How should I configure internal and external DNS servers

[Verne Britton]

you haven’t mentioned your firewall or router config between the private corporate network and the public internet (or I missed it).

Cisco firewalls and I bet others too, have a very interesting and powerful capability – to examine and edit/change packet data (payload data) on the fly in real-time --- that is, when the firewall sees a dns lookup/resolver response coming in from the outside, and notices the internal dns ‘answer’ is for an IP address that is mapped/translated in the firewall to a 10. address (just an example), the firewall will edit that payload and replace the public IP with the 10. address so the internal PC (or helper/resolver internal server) that asked for the dns lookup will receive the correct 10. answer – to avoid NAT hairpinning (?) -- or so I’m told by my co-workers  😊

what this means, if your equipment has this feature, is internal lookups can go to your public dns server and will receive the appropriate internal answer while outside lookups for the same entry will receive a public IP answer – thus you don’t need a duplicate zone internally just to hand out 10. addresses for internal servers/equipment for use by internal PCs.  Did you mention you have internal entries for internal use that don’t exist in the public zone --- if so then decide if exposing just the internal name and its matching 10. address to the public is an issue – note that external users, assuming they can guess that secret internal name, will only learn the 10. address but hopefully your routers and firewalls, as well as the outside user’s equipment, will never allow an outside user to connect into your internal network using a 10. address for that connection.  That is, its possible there is no harm in having your public zone containing the secret name and its 10. address.

Also bind supports ACLs on many items (I forget which one for resolution exactly) so if you want, you can have your public dns servers be used as resolvers by all your internal 10. PCs while not allowing resolution by any outside aka public connections (that is, only serving its authoritative zones to public queries).  Not sure if your optional goal was to eliminate any internal dns server of any type. And as others have said I believe, if you do wish to use bind internally as just a resolver, that’s very easy to set up – plus it lets you not have to learn and manage two different dns packages.


Verne Britton

From: bind-users <bind-users-bounces at lists.isc.org> On Behalf Of Nick Howitt via bind-users [*]
Sent: Saturday, November 4, 2023 3:42 PM
To: bind-users at lists.isc.org
Subject: Re: How should I configure internal and external DNS servers

Thanks for the reply. Interesting.
Option A - It works but I would like to stop maintaining two different servers with the same data.
Option B - I have no chance of getting the company to agree to IPv6.
Option C - From your summary, does not appear to remove the requirement to maintain the data twice
Option D - No chance of re-zoning internally. It would be a long term project like IPv6.
Option E - Agreed. Does not appear to simplify anything
Option F - Looks really interesting. I'll investigate further
Option G - Yes it would be trivial with DNSMasq internally. I don't think I have any chance of pushing this through. Also DNSMasq does not support replication (but it could be scripted). I could look for other solutions but I doubt I would get anywhere in the company.

I'll spend some time investigating option F, thanks.

Nick
On 04/11/2023 02:03, Nick Tait via bind-users wrote:

Hi Nick.

Your current set-up sounds like a fairly common configuration. And depending on your requirements there are a number of options that you might consider.

But let's start with requirements: I've made some assumptions - please advise if I've got any of this wrong?:

  *   You have two distinct sets of authoritative servers, which don't overlap in any way currently. E.g. Servers A (primary/master), B & C (secondaries/slaves) are authoritative for internal zone ("Bind-internal"); Servers C (primary), D & E (secondaries) are authoritative for external zone ("Bind-external").
  *   The records in Bind-external are a subset of those in Bind-internal. In other words, for every resource record (not including SOA & NS records) in Bind-external, there is an identical record in Bind-internal.
  *   Do you have another set of servers that act as recursive resolvers in your network currently, or do A, B and/or C fulfil that role currently? (I'm going to assume that A, B & C are used as recursive resolvers on your internal network for now. It probably doesn't make a huge difference either way but it is just an extra factor that needs to be taken into account.)
  *   You are not using DNSSEC to sign your zones.
  *   Your zone structure is more-or-less flat currently. i.e. You don't have any delegations to sub-zones.
  *   Your primary reason for having separate authoritative servers is for privacy, rather than simply being a workaround for IPv4 Network Address Translation.

There are a few options worth considering, and I should point out that some of these won't fit your requirements, in which case you can immediately rule them out. But I believe it is important that the decision to rule them out is a conscious one, so you are fully aware of the scope/limitations of the solution you end up choosing.

Option A: Keep using separate sets of authoritative servers

What you have currently is not a bad configuration. Sure, there is additional overhead of having to maintain two separate versions of the zone, but it is easy to understand and troubleshoot. If your zones are small and are updated infrequently, then this is probably the best solution. However the fact you are looking for a better solution suggests this isn't the case...

