A firewall is an information technology (IT) security device which is configured to
permit, deny or proxy data connections set and configured by the organization's security
policy. Firewalls can either be hardware and/or software based.
A firewall's basic task is to control traffic between computer networks with different
zones of trust. Typical examples are the Internet which is a zone with no trust and an
internal network which is (and should be) a zone with high trust. The ultimate goal is to
provide controlled interfaces between zones of differing trust levels through the
enforcement of a security policy and connectivity model based on the least privilege
principle and separation of duties.
A firewall is also called a Border Protection Device (BPD) in certain military contexts
were a firewall separates networks by creating perimeter networks in a DMZ. In a BSD
context they are also known as a packet filter. A firewall's function is analogous to
firewalls in building construction.
Proper configuration of firewalls demands skill from the firewall administrator. It requires
considerable understanding of network protocols and of computer security. Small mistakes
can render a firewall worthless as a security tool.
Firewall technology emerged in the late 1980s when the Internet was a fairly new
technology in terms of its global use and connectivity. The original idea was formed in
response to a number of major internet security breaches, which occurred in the late
1980s. In 1988 an employee at the NASA Ames Research Center in California sent a
memo by email to his colleagues that read, "We are currently under attack from an
Internet VIRUS! It has hit Berkeley, UC San Diego, Lawrence Livermore, Stanford, and
NASA Ames." This virus known as the Morris Worm was carried by e-mail. The Morris
Worm was the first large scale attack on Internet security; the online community was
neither expecting an attack nor prepared to deal with one.
First generation - packet filters
The first paper published on firewall technology was in 1988, when Jeff Mogul from
Digital Equipment Corporation (DEC) developed filter systems known as packet filter
firewalls. This fairly basic system was the first generation of what would become a
highly evolved and technical internet security feature. At AT&T Bill Cheswick and Steve
Bellovin were continuing their research in packet filtering and developed a working
model for their own company based upon their original first generation architecture.
Second generation - circuit level
From 1980-1990 two colleagues from AT&T Bell Laboratories, Dave Presetto and
Howard Trickey, developed the second generation of firewalls known as circuit level
Third generation - applicaton layer
Publications by Gene Spafford of Purdue University, Bill Cheswick at AT&T
Laboratories and Marcus Ranum described a third generation firewall known as
application layer firewall, also known as proxy based firewalls. Marcus Ranum's work
on the technology spearheaded the creation of the first commercial product. The product
was released by DEC who named it the SEAL product. DEC’s first major sale was on
June 13, 1991 to a chemical company based on the East Coast of the USA.
In 1992, Bob Braden and Annette DeSchon at the University of Southern California
(USC) were developing their own fourth generation packet filter firewall system. The
product known as “Visas” was the first system to have a visual integration interface with
colours and icons, which could be easily implemented to and accessed on a computer
operating system such as Microsoft's Windows or Apple's MacOS. In 1994 an Israeli
company called Check Point Software Technologies built this into readily available
software known as FireWall-1.
A second generation of proxy firewalls was based on Kernel Proxy technology. This
design is constantly evolving but its basic features and codes are currently in widespread
use in both commercial and domestic computer systems. Cisco, one of the largest internet
security companies in the world released their PIX product to the public in 1997.
The new Next Generation Firewalls leverage their existing deep packet inspection engine
by sharing this functionality with an Intrusion-prevention system (IPS).
There are three basic types of firewalls depending on:
1. Whether the communication is being done between a single node and the
network, or between two or more networks.
2. Whether the communication is intercepted at the network layer, or at the
3. Whether the communication state is being tracked at the firewall or not.
With regard to the scope of filtered communications there exist:
1. Personal firewalls, a software application which normally filters traffic entering or
leaving a single computer.
2. Network firewalls, normally running on a dedicated network device or computer
positioned on the boundary of two or more networks or DMZs (demilitarized
zones). Such a firewall filters all traffic entering or leaving the connected
The latter definition corresponds to the conventional, traditional meaning of "firewall" in
In reference to the layers where the traffic can be intercepted, three main categories of
1. Network layer firewalls. An example would be iptables.
2. Application layer firewalls. An example would be TCP Wrappers.
3. Application firewalls. An example would be restricting ftp services through
These network-layer and application-layer types of firewall may overlap, even though the
personal firewall does not serve a network; indeed, single systems have implemented
There's also the notion of application firewalls which are sometimes used during wide
area network (WAN) networking on the world-wide web and govern the system software.
An extended description would place them lower than application layer firewalls, indeed
at the Operating System layer, and could alternately be called operating system firewalls.
