NetworkManager interferes with hostname configuration

Question

I'm running an arch system with KDE4/Plasma, wpa_supplicant, networkmanager, systemd ...

# cat /proc/version  
Linux version 5.0.0-arch1-1-ARCH (builduser@heftig-18825) (gcc version 8.2.1 20181127 (GCC)) #1 SMP PREEMPT Mon Mar 4 14:11:43 UTC 2019

The content of my /etc/hostname reads localhost. After boot, the shell command hostname now outputs localhost. More precisely:

# hostnamectl 
   Static hostname: localhost
Transient hostname: localhost.localdomain
         Icon name: computer-laptop
           Chassis: laptop
        Machine ID: 7e0a101cd2f0406497a6e4354fc9b3b7
           Boot ID: a1424a0995da4e84b1e55b7f79df957e
  Operating System: Arch Linux
            Kernel: Linux 5.0.0-arch1-1-ARCH
      Architecture: x86-64

When I turn on WiFi, networkmanager connects to a WiFi network and then the hostname changes. For instance:

# hostnamectl 
   Static hostname: localhost
Transient hostname: localhost.localdomain
         Icon name: computer-laptop
           Chassis: laptop
        Machine ID: 7e0a101cd2f0406497a6e4354fc9b3b7
           Boot ID: a1424a0995da4e84b1e55b7f79df957e
  Operating System: Arch Linux
            Kernel: Linux 5.0.0-arch1-1-ARCH
      Architecture: x86-64

The shell command hostname now outputs localhost.localdomain instead of localhost.

As a consequence, the KDE lock-screen cannot be unlocked and I cannot start any X applications from the terminal in KDE (or any other desktop). A typical error message is this:

$ gvim
Invalid MIT-MAGIC-COOKIE-1 keyE233: cannot open display

When I issue hostnamectl set-hostname localhost as root, the behavior resumes to normal.

In some other WiFis, the hostname after connect is not localhost.localdomain but something even more random (it seems to be a hostname determined by the WiFi provider, mostly in big corporate networks). Why does a WiFi provider have the power to set my hostname? Can this be changed somehow?

Answer 1

Ivanivan's answer (tuning dhcpcd.con), though plausible, didn't work in my case. So I suspect it is not about DHCP. I stumbled upon this post which told me that the problem is not about DHCP but about NetworkManager. Adding the following to /etc/NetworkManager/NetworkManager.conf solved the problem for me:

[main]
plugins=keyfile 
hostname-mode=none 

See man 5 NetworkManager.conf for details on the hostname-mode option. Setting it to none prevents NetworkManager from setting a transient hostname which is what happened in my case.

Answer 2

Several potential ways to fix it. It could be that you have a localhost entry in /etc/hosts (like everything else on the planet) but you do not have a localhost.localdomain entry. Adding one, also pointing to 127.0.0.1, may be a fix for your "at home" issue - but not for the "random" seeming ones you get on the "large corporate" networks.

The "better" option though is to check your dhcp client configuration. In /etc/dhcp/dhclient.conf (or it may be dhcpcd.conf depending on distro) look for the request line(s) -

request subnet-mask, broadcast-address, time-offset, routers,
    domain-name, domain-name-servers, domain-search, host-name,
    dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,
    netbios-name-servers, netbios-scope, interface-mtu,
    rfc3442-classless-static-routes, ntp-servers;

And simply remove the host-name option.