A LAND attack is a DoS (Denial of Service) attack that consists of sending a special poison spoofed packet to a computer, causing it to lock up.
The attack involves sending a spoofed TCP SYN packet (connection initiation) with the target host's IP address and an open port as both source and destination.
The reason a land attack works is because it causes the machine to reply to itself continuously.
Definition " A "LAND" attack involves IP packets where the source and destination address are set to address the same device. "
Example (first land attack). It involved sending a spoofed ICMP message to the chargen (character generator) port on a UNIX system. The Character generator would spit out a packet back to the echo port. The echo port would send data back to the chargen and so on, until the resources of the machine were consumed.
Defense: Most firewalls should intercept the poison packet thus protecting the host from this attack. Some operating systems released updates fixing this security hole.