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I have 3 machines:

  • Machine A - local network
  • Machine B - local network
  • Machine C - external network

Local network has a dynamic IP and Dyndns is set up to forward it.

I have ssh on Machine B listening on port 22. Router is set to forward port 22 to Machine B. All works fine:

  • Machine A can ssh to Machine B
  • Machine C can ssh to Machine B

Next I set up ssh on Machine B listening to port 6234. Router is set to forward 6234 to Machine B.

  • Machine A can ssh to Machine B -p 6234
  • Machine C gets "Connection timed out"

WhatsMyIp.org port scanner reports port 6234 is open.

Note: the same thing happens using httpd - port 80 works fine, but other ports do not.

So, how can I find out what is blocking the port?

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So.. the whatsmyip portscanner can access everything. What does running nmap -p6234 machineb.dyndns.org tell you when you run it on machine C? If the port is not displayed as open, machine C is probably getting firewalled. – Paul Hänsch Oct 29 '12 at 19:53
nmap -p6234 bignall.dyndns.org -P0 Starting Nmap 4.11 ( insecure.org/nmap ) at 2012-10-30 07:32 CDT Interesting ports on xx-xx-xxx-xxx.client.mchsi.com (xx.xx.xxx.xxx): PORT STATE SERVICE 6234/tcp filtered unknown Nmap finished: 1 IP address (1 host up) scanned in 12.222 seconds – Rosina Oct 30 '12 at 12:36
Well I tried to get a pre/code formatted version but seems it isn't working correctly ;). At any rate, nmap says the port is filtered. Any suggestions on finding out who is filtering it? My first guess would be my ISP. – Rosina Oct 30 '12 at 12:40

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