Multi-master (HA)

Barry S. Finkel bsfinkel at
Thu May 8 14:13:32 UTC 2014

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.  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.

Another problem that I saw when I had BIND servers slaving AD
zones was this - during patching of the DCs, the zone serial number
might decrease.  In most cases, after the DC patching was complete,
the serial number reverted to the proper value.  I was not allowed to
open a trouble ticket with MS to determine why the zone serial on the
DC was decreasing.  The Windows support group did not see this as a
problem.  It might not have been a problem, as I saw many times where
the zone serial number changed in an AD zone when the zone contents had
not changed.  This just meant more unnecessary zone transfers from the
master to the slave.

--Barry Finkel

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