The remote DNS server allows zone transfers. DNS zone transfer, also sometimes known by its (most common) opcode mnemonic AXFR, is a type of DNS transaction. It is one of the many mechanisms available for administrators to employ for replicating the databases containing the DNS data across a set of DNS servers.
DNS zone transfers have several potential security issues. The data contained in an entire DNS zone may be sensitive in nature. Individually, DNS records are not sensitive, but if a malicious entity obtains a copy of the entire DNS zone for a domain, they may have a complete listing of all hosts in that domain. That makes the job of a computer hacker much easier. A computer hacker needs no special tools or access to obtain a complete DNS zone if the name server is promiscuous and allows anyone to do a zone transfer.
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