Domain Name System (DNS)

The Domain Name System (DNS) is a hierarchical naming system that serves as a directory of networked hosts and resources. Information in the directory maps network names to data and is maintained in logical entries known as resource records. The DNS hierarchy begins with the root domain "." at the top and branches downward to multiple next-level domains.

When a system needs to perform name resolution using a DNS server, it begins by sending queries to the servers listed in /etc/resolv.conf in order, until it gets a response or runs out of servers.

host -v -t A baidu.com
# host -v -t A baidu.com
Trying "baidu.com"
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58473
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;baidu.com.			IN	A

;; ANSWER SECTION:
baidu.com.		375	IN	A	123.125.114.144
baidu.com.		375	IN	A	180.149.132.47
baidu.com.		375	IN	A	220.181.57.217
baidu.com.		375	IN	A	111.13.101.208

Received 91 bytes from 10.72.17.5#53 in 5 ms

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