Option B: Merge the authoritative zones and use IPv6 exclusively for internal hosts

I only included this because the idea had been put forward already. But even if the logistics of assigning public IPv6 addresses to your internal hosts was palatable to you, you'd also want to think about whether you are comfortable making that information (i.e. the IPv6 addresses used for internal servers) publicly available? I think most organisations wouldn't want to do that?

Option C: Merge servers but use views to serve separate (existing) zone files

If your goal was consolidation of servers while keeping the existing internal and external zones separate, then this might be worth looking at. But you haven't mentioned consolidation as a requirement so I'm going to skip over this one. Also it doesn't solve the problem of having multiple zones to maintain.

Option D: Simple delegation

Depending on whether there is opportunity to do some zone refactoring, you might consider something like this...

  *   In Bind-external, create a new zone: internal.example.com
  *   Use permissions (e.g. allow-query) to limit access to internal.example.com to only internal clients
  *   For each zone record in Bind-internal that doesn't exist in Bind-external, create a CNAME record in Bind-external that points to the same name in internal.example.com zone.
  *   You can then get rid of Bind-internal zone. (The servers could still be used as recursive resolvers though.)

Then, if x.example.com was a name that was previously defined only in Bind-internal:

  *   Internally if you attempt to resolve x.example.com, the result will be a CNAME that points to x.internal.example.com, which resolves to the 10.x.x.x IP address.
  *   Externally if you attempt to resolve x.example.com, the result will be a CNAME that points to x.internal.example.com, which will result in some sort of access denied error.
One possible concern with this idea is that even though an external client can't retrieve the IP address of an internal server, the CNAME + access denied error tells them that the name does still exist.

Option E: Split views and delegation

If you liked the general idea of option D, but didn't like the bit where externally attempting to resolve internal host names resulted in an access denied error, then you could look at doing something with views. However this pretty much has the same problem that you started with, where you end up maintaining two versions of the example.com zone, so I'm not going to bother going deeper into this one.

Option F: Response Policy Zones

I saved this one until last because I think this is the most interesting. If you haven't heard of Response Policy Zones (aka RPZs) before, they basically allow you to override the response to a DNS query. You could make use of this feature as follows:

  *   No changes to Bind-external.
  *   Change Bind-internal so that it isn't authoritative for example.com, but has a Response Policy Zone that contains entries for each of the names that previously only existed in Bind-internal, that returns the internal IP address.
  *   The Bind-internal servers would be used as recursive resolvers on the internal network.

Then, if x.example.com was a name that was previously defined only in Bind-internal:

  *   Internally if you attempt to resolve x.example.com, the query will be received by the Bind-internal servers, which will ask the Bind-external servers (because they are authoritative for the zone). The answer from the Bind-external server will be NXDOMAIN, but the Bind-internal server will override the result and return the 10.x.x.x IP address instead.
  *   Externally if you attempt to resolve x.example.com, the query will be received by the Bind-external servers, which will return NXDOMAIN.

By default RPZs are only used for recursive queries, and only if it won't break DNSSEC. But there are configuration options you can look at to change this behaviour.

The main draw-back I see with this option is the complexity it creates.

Option G: Use something other than BIND (e.g. DNSMasq)

...Actually, if we're considering all the options this needs to be included. It may turn out that there is an easier way to achieve your goal that doesn't use BIND.



I'm sure there are other options that I haven't thought of, but hopefully you might find these ideas useful?

Nick.


On 4/11/23 04:51, Nick Howitt via bind-users wrote:
Hi,

I am fairly new to bind but I am thinking my company's use of it is sub-optimal. We have two bind masters (and a few slaves), one for internal use so all our internal servers point to it or its slaves as their DNS resolvers. I will call the internal one bind-internal and the external one bind-external.

Bind-internal is set up as authoritative for the domain example.com.
Bind-external is also set up as authoritative for example.com.

Bind-internal has all sorts of entries resolving in the 10.30, 10.40 and other private ranges, but it also has entries resolving to our public IP's e.g. demo.example.com resolves to 1.2.3.4 (terminated by an F5), which is one of our public ips (munged). As this site is externally accessible as well, we also have to put an identical entry in bind-external so we end up having many identical entries in bind-internal and bind-external. We also have some other domains covered by bind-internal with external IPs, but externally they are covered by the domain host's DNS and they have the same issue where in bind-internal we have some public IP's which are also in the domain host's DNS for external access.

I have a feeling this is a sub-optimal setup, having to maintain external IPs in both bind-internal and bind-external. Does it make sense to stop bind-internal from being authoritative and make it a resolver/caching name server? This way, if it does not find an entry in bind-internal it will then go out to either bind-external or the domain host's DNS to get the answer from the authoritative servers and then there is no need to maintain external IPs in bind internal.

TIA,

Nick



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