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NAT Loopback - How To

Posted on 2014-02-09
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Last Modified: 2014-02-21
I have a web server running on my LAN and have setup port forwarding etc. on my router so it can be accessed from the outside world using a valid URL: for example http://www.myexternaladdress.com:xxxx/mypage.html

Everything from outside works great, but I cannot access the server when on my LAN using the external address.

I know I can use the LAN address but I want to be able to use the one address from wherever I am, I think this is called NAT loopback but I'm no expert here.

How do I divert LAN traffic for the external web address to the internal web address?

I'm using a Comtrend AR5381u Router

Thanks
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Question by:BrianFord
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Expert Comment

by:Dan Craciun
ID: 39845814
That is called hairpin NAT, from what I know.

Does your router support CLI? It uses iptables?

From what consumer routers I've seen, if it does not support hairpin NAT out-of-the-box, your only chance is to modify it via CLI or a firmware update.

HTH,
Dan
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Expert Comment

by:Dan Craciun
ID: 39845829
If you can access and modify iptables, here's a thread on how to create a rule for hairpin:
http://superuser.com/questions/663820/port-forwarding-from-inner-network-to-inner-network-hairpin-nat

Basically, you add a rule to your firewall to route internal traffic on port 80 to 192.168.1.2 (change with the IP of your server)
/ip firewall nat
add chain=dstnat dst-address-type=local protocol=tcp dst-port=80 \
  action=dst-nat to-address=192.168.1.2 to-port=80

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and then add the hairpin:
/ip firewall nat
add chain=srcnat src-address=192.168.1.0/24 \
  dst-address=192.168.1.2 protocol=tcp dst-port=80 \
  out-interface=bridge-local action=masquerade

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HTH,
Dan
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Author Comment

by:BrianFord
ID: 39845866
not sure I can access the iptables, this is an isp provided router, sounds like I may to just use different URL's :-(
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Expert Comment

by:Jelcin
ID: 39845896
You could also edit the hosts file in your windows. There you can just type that the external adress should be mapped to the internal...

http://www.rackspace.com/knowledge_center/article/how-do-i-modify-my-hosts-file
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Author Comment

by:BrianFord
ID: 39845902
I thought about that but I'm building iOS app that needs to access a web service running on my web server so that's why I was trying to access using a single address and not  2
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Expert Comment

by:pergr
ID: 39846719
It might be easier to configure your DNS server to give the private address when the request comes from the inside.
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Author Comment

by:BrianFord
ID: 39876925
That's what I'd like to do but not sure how to do that, unfortunately not much in the manual on how to set that up,  I'm an ASUS RT-68U

Guess I'll be surfing Google again later today :)
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pergr earned 500 total points
ID: 39877721
Perhaps you should run a dns server too on the same machine as the Web server.
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Author Comment

by:BrianFord
ID: 39878504
I think I'm going  set that up, I'm not DNS expert, do have any pointers for setting up the hairpin on Windows Server 2003?
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Author Closing Comment

by:BrianFord
ID: 39878516
Thanks for the advice, I setup a DNS server on the web server and after a quick Google search was able to implement the hairpin

Thank You
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