IP address spoofing
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In computer networking, 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.
The basic protocol for sending data over the Internet network and many other computer networks is the Internet Protocol ("IP"). The header of each IP packet contains, among other things, the numerical source and destination address of the packet. The source address is normally the address that the packet was sent from. By forging the header so it contains a different address, an attacker can make it appear that the packet was sent by a different machine. The machine that receives spoofed packets will send a response back to the forged source address, which means that this technique is mainly used when the attacker does not care about the response or the attacker has some way of guessing the response.
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.
Legitimate uses of IP spoofing 
Spoofed IP packets are not a priori 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) can use IP spoofing, allowing each vuser its own "return address", as well.
Services vulnerable to IP spoofing 
Configuration and services that are vulnerable to IP spoofing:
- RPC (Remote procedure call services)
- Any service that uses IP address authentication
- The X Window System
- The R services suite (rlogin, rsh, etc.)
Defense against spoofing attacks 
Packet filtering is one defense against IP spoofing attacks. The gateway to a network usually performs ingress filtering, which is blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally the gateway would also perform egress filtering on outgoing packets, which is blocking of packets from inside the network with a source address that is not inside. This prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines.
It is also recommended to design network protocols and services so that they do not rely on the IP source address for authentication.
Upper layers 
Some upper layer protocols provide their own defense against IP spoofing attacks. For example, Transmission Control Protocol (TCP) uses sequence numbers negotiated with the remote machine to ensure that arriving packets are part of an established connection. Since the attacker normally can't see any reply packets, the sequence number must be guessed in order to hijack the connection. The poor implementation in many older operating systems and network devices, however, means that TCP sequence numbers can be predicted.
Other definitions 
The term spoofing is also sometimes used to refer to header forgery, the insertion of false or misleading information in e-mail or netnews headers. Falsified headers are used to mislead the recipient, or network applications, as to the origin of a message. This is a common technique of spammers and sporgers, who wish to conceal the origin of their messages to avoid being tracked down.
See also 
- Egress filtering
- Ingress filtering
- Network address translation
- Reverse path forwarding
- RFC 1948
- RFC 1948, Defending Against Sequence Number Attacks, May 1996
- Router (includes a list of manufacturers)
- Spoofed URL
- Tanase, Matthew (March 11, 2003). "IP Spoofing: An Introduction". The Security Blog. Retrieved February 10, 2012.