The focus of this chapter is on stateful firewalls, a type of firewall that attempts to track the state of network connections when filtering packets. The stateful firewall's capabilities are somewhat of a cross between the functions of a packet filter and the additional application-level protocol intelligence of a proxy. Because of this additional protocol knowledge, many of the problems encountered when trying to configure a packet-filtering firewall for protocols that behave in nonstandard ways (as mentioned in Chapter 2, "Packet Filtering") are bypassed.
This chapter discusses stateful filtering, stateful inspection, and deep packet inspection, as well as state when dealing with various transport and application-level protocols. We also demonstrate some practical examples of how several vendors implement state tracking as well as go over examples of such firewalls.