What is Firewall? Top 5 Firewall Applications:

A firewall is a part of a computer system or network that is designed to block
unauthorized access while permitting authorized communications. It is a device or set of
devices configured to permit, deny, encrypt, decrypt, or proxy all (in and out) computer
traffic between different security domains based upon a set of rules and other criteria.
Firewalls can be implemented in either hardware or software, or a combination of both.
Firewalls are frequently used to prevent unauthorized Internet users from accessing
private networks connected to the Internet, especially intranets. All messages entering or
leaving the intranet pass through the firewall, which examines each message and blocks
those that do not meet the specified security criteria.
There are several types of firewall techniques:
1. Packet filter: Looks at each packet entering or leaving the network and accepts or
rejects it based on user-defined rules. Packet filtering is fairly effective and
transparent to users, but it is difficult to configure. In addition, it is susceptible to
IP spoofing.
2. Application gateway: Applies security mechanisms to specific applications, such
as FTP and Telnet servers. This is very effective, but can impose a performance
degradation.
3. Circuit-level gateway: Applies security mechanisms when a TCP or UDP
connection is established. Once the connection has been made, packets can flow
between the hosts without further checking.
4. Proxy server: Intercepts all messages entering and leaving the network. The proxy
server effectively hides the true network addresses.

Top 5 Firewall Applications:
1. Tiny Personal Firewall
2. ZoneAlarm
3. NetWatcher 2000
4. ConSeal PC Firewall
5. Sygate Personal Firewall
How firewall works:
Every firewall have rules to allow/deny ports for incoming and outgoing communication.
Why Firewall Security?
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 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
from a trusted source or even from inside the network! Most firewall products