
+                log_warning_errno(r, "Failed to retrieve system hostname from kernel command line, ignoring: %m");
+        else if (r > 0) {
+                if (hostname_is_valid(b, true)) {
+                        hn = b;
+                        source = HOSTNAME_TRANSIENT;
+                } else  {
+                        log_warning("Hostname specified on kernel command line is invalid, ignoring: %s", b);
+                        b = mfree(b);
+                }
+        }
+
+        if (!hn) {
+                r = read_etc_hostname(NULL, &b);
+                if (r < 0) {
+                        if (r == -ENOENT)
+                                enoent = true;
+                        else
+                                log_warning_errno(r, "Failed to read configured hostname: %m");
+                } else {
+                        hn = b;
+                        source = HOSTNAME_STATIC;
+                }
+        }
+
+        if (isempty(hn)) {
+                /* Don't override the hostname if it is already set and not explicitly configured */
+
+                char buf[HOST_NAME_MAX + 1] = {};
+                if (get_hostname_filtered(buf)) {
+                        log_debug("No hostname configured, leaving existing hostname <%s> in place.", buf);
+                        return 0;
+                }
+
+                if (enoent)
+                        log_info("No hostname configured, using fallback hostname.");
+
+                hn = FALLBACK_HOSTNAME;
+                source = HOSTNAME_FALLBACK;
+
+        }
+
+        r = sethostname_idempotent_full(hn, really);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to set hostname to <%s>: %m", hn);
+        if (r == 0)
+                log_debug("Hostname was already set to <%s>.", hn);
+        else
+                log_info("Hostname %s to <%s>.",
+                         really ? "set" : "would have been set",
+                         hn);
+
+        if (really)
+                hostname_update_source_hint(hn, source);
+
+        return r;
+}
+
+static const char* const hostname_source_table[] = {
+        [HOSTNAME_STATIC]    = "static",
+        [HOSTNAME_TRANSIENT] = "transient",
+        [HOSTNAME_FALLBACK]  = "fallback",
+};
+
+DEFINE_STRING_TABLE_LOOKUP_TO_STRING(hostname_source, HostnameSource);
diff --git a/src/shared/hostname-setup.h b/src/shared/hostname-setup.h
new file mode 100644
index 0000000000..022f0eb835
--- /dev/null
+++ b/src/shared/hostname-setup.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdbool.h>
+#include <stdio.h>
+
+typedef enum HostnameSource {
+        HOSTNAME_STATIC,     /* from /etc/hostname */
+        HOSTNAME_TRANSIENT,  /* a transient hostname set through systemd, hostnamed, the container manager, or otherwise */
+        HOSTNAME_FALLBACK,   /* the compiled-in fallback was used */
+        _HOSTNAME_INVALID = -1,
+} HostnameSource;
+
+const char* hostname_source_to_string(HostnameSource source);
+int sethostname_idempotent(const char *s);
+
+int shorten_overlong(const char *s, char **ret);
+
+int read_etc_hostname_stream(FILE *f, char **ret);
+int read_etc_hostname(const char *path, char **ret);
+
+bool get_hostname_filtered(char ret[static HOST_NAME_MAX + 1]);
+void hostname_update_source_hint(const char *hostname, HostnameSource source);
+int hostname_setup(bool really);
diff --git a/src/shared/machine-image.c b/src/shared/machine-image.c
index 671a56b9e9..0b148c067a 100644
--- a/src/shared/machine-image.c
+++ b/src/shared/machine-image.c
@@ -22,7 +22,7 @@
 #include "fd-util.h"
 #include "fs-util.h"
 #include "hashmap.h"
-#include "hostname-util.h"
+#include "hostname-setup.h"
 #include "id128-util.h"
 #include "lockfile-util.h"
 #include "log.h"
diff --git a/src/shared/meson.build b/src/shared/meson.build
index b43fe9c6d9..b4c77513e2 100644
--- a/src/shared/meson.build
+++ b/src/shared/meson.build
@@ -115,6 +115,8 @@ shared_sources = files('''
         gpt.h
         group-record.c
         group-record.h
+        hostname-setup.c
+        hostname-setup.h
         id128-print.c
         id128-print.h
         idn-util.c
diff --git a/src/test/meson.build b/src/test/meson.build
index 0cfc709f44..9254f18a23 100644
--- a/src/test/meson.build
+++ b/src/test/meson.build
@@ -105,17 +105,6 @@ tests += [
           libmount,
           libblkid]],
 
