[systemd-devel] systemd-hostnamed/hostnamectl and transient hostname change

[Thomas HUMMEL]

Hello,

I hope I'm not on the wrong list to ask this. On CentOS 8.1 
x86_64/systemd-239-18.el8_1.4.x86_64 I'm experiencing the following:

Starting with no static hostname and a transient hostname set by 
dracut/initrd (this is a PXE booted stateless osimage and 
'hostname-mode' is set to 'none' in NetworkManager.conf) :

[root at maestro-1000 ~]# cat /proc/sys/kernel/hostname
maestro-1000

[root at maestro-1000 ~]# dbus-send --print-reply --system 
--dest=org.freedesktop.hostname1 /org/freedesktop/hostname1 
org.freedesktop.DBus.Properties.Get string:'org.freedesktop.hostname1' 
string:'Hostname'
method return time=1587394362.680900 sender=:1.73 -> destination=:1.72 
serial=3 reply_serial=2
    variant       string "maestro-1000"

[root at maestro-1000 ~]# dbus-send --print-reply --system 
--dest=org.freedesktop.hostname1 /org/freedesktop/hostname1 
org.freedesktop.DBus.Properties.Get string:'org.freedesktop.hostname1' 
string:'StaticHostname'
method return time=1587394371.482559 sender=:1.73 -> destination=:1.74 
serial=5 reply_serial=2
    variant       string ""

[root at maestro-1000 ~]# cat /etc/hostname
cat: /etc/hostname: No such file or directory

[root at maestro-1000 ~]# hostnamectl status
    Static hostname: n/a
Transient hostname: maestro-1000
          Icon name: computer-server
            Chassis: server
         Machine ID: d1996816500e4d5ca3fe8b20af23cec1
            Boot ID: 4cc11d011b35465fa9a2ce2737719e78
   Operating System: CentOS Linux 8 (Core)
        CPE OS Name: cpe:/o:centos:centos:8
             Kernel: Linux 4.18.0-147.5.1.el8_1.x86_64
       Architecture: x86-64

I setup a static hostname with hostnamectl(1) command using only the 
--static flag:

# hostnamectl --static set-hostname toto
[root at maestro-1000 ~]# cat /proc/sys/kernel/hostname
toto

As expected a /etc/hostname file is created:

[root at maestro-1000 ~]# cat /proc/sys/kernel/hostname
toto

and the change is visible on dbus or via hostnamectl
[root at maestro-1000 ~]# dbus-send --print-reply --system 
--dest=org.freedesktop.hostname1 /org/freedesktop/hostname1 
org.freedesktop.DBus.Properties.Get string:'org.freedesktop.hostname1' 
string:'StaticHostname'
method return time=1587394555.550333 sender=:1.80 -> destination=:1.83 
serial=8 reply_serial=2
    variant       string "toto"

[root at maestro-1000 ~]# hostnamectl status
    Static hostname: toto
Transient hostname: maestro-1000
          Icon name: computer-server
            Chassis: server
         Machine ID: d1996816500e4d5ca3fe8b20af23cec1
            Boot ID: 4cc11d011b35465fa9a2ce2737719e78
   Operating System: CentOS Linux 8 (Core)
        CPE OS Name: cpe:/o:centos:centos:8
             Kernel: Linux 4.18.0-147.5.1.el8_1.x86_64
       Architecture: x86-64

At this point, the transient hostname is unchanged (which is what I'd 
expect) as seen with hostnamectl above or directly asking dbus:

[root at maestro-1000 ~]# dbus-send --print-reply --system 
--dest=org.freedesktop.hostname1 /org/freedesktop/hostname1 
org.freedesktop.DBus.Properties.Get string:'org.freedesktop.hostname1' 
string:'Hostname'
method return time=1587394568.997699 sender=:1.80 -> destination=:1.85 
serial=9 reply_serial=2
    variant       string "maestro-1000"

but a moment later, it get sets to 'toto':

[root at maestro-1000 ~]# dbus-send --print-reply --system 
--dest=org.freedesktop.hostname1 /org/freedesktop/hostname1 
org.freedesktop.DBus.Properties.Get string:'org.freedesktop.hostname1' 
string:'Hostname'
method return time=1587394788.123612 sender=:1.99 -> destination=:1.98 
serial=3 reply_serial=2
    variant       string "toto"

So my questions are

1. why does the transient hostname change while I stated --static only 
while running hostnamectl ?

2. why does the change take some time to appear on dbus ?

3. what is supposed to happen the other way around ? i.e. if I change 
the transient hostname (hostname(1) command or writing to 
/proc/sys/kernel/hostname) : is dbus/hostnamectl supposed to see the 
change ? When and how ?

Note : as a Centos 8.1 standard install I've got of course the 
systemd-hostnamed service "enabled" (actually static) but I did not ran 
it myself and NetworkManager (hostname-mode=none) does not manage the 
transient hostname. It only uses this service as a proxy to get the 
'original' hostname.

Thanks for your help

--
Thomas HUMMEL