Criminals have long employed the tactic of masking their true identity, from disguises to aliases to caller-id blocking. It should come as no surprise then, that criminals who conduct their nefarious activities on networks and computers should employ such techniques. IP spoofing is one of the most common forms of on-line camouflage. In IP spoofing, an attacker gains unauthorized access to a computer or a network by making it appear that a malicious message has come from a trusted machine by “spoofing” the IP address of that machine.

The concept of IP spoofing, was initially discussed in academic circles in the 1980's. While known about for sometime, it was primarily theoretical until Robert Morris, whose son wrote the first Internet Worm, discovered a security weakness in the TCP protocol known as sequence prediction. Stephen Bellovin discussed the problem in-depth in Security Problems in the TCP/IP Protocol Suite, a paper that addressed design problems with the TCP/IP protocol suite. Another infamous attack, Kevin Mitnick's Christmas Day crack of Tsutomu Shimomura's machine, employed the IP spoofing and TCP sequence prediction techniques. While the popularity of such cracks has decreased due to the demise of the services they exploited, spoofing can still be used and needs to be addressed by all security administrators.

An attacker might spoof a trusted IP address when communicating to a host in order to gain unauthorized access on that host. There are a variety of tools that can be found on the Internet to do IP spoofing. Using IP spoofing, attackers can also initiate Denial of Service by sending data with the source IP spoofed to the attacked IP address. The receiver then sends back replies that can contain large amounts of data to the attacked IP address resulting in a flood attack to that address. Sending data using the spoofed IP address to many hosts will result in a Distributed Denial-of-Service attack.