A firewall is simply a program or hardware device that filters the information coming through the Internet connection into your private network or computer system. If an incoming packet of information is flagged by the filters, it is not allowed through.
Firewalls use one or more of three methods to control traffic flowing in and out of the network:
* 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.
* Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.
* 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.

DNS Hijacking mean hijacker spoof the ip of any DNS query. Lets example. user send a DNS requset for YAHOO.com and Hijacker hijack this packet & answer on behalf of DNS and give the wrong IP Assres.