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DNS Zone Management, Contacts and Zone Transfers
(Page 3 of 3)
Zone Transfer Mechanism
When a zone transfer is required,
it is accomplished using a DNS query sent to the master server using
the regular DNS query/response messaging method used for name resolution
(discussed in the next section). A special DNS question type, called
AXFR (address transfer) is used to initiate the zone transfer.
The server will then transfer the resource records for the zone using
a series of DNS response messages (assuming that the server that requested
the transfer is authorized to do so). Since it's important that zone
transfers be received reliably, and since the amount of data to be sent
is large and needs to be managed, a TCP session must first be established
and used for zone transfers. This is in contrast to the simpler UDP
transport used for regular DNS messages. This is discussed in the
topic discussing the use of UDP and TCP for DNS.
Once the zone transfer is complete,
the slave name server will update its database and return to regular
operation. It will continue to perform regular polls of the master server
every Refresh seconds. If it has a problem with a regular poll,
it will try again after Retry seconds. Finally, if an amount
of time equal to Expires seconds elapses, the master name server
will stop serving data from the zone until it re-establishes contact
with the primary name server.
 Key Concept: Slave name servers do not have their DNS information managed directly by an administrator. Instead, they obtain information from their master name server on a periodic basis through a process called a zone transfer. Several fields in the Start Of Authority DNS resource record control the zone transfer process, including specifying how often transfers are done and how slave name servers handle problem conditions such as an inability to contact the master server.
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Note that the DNS Notify feature
is an enhancement to the basic zone status check / zone transfer model.
It allows the master server to notify a slave server when the master’s
database has changed. Another new feature allows only part of a zone
to be transferred instead of the entire zone. See
the topic on DNS name server enhancements for more.
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