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FIREWALL

FIREWALL
A firewall is a hardware or software device which is configured to permit, deny, or proxy data through a computer network which has different levels of trust. The most common boundary where firewalls are applied is between an organization’s internal network and the interne t. This report will provide readers with a resource for understanding firewall design principles used in network security.

So , 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.
Firewall Architectures
There are two classes of firewall architectures:
1)single layer and,
2) multiple layer.

In a single layer architecture, one host is allocated all firewall functions. This method is usually chosen when either cost is a key factor or if there are only two networks to connect. The advantage to this architecture is any changes to the firewall need only to be done at a single host.
•The biggest disadvantage of the single layer approach it provides single entry point.
If this entry point is breached, the entire network becomes vulnerable to an intruder.
In a multiple layer architecture the firewall functions are distributed among two or more hosts normally connected in series. This method is more difficult to design and manage, it is also more costly, but can provide significantly greater security by diversifying the firewall defense. A common design approach for this type of architecture using two firewall hosts with a demilitarized network (DMZ) between them separating the Internet and the internal network.
How does a firewall work?
There are two access denial methodologies used by firewalls. A firewall may allow all traffic through unless it meets certain criteria, or it may deny all traffic unless it meets certain criteria (see figure ). The type of criteria used to determine whether traffic should be allowed through varies from one type of firewall to another. Firewalls may be concerned with the type of traffic, or with source or destination addresses and ports. They may also use complex rule bases that analyse the application data to determine if the traffic should be allowed through. How a firewall determines what traffic to let through depends on which network layer it operates at. A discussion on network layers and architecture follows.
What different types of firewalls are?
Firewalls fall into four broad categories: 1)packet filters, 2)circuit level gateways, 3)application level gateways and 4) stateful multilayer inspection firewalls.
Packet filtering firewalls work at the network level of the OSI model, or the IP layer of TCP/IP. They are usually part of a router. A router is a device that receives packets from one network and forwards them to another network. In a packet filtering firewall each packet is compared to a set of criteria before it is forwarded. Depending on the packet and the criteria, the firewall can drop the packet, forward it or send a message to the originator.
Circuit level gateways work at the session layer of the OSI model, or the TCP layer of TCP/IP. They monitor TCP handshaking between packets to determine whether a requested session is legitimate. Information passed to remote computer through a circuit level gateway appears to have originated from the gateway. This is useful for hiding information about protected networks. Circuit level gateways are relatively inexpensive and have the advantage of hiding information about the private network they protect. On the other hand, they do not filter individual packets.
Application level gateways, also called proxies, are similar to circuit-level gateways except that they are application specific. They can filter packets at the application layer of the OSI model. Incoming or outgoing packets cannot access services for which there is no proxy. In plain terms, an application level gateway that is configured to be a web proxy will not allow any ftp, gopher, telnet or other traffic through. Because they examine packets at application layer, they can filter application specific commands such as http:post and get, etc.
Stateful multilayer inspection firewalls combine the aspects of the other three types of firewalls. They filter packets at the network layer, determine whether session packets are legitimate and evaluate contents of packets at the application layer. They allow direct connection between client and host, alleviating the problem caused by the lack of transparency of application level gateways. They rely on algorithms to recognize and process application layer data instead of running application specific proxies. Stateful multilayer inspection firewalls offer a high level of security, good performance and transparency to end users. They are expensive however, and due to their complexity are potentially less secure than simpler types of firewalls if not administered by highly competent personnel

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