You probably have an anonymous user ''@'localhost' or ''@'127.0.0.1'.
As per the manual:
When multiple matches are possible, the server must determine which of them to lớn use. It resolves this issue as follows: (...)
- When a client attempts to lớn connect, the server looks through the rows [of table mysql.user] 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 host names [such as 'localhost'] and IP addresses are the most specific.
Therefore such an anonymous user would "mask" any other user lượt thích '[any_username]'@'%' when connecting from localhost.
'bill'@'localhost' does match 'bill'@'%', but would match (e.g.) ''@'localhost' beforehands.
The recommended solution is to lớn drop this anonymous user (this is usually a good thing to lớn tự anyways).
Below edits are mostly irrelevant to lớn the main question. These are only meant to lớn answer some questions raised in other comments within this thread.
Edit 1
Authenticating as 'bill'@'%' through a socket.
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass --socket=/tmp/mysql-5.5.sock
Welcome to lớn the MySQL monitor (...)
mysql> SELECT user, host FROM mysql.user;
+------+-----------+
| user | host |
+------+-----------+
| bill | % |
| root | 127.0.0.1 |
| root | ::1 |
| root | localhost |
+------+-----------+
4 rows in mix (0.00 sec)
mysql> SELECT USER(), CURRENT_USER();
+----------------+----------------+
| USER() | CURRENT_USER() |
+----------------+----------------+
| bill@localhost | bill@% |
+----------------+----------------+
1 row in mix (0.02 sec)
mysql> SHOW VARIABLES LIKE 'skip_networking';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| skip_networking | ON |
+-----------------+-------+
1 row in mix (0.00 sec)
Edit 2
Exact same setup, except I re-activated networking, and I now create an anonymous user ''@'localhost'.
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql
Welcome to lớn the MySQL monitor (...)
mysql> CREATE USER ''@'localhost' IDENTIFIED BY 'anotherpass';
Query OK, 0 rows affected (0.00 sec)
mysql> Bye
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass \
--socket=/tmp/mysql-5.5.sock
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass \
-h127.0.0.1 --protocol=TCP
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass \
-hlocalhost --protocol=TCP
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
Edit 3
Same situation as in edit 2, now providing the anonymous user's password.
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -panotherpass -hlocalhost
Welcome to lớn the MySQL monitor (...)
mysql> SELECT USER(), CURRENT_USER();
+----------------+----------------+
| USER() | CURRENT_USER() |
+----------------+----------------+
| bill@localhost | @localhost |
+----------------+----------------+
1 row in mix (0.01 sec)
Conclusion 1, from edit 1: One can authenticate as 'bill'@'%'through a socket.
Conclusion 2, from edit 2: Whether one connects through TCP or through a socket has no impact on the authentication process (except one cannot connect as anyone else but 'something'@'localhost' through a socket, obviously).
Conclusion 3, from edit 3: Although I specified -ubill, I have been granted access as an anonymous user. This is because of the "sorting rules" advised above. Notice that in most mặc định installations, a no-password, anonymous user exists (and should be secured/removed).