What is a Firewall?

A firewall is used to keep the internal network segment secure from intruders. It was develop by Sun Microsystems and CheckPoint Software Technologies .

What does a Firewall do?

A firewall ensures that all packets coming into the internal network does not consist of malicious contents. This is done by inspecting each packet and decides, based on your rules, whether they should be allowed. It helps to prevent hackers break in and filter email viruses. To protect trusted networks from invasion, many firewalls are deployed at either the network level or on the PC.

Why do we need a Firewall in a business environment?

Every business nowadays relies on Internet to retrieve the latest information and establish a competitive advantage. Firewall helps business to protect their intangible assets, while monitoring Internet usage.

Figure: Firewall

How does a Firewall work?

There are three technologies built on a firewall, they are Packet Filtering, Application Layer Gateways and Stateful Inspection.

Denial of Service (DoS)

All of the varieties of hacker attacks, we introduce the more common ones here: Denial of Service (DoS)

Denial of Service is a type of attack on a network that is designed to bring the network to its knees by flooding it with useless traffic. The servers will be so busy accepting or denying requests and thus cannot provide normal services to legitimate users. Many DoS attacks, such as the IP Spoofing, Ping of Death, SYN Flood and Teardrop attacks, exploit limitations in the TCP/IP protocols.

The three common DoS attacks


IP Spoofing：A technique used to gain unauthorized access to computers, whereby the intruder sends messages to a computer with an IP address indicating that the message is coming from a trusted host. To engage in IP spoofing, a hacker must first use a variety of techniques to find an IP address of a trusted host and then modify the packet headers so that it appears that the packets are coming from that host.

Ping of Death：Attacks operating systems that fail to handle packets that are larger than their acceptable sizes. This is done by “pinging” the target computer with an illegitimate size packet i.e., over 65535 bytes in size). This will make the operating system hangs when it tries to respond to it.

Teardrop ： This attack relies on system vulnerability that causes many systems to crash when they receive IP packets that have been corrupted so that the data overlaps. Teardrop attack exploits weakness in the reassemble of the IP packet fragments. As data is transmitted through a network, IP packets are often broken up into smaller chunks. Each fragment looks like the original packet except that it contains an offset field. The Teardrop program creates a series of IP fragments with overlapping offset fields. When these fragments are reassembled at the destination, some systems will crash, hang, or reboot