4.5 Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on these conditions:

• Your identity and whether you can verify it by supplying the proper credentials.

• Whether your account is locked or unlocked.

The server checks credentials first, then account locking state. A failure at either step causes the server to deny access to you completely. Otherwise, the server accepts the connection, and then enters Stage 2 and waits for requests.

The server performs identity and credentials checking using columns in the user table, accepting the connection only if these conditions are satisfied:

• The client host name and user name match the Host and User columns in some user table row. For the rules governing permissible Host and User values, see Section 4.4, “Specifying Account Names”.

• The client supplies the credentials specified in the row (for example, a password), as indicated by the authentication_string column. Credentials are interpreted using the authentication plugin named in the plugin column.

• The row indicates that the account is unlocked. Locking state is recorded in the account_locked column, which must have a value of 'N'. Account locking can be set or changed with the CREATE USER or ALTER USER statement.

Your identity is based on two pieces of information:

• Your MySQL user name.

• The client host from which you connect.

If the User column value is nonblank, the user name in an incoming connection must match exactly. If the User value is blank, it matches any user name. If the user table row that matches an incoming connection has a blank user name, the user is considered to be an anonymous user with no name, not a user with the name that the client actually specified. This means that a blank user name is used for all further access checking for the duration of the connection (that is, during Stage 2).

The authentication_string column can be blank. This is not a wildcard and does not mean that any password matches. It means that the user must connect without specifying a password. The authentication method implemented by the plugin that authenticates the client may or may not use the password in the authentication_string column. In this case, it is possible that an external password is also used to authenticate to the MySQL server.

Nonblank password values stored in the authentication_string column of the user table are encrypted. MySQL does not store passwords as cleartext for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the password hashing method implemented by the account authentication plugin). The encrypted password then is used during the connection process when checking whether the password is correct. This is done without the encrypted password ever traveling over the connection. See Section 4.1, “Account User Names and Passwords”.

From the MySQL server's point of view, the encrypted password is the real password, so you should never give anyone access to it. In particular, do not give nonadministrative users read access to tables in the mysql system database.

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

User Value	Host Value	Permissible Connections
'fred'	'h1.example.net'	fred, connecting from h1.example.net
''	'h1.example.net'	Any user, connecting from h1.example.net
'fred'	'%'	fred, connecting from any host
''	'%'	Any user, connecting from any host
'fred'	'%.example.net'	fred, connecting from any host in the example.net domain
'fred'	'x.example.%'	fred, connecting from x.example.net, x.example.com, x.example.edu, and so on; this is probably not useful
'fred'	'198.51.100.177'	fred, connecting from the host with IP address 198.51.100.177
'fred'	'198.51.100.%'	fred, connecting from any host in the 198.51.100 class C subnet
'fred'	'198.51.100.0/255.255.255.0'	Same as previous example

It is possible for the client host name and user name of an incoming connection to match more than one row in the user table. The preceding set of examples demonstrates this: Several of the entries shown match a connection from h1.example.net by fred.

When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows:

• Whenever the server reads the user table into memory, it sorts the rows.

• When a client attempts to connect, the server looks through the rows in sorted order.

• The server uses the first row that matches the client host name and user name.

The server uses sorting rules that order rows with the most-specific Host values first:

• Literal IP addresses and host names are the most specific.

• The specificity of a literal IP address is not affected by whether it has a netmask, so 198.51.100.13 and 198.51.100.0/255.255.255.0 are considered equally specific.

• The pattern '%' means “any host” and is least specific.

• The empty string '' also means “any host” but sorts after '%'.

Non-TCP (socket file, named pipe, and shared memory) connections are treated as local connections and match a host part of localhost if there are any such accounts, or host parts with wildcards that match localhost otherwise (for example, local%, l%, %).

Rows with the same Host value are ordered with the most-specific User values first. A blank User value means “any user” and is least specific, so for rows with the same Host value, nonanonymous users sort before anonymous users.

For rows with equally-specific Host and User values, the order is nondeterministic.

To see how this works, suppose that the user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads the table into memory, it sorts the rows using the rules just described. The result after sorting looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a client attempts to connect, the server looks through the sorted rows and uses the first match found. For a connection from localhost by jeffrey, two of the rows from the table match: the one with Host and User values of 'localhost' and '', and the one with values of '%' and 'jeffrey'. The 'localhost' row appears first in sorted order, so that is the one the server uses.

Here is another example. Suppose that the user table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| h1.example.net |          | ...
+----------------+----------+-

The sorted table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| h1.example.net |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

The first row matches a connection by any user from h1.example.net, whereas the second row matches a connection by jeffrey from any host.

Note

It is a common misconception to think that, for a given user name, all rows that explicitly name that user are used first when the server attempts to find a match for the connection. This is not true. The preceding example illustrates this, where a connection from h1.example.net by jeffrey is first matched not by the row containing 'jeffrey' as the User column value, but by the row with no user name. As a result, jeffrey is authenticated as an anonymous user, even though he specified a user name when connecting.

If you are able to connect to the server, but your privileges are not what you expect, you probably are being authenticated as some other account. To find out what account the server used to authenticate you, use the CURRENT_USER() function. (See Information Functions.) It returns a value in user_name@host_name format that indicates the User and Host values from the matching user table row. Suppose that jeffrey connects and issues the following query:

mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| @localhost     |
+----------------+

The result shown here indicates that the matching user table row had a blank User column value. In other words, the server is treating jeffrey as an anonymous user.

Another way to diagnose authentication problems is to print out the user table and sort it by hand to see where the first match is being made.