Because you are using MySQL to authenticate users, you need to restrict network access to Port 3306.
I suggest to just bind MySQL only to the loopback interface 127.0.0.1. This makes sure nobody can connect to your MySQL Daemon via the network.
Edit /etc/init.d/mysql.server and change line 107 as following:
Original line:
$bindir/safe_mysqld --datadir=$datadir --pid-file=$pid_file& |
Changed line:
$bindir/safe_mysqld --datadir=$datadir --pid-file=$pid_file \ --bind-address=127.0.0.1& |
(Re-)start your MySQL-Daemon by issuing /etc/init.d/mysql.server start
To ensure the configuration change was successful issue: netstat -an|grep LISTEN. The Output should be looking similar to this:
bond:~ # netstat -an|grep LISTEN tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN |
This step is only necessary if you run the mysql sever on another host than the mailserver. It allows you to securely connect from another host by allowing only defined IP adresses.
The example used is from the view of the host serving the MySQL database. Lets assume your mailserver has the IP 192.168.0.100 and the MySQL host has 192.168.0.200
Edit /etc/rinetd.conf and add:
192.168.0.200 3306 127.0.0.1 3306 allow 192.168.0.100 |
This means: The MySQL host is listening on 192.168.0.200 port 3306. If 192.168.0.100 is attempting a connection, it is forwared to 127.0.0.1:3306. All other hosts are rejected.