7.Locks HexConnections

When a normal TCP connection starts, a destination host receives a SYN (synchronize/start) packet from a source host and sends back a SYN ACK (synchronize acknowledge). The destination host must then hear an ACK (acknowledge) of the SYN ACK before the connection is established. This is referred to as the "TCP three-way handshake."

While waiting for the ACK to the SYN ACK, a connection queue of finite size on the destination host keeps track of connections waiting to be completed. This queue typically empties quickly since the ACK is expected to arrive a few milliseconds after the SYN ACK.

The TCP SYN flood attack exploits this design by having an attacking source host generate TCP SYN packets with random source addresses toward a victim host. The victim destination host sends a SYN ACK back to the random source address and adds an entry to the connection queue. Since the SYN ACK is destined for an incorrect or nonexistent host, the last part of the "three-way handshake" is never completed, and the entry remains in the connection queue until a timer expires, typically for about one minute.

By generating phony TCP SYN packets from random IP addresses at a rapid rate, it is possible to fill up the connection queue and deny TCP services such as e-mail, file Transfer, or WWW to legitimate users.

There is no easy way to trace the originator of the attack because the IP address of the source is forged.


A TCP SYN flood is an attack based on bogus TCP connection requests created with a spoofed source IP address and sent to the attacked system. Connections are not completed, thus soon it will fill up the connection request table of the attacked system, preventing it from accepting any further valid connection request.

The source host for the attack sends a SYN packet to the target host. The target host replies with a SYN/ACK back to the legitimate user of the forged IP source address.

Since the spoofed source IP address is unreachable, the attacked system will never receive the corresponding ACK packets in return, and the connection request table on the

The attacked system will soon be filled up. The attack works if the spoofed source IP address is not reachable by the attacked system. If the spoofed source IP address were reachable by the attacked system, then the legitimate owner of the source IP address would respond with an RST packet back to the target host, closing the connection and defeating the attack.

TCP SYN flood is a denial of service attack that sends a host more TCP SYN packets than the protocol implementation can handle.

This is a resource starvation DoS attack because once the connection table is full; the server cannot service legitimate requests.

TCP SYN Flood Protection

Apply Operating System fixes

Systems periodically check incomplete connection requests and randomly clear connections that have not completed a three-way handshake. This will reduce the likelihood of a complete block due to a successful SYN attack and allow legitimate client connections to proceed.

  • Configure TCP SYN traffic rate limiting
  • Install IDS (Intrusion Detection Systems) capable of detecting TCP SYN flood attacks

Filter Network Traffic

Use circuit-level firewalls (stateful inspection) to monitor the handshake of each new connection and maintain the state of established TCP connections. The filtering system must be able to distinguish harmful uses of a network service from legitimate uses.

Static packet filtering (stateless) does not protect from TCP SYN flood attacks.