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A normal connection between a user (Alice) and a server. The three-way handshake is correctly performed.

SYN Flood. The attacker (Bob) sends several packets but does not send the "ACK" back to the server. The connections are hence half-opened and eat the server resources. Alice, a legitimate user, tries to connect but the server refuses to open a connection resulting in a denial of service.

A SYN flood is a form of denial-of-service attack in which an attacker sends a succession of SYN requests to a target's system.

When a client attempts to start a TCP connection to a server, the client and server exchange a series of messages which normally runs like this:

1. The client requests a connection by sending a SYN (synchronize) message to the server.
2. The server acknowledges this request by sending SYN-ACK back to the client, which,
3. Responds with an ACK, and the connection is established.

This is called the TCP three-way handshake, and is the foundation for every connection established using the TCP protocol.

This is a well known type of attack and is generally not effective against modern networks. It works if a server allocates resources after receiving a SYN, but before it has received the ACK.

There are two methods, but both involve the server not receiving the ACK. A malicious client can skip sending this last ACK message. Or by spoofing the source IP address in the SYN, the server sends the SYN-ACK to the falsified IP address, and never receives the ACK. In both cases the server will wait for the acknowledgement for some time, as simple network congestion could also be the cause of the missing ACK.

If these half-open connections bind resources on the server, it may be possible to take up all these resources by flooding the server with SYN messages. Once all resources set aside for half-open connections are reserved, no new connections (legitimate or not) can be made, resulting in denial of service. Some systems may malfunction badly or even crash if other operating system functions are starved of resources this way.

Reflector routers can also be used as attackers, instead of client machines.

Countermeasures include SYN cookies or limiting the number of new connections from a source per timeframe.

• Official CERT advisory on SYN Attacks