Lastly, depending on whether the firewalls keeps track of the state of network
connections or treats each packet in isolation, two additional categories of firewalls exist:
1. Stateful firewalls
2. Stateless firewalls
Network layer firewalls operate at a (relatively) low level of the TCP/IP protocol stack as
IP-packet filters, not allowing packets to pass through the firewall unless they match the
rules. The firewall administrator may define the rules; or default built-in rules may apply
(as in some inflexible firewall systems).
A more permissive setup could allow any packet to pass the filter as long as it does not
match one or more "negative-rules", or "deny rules". Today network firewalls are built
into most computer operating systems and network appliances.
Modern firewalls can filter traffic based on many packet attributes like source IP address,
source port, destination IP address or port, destination service like WWW or FTP. They
can filter based on protocols, TTL values, netblock of originator, domain name of the
source, and many other attributes.
Application-layer firewalls work on the application level of the TCP/IP stack (i.e., all
browser traffic, or all telnet or ftp traffic), and may intercept all packets traveling to or
from an application. They block other packets (usually dropping them without
acknowledgement to the sender). In principle, application firewalls can prevent all
unwanted outside traffic from reaching protected machines.
By inspecting all packets for improper content, firewalls can even prevent the spread of
the likes of viruses. In practice, however, this becomes so complex and so difficult to
attempt (given the variety of applications and the diversity of content each may allow in
its packet traffic) that comprehensive firewall design does not generally attempt this
The XML firewall exemplifies a more recent kind of application-layer firewall.
A proxy device (running either on dedicated hardware or as software on a general-
purpose machine) may act as a firewall by responding to input packets (connection
requests, for example) in the manner of an application, whilst blocking other packets.
Proxies make tampering with an internal system from the external network more difficult
and misuse of one internal system would not necessarily cause a security breach
exploitable from outside the firewall (as long as the application proxy remains intact and
properly configured). Conversely, intruders may hijack a publicly-reachable system and
use it as a proxy for their own purposes; the proxy then masquerades as that system to
other internal machines. While use of internal address spaces enhances security, crackers
may still employ methods such as IP spoofing to attempt to pass packets to a target
Network address translation
Firewalls often have network address translation (NAT) functionality, and the hosts
protected behind a firewall commonly have addresses in the "private address range", as
defined in RFC 1918. Firewalls often have such functionality to hide the true address of
The Middlebox Communication (midcom) Working Group of the Internet Engineering
Task Force (IETF) is working on standardizing protocols for managing firewalls and
• Middlebox Communications (MIDCOM) Protocol Semantics
Working Of Firewall
If you have been using the Internet for any length of time, and especially if you work at a
larger company and browse the Web while you are at work, you have probably heard the
term firewall used. For example, you often hear people in companies say things like, "I
can't use that site because they won't let it through the firewall."
If you have a fast Internet connection into your home (either a DSL connection or a cable
modem), you may have found yourself hearing about firewalls for your home network as
well. It turns out that a small home network has many of the same security issues that a
large corporate network does. You can use a firewall to protect your home network and
family from offensive Web sites and potential hackers.
Basically, a firewall is a barrier to keep destructive forces away from your property. In
fact, that's why its called a firewall. Its job is similar to a physical firewall that keeps a
fire from spreading from one area to the next. As you read through this article, you will
learn more about firewalls, how they work and what kinds of threats they can protect you
What It Does
A firewall is simply a program or hardware device that filters the Information coming
through the Internet connection into your private network or computer system. If an
incoming packet of information is flagged by the filters, it is not allowed through.
If you have read the article How Web Servers Work, then you know a good bit about how
data moves on the Internet, and you can easily see how a firewall helps protect computers
inside a large company. Let's say that you work at a company with 500 employees. The
company will therefore have hundreds of computers that all have network cards connecting
them together. In addition, the company will have one or more connections to the Internet
through something like T1 or T3 lines. Without a firewall in place, all of those hundreds of
computers are directly accessible to anyone on the Internet. A person who knows what he
or she is doing can probe those computers, try to make FTP connections to them, try to
make telnet connections to them and so on. If one employee makes a mistake and leaves a
security hole, hackers can get to the machine and exploit the hole.
With a firewall in place, the landscape is much different. A company will place a firewall at
every connection to the Internet (for example, at every T1 line coming into the company).
The firewall can implement security rules. For example, one of the security rules inside the
company might be:
Out of the 500 computers inside this company, only one of them is permitted to
receive public FTP traffic. Allow FTP connections only to that one computer and
prevent them on all others.
A company can set up rules like this for FTP servers, Web servers, Telnet servers and so
on. In addition, the company can control how employees connect to Web sites, whether
files are allowed to leave the company over the network and so on. A firewall gives a
company tremendous control over how people use the network.
