What is a firewall
A firewall is a barrier to keep destructive forces away from your network.It is simply a program or hardware device that filters the information coming through the Internet connection into your private network or computer system. ![]() Firewalls use one or more of three methods to control traffic flowing in and out of the network: 1-Packet filtering -Packets (small chunks of data) are analyzed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded. ![]() How Packet Filters Work ![]() 2-Proxy service - When a client issues a request from the untrusted network, a connection is established with the application gateway/proxy.The proxy determines if the request is valid (by comparing it to any rules or filters) and then sends a new request on behalf of the client to the destination. By using this method, a direct connection is never made from the trusted network to the untrusted network and the request appears to have originated from the application gateway/proxy.The request is answered in the same manner ![]() How Proxy Firewalls Work ![]() 3-Stateful inspection -A newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics,then incoming information is compared to these characteristics. If the comparison yields a reasonable match,the information is allowed through. Otherwise it is discarded. This method can make decisions based on one or more of the following: -Source IP address -Destination IP address -Protocol type (TCP/UDP) -Source port -Destination port -Connection state How Stateful Firewalls Work ![]() Resources:http://www.howstuffworks.com/firewall.htmhttp://www.more.net/technical/netserv/tcpip/firewalls/index.html http://www.syngress.com/solutions Book : Configuring Checkpoint NGX VPN-1/FIREWALL-1 by Robert Stephens ,Barry J. Stiefel ,Stephen Watkins,Simon Desmeules,Eli Faskha |