The first packet sent across the network when setting up a TCP connection. For example, when an individual contacts the University of Ontario Institute of Technology’s Website at http://www.uoit.ca, the first packet that the individual’s system sends is a SYN packet to the HTTP port 80 on www.uoit.ca. The browser tells the Web server that it wants to connect.
Most packet-filtering firewalls operate by blocking SYN packets, which then cause the connections to not be initiated. An individual can still scan behind the firewalls using ACK or FIN packets, but he or she will not be able to connect to any of those machines.
See Also: HTTP (HyperText Transfer Protocol); TCP/IP or Transmission Control Protocol/ Internet Protocol.
Graham, R. Hacking Lexicon. [Online, 2001.] Robert Graham Website. http://www.linuxsecurity.com/resource_files/documentation/hacking-dict.html.