NAT help

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May 29th, 2007
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I set up a 831 router for a client that does web developement and runs a email server on their internal network. They have a static IP address on the outside interface of the router and everything on the inside is on the network. I have setup nat overloading of the outside interface with static nat mappings to foward smtp and webmail to the server for the external clients and all this works fine. The problem is that they also access the server by its FQDN from the clients on the internal network. This does not work. It seems that since the traffic is not coming in on the outside interface the router is not redirecting it. I have read the "NAT on a Stick" article as suggested on another thread and do not see how it will help. I have also seen a bunch of discussion on this topic with no clear-cut solution. Has anyone got it to work correctly? Is really agrravating that the Sonicwall and Netgear routers that were in place before the Cisco were able to do this with ease, but I'm having so many issues with the Cisco.

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Overall Rating: 3 (1 ratings)
Collin Clark Tue, 05/29/2007 - 12:32
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The problem is hairpin routing with the firewall. The easiest way to fix it is to change your internal DNS/host file. On the inside, instead of having pointing to the public address, have it point to the private address.

HTH and please rate.

joeatcheckpoint Tue, 05/29/2007 - 13:58
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ceclark, Thanks for the input I have thought of that but the fact that I don't manage the network, they have some MACs (I have no experience with them), and some stuff like the webmail is on a nonstandard port that is redirected to the mail server by a external web server. The internal users have been use to typing in to get to their web mail. The external web server then changes that to which sends it to the internal mail server. I don't know of anyway to redirect a port using internal DNS or a host file. I guess the biggest problem is the "It worked with the old router".

a.cruea1980 Thu, 06/07/2007 - 12:17
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The first thing I'd check is that the thing is redirecting to a port. . .if it is, you can set up a route-map I believe. Like, let's say your clients try to hit webmail and it gets redirected to port 8080. . .your route-map should allow for an access-list that you can match traffic with and do that.

It's a hack, but it just might work provided your clients ARE getting redirected. You should be able to then tell the route-map for that port to go to a certain server instead of going out the interface.

It sounds like the same problem you get when you try to NAT a server with a VPN'd connection. . .you'll probably have to deny the traffic on that port from trying to go out the interface so your route-map can hit it.


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