Newbie: User and Host question

Today on MySql Forums, there was a question in the newbie section about two users — I have a doubt on db host and db user relationship . What does this mean ?

New DBAs are often confused by the quirky methods of authentication that MySQL uses. Heck, extremely experienced MySQL DBAs can get confused.

From the manual, 6.2.4. Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, and then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

The client host from which you connect

Your MySQL user name

So the first user, Tom, is allowed to connect from ‘%’ and ‘%’ is a wildcard for any system. And the second user is only allowed to connect from a host with the address ’127.0.0.1′. Usually systems have a network loop-back (think short circuit) assigned to 127.0.0.1 and uses that for its own traffic internally¹.

Also from the same manual page (abbreviated) to provide a little more clarity.

The following table shows how various combinations of Host and User values in the user table apply to incoming connections.

Host Value	User Value	Permissible Connections
‘thomas.loc.gov’	‘fred’	fred, connecting from thomas.loc.gov
‘thomas.loc.gov’	”	Any user, connecting from thomas.loc.gov
‘%’	‘fred’	fred, connecting from any host
‘%’	”	Any user, connecting from any host
‘%.loc.gov’	‘fred’	fred, connecting from any host in the loc.gov domain

It helps to occasionally re-read the The MySQL Access Privilege System of the manual to help remember how users get into the system as well as the edge cases. It is all too easy to set up multiple users with the same user name value but different privileges depending on where they connect.

And thanks to all who take the time to answer questions on the forums!

1. Hugh simplification used here for brevity.