Access denied for user 'www-data'@'localhost - how to deal with that?

Question

I face with a strange problem yesterday. I have server running Debian with installed PHP 4.4.4-8 and mysql 5.5.9. That server serves a several websites. For some reason randomly I get that error "Access denied for user 'www-data'@'localhost' (using password: NO)" when I try to load the webpage. If I hit refresh the page loads normally, but afer several clicks that message appears again. Username which that page use to connect to mysql server is not www-data.

Does anyone has faced similar problem ?

Answer 1

For absent-minded people, this error may happen when mysql_query() is called after mysqli_connect(), when it should be mysqli_query().

Answer 2

www-data is the Debian user that runs apache and php. If you attempt a query when you don't have a valid connection, php/mysql will attempt to create a connection using <unix-user>@localhost with no password. This is where www-data@localhost (using password:NO) is coming from.

The most likely reason that this has started happening now (though it has been running fine for 2-years prior) is that your db load has increased to the point where some connections are unable to succeed (probably due to max_connections, or max_user_connections; though this can also result from other limits like memory, threads, etc). When this happens, your call to mysql_connect will emit an error message, and return FALSE. If you fail to detect this failure, then your next mysql call (probably mysql_query, or mysql_select_db) will attempt the connection to www-data@localhost -- thus causing the problem you're seeing.

I suggest enabling error reporting, and error display (as suggested by @DarkMantis) :

ini_set('error_reporting', E_ALL|E_STRICT);
ini_set('display_errors', 1);

Also, be sure that your call to mysql_connect is not preceded by a @ sign; and make sure to check the return value. It should look something like this:

$cxn = mysql_connect('localhost','yourusername','yourpassword');
if( $cxn === FALSE ) {  die('mysql connection error: '.mysql_error()); }