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I set up a server which is connected to my router. The router forwards a port of my server. So when I scan that port using http://www.t1shopper.com/tools/port-scan/ I got:

188.143.126.14 is responding on port 10415 ()

In other words the handshake was successful.
But when I try to ssh to that port it fails - because the SYN ACK not arrives to the client and start retransmitting the SYN. Although SYN ACK was sent out by the server. I compared the working case and the wrong case but I have not found any differences. The only difference is that SYN ACK arrives in the first case, but not in the second one.

I can reach the ssh on the subnet. There is no firewall neither on the router nor on the server.

What could be the problem?

Update: and the other difference is that the scanning was done from outside, but the ssh connection was made from the subnet using the public IP. When I used the local IP it worked. But now I tried to connect using my smarthones data - and this time it worked. So it seems that initiating the connection from inside with the public IP causing some routing issue.

bb@lenovo:~$ ssh balazs@188.143.126.14 -p 10415
ssh: connect to host 188.143.126.14 port 10415: Connection timed out
bb@lenovo:~$ ssh balazs@192.168.1.70 -p 10415
balazs@192.168.1.70's password:
laplasz
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  • Can you reach the port on the machine itself via localhost? Can you reach the port via another machine on the same subnet? Is there a port checker on the router itself you can check? Are there firewalls in place? There are a lot of unknowns here. – jski Jul 16 '14 at 21:50
  • updated - so if a firewall would set up then the scan would either work from outside i think – laplasz Jul 16 '14 at 21:54
  • Are you absolutely certain that your external firewall rule (from WAN into your LAN) is direct NAT from port 10415 to port 10415? And your sure your firewall rules aren't different for traffic from the local network as opposed to traffic from anywhere else? – jski Jul 16 '14 at 22:31
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    To connect to the WAN IP from the LAN and have the port forwarding work, your router needs to support and/or be configured for NAT-Loopback - http://en.wikipedia.org/wiki/Network_address_translation#NAT_loopback – TessellatingHeckler Jul 16 '14 at 22:51
  • @TessellatingHeckler thanks! I did not know that. I have been looking the answer for weeks. – laplasz Jul 17 '14 at 06:30

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