THE FQDN
       The  FQDN  (Fully Qualified Domain Name) of the system is the name that
       the resolver(3) returns for the host name, such as, ursula.example.com.
       It  is  usually  the hostname followed by the DNS domain name (the part
       after the first dot).  You can check the FQDN using hostname --fqdn  or
       the domain name using dnsdomainname.

       You cannot change the FQDN with hostname or dnsdomainname.

       The  recommended  method of setting the FQDN is to make the hostname be
       an alias for the fully qualified name using /etc/hosts,  DNS,  or  NIS.
       For  example,  if  the  hostname was "ursula", one might have a line in
       /etc/hosts which reads

              127.0.1.1    ursula.example.com ursula

       Technically: The FQDN is the name getaddrinfo(3) returns for  the  host
       name returned by gethostname(2).  The DNS domain name is the part after
       the first dot.

       Therefore it depends on the configuration of the resolver  (usually  in
       /etc/host.conf) how you can change it. Usually the hosts file is parsed
       before DNS or NIS,  so  it  is  most  common  to  change  the  FQDN  in
       /etc/hosts.