Comparison of Firewall Intrusion Prevention and Antivirus Technologies

Network attacks are increasing both in sheer number as well as complexity. In recent news,
we have seen how viruses, worms and other attacks can cause major business disruptions and
cost companies worldwide billions of dollars. For instance, the Blaster worm infected over 1.2
million computers worldwide, and the SoBig.F virus infected over 100,000 computers1.

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Viruses and worms are all examples of what are generally known as malicious programs or
malware for short. A virus is just a program that tells the computer to do something that the
user does not want it to do. It requires a host program to live and infects other files so that it
can “live” longer. A virus can perform destructive actions, such as displaying irritating
messages, overwriting hard drives, or rendering the machine inoperable.

A worm is a program that replicates itself and spreads through network connections to infect
other machines, eating up bandwidth and storage space and slowing computers down. Some
worms use email to send messages to other users, while others use application vulnerabilities
to replicate via the network. The distinction between viruses and worms is beginning to blur,
as many viruses today also use email as their means of propagation.

“Blended threats”, such as Code Red and Nimda, are sophisticated attacks that use multiple
methods and techniques to propagate and inflict damage, thus spreading very rapidly and
causing significant productivity disruptions. Blended threats can be part virus, part worm,
and part backdoor2.

Widely connected enterprise networks and the Internet have enabled viruses, worms and
blended threats to make use of computer networks for propagation, significantly increasing
the speed of infection and damage. The Internet, with its ease of sharing and downloading of
files, has also increased the risk of infection to the average user. A user may infect a computer
by an action as simple as clicking on a downloaded file or an email attachment.

1 “Many more worms will wriggle into our future”, San Francisco Chronicle, September 4, 2003
2 A backdoor is a program that allows someone to take control of another user’s PC via a network.

It basically sets the computer open to remote control and unauthorized access.

Technologies for Network Security
In this paper, we look at network security technologies that are used to protect computer
networks and mitigate the risks associated with viruses, worms and other network attacks.
For effective protection of computer networks, most organizations are deploying multiple
layers of security, including firewall, intrusion prevention and gateway antivirus
technologies. Understanding the different kinds of protection provided by each technology is
helpful in deciding what systems are required for each network.

Network security technologies can be broadly classified into four categories:

Packet level protection, such as routers’ Access Control Lists (ACL) or stateless firewalls
Session level protection, such as stateful inspection firewalls
Application level protection, such as proxy firewalls and intrusion prevention systems

File level protection, such as gateway antivirus systems

Figure 1 compares the four categories of network security technologies. Evaluation of each
category by coverage of protocols/applications, level of protection, and relative performance
enables organizations to choose the appropriate network security technologies to protect their

Figure 1. Comparison of network security technology categories

Packet Level Protection

Packet level protection, also known as packet filtering, is one of the most widely used means
of controlling access to a network. The concept is simple: determine whether a packet is
allowed by comparing some basic pieces of information in the packet headers. Cisco IOS
Access Control List (ACL) is one of the most used packet filters. IPChains is also a popular
packet filter application, which comes bundled with many versions of Linux.

Two-way communication presents a challenge for network security based on packet filtering.
If one blocks all incoming traffic, one prevents responses to outgoing traffic from coming in,
disrupting communication. Consequently, one has to open two holes, one for outgoing traffic
and one for incoming traffic, without enforcing any association of the incoming traffic with
existing outgoing connections in the network. Packet filtering thus can allow in crafted
malicious packets that appear to be part of existing sessions, causing damage to protected

Packet filtering devices do not track dynamic protocols, where a server and a client negotiate
a random port for data transmission. Examples of protocols that use dynamic ports include
FTP, RPC, and H.323. To enable these applications to pass through packet filtering systems,
one has to open a very large hole, significantly reducing the security protection provided by
packet-filtering systems. For instance, in order to allow in standard FTP, one must let through
any traffic with a destination port greater than 1,023 (1,023 – 65,500) and source port of 20,
thus opening a significant security hole in the network.

Session Level Protection

Session level protection technologies control the flow of traffic between two or more networks
by tracking the state of sessions and dropping packets that are not part of a session allowed
by a predefined security policy. Firewalls that implement session-level protection keep state
information for each network session and make allow/deny decisions based on a session state
table. The most common systems for session level protection are stateful inspection firewalls.

Note that session level protection technologies are “session based,” meaning that firewalls go
beyond individual TCP connections to involve many such connections. Session-level firewalls
support dynamic protocols by identifying port change instructions in client-server
communication and comparing future sessions against these negotiated ports. For instance, to
track FTP sessions, the firewall inspects the control connection, used for issuing commands
and negotiating dynamic ports, and then allows in various data connections for transferring

Because session level protection provides all the benefits of packet level protection without
the limitations, it renders packet level protection unnecessary for most networks.

