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I have a server that is set up to listen to a specific port (1433), and normally there is no problem accessing that port from anywhere. From one of the locations I work, however, I can't connect to that port specifically. No problems with other ports like 80, 443 or 22, just that one.

I suspect the ISP or the local router (to which I have no easy access), but in order to troubleshoot I'd like to pinpoint the culprit. Is there a tool that could help me see where in the route the port is blocked, kind of like traceroute but port-specific?

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up vote 1 down vote accepted

In the *NIX world there is tcptraceroute:

sudo tcptraceroute <host/ip> <tcpport>

I'm going to guess, based on the fact that you mention port 1433 which is commonly used by MS SQL, that you're using Windows.

There's an app for Windows called tracetcp which appears to do the same, but I haven't used it myself.

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I'm running both Windows and Linux clients, but the service runs on an Ubuntu server and it's not related to MSSQL. Thanks for the tip, I'll test it when I get back there later today. – Zano Jun 23 '10 at 12:41
Has nothing to do with redhat linux. – Denys S. Jun 9 '11 at 15:20

If you can connect to the server from anywhere but at location A, it's probably location A's firewall or their service provider. If it's that location's firewall you should be able to tell by talking to the site admin or looking at the firewall rules, or you can see if you can connect a system on that network on the other side of the firewall and see if the connection works.

If not, then you have it at their ISP, most likely, and you can contact their network op center and ask them about it. If it's a commercial connection they should be willing to work with you. If not, you may have to look at a VPN solution or a proxy solution or change the port your system is listening to.

You might be able to test if you have more than one location that can listen to that port using nmap to see if that port is coming up as blocked no matter where you go, but if you can just remove the router/firewall from the equation and try it directly from the site without interference that should tell you if it's the ISP or your site's router in particular.

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