The document discusses Linux iptables firewall. Iptables is the default firewall package for Linux and runs inside the Linux kernel. It has three built-in tables (filter, nat, mangle) that are used to filter, alter, and inspect packets. Iptables uses built-in chains and user-defined rules to allow or deny traffic based on packet criteria like source/destination, protocol, interface etc. Common iptables commands and options are also explained.