We attempt to export a X11 display of a server to a remote computer. However the display does not appear on the remote host.
We need to identify where it fails.
Check the client firewall
Obvious… but so necessary….
– Check (and temporary deactivate if possible) the firewall of the client.
Check that display is well exported on the server
Yes this is evident but has to be checked first…
Verify that the client is listening
- Check on the client that MOBAXterm or Exceed or another X server is effectively listening to port 6000
- Use
netstatcommand to see if something is listening on portnetstat –ano
Traceroute from server to the client
This could show an intermediate firewall or netxork equipment that may filter the traffic
traceroute -p 6000 target_ip
Test connectivity with Nmap X11 script
Nmap provides X11 testing scripts to ease debug.
I did some tests to compare script results with 2 known working machines on the same subnet (that work) and the real server->client setup.
Results show differences. In the second attempt, Nmap indicates that X11 traffic may be filtered and X11 session cannot be started.
Attempt connection between working LAN machines:
nmap -Pn -p6000 --script x11-access target_IP_address_that_works
Starting Nmap 5.51 ( http://nmap.org ) at 2015-02-19 16:37 CET
Nmap scan report for target_host_that_works (target_IP_address_that_works)
Host is up (0.0011s latency).
PORT STATE SERVICE
6000/tcp open X11
|_x11-access: X server access is granted
Nmap done: 1 IP address (1 host up) scanned in 0.08 seconds
Attempt connection to the real client computer
nmap -Pn -p6000 --script x11-access target_IP_address
Note the filtered X11 words in result. Chances are high that a firewall is blocking our destination port.
Starting Nmap 5.51 ( http://nmap.org ) at 2015-02-19 16:37 CET
Nmap scan report for target_computer.corp (target_IP_address)
Host is up.
PORT STATE SERVICE
6000/tcp filtered X11
Nmap done: 1 IP address (1 host up) scanned in 2.08 seconds
Re-check the client firewall
Yes, yes, already said…. but the gap between theory and practice ….just redo it….