Multi-master (HA)

Dave Warren davew at
Thu May 8 22:37:52 UTC 2014

On 2014-05-08 07:13, Barry S. Finkel wrote:
> On 2014-05-07 15:06, Lawrence K. Chen, P.Eng. wrote:
>>> OTOH, the idea of multi-master is intriguing.....the only down side 
>>> I see, is hat I
> >> have one really powerful server for my current master....(Sun Fire 
> X4170)....and my
> >> other servers are weak leftovers....just passed EOL last year.
> >> And, have all the servers doing full DNSSEC signing could be
> >> interesting.
>>> It also raises the question of how does the outside world cope with 
>>> all the servers
> >> having identical zones...signed on slightly different times, etc.
> >> (especially since I'm using unix timestamp for zone serial....avoids
> >> issues of multiple admins incrementing serial without
> >> noticing others and/or collisions with DNSSEC's
> >> incrementing of serials.)
> Dave Warren replied:
>> I wouldn't expect any real issues here, Windows DNS has done multimaster
>> DNS since Windows 2000. In the case of Windows, dynamic updates (via
>> client or GUI) can be done at any location, the serial numbers are
>> incremented automatically, but the zones and servers may vary from each
>> other for a brief period of time.
>> So for example, DC1 and DC2 may start with serial 100, DC1 will receive
>> 2 changes and be up to 102, DC2 will give 5 different changes and be up
>> to 105. When Active Directory synchronization happens outside of DNS,
>> the two sides merge changes together, and set the serial to the higher
>> of the two plus one, so the serial would be 106. To the outside world,
>> records can appear/disappear for a brief period while the servers drift
>> out of sync, similar to what could happen in a BIND configuration
>> without notifies as resolvers hit the two DNS servers round-robin.
>> The only thing that causes issues is if you use DNS to create a
>> non-Active Directory slave. BIND will throw errors because it will see
>> serial 100, 101, 102, then get a notify from the second server about
>> 101. However, the slave will still sync up once the AD servers sync to
>> 106. The fix here is to configure BIND to only slave off of one master
>> or the other, not both.
>> While there might be other factors involved in turning BIND into a true
>> multi-master solution, I wouldn't expect zones drifting out of sync or
>> having minor differences to be a big factor since it happens in the wild
>> already.
> As I have written before, see MS article 282826.  If one is going
> to slave an MS AD DNS server, one has to choose ONLY ONE AD DNS
> Server as a master.  As I see it, there is no way that AD can
> choose a zone serial number from among all of the AD DNS Servers.
> Assuming that a zone has the same contents and same serial number,
> say n, on all Domain Controllers.  Then, one Windows machine sends
> a DDNS update for the zone to DC1 at the same time that another Windows
> machine sends a different DDNS update for that zone to DC2.  Now,
> each DC has serial number n+1 and different contents.  When AD
> synchronizes the zone contents and serial number under the covers,
> what serial number can it choose?  It can't choose n+1, as that
> serial number has already been used.  It can't choose n+2, as it
> does not know if another DDNS for the same zone has arrived before
> the synchronization has taken place.

n+2 works fine, the situation is no worse off than it was with two 
servers each at n+1 and being slightly out of sync. At the n+2 step, the 
zones are closer to being in sync then they were. The logic that MS DNS 
uses is to always set the serial number to the highest seen anywhere +1 
and it works very well internally.

Even if you don't follow the advice in 282826, it actually works 
surprisingly well; as AD syncs up (which tends to happen very quickly 
for DNS servers in the same site, slower with intra-site replication), 
the changes merge together, the serial increments and BIND gets the 
latest zone. You obviously have to use AXFR rather than IFXR, and you 
have to accept that newly added records will appear and disappear from 
the BIND zone when/if BIND flips between AD masters, but the effects are 
understandable and manageable.

(I'm not advocating slaving off of multiple AD masters, I agree 
completely with 282826 -- it's a dumb idea. But I've seen it done and if 
you ignore BIND's logs and understand that newly added records need to 
propagate before they will exist reliably, it works well in production)

But again, the point of this isn't "how to integrate MS DNS and BIND", 
it's "What happens, in the real world, if multi-master authoritative 
servers were to serve ever-so-slightly-different-versions of the same 
zone with the same serial", and the answer is that this is already 
battle-tested in the real world and it works very well, outside of 
slaves which aren't aware of this design or aren't part of the 
multi-master configuration.

> IIRC, 282826 says that if a
> DC is not used as a master for a BIND slave, then its zone serial
> number is not important.

Indeed -- And that's my point, the situation where slightly different 
versions of the same zone are being served by different DNS servers 
already happens in the real world and most things work fine, except for 
the one big issue of slaving off of such a server.

Dave Warren

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