Re: hostname-mode : short vs fqdn name

[Beniamino Galvani <bgalvani redhat com>]

On Wed, Apr 08, 2020 at 04:18:32PM +0200, Thomas HUMMEL wrote:
> Hello, thanks for your answer
>
> On 4/8/20 10:25 AM, Beniamino Galvani wrote:
>
> > > - step2 (I guess) puts HOSTNAME=maestro-1000 in /etc/sysconfig/network
> >
> > AFAIK, NetworkManager doesn't do that.
>
> Although I saw that it was the postscript which wrote into this file, I
> thought NetworkManager was considering it because it is mentionned in man 5
> nm-settings-ifcfg-rh

Yes, the file is used for other variables but not for HOSTNAME.


> > Yes, it uses the SetHostname D-Bus call of systemd-hostnamed, which
> > changes the transient (kernel) hostname.
>
> Talking about D-Bus, I see this in the begining of all 3 (default, none,
> dhcp) traces :
>
> Apr 08 14:53:29 maestro-1000.maestro.pasteur.fr NetworkManager[25235]:
> <trace> [1586350409.2521] policy: get-hostname:
> "maestro-1000.maestro.pasteur.fr" (from dbus)
> Apr 08 14:53:29 maestro-1000.maestro.pasteur.fr NetworkManager[25235]:
> <trace> [1586350409.2521] policy: hostname-original: set to
> "maestro-1000.maestro.pasteur.fr"
>
> how is this hostname fully qualified who sets it ? The original hostname
> always seems to be this one.

I don't know, it is set before NM starts.

> > > a) default : what's described above (fqdn)
> > > b) none : hostname == short name
> > > c) dhcp : same as a) (fqdn)
> > >
> > > I can elaborate on that like this :
> > >
> > > a) as man said NM sets hostname "with the one provided via DHCP", which
> is
> > > first the short one (step1) then as a static profile is activated, it
> falls
> > > back to "hostname from the reverse lookup of the IP address" (which DNS
> > > resolves in the fqdn)
> >
> > Right.
>
> > > b) : NM just don't play with the transient name and sets a static
> hostname
> > > coming from /etc/sysconfig/network
> >
> > NM sets the transient host name. It doesn't know about
> > /etc/sysconfig/network. The hostname is determined in this way:
>
> But man says "none: NetworkManager will not manage the transient hostname
> and will never set it."
> Makes me wonder what exactly is the transient hostname.

It's the kernel hostname, the one used among other things in the journal.

> Besides, trace for 'none' confirms hostname is "unmanaged"
>
> So where does the short hostname I see in this case b) come from if not dhcp
> (none) neither the sysconfig/network file ?

Good question. It is not NM that sets it.

> > https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/1.20.0/src/nm-policy.c#L744
>
> ok. But what does exactly "set outside of NetworkManager" mean ?
>
>
> > No, it only takes the short hostname from DHCP.
>
> ok, makes sens as this is what's sent
>
>
> > I don't know why it is setting the FQDN in your case. Could you please
> > put level=trace in the [logging] section of
> > /etc/NetworkManager/NetworkManager.conf, disabled journald
> > ratelimiting, reproduce the issue and attach the journal log of a boot
> > with hostname-mode=dhcp?
>
> Please find the 3 traces here:
>
> http://dl.pasteur.fr/fop/P3H6bSGV/NM-hostname-mode.tar.gz
>
> So basically, I still don't understand the 2 following cases
>
> - none (end up with short name) : if dhcp, NM seems to still manage the
> hostname despite none

In the 'none' log, the FQDN is set before NM starts. Around 14:53:41
someone adds a new connection and activates it, disconnecting the
previous DHCP connection. At the same time the kernel hostname is
changed externally to NM. I can't say who does this.

> - dhcp (end up with fqdn).

In this case the short hostname got via DHCP is set by NM at
15:05:49. But later, somebody adds and activates a new connection
'xcat-enp33s0f0' with static addresses and so the DHCP hostname is
removed, restoring the initial one.

Beniamino