Firewalls use one or more of three methods to control traffic flowing in and out of the
1. Packet filtering - Packets (small chunks of data) are analyzed against a set of
filters. Packets that make it through the filters are sent to the requesting system
and all others are discarded.
2. Proxy service - Information from the Internet is retrieved by the firewall and then
sent to the requesting system and vice versa.
3. Stateful inspection - A newer method that doesn't examine the contents of each
packet but instead compares certain key parts of the packet to a database of
trusted information. Information traveling from inside the firewall to the outside is
monitored for specific defining characteristics, then incoming information is
compared to these characteristics. If the comparison yields a reasonable match,
the information is allowed through. Otherwise it is discarded.
Making the Firewall Fit
1. Firewalls are customizable. This means that you can add or remove filters based
on several conditions. Some of these are:
2. IP addresses - Each machine on the Internet is assigned a unique address called an
IP address. IP addresses are 32-bit numbers, normally expressed as four "octets"
in a "dotted decimal number." A typical IP address looks like this: 220.127.116.11.
For example, if a certain IP address outside the company is reading too many files
from a server, the firewall can block all traffic to or from that IP address.
3. Domain names - Because it is hard to remember the string of numbers that make
up an IP address, and because IP addresses sometimes need to change, all servers
on the Internet also have human-readable names, called domain names. For
example, it is easier for most of us to remember www.howstuffworks.com than it
is to remember 18.104.22.168. A company might block all access to certain
domain names, or allow access only to specific domain names.
4. Protocols - The protocol is the pre-defined way that someone who wants to use a
service talks with that service. The "someone" could be a person, but more often it
is a computer program like a Web browser. Protocols are often text, and simply
describe how the client and server will have their conversation. The http in the
Web's protocol. Some common protocols that you can set firewall filters for
1. IP (Internet Protocol) - the main delivery system for information over the
2. TCP (Transmission Control Protocol) - used to break apart and rebuild
information that travels over the Internet
3. HTTP (Hyper Text Transfer Protocol) - used for Web pages
4. FTP (File Transfer Protocol) - used to download and upload files
5. UDP (User Datagram Protocol) - used for information that requires no response,
such as streaming audio and video
6. ICMP (Internet Control Message Protocol) - used by a router to exchange the
information with other routers
7. SMTP (Simple Mail Transport Protocol) - used to send text-based information (e-
8. SNMP (Simple Network Management Protocol) - used to collect system
information from a remote computer
9. Telnet - used to perform commands on a remote computer
A company might set up only one or two machines to handle a specific protocol and ban
that protocol on all other machines.
Ports - Any server machine makes its services available to the Internet using
numbered ports, one for each service that is available on the server (see How Web
Servers Work for details). For example, if a server machine is running a Web (HTTP)
server and an FTP server, the Web server would typically be available on port 80, and the
FTP server would be available on port 21. A company might block port 21 access on all
machines but one inside the company.
Specific words and phrases - This can be anything. The firewall will sniff (search
through) each packet of information for an exact match of the text listed in the filter. For
example, you could instruct the firewall to block any packet with the word "X-rated" in
it. The key here is that it has to be an exact match. The "X-rated" filter would not catch
"X rated" (no hyphen). But you can include as many words, phrases and variations of
them as you need. Some operating systems come with a firewall built in. Otherwise, a
software firewall can be installed on the computer in your home that has an Internet
connection. This computer is considered a gateway because it provides the only point of
access between your home network and the Internet.
With a hardware firewall, the firewall unit itself is normally the gateway. A good
example is the Linksys Cable/DSL router. It has a built-in Ethernet card and hub.
Computers in your home network connect to the router, which in turn is connected to
either a cable or DSL modem. You configure the router via a Web-based interface that
you reach through the browser on your computer. You can then set any filters or
Hardware firewalls are incredibly secure and not very expensive. Home versions that
include a router, firewall and Ethernet hub for broadband connections can be found for
well under $100.
What It Protects You From
There are many creative ways that unscrupulous people use to access or abuse
Remote login - When someone is able to connect to your computer and control it in
some form. This can range from being able to view or access your files to actually
running programs on your computer.
Application backdoors - Some programs have special features that allow for remote
access. Others contain bugs that provide a backdoor, or hidden access, that provides
some level of control of the program.
SMTP session hijacking - SMTP is the most common method of sending e-mail over
the Internet. By gaining access to a list of e-mail addresses, a person can send
unsolicited junk e-mail (spam) to thousands of users. This is done quite often by
redirecting the e-mail through the SMTP server of an unsuspecting host, making the
actual sender of the spam difficult to trace.
