Software/systemd/hostnamed

hostnamed

systemd 25 and newer include systemd-hostnamed. This is a tiny daemon that can be used to control the host name and related machine meta data from user programs. It currently offers access to four variables:

The daemon is accessible via D-Bus: 

$ gdbus introspect --system --dest org.freedesktop.hostname1 --object-path /org/freedesktop/hostname1 
node /org/freedesktop/hostname1 {
  interface org.freedesktop.hostname1 {
    methods:
      SetHostname(in  s name,
                  in  b user_interaction);
      SetStaticHostname(in  s name,
                        in  b user_interaction);
      SetPrettyHostname(in  s name,
                        in  b user_interaction);
      SetIconName(in  s name,
                  in  b user_interaction);
    signals:
    properties:
      readonly s Hostname = 'dhcp-192-168-47-11';
      readonly s StaticHostname = 'lennarts-computer';
      readonly s PrettyHostname = 'Lennart's Computer';
      readonly s IconName = 'computer-laptop';
  };
  interface org.freedesktop.DBus.Properties {
  };
  interface org.freedesktop.DBus.Introspectable {
  };
  interface org.freedesktop.DBus.Peer {
  };
};

Whenever the hostname or other meta data is changed via the daemon PropertyChanged signals are sent out to which clients can subscribe. Changing a hostname using this interface is authenticated via PolicyKit.

A couple of notes on the semantics:

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

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

The pretty host name is a free-form UTF8 host name for presentation to the user. UIs 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 host name 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 SMBIOS/DMI are 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 returns the empty string. In that case an application should fall back to a replacement icon, for example "computer". If this property is set to the empty string this automatic fallback name selection is enabled again.

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

Applications may bypass the daemon to read the hostname data if notifications of host name changes are not necessary. Use gethostname(), /etc/hostname (possibly with per-distribution fallbacks), and /etc/machine-data for that. For more information on these files and syscalls see the respective man pages.

The user_interaction boolean parameters can be used to control wether PolicyKit should interactively ask the user for authentication credentials if it needs to.

The PolicyKit action for SetHostname() is org.freedesktop.hostname1.set-hostname. For SetStaticHostname() and SetPrettyHostname() it is org.freedesktop.hostname1.set-static-hostname. For SetIconName() it is org.freedesktop.hostname1.set-machine-info.

This is inspired by, but not the same as David Zeuthen's xdg-hostname: http://people.freedesktop.org/~david/xdg-hostname/

Also see David's original Fedora feature page about this: http://fedoraproject.org/wiki/Features/BetterHostname

The sources for hostnamed are available in git for review: http://cgit.freedesktop.org/systemd/tree/src/hostnamed.c

Here are three examples 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 for them we recommend using hostnamed in combination with nss-myhostname: http://0pointer.de/lennart/projects/nss-myhostname/

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

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

Here are a couple of example conversions:

Oh, and 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.

This D-Bus interface follows the usual interface versioning guidelines.