Re: Hostname Management

[Mark Corbin <mark dibsco co uk>]

Many thanks Thomas - very helpful.

On 23/07/2021 20:42, Thomas Haller wrote:
> On Fri, 2021-07-23 at 17:24 +0100, Mark Corbin wrote:
> > Hello
> >
> > I'm trying to control a device hostname independently of
> > NetworkManager,
> > but am unable to disable NetworkManager's hostname management. I'm
> > using
> > NM version 1.28.2
> >
> > I've added the following to the configuration file:
> >
> > [main]
> > hostname-mode=none
>
> that's right. This is documented in `man NetworkManager.conf`.
>
> > When NM starts at boot I see the following messages:
> >
> > hostname: hostname: using hostnamed
> > hostname: hostname changed from (none) to "<my hostname>"
>
> that's only informational. It doesn't mean that NetworkManager actively
> changes the hostname of your system.
>
>
> > Using nmcli to set the hostname triggers a DBus call to set the static
> > (which fails) and transient (which succeeds) hostnames:
>
> `nmcli general hostname` sets the static hostname.
>
> That is mostly unrelated to `[main].hostname-mode`, which is about the
> transient hostname.
>
> This comes from a time before hostnamectl. Today there is little reason
> to call this. Just use hostnamectl to set the static hostname.
>
> Setting the transient hostname on the other hand still makes sense and
> (depending on your environment) is NetworkManager's job. This way you
> can get the (transient) hostname from DHCP or via reverse DNS lookup.
> But it doesn't seem that you want that (which is sensible. Just set a
> static hostname instead).
>
> > Jul 23 16:11:03 OldHostname dbus-daemon[1368]: [system] Activating via
> > systemd: service name='org.freedesktop.hostname1'
> > unit='dbus-org.freedesktop.hostname1.service' requested by ':1.4'
> > (uid=0
> > pid=1472 comm="/usr/sbin/NetworkManager --no-daemon ")
> > Jul 23 16:11:03 OldHostname audit: AUDIT1334 prog-id=35 op=LOAD
> > Jul 23 16:11:03 OldHostname audit: AUDIT1334 prog-id=36 op=LOAD
> > Jul 23 16:11:03 OldHostname kernel: audit: type=1334
> > audit(1627056663.748:120): prog-id=35 op=LOAD
> > Jul 23 16:11:03 OldHostname kernel: audit: type=1334
> > audit(1627056663.748:121): prog-id=36 op=LOAD
> > Jul 23 16:11:03 OldHostname dbus-daemon[1368]: [system] Successfully
> > activated service 'org.freedesktop.hostname1'
> > Jul 23 16:11:03 NewHostname systemd-hostnamed[10804]: Failed to write
> > static host name: Device or resource busy
> > Jul 23 16:11:03 NewHostname NetworkManager[1472]: <warn>
> > [1627056663.9942] hostname: could not set hostname:
> > GDBus.Error:System.Error.EBUSY: Failed to set static hostname: Device
> > or
> > resource busy
> > Jul 23 16:11:03 NewHostname NetworkManager[1472]: <info>
> > [1627056663.9944] audit: op="hostname-save" arg="NewHostname" pid=10800
> > uid=0 result="fail" reason="Saving the hostname failed."
> >
> > I was hoping that nmcli would return "set-hostname: hostname is
> > unmanaged" when hostname-mode set to 'none'.
>
> `nmcli general hostname` calls SetHostname on D-Bus, which ends up at
> this code:
>
>    
> https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/888735838382cf45530914b34d383277ff4ddbe4/src/core/nm-hostname-manager.c#L330
>
>
> > I have two questions:
> > 1) Can I configure NM so that it doesn't make any attempts to change
> > either the static or transient device hostname?
>
> - NM only sets the static hostname if you call the respective API.
> Don't don't do that, if you don't want that.
>
> - to avoid NM setting the transient hostname, hostname-mode=none would
> work. Alternatively just configure a static hostname.
>
> > 2) If I change the hostname externally from NM should the NM service
> > be
> > restarted?
>
> No. NM listens to signals from systemd-hostnamed or watches
> /etc/hostname file. It would pick up the change automatically.
>
>
>
> best,
> Thomas