ドメイン登録したらAPACHEの再起動でワーニングの対処

As the following warning appeared after introducing of a domain name, I fixed it
by modifying of /etc/hosts.

Warning:

apache2: Could not reliably determine the server’s fully qualified domain name,
using 127.0.1.1 for ServerName

Fix:

sudo nano /etc/hosts

before
127.0.1.1 host_name

after
127.0.1.1 domain_name host_name

Then restart apache.