What is the difference between localhost and localhost.localdomain on MySQL

This is one of the things that confused me for a long time because it isn’t mentioned in a lot of MySQL documentation. In MySQL in Linux, you can do the following:

mysql -u root -p
select user, host from mysql.user;

You may get the following output returned:

+------+-----------------------+

| user | host                  |

+------+-----------------------+

|      | localhost             |

| root | localhost             |

|      | localhost.localdomain |

| root | localhost.localdomain |

+------+-----------------------+

4 rows in set (0.00 sec)

What is the differences between localhost and localhost.localdomain? Both localhost and localhost.localdomain refers to the local computer. The difference is as followed:

• The host localhost allows connection to mysql through the UNIX socket.
• The host localhost.localdomain allows connection to mysql through TCP/IP.

When you type mysql and don’t specify a host, it will connect via UNIX socket. If you type mysql -h localhost.localdomain, it will connect via TCP/IP. You can check which which connection you are at when you type the command status. The line connection will list either UNIX socket or TCP/IP.

Connecting through the UNIX socket has a lower overhead than through TCP/IP, but not every client can connect to a UNIX socket.