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Lesson 21
A firewall is a combination of hardware and software that sits between the internet and internal network of
an organization to protect the network from outside attack (Fig. 1). It can examine the data entering or
leaving from the network and can filter the data according to certain rules, thus, protects the network from
an attack. There are three main types of firewalls detailed as follows:
Fig. 1
Packet filter firewall
It uses a set of rules to determine whether outgoing or incoming data packets are allowed to pass through
the firewall. For example, we can, as a rule, specify IP addresses of sending devices such that packets from
these IP addresses are not allowed to enter the network. The Firewall would stop them from entering. A
packet filter firewall is the simplest type of firewalls which operates at data link and network layers of the
OSI model.
Circuit level firewall
It is quite similar to the packet filter firewall. It also works on the basis of a set of rules for filtering packets
but operates at the transport layer of the OSI Model so has greater functionality. As a rule, the higher the
layer of OSI model where a firewall operates, the more sophisticated is the firewall. It can make packets sent
from internal network to a destination outside the firewall appear as if they originated at the firewall. Thus
information regarding hosts on the internal network remains secret. It can also determine whether TCP/IP
connection between a host and a machine outside firewall has been properly established. Thus it can cut off
any connection which has been hijacked by a hacker trying to pass through the firewall.
Application gateway firewall
It operates at application layer of the OSI Model. It uses strong user authentication to verify identity of a
host attempting to connect to the network using application layer protocols such us FTP. In contrast to
packet filter firewall, it filters the requests rather than packets entering/leaving the network. It can block any
outgoing HTTP or FTP requests. It can prevent employees of a company inside a firewall from
downloading potentially dangerous programs from the outside. In other words, this type of firewall is used
to control connections thus employees of a company can be restricted from connecting to certain web sites.
We can combine circuit level capabilities with application gateway services to form Hybrid type of a firewall.
Proxy server
A proxy server sits between an internal trusted network and the untrusted network, that is, internet, as you
can see in Fig. 2 below.
Fig. 2
Mainly, it can do three things:
An http request from the browser goes to proxy server. It can affix its own IP address instead of IP
address of the requesting machine; thus, it hides the information of the host. It downloads the
requested page itself and afterwards supplies it to the user.
It can also act as a firewall filtering requests for certain web pages.
An important job it can do is to speed up the processing of http requests by caching web pages.
Caching means that it can store the requested web pages in its memory (cache memory) for a certain
period. The advantage of caching is that for subsequent web page requests the time of supply of the
web pages is reduced. Instead of sending the request to actual web server, the proxy server can quickly
supply the web page stored in its cache memory, thus, it saves the time of downloading the page.
Virtual private network (VPN)
Suppose that a client is sitting at a local branch network of a company and wants to become part of a
bigger, head office network of that company located far away. One option for him is to set up a dial up
connection, which means that he can be connected to a server machine lying in the head office network
through a direct telephone line. That server machine may be called a Remote Access Server (RAS) and
the client may be called a Remote Access Client (RAC). Remote access is a two way process so both RAS
and RAC must be configured, first. Some windows operating systems provide the facility to configure the
RAS and RAC. Basically, the client specifies the phone no. of RAS while configuring. After both RAS and
RAC are configured, the client enters identification information (password etc.) and clicks at "Dial".
Accordingly, phone no. of RAS is dialed and connection with RAS is setup. Once clients are connected to
RAS, they can access the remote company network and its resources ­ servers, printers etc. A protocol,
Point to Point Protocol (PPP), is used to set up the dial up connection between RAC and RAS for exchange
of data packets.
A VPN provides another option of remote access. It is defined as a secure, dedicated point to point
connection over the internet. In VPN we use internet infrastructure for connection instead of a special
telephone line. Both RAS (also called tunnel server) and RAC (also called tunnel client) are connected to the
internet. Initially, both are configured for VPN. IP address of tunnel server must be specified during the
configuration of tunnel client (instead of phone no.). The option of VPN is available if we explore the menu
`Internet Options'. We can enable VPN, thus. Similarly, tunnel server should also be configured so that a
client's request for access can be authenticated. VPN connections or tunnels are managed by Point to Point
Tunneling Protocol (PPTP) which due to encryption provides secure transport of private communications
over the public internet. A VPN connection thus can be created between the branch office and the
corporate head office.
VPN is a cost saving measure as compared to simple remote access using dial up connection. In VPN one
makes a local call to the ISP and then using ISP's infrastructure, routers etc. one is connected to the
internet. In other words a client can become part of the remote network through the internet. Note that a
tunnel client just incurs the cost of a local call to the ISP and yet he can remain part of the remote corporate
network for many hours. On the other hand, in case of dial up connection for remote access one has to pay
the cost of a long distance call for as many no. of hours as one wants to be connected to the remote
corporate network. This is going to be very expensive. VPN is the example of an extranet. You know that
when two or more intranets are connected to each other they form an extranet. A manufacturing company
thus can be connected to its suppliers of raw material and its distributors through VPN.
Security ­ the biggest challenge
There is a consensus that the issue of computer and data security is the biggest hurdle in the growth of e-
commerce. Web servers also face this security threat. Programs that run on a server have the potential to
damage databases, abnormally terminate server software or make changes in the information placed there. A
number of international organizations have been formed to share information and combat security threats
to computers and computer networks. The names of two such organizations are worth-mentioning:
Computer Emergency Response Team (CERT)
Systems Administrator, Audit, Network and Security Institute (SANS Institute)
The best response that the experts have come up with to tackle the security issue is in terms of
Cryptography is the technique of converting a message into unintelligible or non-understandable form such
that even if some unauthorized or unwanted person intercepts the message he/she would still not be able to
make any sense out of it. Cryptography is thousands of years old.
Techniques used for cryptography Substitution In substitution we replace each letter in the message with
another to make the message non-understandable. For example, each letter "a" in the message can be
replaced with letter "d" and letter "b" with letter "e" and so on. Transposition It is based on scrambling the
characters in a message. A transposition system may first write a message into a table row by row then the
message can be read and rewritten column by column to make it scrambled (see Fig. 3).
Fig. 3
Historically, cryptography has long been used as a military technology. Julis Ceaser used a simple
transposition cipher to scramble messages to give instructions to his commanders in the battlefield.
Similarly, Hitler used Enigma encryption cipher to scramble messages sent by radio to German armies and
u-boats during the Second World War. Cryptography has also been used for non-military purposes over the
centuries. There are records of people using cryptography to protect religious secrets and to hide secrets of
science and industry. In recent years, the use of cryptography in business and commerce appears to have
surpassed its earlier use. It has made the rapid commercialization of internet possible. Without
cryptography, it is doubtful that banks, businesses and individuals would feel safe doing business online.