For a long time, databases were relatively self-contained; most accesses to a database system were from the same machine that was running the software. These days, databases are very rarely self-contained. Instead, they are the data storage for larger, distributed systems; sales information systems, e-commerce systems, even large electronic mail systems all use databases and communicate with them over networks.
This makes secure remote communication with databases more important than ever. Unfortunately, database communication protocols tend to be proprietary and different for each database manufacturer. Furthermore, they've only recently been designed with any concern for security. It is unwise to pass database transactions unprotected across the Internet. Chapter 23, discusses database protocols and ways to configure databases to function with your firewall.