Multi-master (HA)

Dave Warren davew at
Thu May 8 07:01:14 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.)

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 

Dave Warren

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