I am new to this so apologies if this is the wrong place for such a question.

I'm testing out a simple "hello world" website hosted at home. As we have not been assigned a static IP address by our ISP I've set up a subdomain with a dynamic DNS provider to point at the home router - let's call this subdomain mysubdomain.ddnsprovider.net. I also have a domain which I own (and which was set up with a different provider than that to the dynamic DNS subdomain) - let's call that mymaindomain.com. A DNS CNAME record for mymaindomain.com is set up to point at mysubdomain.ddnsprovider.net.

I can successfully browse to mymaindomain.com and see my "hello world" page from anywhere outside of my home LAN - but if I browse to mymaindomain.com from any machine at home (i.e. within the LAN) I get "Server not found"/"web page not available" etc errors.

I understand from various searches that this looks like a NAT loopback issue and as such is likely best solved by getting a new router that supports NAT loopback. However, I can see my "hello world" site if I browse to mysubdomain.ddnsprovider.net from anywhere at all - including from within my LAN. From what (little) I understand about NAT loopback, I shouldn't be able to connect to a site hosted within my LAN via an external address (i.e. mysubdomain.ddnsprovider.net) at all if lack of support for NAT loopback by my router is the cause of the issue.

So, before I go out and buy a new router, my question is:

Is this definitely a NAT loopback issue (how can I confirm that)? And if so, what's the reason I can see my website from within the LAN via mysubdomain.ddnsprovider.net? If it isn't NAT loopback what is it likely to be and what's the best solution?

FWIW - I'm in the UK and am using a BT home hub 3 wireless router connected to a BT Openreach modem (Infinity fibre). Does anyone know whether the lack of NAT loopback support is inherent in the router, or modem, or both? - i.e. is replacing just the router likely to solve the problem or is it likely to persist unless I replace both of them (if that's possible).

Any light that anyone can shed on this and any advice would be very gratefully received.

closed as off-topic by Ron Maupin, Craig Constantine Oct 12 '15 at 10:55

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "NE is a site for to ask and provide answers about professionally managed networks in a business environment. Your question falls outside the areas our community decided are on topic. Please visit the help center for more details. If you disagree with this closure, please ask on Network Engineering Meta." – Ron Maupin, Craig Constantine
If this question can be reworded to fit the rules in the help center, please edit the question.

  • Questions about home networking, and consumer-grade equipment are both specifically off-topic on this forum. Please see the Help Center for which types of questions are allowed, and which types are not. You can try on Super User. – Ron Maupin Oct 11 '15 at 20:18
  • @Ron Maupin - I think the question is OK - NAT hairpining is also an issue in the enterprise space.. – kaisero Oct 11 '15 at 20:41
  • @kaisero, the question specifically mentions that it is a home network and the consumer-grade equipment. Both of those conditions are specifically off-topic. Questions reworded to fit the requirements ore OK, but, as it stands, the question is off-topic. If you disagree, you may take it up in Meta, but the question has already been asked in Super User. – Ron Maupin Oct 11 '15 at 21:05

You have diagnosed the problem correctly. The problem is that you try to access your webserver which is located within your local network by its public ip address.

Your router will not NAT your DST ip address back to your private ip address since traffic is originating from within your network. To work around this issue you would need NAT hairpining also called NAT loopback as you stated above already.

To verifiy why you can access your webserver with the URL mysubdomain.ddnsprovider.net could you please do a nslookup via Windows Powershell/CMD and post the result.

If both your domain and ddnsprovider.net ip address matches the problem might not be related to NAT hairpining.

  • Thanks kaisero - I get the following (redacted) from nslookup: $ nslookup mysubdomain.ddnsprovider.net Server: Address: Non-authoritative answer: Name: mysubdomain.ddnsprovider.net Address: xx.xx.xxx.xxx <- correct IP – rfrank Oct 11 '15 at 20:47
  • by correct IP you mean your public IP address? – kaisero Oct 12 '15 at 5:53

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