hostname-mode=default and hostname set outside NM

[Thomas HUMMEL <thomas hummel pasteur fr>]

Hello,

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 NetworkManager."

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 
externally
                 * to NM or if there wasn't one, 'localhost.localdomain'
                 */
                _set_hostname (self, priv->orig_hostname, "no default 
device");
                return;
        }

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 ?

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.

Thanks for your help

--
Thomas HUMMEL