Operating system bugs - Like applications, some operating systems have backdoors.
Others provide remote access with insufficient security controls or have bugs that an
experienced hacker can take advantage of.
Denial of service - You have probably heard this phrase used in news reports on the
attacks on major Web sites. This type of attack is nearly impossible to counter. What
happens is that the hacker sends a request to the server to connect to it. When the server
responds with an acknowledgement and tries to establish a session, it cannot find the
system that made the request. By inundating a server with these unanswerable session
requests, a hacker causes the server to slow to a crawl or eventually crash.
E-mail bombs - An e-mail bomb is usually a personal attack. Someone sends you
the same e-mail hundreds or thousands of times until your e-mail system cannot
accept any more messages.
Macros - To simplify complicated procedures, many applications allow you to
create a script of commands that the application can run. This script is known as a
macro. Hackers have taken advantage of this to create their own macros that,
depending on the application, can destroy your data or crash your computer.
Viruses - Probably the most well-known threat is computer viruses. A virus is a
small program that can copy itself to other computers. This way it can spread quickly
from one system to the next. Viruses range from harmless messages to erasing all of
Spam - Typically harmless but always annoying, spam is the electronic equivalent
of junk mail. Spam can be dangerous though. Quite often it contains links to Web
sites. Be careful of clicking on these because you may accidentally accept a cookie
that provides a backdoor to your computer.
• Redirect bombs - Hackers can use ICMP to change (redirect) the path
information takes by sending it to a different router. This is one of the ways that a
denial of service attack is set up.
• Source routing - In most cases, the path a packet travels over the Internet (or any
other network) is determined by the routers along that path. But the source
providing the packet can arbitrarily specify the route that the packet should travel.
Hackers sometimes take advantage of this to make information appear to come
from a trusted source or even from inside the network! Most firewall products
disable source routing by default.
Some of the items in the list above are hard, if not impossible, to filter using a firewall.
While some firewalls offer virus protection, it is worth the investment to install anti-virus
software on each computer. And, even though it is annoying, some spam is going to get
through your firewall as long as you accept e-mail.
The level of security you establish will determine how many of these threats can be
stopped by your firewall. The highest level of security would be to simply block
everything. Obviously that defeats the purpose of having an Internet connection. But a
common rule of thumb is to block everything, then begin to select what types of traffic
you will allow. You can also restrict traffic that travels through the firewall so that only
certain types of information, such as e-mail, can get through. This is a good rule for
businesses that have an experienced network administrator that understands what the
needs are and knows exactly what traffic to allow through. For most of us, it is probably
better to work with the defaults provided by the firewall developer unless there is a
specific reason to change it.
One of the best things about a firewall from a security standpoint is that it stops anyone
on the outside from logging onto a computer in your private network. While this is a big
deal for businesses, most home networks will probably not be threatened in this manner.
Still, putting a firewall in place provides some peace of mind.
Proxy Servers and DMZ
A function that is often combined with a firewall is a proxy server. The proxy
server is used to access Web pages by the other computers. When another computer
requests a Web page, it is retrieved by the proxy server and then sent to the
requesting computer. The net effect of this action is that the remote computer
hosting the Web page never comes into direct contact with anything on your home
network, other than the proxy server.
Proxy servers can also make your Internet access work more efficiently. If you
access a page on a Web site, it is cached (stored) on the proxy server. This means
that the next time you go back to that page, it normally doesn't have to load again
from the Web site. Instead it loads instantaneously from the proxy server.
There are times that you may want remote users to have access to items on your
network. Some examples are:
• Web site
• Online business
• FTP download and upload area
In cases like this, you may want to create a DMZ (Demilitarized Zone). Although
this sounds pretty serious, it really is just an area that is outside the firewall. Think
of DMZ as the front yard of your house. It belongs to you and you may put some
things there, but you would put anything valuable inside the house where it can be
Setting up a DMZ is very easy. If you have multiple computers, you can choose to
simply place one of the computers between the Internet connection and the firewall.
Most of the software firewalls available will allow you to designate a directory on
the gateway computer as a DMZ.
Once you have a firewall in place, you should test it. A great way to do this is to go
to www.grc.com and try their free Shields Up! security test. You will get immediate
feedback on just how secure your system is!
A firewall is an information technology (IT) security device which is configured to
permit, deny or proxy data connections set and configured by the organization's security
policy. Firewalls can either be hardware and/or software based. A firewall is also called a
Border Protection Device (BPD) in certain military contexts were a firewall separates
networks by creating perimeter networks in a DMZ. In a BSD context they are also
known as a packet filter. A firewall's function is analogous to firewalls in building