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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