-        [['src/test/test-hostname.c'],
-         [libcore,
-          libshared],
-         [threads,
-          librt,
-          libseccomp,
-          libselinux,
-          libmount,
-          libblkid],
-         '', 'unsafe'],
-
         [['src/test/test-dns-domain.c'],
          [libcore,
           libshared],
@@ -339,6 +328,10 @@ tests += [
          [],
          []],
 
+        [['src/test/test-hostname-setup.c'],
+         [],
+         []],
+
         [['src/test/test-hostname-util.c'],
          [],
          []],
diff --git a/src/test/test-hostname-setup.c b/src/test/test-hostname-setup.c
new file mode 100644
index 0000000000..55996500f3
--- /dev/null
+++ b/src/test/test-hostname-setup.c
@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <unistd.h>
+
+#include "alloc-util.h"
+#include "fileio.h"
+#include "hostname-setup.h"
+#include "string-util.h"
+#include "tests.h"
+#include "tmpfile-util.h"
+
+static void test_read_etc_hostname(void) {
+        char path[] = "/tmp/hostname.XXXXXX";
+        char *hostname;
+        int fd;
+
+        fd = mkostemp_safe(path);
+        assert(fd > 0);
+        close(fd);
+
+        /* simple hostname */
+        assert_se(write_string_file(path, "foo", WRITE_STRING_FILE_CREATE) == 0);
+        assert_se(read_etc_hostname(path, &hostname) == 0);
+        assert_se(streq(hostname, "foo"));
+        hostname = mfree(hostname);
+
+        /* with comment */
+        assert_se(write_string_file(path, "# comment\nfoo", WRITE_STRING_FILE_CREATE) == 0);
+        assert_se(read_etc_hostname(path, &hostname) == 0);
+        assert_se(hostname);
+        assert_se(streq(hostname, "foo"));
+        hostname = mfree(hostname);
+
+        /* with comment and extra whitespace */
+        assert_se(write_string_file(path, "# comment\n\n foo ", WRITE_STRING_FILE_CREATE) == 0);
+        assert_se(read_etc_hostname(path, &hostname) == 0);
+        assert_se(hostname);
+        assert_se(streq(hostname, "foo"));
+        hostname = mfree(hostname);
+
+        /* cleans up name */
+        assert_se(write_string_file(path, "!foo/bar.com", WRITE_STRING_FILE_CREATE) == 0);
+        assert_se(read_etc_hostname(path, &hostname) == 0);
+        assert_se(hostname);
+        assert_se(streq(hostname, "foobar.com"));
+        hostname = mfree(hostname);
+
+        /* no value set */
+        hostname = (char*) 0x1234;
+        assert_se(write_string_file(path, "# nothing here\n", WRITE_STRING_FILE_CREATE) == 0);
+        assert_se(read_etc_hostname(path, &hostname) == -ENOENT);
+        assert_se(hostname == (char*) 0x1234);  /* does not touch argument on error */
+
+        /* nonexisting file */
+        assert_se(read_etc_hostname("/non/existing", &hostname) == -ENOENT);
+        assert_se(hostname == (char*) 0x1234);  /* does not touch argument on error */
+
+        unlink(path);
+}
+
+static void test_hostname_setup(void) {
+        hostname_setup(false);
+}
+
+int main(int argc, char *argv[]) {
+        test_setup_logging(LOG_DEBUG);
+
+        test_read_etc_hostname();
+        test_hostname_setup();
+
+        return 0;
+}
diff --git a/src/test/test-hostname-util.c b/src/test/test-hostname-util.c
index c7a63bd047..24c8ed9e3b 100644
--- a/src/test/test-hostname-util.c
+++ b/src/test/test-hostname-util.c
@@ -6,8 +6,8 @@
 #include "fileio.h"
 #include "hostname-util.h"
 #include "string-util.h"
+#include "tests.h"
 #include "tmpfile-util.h"
-#include "util.h"
 
 static void test_hostname_is_valid(void) {
         assert_se(hostname_is_valid("foobar", 0));
@@ -91,55 +91,6 @@ static void test_hostname_cleanup(void) {
         assert_se(streq(hostname_cleanup(s), "xxxx.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"));
 }
 
