Introduction¶

systemd-hostnamed.service(8) is a system service that can be used to control the hostname and related machine metadata from user programs. This page describes the hostname semantics and the D-Bus interface.

The D-Bus API¶

The service exposes the following interfaces on the bus:

node /org/freedesktop/hostname1 {
  interface org.freedesktop.hostname1 {
    methods:
      SetHostname(in  s hostname,
                  in  b interactive);
      SetStaticHostname(in  s hostname,
                        in  b interactive);
      SetPrettyHostname(in  s hostname,
                        in  b interactive);
      SetIconName(in  s icon,
                  in  b interactive);
      SetChassis(in  s chassis,
                 in  b interactive);
      SetDeployment(in  s deployment,
                    in  b interactive);
      SetLocation(in  s location,
                  in  b interactive);
      GetProductUUID(in  b interactive,
                     out ay uuid);
    properties:
      readonly s Hostname = '...';
      readonly s StaticHostname = '...';
      readonly s PrettyHostname = '...';
      readonly s IconName = '...';
      readonly s Chassis = '...';
      readonly s Deployment = '...';
      readonly s Location = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s KernelName = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s KernelRelease = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s KernelVersion = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s OperatingSystemPrettyName = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s OperatingSystemCPEName = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly s HomeURL = '...';
  };
  interface org.freedesktop.DBus.Peer { ... };
  interface org.freedesktop.DBus.Introspectable { ... };
  interface org.freedesktop.DBus.Properties { ... };
};
    

Whenever the hostname or other metadata is changed via the daemon, PropertyChanged signals are sent out to subscribed clients. Changing a hostname using this interface is authenticated via polkit.

Semantics¶

The static (configured) hostname is the one configured in /etc/hostname. It is chosen by the local user. It is not always in sync with the current hostname as returned by the gethostname(3) system call. If no hostname is configured this property will be the empty string. Setting this property to the empty string will remove /etc/hostname. This property should be an internet-style hostname, 7-bit lowercase ASCII, no special chars/spaces.

The transient (dynamic) hostname is the one configured via the kernel's sethostname(3). It can be different from the static hostname if DHCP or mDNS have been configured to change the name based on network information. This property is never empty. If no hostname is set this will default to "localhost" (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 "localhost". This property should be an internet-style hostname, 7-bit lowercase ASCII, no special chars/spaces.

The pretty hostname 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 "Lennart’s Computer" the latter should be "lennarts-computer". 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.

The icon name 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. "computer-laptop" vs. "computer-desktop" 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 "computer". If this property is set to the empty string, the automatic fallback name selection is enabled again.

The chassis type should be one of the currently defined chassis types: "desktop", "laptop", "server", "tablet", "handset", as well as the special chassis types "vm" and "container" 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.

Note that systemd-hostnamed starts only on request and terminates after a short idle period. This effectively means that PropertyChanged 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.

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 PropertyChanged messages from systemd-hostnamed. To accomplish this, open /proc/sys/kernel/hostname and poll(3) for SIGHUP 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) hostname.

Applications may read the hostname data directly if hostname change notifications are not necessary. Use gethostname(3), /etc/hostname (possibly with per-distribution fallbacks), and machine-info(3) for that. For more information on these files and syscalls see the respective man pages.

Recommendations¶

Here are three examples that show how the pretty hostname and the icon name should be used:

To properly handle name lookups with changing local hostnames without having to edit /etc/hosts, we recommend using systemd-hostnamed in combination with nss-myhostname(3).

A client that wants to change the local hostname for DHCP/mDNS should invoke SetHostname("newname", false) as soon as the name is available and afterwards reset it via SetHostname("").

Here are some recommendations to follow when generating a static (internet) hostname from a pretty name:

Note that while systemd-hostnamed applies some checks to the hostname you pass they are much looser than the recommendations above. For example, systemd-hostnamed will also accept "_" in the hostname, but we recommend not using this to avoid clashes with DNS-SD service types. Also systemd-hostnamed allows longer hostnames, but because of the DNS label limitations, we recommend not making use of this.

Here are a couple of example conversions:

Of course, an already valid internet hostname label you enter and pass through this conversion should stay unmodified, so that users have direct control of it, if they want — by simply ignoring the fact that the pretty hostname is pretty and just edit it as if it was the normal internet name.

Versioning¶

These D-Bus interfaces follow the usual interface versioning guidelines.

Examples¶

$ gdbus introspect --system \
  --dest org.freedesktop.hostname1 \
  --object-path /org/freedesktop/hostname1
      

See also¶

David Zeuthen's original Fedora Feature page about xdg-hostname