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I use my next door WiFi for a small charge as I am unable to install my own broadband connection. I have a 300Mbps WISP router connected with a fixed IP to my next doors router (192.168.0.2).

My WISP router has its own firewall, so was wondering if it would be safe to place my router on the neighbour's DMZ, and would this cause them any issues. I want to do this because I want to be able to access services on 'my network' from the Internet.

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Yes that would be fine. However what ever ports you expose on your router your neighbor will be able to access as well. To minimize risks you could block access to that port from your neighbors ip range and also your external ip. This would minimize risks of them discovering your services however would not stop them from connecting from another ip or preforming a port scan from a remote host. To clarify things setting up a dmz will expose your router to the Internet the computers behind your router will be fine unless you have ports forwarded. For further questions I would go to Super User forum as your question only just makes it on the on-topic list for this forum

  • You'd have to block access from your neighbor's IP range and block access from (and track) your own public IP. I'm pretty sure there's really no good way to do that. – Iszi Jun 11 '15 at 16:40
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Regarding the question around creating issues for your neighbour, it will depend heavily on how the DMZ feature is implemented on your neighbour's network.

One classical DMZ model uses public IP addresses, one per DMZ host, sometimes via NAT, sometimes directly. Another approach uses private address space and port forwards, either forwarding all ports to one host with overrides where a different host requires a specific port.

In your case, on the (assumedly) residential router used by your neighbour, it's likely that it has a single public IP and DMZ mode exposes all ports (possibly filtered down to a smaller list) on that IP to a single internal address assigned to the "DMZ Host".

The impact of all this on your neighbour if you set the external address of your router as the DMZ host on their router is that they won't be able to do any port forwards to equipment on their internal network.

If their router is more sophisticated and allows for additional forwards to other hosts for specific ports, then it will only be an issue if you are wanting to forward the same port they that want.

You'll also need to consider the effects of UPNP here too, if your neighbour is running apps that instruct their router to temporarily listen on a given port and forward traffic back to them, it will either fail or will interfere with your forwards as your router won't be aware of these and likely will not be forwarding UPNP information on.

Another option available to you if DMZ is not possible, is to run a VPN service and forward just that port on your neighbour's router. Alternatively forward ssh, then tunnel everything else you'll require.

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