-static void test_read_etc_hostname(void) {
-        char path[] = "/tmp/hostname.XXXXXX";
-        char *hostname;
-        int fd;
-
-        fd = mkostemp_safe(path);
-        assert(fd > 0);
-        close(fd);
-
-        /* simple hostname */
-        assert_se(write_string_file(path, "foo", WRITE_STRING_FILE_CREATE) == 0);
-        assert_se(read_etc_hostname(path, &hostname) == 0);
-        assert_se(streq(hostname, "foo"));
-        hostname = mfree(hostname);
-
-        /* with comment */
-        assert_se(write_string_file(path, "# comment\nfoo", WRITE_STRING_FILE_CREATE) == 0);
-        assert_se(read_etc_hostname(path, &hostname) == 0);
-        assert_se(hostname);
-        assert_se(streq(hostname, "foo"));
-        hostname = mfree(hostname);
-
-        /* with comment and extra whitespace */
-        assert_se(write_string_file(path, "# comment\n\n foo ", WRITE_STRING_FILE_CREATE) == 0);
-        assert_se(read_etc_hostname(path, &hostname) == 0);
-        assert_se(hostname);
-        assert_se(streq(hostname, "foo"));
-        hostname = mfree(hostname);
-
-        /* cleans up name */
-        assert_se(write_string_file(path, "!foo/bar.com", WRITE_STRING_FILE_CREATE) == 0);
-        assert_se(read_etc_hostname(path, &hostname) == 0);
-        assert_se(hostname);
-        assert_se(streq(hostname, "foobar.com"));
-        hostname = mfree(hostname);
-
-        /* no value set */
-        hostname = (char*) 0x1234;
-        assert_se(write_string_file(path, "# nothing here\n", WRITE_STRING_FILE_CREATE) == 0);
-        assert_se(read_etc_hostname(path, &hostname) == -ENOENT);
-        assert_se(hostname == (char*) 0x1234);  /* does not touch argument on error */
-
-        /* nonexisting file */
-        assert_se(read_etc_hostname("/non/existing", &hostname) == -ENOENT);
-        assert_se(hostname == (char*) 0x1234);  /* does not touch argument on error */
-
-        unlink(path);
-}
-
 static void test_hostname_malloc(void) {
         _cleanup_free_ char *h = NULL, *l = NULL;
 
@@ -158,12 +109,10 @@ static void test_fallback_hostname(void) {
 }
 
 int main(int argc, char *argv[]) {
-        log_parse_environment();
-        log_open();
+        test_setup_logging(LOG_INFO);
 
         test_hostname_is_valid();
         test_hostname_cleanup();
-        test_read_etc_hostname();
         test_hostname_malloc();
 
         test_fallback_hostname();
diff --git a/src/test/test-hostname.c b/src/test/test-hostname.c
deleted file mode 100644
index 1a925f253c..0000000000
--- a/src/test/test-hostname.c
+++ /dev/null
@@ -1,14 +0,0 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
-
-#include "hostname-setup.h"
-#include "util.h"
-
-int main(int argc, char* argv[]) {
-        int r;
-
-        r = hostname_setup();
-        if (r < 0)
-                log_error_errno(r, "hostname: %m");
-
-        return 0;
-}
diff --git a/units/systemd-hostnamed.service.in b/units/systemd-hostnamed.service.in
index d3d0efebd0..222700564e 100644
--- a/units/systemd-hostnamed.service.in
+++ b/units/systemd-hostnamed.service.in
@@ -32,7 +32,7 @@ ProtectKernelLogs=yes
 ProtectKernelModules=yes
 ProtectKernelTunables=yes
 ProtectSystem=strict
-ReadWritePaths=/etc
+ReadWritePaths=/etc /run/systemd
 RestrictAddressFamilies=AF_UNIX
 RestrictNamespaces=yes
 RestrictRealtime=yes