Re: hostname-mode=default and hostname set outside NM

On Wed, Apr 22, 2020 at 08:18:24PM +0200, Thomas HUMMEL wrote:

using NetworkManager-1.20.0-5.el8_1.x86_64 on CentOS 8.1 I tried to figure
out the hostname-mode semantics beyond what the manual says (I've got
another thread for this but somehow unrelated to the following question).

In this post, I'd like to clarify this NetworkManager.conf(5) quote for
default :

"If not present, the hostname will be updated to the last one set outside

For what I saw when tracing, nm-policy reads the hostname-original and
restores this hostname when the DHCP connection gets deactivated.

Looking at the source, it seems to match this part in
nm-policy.c:update_system_hostname() :

if (!priv->default_ac4 && !priv->default_ac6) {
                /* No best device; fall back to the last hostname set
                 * to NM or if there wasn't one, 'localhost.localdomain'
                _set_hostname (self, priv->orig_hostname, "no default

Same kind of mechanism without default fallbacks for hostname-mode=dhcp

What's not clear to me is : what if the transient hostname is changed
outside NetworkManager after hostname-original gets read at NM start ?
Let's say someone issues hostname foobar (instead of hostnamctl --transient
set-hostname foobar) while the DHCP connection is still up : would that
become the new set-out-of-NM hostname wich would be restored when the
connection is deactivated ?

Yes, NM always re-reads the kernel hostname to detect whether it has
changed before trying to set a new one. When the DHCP connection gets
deactivated, NM should notice that the hostname was changed externally
and use it as the 'last one set outside NetworkManager' mentioned in
the man page.

Technically NM could notice it directly issuing gethostname(3) but my
understanding is that it would rather rely on systemd-hostnamed/dbus for
that. The latter seems to have in some cases some delay to catch up with
transient hostanme.

It uses systemd-hostnamed, and falls back to gethostname() in case of
failure [1].



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