FIREWALL

Introduction

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.

History

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

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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

firewalls.

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.

Subsequent generations

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

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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).

Types

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

application layer.

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

networks.

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The latter definition corresponds to the conventional, traditional meaning of "firewall" in

networking.

In reference to the layers where the traffic can be intercepted, three main categories of

firewalls exist:

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

/etc/ftpaccess file

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

both together.

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

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).

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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

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

approach.

The XML firewall exemplifies a more recent kind of application-layer firewall.

Proxies

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

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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..

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

protected hosts.

Management

The Middlebox Communication (midcom) Working Group of the Internet Engineering

Task Force (IETF) is working on standardizing protocols for managing firewalls and

other middleboxes.

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."

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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

from.

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

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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

network:

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.

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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: 216.27.61.137.

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 216.27.61.137. 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

include:

1. IP (Internet Protocol) - the main delivery system for information over the

Internet

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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-

mail)

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.

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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

additional information.

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

unprotected computers:

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

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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

your data.

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

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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

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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

properly secured.

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!

Conclusion

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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

construction.

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