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Re: XFREE86 can block reserved ports
From: abelits () PHOBOS ILLTEL DENVER CO US (Alex Belits)
Date: Wed, 6 Aug 1997 08:35:25 -0700

On Wed, 6 Aug 1997, Willy TARREAU wrote:

Hello, and sorry if it is already known stuff.

   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.

This is one more reason to remove setuid bit from X server. xdm starts
local X server just fine.

It is even possible to run a server on any port <= 1023
to disable local rlogin/rsh from the local host.

Considering the level of security provided by checking outgoing port
number, creating trouble for the use of this feature can be considered a
security enhancement ;-)


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