Firewalls

A firewall is a combination of hardware and software that serves as a gateway between the private network and the Internet. Predefined access and scope of use are required, and all other requests are blocked. An effective firewall should protect both the export and import of data from and to the private network.
If designed effectively, a firewall can look at every piece of data that passes into or out of a private network and decide whether to allow the passage based on the following:
• User identification
• Point of origin
• Point of destination
• The information contents
By careful examination of the packet that is trying to exit from or enter into the private network, a firewall can choose one of the following actions:
• Reject the incoming packet
• Send a warning to the network administrator
• Send a message to the sender of the message that the attempt has failed
• Allow the message to enter the private network
Types of Firewalls

Based on their functions, firewalls have been broadly classified into two groups
Packet filter firewalls control the data traffic (the export and import of data) based on datagrams.The filter examines fields in Internet Protocol (IP) packets such as source and destination IP addresses and source and desti- nation ports. By checking these fields, the packet filter can allow or deny pas- sage of packets. Factors that influence the rejection or acceptance of a datagram are based on the network application requested, protocol type, and the source and destination of the datagram. A packet filter firewall can be programmed to ac- cept or reject a datagram based on the host that initiated the datagram.
Packet filter firewalls may not be very efficient since they have to examine each packet individually, and they may be difficult to install. They may provide a false sense of security, and they usually cannot accurately record all of the ac- tions taking place at the firewall. This makes it difficult for network administra- tors to find out how intruders are trying to break into the private network.
           Application (proxy) firewalls are regarded as more secure and flexible and are therefore more expensive than packet filter firewalls. These firewalls are in- stalled in the host computer. A dedicated PC or a workstation may perform this task. An application firewall controls the private network applications, such as e-mail, Telnet, and FTP at the individual or group level by focusing on the type of action and the time period in which the action is taking place. By concentrat- ing on the time period, these firewalls can be very effective because many unau- thorized attempts take place at night or after hours. Application firewalls are able to log actions that take place at the firewall. By using these data, network ad- ministrators are able to identify potential breaches to security that may be di- rected to the private network. Application firewalls can also filter viruses, a ma- jor security threat in the e-commerce environment. Because application firewalls perform a great deal of work to check for unauthorized activities in a network,they have a relatively slow processing speed. This in turn may bring down the performance of the whole network.
In large organizations, proxy servers are also used in addition to firewalls for
protecting the security and integrity of a network.A proxy server is a server that acts as an intermediary between a workstation user and the Internet so that the organization can ensure security, administrative con trol, and caching services. Proxy servers separate the organization’s network from the outside network, and a firewall server protects the organization’s network from unauthorized outside access.