Firewall

General

Firewalls are systems that establish access control policies among networks. They can block information from entering a network or from getting out of that network, they can permit different users to perform different kinds of operations, according to the user's authorizations.
There are several kinds of firewall implementations which enable each company to decide what exaclly are the characteristics of the security policy that would best fit its special demands.There are two general types of firewalls: Packet Level Firewalls, which examine
packets and decide according to filtering rules whether to pass them to the network. and Application Level Firewalls, which monitor specific applications protocols.

Characteristics

• Filtering Router - Since firewall is actually a filter which decides which packets are authorized to reach a local network from the outside network and which are considered hostile, the simplest way to implement a firewall is by discarding packets that came from certain IP addresses while allowing those packets, whose source IP address is in a predefined list, to access the network. The problem with such a simple firewall is that it is very easy to forge a source IP address. Therefor, this kind of firewalls are not commonly used.

• Proxy Server - In order to achive a better security features, a firewall can use an application level gateway. The gateway is located between two firewalls (routers), each on the other side of the gateway. This firewalls will deliver only information which target is the gateway or information whose source is the gateway. This way, only information that was sent by a local user to the gateway can be reached by an external user and local user which wishes to receive information from the external network must first get loging permissions from the gateway which would fetch the information according to the company's security policy.

1. Circuit Level Proxy - A circuit level proxy verifies, according to predefined secuirty  files, whether a certain IP datagrams are "safe" to deliver through a firewall. After we establish connection it is resposible to deliver packets from and to an external network from a user application.
2. Application Level Proxy - The main difference between Application Level Proxy and Circuit Level Proxy or Filtering router is its ability to "understand" applications specific information structure. This way the Application Level Proxy can examine application's flow of information and decide whether to allow specific application level requests for information, or requests to change information and not only to decide according to "external" charateristics of the information packets.  In order to achive such security level, the firewall must "know" each user application. The main problem is that the Application Level Proxy is not transparent to the user  which must take into consideration that the information may be filtered due to context problems. Another problem is, of course, that applications communication protocol should be supported by the firewall.
 

• IP address
• Routers
• Gateways