NAT issue on ADSL router

Unanswered Question
Jun 8th, 2008
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Dear friends,

A small issue on NAT'ing on a simple network:

The scenario is as follows:

1. Two inside networks viz. and

2. The first inside network is behind ASA firewall

3. The second inside network is behind ADSL Router.

4. The logical flow is <Inside network>--->ASA Inside----->ASA Outside---> network-->ADSL Router-->outside world

5. There is no nating on ASA. All NAT'ing is done only on ADSL router

Problem: when i do a nslookup from for the hostname for, it is giving me the Dialer or public IP address for instead of giving itself. This name resolution is causing a problem because network users are not able to use the services hosted by

The Router is 2811 router and the firewall is ASA 5520

The problem described above vanishes when the server is statically nat'ed to Dialer interface since this server is hosting ftp, smtp services. Once this is done, DNS lookup for the hostname fetches the expected result Otherwise, the public address is fetched which is not desirable when the client resides within the network.

Is it possible that dns lookup requests for hostname result in the internal ip address being fetched (something like DNS doctoring) without modifying the hosts file in all machines?

Both the networks network and network are connected to inside of ADSL router.

Looking forward to your kind suggestions on this.

Thanks a lot


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gautamzone Sun, 06/08/2008 - 11:47
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Dear Thot,

The NAT device is IOS Router. Is there any DNS doctoring that an IOS router can do?

paolo bevilacqua Mon, 06/09/2008 - 05:48
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Yes, you can try the internal DNS server.

"ip dns server". If you have "ip hosts"configured, the Ip for them should be returned, for all others, a quesry is generated and the reply forwarded.


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