Since I enabled comments in this blog, I finally needed to configure a split DNS for my network.

There are various reasons why one needs a split DNS and as it's usually pointed out, the reasons are usually non-technical. In my case the reasons are technical: I have a NAT in my local network that allows me to host this website locally. What causes problems is that the domain name ressukka.net points to the external IP address and that doesn't work from the inside. So split DNS it is.

There are various ways of building a split DNS, one can use the views feature in bind9 or you can set up 2 separate DNS servers that provide different information (and redirect your local resolver to use the internal server). The latter is more secure if the internal zone is sensitive.

I decided to use a hybrid solution. I already knew that PowerDNS Recursor was capable of serving authoritative zones (think pre-cached) so I decided to leverage on that. Setting this up turned out to be simpler than I expected.

First I made a copy of the existing zone and edited it to fit my needs. I changed the IP address of ressukka.net to point to the IP address on the local network. I also adjusted some other entries that pointed to the local network.

Next I modified bind to listen on the external IP address. This can be accomplished by adding a listen-on { 1.2.3.4; }; to the options in the configuration. I also disabled the resolver by adding recursion no;, this forces the bind to work as authoritative only.

Then I installed the PowerDNS Recursor (pdns-recursor package in debian) and configured it to listen on the internal address only (local-address=10.0.0.1) and added the pre-cached zone to the configuration with auth-zones=ressukka.net=/path/to/internal-zone

Now, after restarting both daemons, I had a working split DNS with minimal configuration. I was also able to change the external DNS to authoritative only mode, which is a good idea in any case.

Firewall?
Would it not be easier to configure your NAT properly, to actually have direct access to the external IP from an internal host? Usually the problem is forgetting to add a following rule:
iptables -t nat -A POSTROUTING -d internal_network -j SNAT --to-source internal_ip_of_the_NAT_box
Comment by Aigars Mahinovs Tue Mar 24 02:09:38 2009
Double nat and more

It would be a lot easier if I could just do that and configure the firewall to transparently manage the connections. But due to the wonderful world that is IPv4 I can't.

The firewall I was configuring doesn't actually have the public IP address either. This is again due to technical reasons. If I wanted to configure the firewall to transparently route the traffic back it would first travel through my firewall, then over a slow link to another firewall that would do the NAT then come back the slow link and pass my firewall landing to the host in question. Then the response would do the same trip just to land back to my network. That doesn't sound like too much fun does it? :(

The only solution I was able to come up was to have something similar to Cisco ASA and do a DNS rewrite. This would require me to move the dns to the internal network, but at least I would have a single DNS server.

Comment by ressukka.net Tue Mar 24 06:54:17 2009
Other solution

I use dnsmasq to achieve something similar.

dnsmasq is a recursor+. I can just specify the hostnames that are different, so I don't have to override a whole zone.

Comment by Anonymous Tue Mar 24 17:01:32 2009
Very nice resource
Hello! Very Interesting post! Thank you for such interesting resource! PS: Sorry for my bad english, I'v just started to learn this language ;) See you! Your, Raiul Baztepo
Comment by RaiulBaztepo Sun Mar 29 04:38:24 2009
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