## 6.2.22 SQL-Based Account Activity Auditing 

Applications can use the following guidelines to perform SQL-based auditing that ties database activity to MySQL accounts. 

MySQL accounts correspond to rows in the mysql.user  system table. When a client connects successfully, the server authenticates the client to a particular row in this table. The User and Host  column values in this row uniquely identify the account and correspond to the ’ user_name ’@’ host_name ’  format in which account names are written in SQL statements. 

The account used to authenticate a client determines which privileges the client has. Normally, the CURRENT_USER()  function can be invoked to determine which account this is for the client user. Its value is constructed from the User  and Host  columns of the user  table row for the account. 

However, there are circumstances under which the CURRENT_USER()  value corresponds not to the client user but to a different account. This occurs in contexts when privilege checking is not based the client’s account: 

•Stored routines (procedures and functions) defined with the SQL SECURITY DEFINER  characteristic 

•Views defined with the SQL SECURITY DEFINER  characteristic 

•Triggers and events 

In those contexts, privilege checking is done against the DEFINER  account and CURRENT_USER()  refers to that account, not to the account for the client who invoked the stored routine or view or who caused the trigger to activate. To determine the invoking user, you can call the USER()  function, which returns a value indicating the actual user name provided by the client and the host from which the client connected. However, this value does not necessarily correspond directly to an account in the user  table, because the USER()  value never contains wildcards, whereas account values (as returned by CURRENT_USER() ) may contain user name and host name wildcards. 

For example, a blank user name matches any user, so an account of ’’@’localhost’  enables clients to connect as an anonymous user from the local host with any user name. In this case, if a client connects as user1  from the local host, USER()  and CURRENT_USER()  return different values: 

mysql> SELECT USER(), CURRENT_USER(); +-----------------+----------------+ | USER() | CURRENT_USER() | +-----------------+----------------+ | user1@localhost | @localhost | +-----------------+----------------+ 

The host name part of an account can also contain wildcards. If the host name contains a ’%’  or ’_’  pattern character or uses netmask notation, the account can be used for clients connecting from multiple hosts and the CURRENT_USER()  value does not indicate which one. For example, the account ’user2’@’%.example.com’  can be used by user2  to connect from any host in the example.com domain. If user2  connects from remote.example.com , USER()  and CURRENT_USER()  return different values: 

mysql> SELECT USER(), CURRENT_USER(); +--------------------------+---------------------+ | USER() | CURRENT_USER() | +--------------------------+---------------------+ | user2@remote.example.com | user2@%.example.com | +--------------------------+---------------------+ 

If an application must invoke USER()  for user auditing (for example, if it does auditing from within triggers) but must also be able to associate the USER()  value with an account in the user  table, it is necessary 

