FIREWALL INTRODUCTION

 Firewall

The filrewall is inserted between the premises network and the internet to establish a controlld link and to erect an outer security wall or perimeter. The aim to provide a single choke point where security and audit can be imposed. The firewall may be a single computer system or a set of 2 ore more systems that cooperate to perform the firewall function.
It is a system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both.

Types of Firewall


Packet Filtering Router: This applies a set of rules to each incoming IP packet and then forwards or discards the packet. This is configured to filter packets going in both directions and filtering rules are based on information contained in a network packet that are-

• Source IP address
• Destination IP address
• Source and destination transport level address
• IP protocol field
• Interface

Application Level Gateway: Application level gateway also called proxy server. The user contacts the gateway using a TCP/IP application, such as Telnet or FTP, and gateway asks the user for the name of remote host to be accessed. When the user responds and provides a valid ID and Password, the gateway contacts the application on the remote host and relays TCP segments containing the application data between the two end points. If the gateway does not implement the proxy code for a specific application, the server is not supported and can not be forwarded across the firewall.

Circuit Level Gateway: This can be a stand-alone system or it can be specialized function performed by an application level gateway for certain applications. A circuit level gateway doesn't permit an end to end TCP connection, rather the gateway sets up two TCP connections one between itself and TCP user on an inner host and one between itself and a TCP user on an outside host. Once the two connections are established, the gateway typically re;lays TCP segments from one connection to the other without examining the content.