Application Level Protection

Application level protection technologies monitor network traffic and dynamically analyze it
for signs of attacks and intrusions. Within the network security infrastructure, two common
technologies for application level protection are proxy firewalls and Intrusion Prevention
Systems (IPS).

Proxy firewalls are network systems that act on behalf of the client accessing a network
service and shield the client and the server from direct peer-to-peer connection. The client
establishes a connection with the proxy server, and the proxy server establishes a connection
with the destination server. The proxy then forwards the data between the parties.

IPS are network devices that can accept or deny traffic based on IP addresses,
protocol/service, and application level analysis and verification. IPS receive traffic from the
network, reassemble the traffic streams and look at application primitives and commands to
detect suspicious fields that warrant some predefined action. These actions vary from logging
suspicious events to dropping the connection completely.

Proxy firewalls and IPS examine control and data fields within the application flow to verify
that the actions are allowed by the security policy and do not represent a threat to end
systems. By understanding application-level commands and primitives, they can identify
content out of the norm and content that represents a known attack or exploit. Proxy firewalls
and IPS perform IP de-fragmentation and TCP stream reassembly as well as eliminating
ambiguity within traffic, which can be used by malicious users trying to conceal their actions.

Proxy firewalls usually support the common Internet applications, including HTTP, FTP,
telnet, rlogin, email and news. Yet, a new proxy must be developed for each new application
or protocol to pass through the firewall, and custom software and user procedures are
required for each application.

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IPS generally support a wider range of protocols and applications, including those required to
protect the network against attacks from the Internet. New applications can be allowed
through an IPS without requiring changes to the user workstations. In this way, IPS are more
transparent to the network than proxy firewalls.

Proxy firewalls and IPS can detect certain viruses or Trojans by looking at application service
fields. For instance, IPS can look at the subject field, attachment name, or attachment type
within email traffic to detect characteristics of known viruses. However, application level
protection does not do a detailed analysis at the file level, which is also required to detect the
large number of viruses in existence.

File Level Protection

File level protection provides the ability to extract files within traffic and inspect them to
detect malware, including viruses, worms or Trojans3. A common technology for file level
protection in a network is gateway antivirus.

An antivirus system looks for virus signatures – a unique string of bytes that identifies a virus
– and zaps the virus from the file. Most antivirus scanning systems catch not only the initial
virus but also many of its variants, since the signature code usually remains intact.

Gateway antivirus systems scan files that are embedded in network traffic, including files in
HTTP traffic (web downloads) and files in email traffic (attachments). If an infected file is
detected, a gateway antivirus system removes it from the traffic, so it does not affect other
users. To scan files within network traffic, gateway antivirus must understand a broad range
of file encoding protocols (i.e., MIME, uucode, Base64) and file compression algorithms.
Since there are over 60,000 known viruses, gateway antivirus systems must be able to conduct
thorough scans. Constant updates to the virus pattern file are required for effective protection
against new virus outbreaks, since new viruses are continuously being uncovered4.

3 A Trojan is a program that performs some unwanted action while pretending to be useful.
When loaded, a Trojan can capture information from systems, such as user names
and passwords.
4 ICSA estimates that more than 400 viruses are discovered each week.

Since the application streams that are scanned for viruses must be completely reassembled by
the gateway antivirus system as the traffic crosses the network, users or servers might
experience a slight delay in the scanned streams. Administrators usually have granular
control of the traffic and file types that warrant scanning.

Antivirus typically scans files in email and web traffic, mainly inspecting communication
from servers to clients. Viruses are aimed at damaging end user systems, but use various
email and web servers to propagate. Consequently, it is important to detect viruses while
they are being uploaded to or downloaded from servers.


Network security technologies at the network level, session level, application level and file
level are used by organizations to add layers of protection in the network against viruses,
worms and other network attacks. Note that session level protection offers the security of
packet level protection without the limitations, so it makes packet level protection
unnecessary for most networks.

Figure 2 illustrates the inspection functions that take place as the packets are analyzed by
stateful firewall for session level protection, Intrusion Prevention Systems (IPS) for application
level protection and gateway antivirus for file level protection.

Figure 2. Network packets are inspected by session level protection, application level
protection and file level protection technologies in order to defend the network from viruses,
worms and other network attacks.

Organizations must deploy multiple security technologies to protect networks against viruses,
worms and other sophisticated attacks. Stateful inspection firewalls offer protection at the
session level, proxy firewalls and Intrusion Prevention Systems at the application level, and
gateway antivirus at the file level. Investment on all these levels of protection is required for
most enterprises for effective protection of computer networks.