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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Each list difference sequences is preceded by the server’s current version of the<br />

SOA <strong>and</strong> represents one update to the zone. Similarly, each difference<br />

sequence is preceded by an SOA version (indicating in which versions<br />

correspond to each change, <strong>and</strong> the difference sequences are ordered oldest to<br />

newest. Upon receiving this message, a server can update its zone by tracking<br />

the version history listed in the IXFR answer section.<br />

For example, assume a server has the following zone:<br />

MYZONE.MYDIV.MYCORP IN SOA MYHOST.MYDIV.MYCORP (<br />

1 600 600 3600000 614800)<br />

IN NS MYHOST.MYDIV.MYCORP<br />

MYHOST.MYDIV.MYCORP IN A 10.1.2.3<br />

OTHERHOST.MYDIV.MYCORP IN A 10.2.3.4<br />

Otherhost.mydiv.mycorp is removed, <strong>and</strong> in version 2, thishost.mydiv.mycorp is<br />

added, leaving the zone as:<br />

MYZONE.MYDIV.MYCORP IN SOA MYHOST.MYDIV.MYCORP (<br />

2 600 600 3600000 614800)<br />

IN NS MYHOST.MYDIV.MYCORP<br />

MYHOST.MYDIV.MYCORP IN A 10.1.2.3<br />

THISHOST.MYDIV.MYCORP IN A 10.2.3.5<br />

If the server receives an IXFR query, it sends back the following answer section:<br />

MYZONE.MYDIV.MYCORP IN SOA serial=2<br />

MYZONE.MYDIV.MYCORP IN SOA serial=1<br />

OTHERHOST.MYDIV.MYCORP IN A 10.1.2.4<br />

MYZONE.MYDIV.MYCORP IN SOA serial=2<br />

THISHOST.MYDIV.MYCORP IN A 10.2.3.5<br />

MYZONE.MYDIV.MYCORP IN SOA serial=2<br />

Note: If a server received an IXFR query, but incremental zone transfers are<br />

not available, it will send back the entire zone in the reply.<br />

12.2.3 Prompt notification of zone transfer<br />

RFC 1996 introduces the NOTIFY DNS message type, which is used by a<br />

master server to inform subordinate servers that an update has taken place <strong>and</strong><br />

that they should initiate a query to discover the new data. The NOTIFY message<br />

uses the DNS message format, but only a subset of the available fields (unused<br />

fields are filled with binary zeros). The message is similar to a QUERY message,<br />

<strong>and</strong> can contain the name of the RRs that have been updated. Upon receipt of a<br />

NOTIFY message, the subordinate returns a response. The response contains<br />

no useful information, <strong>and</strong> only serves to alert the master server of receipt of the<br />

Chapter 12. Directory <strong>and</strong> naming protocols 457

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