The libbind functions have been separated from the BIND suite as of
BIND 9.6.0. Originally from older versions of BIND, they have been
continually maintained and improved but not installed by default with
BIND 9. This standard resolver library contains the same historical
functions and headers included with many Unix operating systems.
In fact, most implementations are based on the same original code.

ISC's libbind provides the standard resolver library, along with header
files and documentation, for communicating with domain name servers,
retrieving network host entries from /etc/hosts or via DNS, converting
CIDR network addresses, performing Hesiod information lookups, retrieving
network entries from /etc/networks, implementing TSIG transaction/request
security of DNS messages, performing name-to-address and address-to-name
translations, and utilizing /etc/resolv.conf for resolver configuration.