NAT issue on ADSL router

Unanswered Question
Jun 8th, 2008

Dear friends,

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

The scenario is as follows:

1. Two inside networks viz. 172.16.0.0/24 and 192.168.1.0/24.

2. The first inside network 172.16.0.0/24 is behind ASA firewall

3. The second inside network 192.168.1.0/24 is behind ADSL Router.

4. The logical flow is <Inside 172.16.0.0 network>--->ASA Inside----->ASA Outside--->192.168.1.0/24 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 192.168.1.0/24 for the hostname for 172.16.0.2, it is giving me the Dialer or public IP address for 172.16.0.2 instead of giving 172.16.0.2 itself. This name resolution is causing a problem because 192.168.1.0 network users are not able to use the services hosted by 172.16.0.2

The Router is 2811 router and the firewall is ASA 5520

The problem described above vanishes when the server 172.16.0.2 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 172.16.0.2. 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 xyz.com result in the internal ip address being fetched (something like DNS doctoring) without modifying the hosts file in all 192.168.1.0/24 machines?

Both the networks 192.168.1.0/24 network and 172.16.0.0/24 network are connected to inside of ADSL router.

Looking forward to your kind suggestions on this.

Thanks a lot

Gautam

I have this problem too.
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gautamzone Sun, 06/08/2008 - 11:47

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

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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