Rick Yune was on a series that cancelled on December 13.
DNS zone transfer , also sometimes known by the inducing DNS query type AXFR , is a type of DNS transaction .. DNS. Domain Name System. transaction. Database transaction. zone. DNS zone. It is one of the many mechanisms available for administrators to replicate DNS databases across a set of DNS servers .. DNS. Domain Name System. replicate. distributed database. A zone transfer uses the Transmission Control Protocol -LRB- TCP -RRB- for transport , and takes the form of a client -- server transaction .. transaction. Database transaction. Transmission Control Protocol. Transmission Control Protocol. zone. DNS zone. The client requesting a zone transfer may be a slave server or secondary server , requesting data from a master server , sometimes called a primary server .. zone. DNS zone. The portion of the database that is replicated is a zone .. zone. DNS zone. Zone transfer comprises a preamble followed by the actual data transfer .. The preamble comprises a lookup of the Start of Authority -LRB- SOA -RRB- resource record for the `` zone apex '' , the node of the DNS namespace that is at the top of the `` zone '' .. DNS. Domain Name System. zone. DNS zone. Start of Authority. Start of Authority. The fields of this SOA resource record , in particular the `` serial number '' , determine whether the actual data transfer need to occur at all .. The client compares the serial number of the SOA resource record with the serial number in the last copy of that resource record that it has .. If the serial number of the record being transferred is greater , the data in the zone are deemed to have `` changed '' -LRB- in some fashion -RRB- and the slave proceeds to request the actual zone data transfer .. zone. DNS zone. If the serial numbers are identical , the data in the zone are deemed not to have `` changed '' , and the client may continue to use the copy of the database that it already has , if it has one .. zone. DNS zone. The actual data transfer process begins by the client sending a query -LRB- opcode 0 -RRB- with the special query type AXFR -LRB- value 252 -RRB- over the TCP connection to the server .. The server responds with a series of response messages , comprising all of the resource records for every domain name in the `` zone '' .. zone. DNS zone. The first response comprises the SOA resource record for the zone apex .. zone. DNS zone. The other data follows in no specified order .. The end of the data is signaled by the server repeating the response containing the SOA resource record for the zone apex .. zone. DNS zone. Some zone transfer clients perform the SOA lookup of the preamble using their system 's normal DNS query resolution mechanism .. DNS. Domain Name System. zone. DNS zone. These clients do not open a TCP connection to the server until they have determined that they need to perform the actual data transfer .. However , since TCP can be used for normal DNS transactions , as well as for zone transfer , other zone transfer clients perform the SOA lookup preamble over the same TCP connection as they then -LRB- may -RRB- perform the actual data transfer .. DNS. Domain Name System. zone. DNS zone. These clients open the TCP connection to the server before they even perform the preamble .. The preceding describes full zone transfer .. zone. DNS zone. Incremental zone transfer differs from full zone transfer in the following respects  :. zone. DNS zone. The client uses the special QTYPE IXFR -LRB- value 251 -RRB- instead of the AXFR QTYPE .. The client sends the SOA resource record for the zone apex that it currently has , if any , in the IXFR message , letting the server know which version of the `` zone '' it believes to be current .. zone. DNS zone. Though the server may respond in the normal AXFR manner with the full data for the zone , it may also instead respond with an `` incremental '' data transfer .. zone. DNS zone. This latter comprises the list of changes to the zone data , in zone serial number order , between the version of the zone that the client reported to the server as having and the version of the zone that is current at the server .. zone. DNS zone. The changes comprise two lists , one of resource records that are deleted and one of resource records that are inserted .. -LRB- A modification to a resource record is represented as a deletion followed by an insertion . -RRB-. Zone transfer is entirely client-initiated .. Though servers can send a NOTIFY message to clients -LRB- that they have been informed about -RRB- whenever a change to the zone data has been made , the scheduling of zone transfers is entirely under the control of the clients .. zone. DNS zone. Clients schedule zone transfers initially , when their databases are empty , and thereafter at regular intervals , in a pattern controlled by the values in the `` refresh '' , `` retry '' , and `` expire '' fields in the SOA resource record of the zone apex .. zone. DNS zone