Configuring the SYN threshold to prevent SYN floods

Configuring the SYN threshold to prevent SYN floods

The preferred primary defense against any type of SYN flood is the DoS anomaly check for tcp_syn_flood threshold. The threshold value sets an upper limit on the number of new incomplete TCP connections allowed per second. If the number of incomplete connections exceeds the threshold value, and the action is set to Pass, the FortiGate unit will allow the SYN packets that exceed the threshold. If the action is set to Block, the FortiGate unit will block the SYN packets that exceed the threshold, but it will allow SYN packets from clients that send another SYN packet.

The tools attackers use to generate network traffic will not send a second SYN packet when a SYN+ACK response is not received from the server. These tools will not “retry.” Legitimate clients will retry when no response is received, and these retries are allowed even if they exceed the threshold with the action set to Block.

SYN proxy

FortiGate units with network acceleration hardware, whether built-in or installed in the form of an add-on module, offer a third action for the tcp_syn_flood threshold. Instead of Block and Pass, you can choose to Proxy the incomplete connections that exceed the threshold value.

When the tcp_syn_flood threshold action is set to f, incomplete TCP connections are allowed as normal as long as the configured threshold is not exceeded. If the threshold is exceeded, the FortiGate unit will intercept incoming SYN packets from clients and respond with a SYN+ACK packet. If the FortiGate unit receives an ACK response as expected, it will “replay” this exchange to the server to establish a communication session between the client and the server, and allow the communication to proceed.

Other flood types

UDP and ICMP packets can also be used for DoS attacks, though they are less common. TCP SYN packets are so effective because the target receives them and maintains a session table entry for each until they time out. Attacks using UDP or ICMP packets do not require the same level of attention from a target, rendering them less effective. The target will usually drop the offending packets immediately, closing the session.

Use the udp_flood and icmp_flood thresholds to defend against these DoS attacks.


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