Understanding Land Attacks

Combining a SYN attack with IP spoofing, a land attack occurs when an attacker sends spoofed SYN packets containing the IP address of the victim as both the destination and the source IP address.

The receiving system responds by sending the SYN-ACK packet to itself, creating an empty connection that lasts until the idle timeout value is reached. Flooding a system with such empty connections can overwhelm the system, causing a denial of service (DoS). See Figure 81.

When you enable the screen option to block land attacks, Junos OS combines elements of the SYN flood defense and IP spoofing protection to detect and block any attempts of this nature.