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The following diagram details the incremental transfer mechanism.
Master DNS
Server
Slave DNS
Server 1
Serial Number 11
Serial Number 10
Serial Number 8
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X
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Serial Number 11
changes
Serial Number 12
changes
Serial Number 10
changes
IX
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XF
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Zone Log File
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Slave DNS
Server 2
Slave DNS
Server 3
Serial Number 12
IXFR and DS Integration
As was mentioned above, IXFR is an order-based protocol, which will send the
zone changes based on differences in the zone serial numbers. In a DS integrated
multi-master environment, changes to a DNS zone can be applied to any master
server. Therefore, different master servers will contain the zone changes applied in
a different order. This can cause problems in situations where a master IXFR server
that provided the zone changes to an IXFR client the last time is not available. If the
IXFR client selects another master server with zone changes applied in a different
order, the integrity of the IXFR client’s zone may be compromised after the
incremental transfer. In this case the server initiating a zone transfer will request
AXFR.
In summary, the DNS server could be a Slave and a Master with respect to the
same zone at the same time. This can happen if the zone is replicated from the
Master, server1, to the Slave, server2, and further from the Master, server2, to the
Slave, server3. (This chain could continue further, but regardless of its length it
obeys the rules described in this Section.) In this scenario the server2 will support
IXFR to the server3 as long as it receives IXFR from the server1.
Dynamic Update
In a conventional DNS implementation, if the authoritative information must be
changed, the network administrator has to edit the appropriate zone file manually.
The Domain Name System was originally designed to support queries of a statically
configured database. While the data was expected to change, the frequency of
those changes was expected to be fairly low, and all updates were made as
external edits to a zone’s primary master file.
The advent of dynamic, automated IP addressing using DHCP and related
Windows 2000 White Paper
15