21st December 2015

CentOS - Hostname via DHCP

I created a CentOS Virtual Machine using static IP addressing. Afterwards, I changed the networking to use instead DHCP, so that the name would be properly obtained from the DHCP server. Eventually, I performed a set of operations, including adding a second NIC and disabling the Network Manager. And, along the way, the hostname reverted to the initial static hostname that I had initially allocated.

I looked for the usual suspects, including /etc/hosts and /etc/hostname, but nothing seemed wrong. I checked the file /etc/sysconfig/network-scripts/network-functions, that contains a function called need_hostname 

need_hostname ()
{
    CHECK_HOSTNAME=$(hostname)
    if [ "$CHECK_HOSTNAME" = "(none)" -o "$CHECK_HOSTNAME" = "localhost" -o \
        "$CHECK_HOSTNAME" = "localhost.localdomain" ]; then
        return 0
    else
        return 1
    fi
}

And the problem was that, on the first line, CHECK_HOSTNAME was initialized with the initial static hostname (centos7).

The command hostamectl gave some additional information: 

   Static hostname: n/a
Transient hostname: centos7
         Icon name: computer
           Chassis: n/a
        Machine ID: 5c9e46abd14b479ab95c2488fc96de55
           Boot ID: 297819ceb8f848ceb2379c9dc629d214
    Virtualization: kvm
  Operating System: CentOS Linux 7 (Core)
       CPE OS Name: cpe:/o:centos:centos:7
            Kernel: Linux 3.10.0-229.20.1.el7.x86_64
      Architecture: x86_64

Now, looking to the function need_hostname above, I realized that any name except (none) or localhost ot localhost.domain would imply that no hostname was required. Ok, time to change the transient hostname: 

hostnamectl set-hostname '' --transient

But this does nothing: that is the transient name, each time the machine is rebooted, is lost. What is needed is to set the static hostname, so the following line should solve this issue: 

    hostnamectl set-hostname localhost --static