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I have a U.S. Robotics router, on which there is a DMZ Host configured. DMZ Host's IP is 192.168.1.115, which is the IP of a machine I would like to open to the Internet. This machine is connected to a router through a (separate) switch and has a static IP configured. There are no open ports on the router and this configuration has worked previously.

For some strange reason, it doesn't work right now (after the server and the router have been restarted - I am not aware of any configuration changes in any of thos). When I open the public IP of the router (on both HTTP and SSH ports) I am connected to my router, not to the DMZ host, so I think the problem is related to the router configuration.

The server is a fresh Debian Jessie installation and has connection to the Internet.

How can I diagnoze it?

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  • Just to clarify, are you running a router behind your US Robotics router - double NATing?
    – Kinnectus
    Jun 11, 2015 at 9:18
  • No, there is only 1 (US Robotics) router, to which a switch is connected.
    – syntagma
    Jun 11, 2015 at 9:22
  • You are connecting to your router when you try from the outside of the network targetting the public IP, or the inside of your network targetting the public IP?
    – Paul
    Jun 11, 2015 at 10:09
  • From inside of my network targetting the public IP. Shouldn't it work the same in this case?
    – syntagma
    Jun 11, 2015 at 10:21

2 Answers 2

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It's quite common that connection to DMZ hosts, using the public IP, from machines on the internal network don't work.

This behaviuor varies based on the devices so there's no general rule.

Regarding your question on how to troubleshoot:

  1. Check from an external host: maybe there is no problem to solve, just the DMZ feature not working from inside
  2. Check that the exposed services are reachable on the INTERNAL IP
  3. Check the presence of firewall features on the router
  4. Check that the target of port forwarding has the default gateway correctly set
  5. Check that the target of port forwarding has no firewall rules preventing connections
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  • Can you explain why is this problem so common?
    – syntagma
    Jun 17, 2015 at 15:38
  • It's a bit more difficult to implement than the standard case and a lot of router firmwares simply dont' implement it. Jun 17, 2015 at 15:44
  • Did you test from outside? Jun 17, 2015 at 15:44
  • Yes, it works from outside but I still would like to understand why it doesn't work from inside of my network.
    – syntagma
    Jun 17, 2015 at 15:53
  • Search for "NAT reflection" here on stackexchange and you'll see you're not alone.. ;-) Jun 17, 2015 at 15:58
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I was trying for 3 days to get DMZ-host to work on my TP-Link Business Router. Finally the problem was found that the DMZ Host did not have the router IP address as default gateway. Pheww!!

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  • Welcome to Superuser. If what you describe is a solution to the problem, could you edit your post and list the steps you took to solve it? That would be helpful for a future visitor :)
    – Joseph
    Mar 3, 2021 at 19:07

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