<P> IP address spoofing involving the use of a trusted IP address can be used by network intruders to overcome network security measures, such as authentication based on IP addresses . 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 on the same network may be able to access the target machine without authentication . </P> <P> IP address spoofing is most frequently used in denial - of - service attacks, where the objective is to flood the target with an overwhelming volume of traffic, and the attacker does not care about receiving responses to the attack packets . Packets with spoofed IP addresses 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 . </P> <P> The use of packets with a false source IP address is not always evidence of malicious intent . For example, in performance testing of websites, hundreds or even thousands of "vusers" (virtual users) may be created, each executing a test script against the website under test, in order to simulate what will happen when the system goes "live" and a large number of users log on at once . </P> <P> Since each user will normally have its own IP address, commercial testing products (such as HP LoadRunner, WebLOAD, and others) can use IP spoofing, allowing each user its own "return address" as well . </P>

Source ip address spoofed (impossible packet)