Summary

This chapter should serve as an introduction to simple Layer 4 DMZs and host-based firewall rule creation. Other protocols, such as FTP, Instant Messaging, or NFS are followed up in later chapters due to their more complex nature. We hope we have also demonstrated the use of the -j RETURN function in such a way that it will help to ease the administrative overhead of creating large complex exceptions to your rules and the process of creating user-defined chains in general. In addition, take note of the use of the -A flag when creating new firewall rules, as mentioned in previous chapters. -A appends rules to the end of the chain. Overuse of the -I flag, which inserts a rule at the beginning of a chain, will invert the rules and can be especially troublesome when combined with -j RETURN constructs.