What is the { SYN, ACK, FIN, RST } bit?
See the FAQ article Debugging TCP/IP. 4.13 – Is it a bad idea to bind() to a particular port in a client program? It’s occasionally justifiable, but most of the time it’s a very bad idea. I’ve only heard of two good uses of this feature. The first is when your program needs to bind to a port in a particular range. Some implementations of the Berkeley “r commands” (e.g. rlogin, rsh, rcp, etc.) do this for security purposes. Because only the superuser on a Unix system can bind to a low-numbered port (1-1023), such an r command tries, sequentially, to bind to one of the ports in this range until it succeeds. This allows the remote server to surmise that if the connection is coming from a low-numbered port, the remote user must be a superuser. (This port range limit also applies on Windows NT derivatives, but not on Windows 95 derivatives.) The second justifiable example is FTP in its “active” mode: the client binds to a random port and then tells the server to connect to that port for the
