在web开发中。总要熟悉的就是http协议。而发起一个http開始前最先要经历的一个过程就是DNS解析。简单说就是域名怎样终于解析到实际serverip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。
和他相应的有个比較传统的命令nslookup,只是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者使用。
实例1（訪问server本地DNSserver查询根域名server）: ~#: 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server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server到终于解析到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DNSserver得到若干各根域名server
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server查询com域名server
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server解析baidu.com的域名server
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:通过选择訪问dns.baidu.com域名server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
