A Quick Overview - IP Spoofing

IP Spoofing (IP address forgery or a host file hijack)

IP address spoofing or IP spoofing is the creation of Internet Protocol (IP) packets with a forged source IP address, with the purpose of concealing the identity of the sender or impersonating another computing system.

IP spoofing is also known as IP address forgery or a host file hijack, is a hijacking technique in which a cracker masquerades as a trusted host to conceal his identity, spoof a Web site, hijack browsers, or gain access to a network. Here's how it works: The hijacker obtains the IP address of a legitimate host and alters packet headers so that the legitimate host appears to be the source.

When IP spoofing is used to hijack a browser, a visitor who types in the URL (Uniform Resource Locator) of a legitimate site is taken to a fraudulent Web page created by the hijacker. For example, if the hijacker spoofed the Instagram Web site, then any Internet user who typed in the URL www.instagram.com would see spoofed content created by the hijacker.

If a user interacts with dynamic content on a spoofed page, the hijacker can gain access to sensitive information or computer or network resources. He could steal or alter sensitive data, such as a credit card number or password, or install malware . The hijacker would also be able to take control of a compromised computer to use it as part of a zombie army in order to send out spam.

Web site administrators can minimize the danger that their IP addresses will be spoofed by implementing hierarchical or one-time passwords and data encryption/decryption techniques. 

Users and administrators can protect themselves and their networks by installing and implementing firewalls that block outgoing packets with source addresses that differ from the IP address of the user's computer or internal network.

IP spoofing is most frequently used in denial-of-service attacks. In such attacks, the goal is to flood the victim with overwhelming amounts of traffic, and the attacker does not care about receiving responses to the attack packets. Packets with spoofed addresses are thus suitable for such attacks. They have additional advantages for this purpose—they are more difficult to filter since each spoofed packet appears to come from a different address, and they hide the true source of the attack. Denial of service attacks that use spoofing typically randomly choose addresses from the entire IP address space, though more sophisticated spoofing mechanisms might avoid unroutable addresses or unused portions of the IP address space. The proliferation of large botnets makes spoofing less important in denial of service attacks, but attackers typically have spoofing available as a tool, if they want to use it, so defenses against denial-of-service attacks that rely on the validity of the source IP address in attack packets might have trouble with spoofed packets. Backscatter, a technique used to observe denial-of-service attack activity in the Internet, relies on attackers' use of IP spoofing for its effectiveness.

IP spoofing can also be a method of attack used by network intruders to defeat network security measures, such as authentication based on IP addresses. This method of attack on a remote system can be extremely difficult, as it involves modifying thousands of packets at a time. This type of attack is most effective where trust relationships exist between machines. 

For example, it is common on some corporate networks to have internal systems trust each other, so that users can log in without a username or password provided they are connecting from another machine on the internal network (and so must already be logged in). By spoofing a connection from a trusted machine, an attacker may be able to access the target machine without an authentication.

Spoofed IP packets are not incontrovertible evidence of malicious intent; however, in performance testing of websites, hundreds or even thousands of "vusers" (virtual users) may be created, each executing a test script against the Web site under test, in order to simulate what will happen when the system goes "live" and a large number of users log on at once.

Since each user will normally have their own IP address, commercial testing products (such as HP's Loadrunner software or Websense etc) can use IP spoofing, allowing each user its own "return address", as well.