internal and external DNS

Posted on 2002-05-02
Last Modified: 2010-03-18
i am using red hat ipchains to set up a firewall. Since the internal machines need to use internal internet service, i have set up a internal DNS
for local network and a external DNS for outside network. Both are using linux red hat. It works
fine. Both outside and inside machines can access my internal internet service using domain name.

But if a new internet service machine is added outside the firewall, i need to configure both external and internal DNS since if I don't add
it to the internal DNS, the machines inside the firewall can't query the IP of the new machine.
I feel very troublesome every time the IP is changed or a new machine is added outside firewall. I need to change both DNS.

Is there anyway that if a domain name can't be queried by the internal DNS, the request can be forwarded to external DNS automatically? If so, i can just change the external DNS. Also, internal machines can access the both internet service inside and outside the firewall without changing the config of internal DNS

or any other suggestions?

Question by:Torus
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Expert Comment

ID: 6987416
Not if you use the same domain name inside and outside of the firewall. Each of the DNS servers believe that they are authoritative for the domain and thus won't forward a request for that domain elsewhere. In a like manner there isn't a client side solution either. A client could have both DNS servers configured, but the first server that it uses has to have data for the inside and outside hosts. If there's no A, CNAME, or PTR record the DNS server will return a 'no data error' and the client won't ask the other server. The only time a second DNS server would be used would be when the first server can't be contacted.

Expert Comment

ID: 6991989
When you create a split horizon (same names, different IPs)for internal and external DNS, you will have to manage the entirety of those domains on both DNS servers.  The best way to manage a split horizon to create a subdomain for the machines that will have different ip's (like  That way you only have to manage a smaller piece of the domain (just the machines with different ip's) on both DNS servers.  Then let your external DNS servers resolve for the rest.

Author Comment

ID: 6997949
Sorry, I don't quite understand how will work.
Can you give me an example or more in detail.

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Expert Comment

ID: 6997968
That way that works is that you have your outside DNS servers configured to be your domain (mydomain.tld) and all outside servers are configured to be in that domain. The outside servers are configured to delegate authority for a sub-domain (inside.mydom.tld) to your inside DNS servers and of course all machines on the inside of the firewall are configured to be in the sub domain. So when an inside host needs to access an outside server it'll ask the inside DNS server, but since those know that they are only authoritative for a sub-domain they'll pass the request up to the outside servers.

Now this works if your Internet accessible servers are all located outside of the firewall. It becomes problematical if your servers are actually inside the firewall with Internet requests being port forwarded or passed through the firewall via static NAT translations. In particular web servers are a problem, especially when using name based virtual hosts. URL's are, of course, hostname sensitive. So if you have aweb server inside of the firewall it effectively has to properly respond to two names, e.g., web.mydomain.tld and web.inside.mydom.tld. SSH servers are also a problem for a similar reason.

Author Comment

ID: 7005101
Well, i think it is not applicable in my situation to set a subdomain internally and domain outside. I think I need
to set to access the services in the same name no matter inside or outside firewall.

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Accepted Solution

jlevie earned 100 total points
ID: 7005163
If you can't use a subdomain on the inside then you have to maintain each of the DNS namespaces like you've been doing. Using a subdomain is the only way you can have the inside DNS forward requests for the external machines.

Author Comment

ID: 7010552

Expert Comment

ID: 7017360
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Expert Comment

ID: 9082052
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Expert Comment

ID: 10004214
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