XFree86, as any X-server, uses TCP ports 6000 and above to listen to, waiting for incoming connections. Any user can choose his display number simply by starting "X :0" or "X :2500" or "X :any_display". The X server automatically chooses its port by adding the display number to 6000. But as the ports are 16-bits coded, port 65536 equals 0, so displays 59536 to 65535 generate listening sockets on ports 0 to 5999. And as the X-server runs suid root, any user can use it to block known ports before a daemon starts using it. For example, it would be possible to use display 59556 = port 20 to prevent ftp server from transfering data with remote systems. It is even possible to run a server on any port <= 1023 to disable local rlogin/rsh from the local host. I have only tested this on XFree86 release 3.3 for Linux ELF, but I think many other X servers running suid root have the same hole. I personaly use a display :65290 on a Sparc under SunOS 4.1.4, which equals port 5754, but as the X server on this system doesn't run suid root, it is impossible to get use of ports 0-1023. Last thing: simple method to convert display number to port number: port = (display + 6000) & 0xFFFF = (display + 6000) if display < 59536 = (display - 59536) if display >= 59536 and now, port to display: display = (port + 59536) & 0xFFFF = (port + 59536) if port < 6000 = (port - 6000) if port >= 6000