[Users] Hostname issue on CentOS 7.x

[Jeffrey Walton]

> We’re facing an issue on many CentOS 7.x containers running cPanel. Clients are getting emails saying that the hostnames are not valid, for example it will show server1   instead of the fully qualified domain name server1.domain.com
>

Two small quick nits...

(1) Use example.com. IANA reserves the name for the purpose.

(2) 'server.example.com' is not a FQDN.

FQDN's end in a dot '.' to indicate the top of the tree. All of these
are fully qualified:

    * server.example.com.
    * localhost.
    * localhost.localdomain.

While the second might look strange, its needed on some OSes like
Solaris. Otherwise, boots take 2x or 3x times longer while the network
stack tries to determine its place in the DNS tree. It spews
complaints to dmesg as its booting about lack of a fqdn.

Also see W. Richard Steven's TCP/IP Illustrated, Volume 1: The Protocols.

Jeff