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Jim Carter's Bugfixes

Debugging XDMCP X-Windows Service

James F. Carter
2011-08-26

XDMCP is the protocol by which an X-server asks an execution server to put a greeter on its screen and then to run a session for the authenticated user. When there's an issue with XDMCP it's always a struggle to debug it from home through multiple firewalls. Here's the procedure I worked out.

In the examples the execution server is called remexe (for remote execution). The machine where you are working is referred to as localhost. RFC 1918 addresses of 192.168.1.1 and 192.168.1.2 are used for the tunnel, but you could substitute any suitable pair of addresses. Display :2 is used but if this is occupied on your machine you could pick any unused display number. The firewall (if any) on the local and remote hosts have to be jiggered to allow ports 177/udp (for XDMCP) and 6002/tcp (for display :2) to be passed by the tunnel. Firewalls on intermediate router(s) only need to let ssh through, port 22/tcp.


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