IXFR (Incremental Zone Transfer)
IXFR, defined in RFC 1995, lets a secondary nameserver request only the records that changed in a zone since the serial number it already has, instead of pulling every record the way a full AXFR transfer does. A secondary sends its current SOA serial in the request; if the primary has kept a journal of changes since that serial, it replies with an ordered list of the specific records removed and added to reach the current version, far cheaper to send and apply on a large zone that only changed a handful of records. If the primary can't reconstruct that incremental diff -- its journal doesn't go back far enough, or it doesn't keep one at all -- RFC 1995 has it fall back to a full zone transfer instead, so an incremental request always gets a usable answer either way. In practice, DNS NOTIFY is what normally prompts a secondary to check its serial in the first place, with IXFR, or AXFR as the fallback, as the transfer that follows.
By AdminKit team, Netbase JSC · Updated