I tried to open a remote XDMCP connection through Knoppix (as root) but it always fails as follows:

AUDIT: Tue Jan 21 02:54:58 2003: 4157 X: client 1 rejected from IP 192.168.1.1 port 36014
Auth name: MIT-MAGIC-COOKIE-1 ID: -1
AUDIT: Tue Jan 21 02:55:13 2003: 4157 X: client 1 rejected from IP 192.168.1.1 port 36015
Auth name: MIT-MAGIC-COOKIE-1 ID: -1
AUDIT: Tue Jan 21 02:55:28 2003: 4157 X: client 1 rejected from IP 192.168.1.1 port 36017
Auth name: MIT-MAGIC-COOKIE-1 ID: -1
AUDIT: Tue Jan 21 02:55:43 2003: 4157 X: client 1 rejected from IP 192.168.1.1 port 36022
Auth name: MIT-MAGIC-COOKIE-1 ID: -1
AUDIT: Tue Jan 21 02:55:58 2003: 4157 X: client 1 rejected from IP 192.168.1.1 port 36046
Auth name: MIT-MAGIC-COOKIE-1 ID: -1

Fatal server error:
XDMCP fatal error: Session failed Session 312096010 failed for display 192.168.1.137:1: cannot open display