在web开发中，总要熟悉的就是http协议，而发起一个http开始前最先要经历的一个过程就是DNS解析。简单说就是域名如何最终解析到实际服务器ip的过程。
而在研究DNS解析和排除DNS解析类故障问题的时候一个强大的工具就是dig。和他对应的有个比较传统的命令nslookup,不过dig作为更新的命令，其强大而简洁可定制的输出也使得运维其开发人员使用。
实例1（访问服务器本地DNSserver查询根域名服务器）: ~#: dig
; <<>> DiG 9.3.6-P1-RedHat-9.3.6-20.P1.el5_8.6 <<>>
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39027
;; flags: qr rd ra; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 13
;; QUESTION SECTION:
;. IN NS
;; ANSWER SECTION:
. 85715
IN NS a.root-servers.net.
. 85715
IN NS l.root-servers.net.
. 85715
IN NS f.root-servers.net.
. 85715
IN NS d.root-servers.net.
. 85715
IN NS e.root-servers.net.
. 85715
IN NS b.root-servers.net.
. 85715
IN NS c.root-servers.net.
. 85715
IN NS m.root-servers.net.
. 85715
IN NS k.root-servers.net.
. 85715
IN NS i.root-servers.net.
. 85715
IN NS j.root-servers.net.
. 85715
IN NS h.root-servers.net.
. 85715
IN NS g.root-servers.net.
实例2(百度首页DNS解析): dig www.baidu.com
; <<>> DiG 9.3.6-P1-RedHat-9.3.6-20.P1.el5_8.6 <<>> www.baidu.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15372
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;www.baidu.com. IN
A
;; ANSWER SECTION:
www.baidu.com. 641
IN CNAME
www.a.shifen.com.
www.a.shifen.com. 110
IN A
180.97.33.107
www.a.shifen.com. 110
IN A
180.97.33.108
;; Query time: 1 msec
;; SERVER: 192.168.100.52#53(192.168.100.52)
;; WHEN: Wed Aug 13 22:33:59 2014
;; MSG SIZE rcvd: 90
实例3(制定使用google域名服务器解析百度首页); dig @8.8.8.8 www.baidu.com
; <<>> DiG 9.3.6-P1-RedHat-9.3.6-20.P1.el5_8.6 <<>> @8.8.8.8 www.baidu.com
; (1 server found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44033
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;www.baidu.com. IN
A
;; ANSWER SECTION:
www.baidu.com. 557
IN CNAME
www.a.shifen.com.
www.a.shifen.com. 299
IN A
180.76.3.151
;; Query time: 615 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Wed Aug 13 22:36:36 2014
;; MSG SIZE rcvd: 74
实例4(使用+trace选项查看从根域名服务器到最终解析到ip的过程): dig www.baidu.com +trace
. 84899
IN NS f.root-servers.net.
. 84899
IN NS d.root-servers.net.
. 84899
IN NS e.root-servers.net.
. 84899
IN NS b.root-servers.net.
. 84899
IN NS c.root-servers.net.
. 84899
IN NS m.root-servers.net.
. 84899
IN NS k.root-servers.net.
. 84899
IN NS i.root-servers.net.
. 84899
IN NS j.root-servers.net.
. 84899
IN NS h.root-servers.net.
. 84899
IN NS g.root-servers.net.
. 84899
IN NS a.root-servers.net.
. 84899
IN NS l.root-servers.net.
;; Received 509 bytes from 192.168.100.52#53(192.168.100.52) in 1 ms
说明：从本地DNS服务器得到若干各根域名服务器
com. 172800
IN NS a.gtld-servers.net.
com. 172800
IN NS e.gtld-servers.net.
com. 172800
IN NS h.gtld-servers.net.
com. 172800
IN NS j.gtld-servers.net.
com. 172800
IN NS i.gtld-servers.net.
com. 172800
IN NS k.gtld-servers.net.
com. 172800
IN NS b.gtld-servers.net.
com. 172800
IN NS m.gtld-servers.net.
com. 172800
IN NS f.gtld-servers.net.
com. 172800
IN NS g.gtld-servers.net.
com. 172800
IN NS c.gtld-servers.net.
com. 172800
IN NS d.gtld-servers.net.
com. 172800
IN NS l.gtld-servers.net.
;; Received 503 bytes from 192.5.5.241#53(f.root-servers.net) in 39 ms
说明：选取f.root-servers.net根域名服务器查询com域名服务器
baidu.com. 172800
IN NS
dns.baidu.com.
baidu.com. 172800
IN NS
ns2.baidu.com.
baidu.com. 172800
IN NS
ns3.baidu.com.
baidu.com. 172800
IN NS
ns4.baidu.com.
baidu.com. 172800
IN NS
ns7.baidu.com.
;; Received 201 bytes from 192.5.6.30#53(a.gtld-servers.net) in 360 ms
说明:选取a.gtld-servers.net的com域名服务器解析baidu.com的域名服务器
www.baidu.com. 1200
IN CNAME
www.a.shifen.com.
a.shifen.com. 1200
IN NS
ns3.a.shifen.com.
a.shifen.com. 1200
IN NS
ns5.a.shifen.com.
a.shifen.com. 1200
IN NS
ns1.a.shifen.com.
a.shifen.com. 1200
IN NS
ns4.a.shifen.com.
a.shifen.com. 1200
IN NS
ns2.a.shifen.com.
;; Received 228 bytes from 202.108.22.220#53(dns.baidu.com) in 206 ms
说明:通过选择访问dns.baidu.com域名服务器解析到www.baidu.com作为www.a.shifen.com的别名;
于是我们再dig下www.a.shifen.com.
[root@local200-65 JakQuan]# dig www.a.shifen.com +short
180.97.33.108
180.97.33.107
linux命令行学习-dig(DNS查询器),布布扣,bubuko.com
原文地址：http://blog.csdn.net/fableboy/article/details/38544085
