Publishing an email server behind a NAT behind a NAT

Posted on 2010-09-18
Last Modified: 2013-11-16
Hello experts,

I have an internet connection over ADSL through a Linksys SOHO router. The external IP address is dynamic. The linksys internal IP address is

Behind Linksys router, there is an ISA server with two LAN interfaces. The external interface is connected to the Linksys router with IP address The internal interface is connected to the office network and has an IP address

There are many services in the internal network which should be published to the Internet (Like SMTP 25, IAX2 4569, RDP 3389....)

The linksys router doesn't accept port forwarding to any destination outside it's local network (

I have tried to forward some port to the ISA, and then republish the desired port through ISA, but it didn't work.

Can anybody give me a solution when there are two NAT firewalls behind each other?
Question by:Muhajreen
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LVL 16

Expert Comment

by:R. Andrew Koffron
ID: 33708465
Why not make the ISA server the DMZ target for the linksys (just forward everything there). than make Publishing rules on the ISA server to hit your internal service providers?

But, if your getting dynamic IP from your ISP how is the incoming traffic getting to the linksys? dynDNS?

Author Comment

ID: 33708542
I am sorry for the mistake, I am using NetGear router, not Linksys.

In NetGear, there is no DMZ option, but there is an option of forwarding ALL traffic ports. I have tried it, but didn't succeed.

I am using CustomDNS service by
LVL 16

Expert Comment

by:R. Andrew Koffron
ID: 33708606
do you have a laptop you can put in the place of the ISA server's external NIC, and test the port forwarding?  enable remote and try an open RDP from an outside network?
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LVL 16

Expert Comment

by:R. Andrew Koffron
ID: 33708614
you could also try forwarding all the ports to your mail server on a very temperary basis to see of the problem is the ISA server or the router.

Author Comment

ID: 33708620
RDP and other services are working from external, but all traffic is forwarded to ISA server external IP It's not being forwarded by ISA itself to the network
LVL 16

Accepted Solution

R. Andrew Koffron earned 500 total points
ID: 33708627
did you add a network for NAT from to ?

Author Comment

ID: 33708669
Thank you !! I have really forgotton to add a NAT rule from to (I think you meant so, not from to

Thank you

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