Find the authoritative server of a domain, by:

host -t ns baidu.com

find actual IP of a sub-domain, by:

dig @dns.baidu.com www.baidu.com

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What is authoritative server of dns?

[]An authoritative name server provides actual answer to your DNS queries such as – mail server IP address or web site IP address (A resource record). It provides original and definitive answers to DNS queries. It does not provides just cached answers that were obtained from another name server. Therefore it only returns answers to queries about domain names that are installed in its configuration system. There are two types of Authoritative Name Servers.

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 From DNS & BIND version 4, learned ip domain resolve. By:

host ip-addr

dns has several records, such as cname, a-record(ipv4), aaaa-record(ipv6), soa(belonging info of domain), ns(name server), ptr(ip-domain-mapping), mx(mail exchange record). All of these can be queried both with host and dig.

DNS DBs are mostly managed in tree-format, see below. (and any domain is ended in ".", domains displayed in address-bar without the last dot are relative domains w.r.t root node.