In computer networking, the term Internet Protocol spoofing (IP spoofing) is the creation of IP packets with a forged (spoofed) source IP address. Actually the term is often used to define another (more complex) attack related to the TCP protocol, where a TCP connection is created using a spoofed IP address, guessing the TCP ISN (Initial Sequence Number) sent by the other peer in the TCP SYN/ACK packet sent as reply to the intial TCP SYN packet. Today IP-based authentication is (or at least should) not be used, but in the past this attack was used in order to gain reserved access where there was some IP-related trusted relationship. This attack is hard against modern and robust TCP/IP stack implementations that are designed in order to generate unguessable ISN numbers.
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