two masters for the same zone

[Barry Finkel]

On Sun, 7 Aug 2005, srg wrote:

> Hello:
> 
> I need the following scenario:
> 
> 1. two or more nameservers (ns1, ns2, ...) MASTERS of the same zone, ie, 
> example.com.
> 2. one or more clients do ddns updates to the nameservers in (1). Those 
> updates will be done sometimes to ns1 and other times to ns2.
> Here comes de problem:
> 3. If a client registers it's name (client1) in ns1's example.com. zone 
> then I need that the name client1.example.com. to be resolved in ns1 AND 
> IN ns2.
> 4. The same if the client registers in ns2's example.com. zone (the name 
> MUST be resolved in ns1 and in ns2).
> 
> In other words, I need that the ddns name registrartion (using nsupdate 
> or any other ddns client) can be done in ns1 and in ns2, and, that the 
> registered name will resolve in BOTH n's no matter the one used to register.
> 
> I have been done a similar configuration but configuring ns2 as a SLAVE 
> (including to do the updates in the slave and then the salve forward the 
> update request to the master), but the problem is that if ns1 goes down 
> then no more clients can be registered.

The problem is with zone serial numbers and zone synchronization.
Assume you have two copies of the zone, both identical with serial
number, say 1000.  At the same time, client1 sends a DDNS update for
the zone to ns1 and client2 sends a DDNS update for the zone to ns2.
After the updates are complete, both ns1 and ns2 now have serial number
1001 in the zone, but the zones have different contents.  How do you
synchronize?  Do you send the ns1 update to ns2 and the ns2 update to
ns1, now creating serial number 1002 in each zone?  What happens when
a second update is sent to ns1 before the syncrhonzation?  Now ns1 has
serial number 1002 and ns2 still has serial number 1001.  What serial
number is used after the synchronization?  I do not have an answer to
this problem.  Microsoft attempts to handle this in w2k(+3) with their
multi-master DDNS (one DNS Server running on each DC), but they 
document potential serial number problems in Q282826.
----------------------------------------------------------------------
Barry S. Finkel
Computing and Information Systems Division
Argonne National Laboratory          Phone:    +1 (630) 252-7277
9700 South Cass Avenue               Facsimile:+1 (630) 252-4601
Building 222, Room D209              Internet: BSFinkel at anl.gov
Argonne, IL   60439-4828             IBMMAIL:  I1004994