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 it 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.
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
Usercolumns in some
usertable row. For the rules governing permissible
Uservalues, see Section 6.2.4, “Specifying Account Names”.
The client supplies the password specified in the row, as indicated by the
Your identity is based on two pieces of information:
Your MySQL user name.
The client host from which you connect.
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.
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).
Password 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. If the
server authenticates a client using a plugin, the authentication
method that the plugin implements may or may not use the password
Password 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
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
function). 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 6.2.1, “Account User Names and Passwords”.
From MySQL'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
Host values in the
user table apply to incoming connections.
||Any user, connecting from
||Any user, connecting from any host|
||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
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
usertable 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
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
198.51.100.0/255.255.255.0are considered equally specific.
'%'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
otherwise (for example,
Rows with the same
Host value are ordered with
User values first. A blank
User value means “any user” and is
least specific, so for rows with the same
value, nonanonymous users sort before anonymous users.
For rows with equally-specific
User values, the order is nondeterministic.
To see how this works, suppose that the
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
jeffrey, two of
the rows from the table match: the one with
User values of
'', and the
one with values of
row appears first in sorted order, so that is the one the server
Here is another example. Suppose that the
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.
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
jeffrey is first matched not by the row
'jeffrey' as the
User column value, but by the row with no
user name. As a result,
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
Section 12.16, “Information Functions”.) It returns a value in
format that indicates the
Host values from the matching
user table row. Suppose that
jeffrey connects and issues the following
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
jeffrey as an anonymous user.
Another way to diagnose authentication problems is to print out
user table and sort it by hand to see where
the first match is being made.