Network Administration: Firewall Protocols

All network traffic into and out of the LAN should pass through a firewall, which prevents unauthorized access to the network. There are four techniques firewalls use to keep unwelcome visitors out of your network. Three of these techniques are described here. The fourth, packet-filtering, is not covered in this article.

Stateful packet inspection (SPI)

Stateful packet inspection, also known as SPI, is a step up in intelligence from simple packet filtering. A firewall with stateful packet inspection looks at packets in groups rather than individually. It keeps track of which packets have passed through the firewall and can detect patterns that indicate unauthorized access.

In some cases, the firewall may hold on to packets as they arrive until the firewall gathers enough information to make a decision about whether the packets should be authorized or rejected.

Stateful packet inspection was once found only on expensive, enterprise-level routers. Now, however, SPI firewalls are affordable enough for small- or medium-sized networks to use.

Circuit-level gateway

A circuit-level gateway manages connections between clients and servers based on TCP/IP addresses and port numbers. After the connection is established, the gateway doesn’t interfere with packets flowing between the systems.

For example, you could use a Telnet circuit-level gateway to allow Telnet connections (port 23) to a particular server and prohibit other types of connections to that server. After the connection is established, the circuit-level gateway allows packets to flow freely over the connection. As a result, the circuit-level gateway can’t prevent a Telnet user from running specific programs or using specific commands.

Application gateway

An application gateway is a firewall system that is more intelligent than a packet-filtering firewall, stateful packet inspection, or circuit-level gateway firewall. Packet filters treat all TCP/IP packets the same. In contrast, application gateways know the details about the applications that generate the packets that pass through the firewall.

For example, a web application gateway is aware of the details of HTTP packets. As a result, it can examine more than just the source and destination addresses and ports to determine whether the packets should be allowed to pass through the firewall.

In addition, application gateways work as proxy servers. Simply put, a proxy server is a server that sits between a client computer and a real server. The proxy server intercepts packets that are intended for the real server and processes them.

The proxy server can examine the packet and decide to pass it on to the real server, or it can reject the packet. Or, the proxy server may be able to respond to the packet itself without involving the real server at all.

For example, web proxies often store copies of commonly used web pages in a local cache. When a user requests a web page from a remote web server, the proxy server intercepts the request and checks whether it already has a copy of the page in its cache. If so, the web proxy returns the page directly to the user. If not, the proxy passes the request on to the real server.

Application gateways are aware of the details of how various types of TCP/IP servers handle sequences of TCP/IP packets to can make more intelligent decisions about whether an incoming packet is legitimate or is part of an attack. As a result, application gateways are more secure than simple packet-filtering firewalls, which can deal with only one packet at a time.

The improved security of application gateways, however, comes at a price. Application gateways are more expensive than packet filters, both in terms of their purchase price and in the cost of configuring and maintaining them. In addition, application gateways slow network performance because they do more detailed checking of packets before allowing them to pass.