DNS DomainNameSystem域名系统，根据域名查出IP地址
1.dig命令可以显示整个查询的过程
root@VM-38-204-ubuntu:~# dig www.sopans.com
//这一段是查询参数和统计
; <<>> DiG 9.10.3-P4-Ubuntu <<>> www.sopans.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1899
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
//这一段是查询内容，查询www.sopans.com的A记录，A是Address意思
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.sopans.com. IN A
//DNS服务器的答复，两条A记录，575是TTL的值（Time to live）缓存时间575秒内不用重新查询
;; ANSWER SECTION:
www.sopans.com. 575 IN A 123.206.7.231
www.sopans.com. 575 IN A 115.159.28.111
//DNS服务器的一些传输信息，本机DNS服务器是10.236.158.106，端口53，返回了75字节
;; Query time: 58 msec
;; SERVER: 10.236.158.106#53(10.236.158.106)
;; WHEN: Wed Nov 28 21:36:03 CST 2018
;; MSG SIZE rcvd: 75
2.
+short参数只返回两条A记录IP地址
root@VM-38-204-ubuntu:~# dig www.sopans.com +short
123.206.7.231
115.159.28.111
3.
本机DNS服务器的IP地址位于/etc/resolv.conf
root@VM-38-204-ubuntu:~# cat /etc/resolv.conf
nameserver 10.236.158.106
nameserver 10.236.158.114
options timeout:1 rotate
4.使用@参数指定DNS服务器地址，例如指定DNS服务器地址为8.8.8.8
dig www.sopans.com @8.8.8.8
5.域名的层级结构
主机名.次级域名.*域名.根域名 www.sopans.com.root ,root可以省略因此会显示成www.sopans.com.
6.使用+trace参数，查看分级查询的过程，
NS记录是管理该级域名的域名服务器，按每一级逐级查询的过程
root@VM-38-204-ubuntu:~# dig www.sopans.com +trace
. 294897 IN NS c.root-servers.net.
. 294897 IN NS d.root-servers.net.
. 294897 IN NS e.root-servers.net.
. 294897 IN NS f.root-servers.net.
. 294897 IN NS g.root-servers.net.
. 294897 IN NS h.root-servers.net.
. 294897 IN NS i.root-servers.net.
. 294897 IN NS a.root-servers.net.
. 294897 IN NS j.root-servers.net.
. 294897 IN NS k.root-servers.net.
. 294897 IN NS l.root-servers.net.
. 294897 IN NS m.root-servers.net.
. 294897 IN NS b.root-servers.net.
;; Received 239 bytes from 10.236.158.106#53(10.236.158.106) in 0 ms
com. 172800 IN NS e.gtld-servers.net.
com. 172800 IN NS b.gtld-servers.net.
com. 172800 IN NS j.gtld-servers.net.
com. 172800 IN NS m.gtld-servers.net.
com. 172800 IN NS i.gtld-servers.net.
com. 172800 IN NS f.gtld-servers.net.
com. 172800 IN NS a.gtld-servers.net.
com. 172800 IN NS g.gtld-servers.net.
com. 172800 IN NS h.gtld-servers.net.
com. 172800 IN NS l.gtld-servers.net.
com. 172800 IN NS k.gtld-servers.net.
com. 172800 IN NS c.gtld-servers.net.
com. 172800 IN NS d.gtld-servers.net.
com. 86400 IN DS 30909 8 2 E2D3C916F6DEEAC73294E8268FB5885044A833FC5459588F4A9184CF C41A5766
com. 86400 IN RRSIG DS 8 1 86400 20181211050000 20181128040000 2134 . FeSgJRqqq/LY82e1pAM38Eiu07pepp53zIY23OlA65VDqA4ynhKWb8Ov OKiWzHhWb=
;; Received 1174 bytes from 198.41.0.4#53(a.root-servers.net) in 224 ms
sopans.com. 172800 IN NS dns2.hichina.com.
sopans.com. 172800 IN NS dns1.hichina.com.
CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN NSEC3 1 1 0 - CK0Q1GIN43N1ARRC9OSM6QPQR81H5M9A NS SOA RRSIG DNSKEY NSEC3PARAM
CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN RRSIG NSEC3 8 2 86400 20181202054332 20181125043332 37490 com. R+QjJnuxe+1Bs7+KvcUsTaFaNlUGt+wxLpwdpF9Mxkccm+Re8YJS2R=
O7K3MHPOSL4GCSK01O8ER0LQ04KLNNUT.com. 86400 IN NSEC3 1 1 0 - O7K5ETU7SVNP269R4U7KIQRE79H62BPM NS DS RRSIG
O7K3MHPOSL4GCSK01O8ER0LQ04KLNNUT.com. 86400 IN RRSIG NSEC3 8 2 86400 20181203062329 20181126051329 37490 com. OKTU40+7GCLv+yWhDXu9GEuklwOKrjTgEVYe51v8C7CgIMiBH0Cysy=
;; Received 886 bytes from 192.43.172.30#53(i.gtld-servers.net) in 362 ms
www.sopans.com. 600 IN A 115.159.28.111
www.sopans.com. 600 IN A 123.206.7.231
;; Received 75 bytes from 140.205.41.23#53(dns1.hichina.com) in 4 ms
7.ns参数和+short参数可以直接查询该级域名的NS记录
root@VM-38-204-ubuntu:~# dig ns +short sopans.com
dns1.hichina.com.
dns2.hichina.com.
8.
A记录：地址记录，域名指向的IP地址
MX记录：邮件记录，使用邮箱时设置的服务器地址
CNAME:别名记录，当前查询的域名跳转到另一个域名，
比如www.baidu.com. 1080 IN CNAME www.a.shifen.com.
这样的好处是，当要变更www.baidu.com指向的IP地址时，只需更改www.a.shifen.com的就可以了，www.baidu.com不需要变动；一旦设置了cname,就不能设置其他记录了
NS记录：管理该级域名的域名服务器
PTR:逆向查询记录，从IP地址查询域名
9.-x参数是查询PTR记录，从IP地址反查域名
dig -x 192.30.252.153