What is a DNS zone transfer?
It is an answer to a DNS query to list all DNS information (such as Name Servers, host names, MX records, CNAME records, glue records (delegation for child Domains), zone serial number, Time To Live (TTL) records, etc) for a Domain. The query can be made from a single host to look up information for the entire Domain. It is also the mechanism Primary and Secondary Name Servers use to update its DNS data. This is one of the vulnerable points where a malicious person can compromise DNS data integrity.