Section 2.1. Some Terminology

Let's get some definitions cleared up before we proceed. These may not be the same definitions you're used to or prefer, but they're the ones I use in this chapter:

Application Gateway (or Application-Layer Gateway)

A firewall or other proxy server possessing application-layer intelligence, e.g., able to distinguish legitimate application behavior from disallowed behavior, rather than dumbly reproducing client data verbatim to servers, and vice versa. Each service that is to be proxied with this level of intelligence must, however, be explicitly supported (i.e., "coded in"). Application Gateways may use packet-filtering or a Generic Service Proxy to handle services for which they have no application-specific awareness.

Bastion host

A system that runs publicly accessible services but is usually not itself a firewall. Bastion hosts are what we put on DMZs (although they can be put anywhere). The term implies that a certain amount of system hardening (see later in this list) has been done, but sadly, this is not always the case.

DMZ (DeMilitarized Zone)

A network, containing publicly accessible services, that is isolated from the "internal" network proper. Preferably, it should also be isolated from the outside world. (It used to be reasonable to leave bastion hosts outside of the firewall but exposed directly to the outside world; as we'll discuss shortly, this is no longer justifiable or necessary.)


A system or network that isolates one network from another. This can be a router, a computer running special software in addition to or instead of its standard operating system, a dedicated hardware device (although these tend to be prepackaged routers or computers), or any other device or network of devices that performs some combination of packet-filtering, application-layer proxying, and other network-access control. In this discussion, the term will generally refer to a single multihomed host.

Generic Service Proxy (GSP)

A proxy service (see later in this list) that has no application-specific intelligence. These are nonetheless generally preferable over packet-filtering, since proxies provide better protection against TCP/IP Stack-based attacks. Firewalls that use the SOCKS protocol rely heavily on GSPs.

Hardened System

A computer on which all unnecessary services have been disabled or uninstalled, all current OS patches have been applied, and in general has been configured in as secure a fashion as possible while still providing the services for which it's needed. This is the subject of Chapter 3.

Internal Network

What we're trying to protect: end-user systems, servers containing private data, and all other systems to which we do not wish the outside world to initiate connections. This is also called the "protected" or "trusted" network.

Multihomed Host

Any computer having more than one logical or physical network interface (not counting loopback interfaces).


Inspecting the IP headers of packets and passing or dropping them based primarily on some combination of their Source IP Address, Destination IP Address, Source Port, and their Destination Port (Service). Application data is not considered; i.e., intentionally malformed packets are not necessarily noticed, assuming their IP headers can be read. Packet-filtering is a necessary part of nearly all firewalls' functionality, but is not considered, by itself, to be sufficient protection against any but the most straightforward attacks. Most routers (and many low-end firewalls) are limited to packet-filtering.

Perimeter Network

The portion or portions of an organization's network that are directly connected to the Internet, plus any "DMZ" networks (see earlier in this list). This isn't a precise term, but if you have much trouble articulating where your network's perimeter ends and your protected/trusted network begins, you may need to re-examine your network architecture.


An intermediary in all interactions of a given service type (ftp, http, etc.) between internal hosts and untrusted/external hosts. In the case of SOCKS, which uses Generic Service Proxies, the proxy may authenticate each connection it proxies. In the case of Application Gateways, the proxy intelligently parses Application-Layer data for anomalies.

Stateful packet-filtering

At its simplest, the tracking of TCP sessions; i.e., using packets' TCP header information to determine which packets belong to which transactions, and thus filtering more effectively. At its most sophisticated, stateful packet-filtering refers to the tracking of not only TCP headers, but also some amount of Application-Layer information (e.g., end-user commands) for each session being inspected. Linux's iptables include modules that can statefully track most kinds of TCP transactions and even some UDP transactions.

TCP/IP Stack Attack

A network attack that exploits vulnerabilities in its target's TCP/IP stack (kernel-code or drivers). These are, by definition, OS specific: Windows systems, for example, tend to be vulnerable to different stack attacks than Linux systems.

That's a lot of jargon, but it's useful jargon (useful enough, in fact, to make sense of the majority of firewall vendors' propaganda!). Now we're ready to dig into DMZ architecture.