$ cat /etc/resolv.conf nameserver 8.8.4.4 nameserver 8.8.8.8
#hosts: db files nisplus nis dns hosts: files dns所以建议把内网的IP和主机名的对应关系写到/etc/hosts文件里。
# host baidu.com baidu.com has address 123.125.114.144 baidu.com has address 220.181.111.85 baidu.com has address 220.181.111.86 baidu.com mail is handled by 20 mx1.baidu.com. baidu.com mail is handled by 20 jpmx.baidu.com. baidu.com mail is handled by 20 mx50.baidu.com. baidu.com mail is handled by 10 mx.mailcdn.baidu.com.2. nslookup
# nslookup baidu.com Server: 8.8.4.4 Address: 8.8.4.4#53 Non-authoritative answer: Name: baidu.com Address: 123.125.114.144 Name: baidu.com Address: 220.181.111.85 Name: baidu.com Address: 220.181.111.863. dig
# dig +trace baidu.com ; <<>> DiG 9.3.6-P1-RedHat-9.3.6-20.P1.el5 <<>> +trace baidu.com ;; global options: printcmd . 19079 IN NS m.root-servers.net. . 19079 IN NS i.root-servers.net. . 19079 IN NS g.root-servers.net. . 19079 IN NS a.root-servers.net. . 19079 IN NS f.root-servers.net. . 19079 IN NS e.root-servers.net. . 19079 IN NS l.root-servers.net. . 19079 IN NS j.root-servers.net. . 19079 IN NS d.root-servers.net. . 19079 IN NS b.root-servers.net. . 19079 IN NS h.root-servers.net. . 19079 IN NS c.root-servers.net. . 19079 IN NS k.root-servers.net. ;; Received 228 bytes from 8.8.4.4#53(8.8.4.4) in 162 ms com. 172800 IN NS h.gtld-servers.net. com. 172800 IN NS j.gtld-servers.net. com. 172800 IN NS a.gtld-servers.net. com. 172800 IN NS m.gtld-servers.net. com. 172800 IN NS e.gtld-servers.net. com. 172800 IN NS k.gtld-servers.net. com. 172800 IN NS l.gtld-servers.net. com. 172800 IN NS d.gtld-servers.net. com. 172800 IN NS c.gtld-servers.net. com. 172800 IN NS i.gtld-servers.net. com. 172800 IN NS f.gtld-servers.net. com. 172800 IN NS g.gtld-servers.net. com. 172800 IN NS b.gtld-servers.net. ;; Received 487 bytes from 202.12.27.33#53(m.root-servers.net) in 174 ms 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 197 bytes from 192.54.112.30#53(h.gtld-servers.net) in 483 ms baidu.com. 600 IN A 220.181.111.86 baidu.com. 600 IN A 123.125.114.144 baidu.com. 600 IN A 220.181.111.85 baidu.com. 86400 IN NS ns3.baidu.com. baidu.com. 86400 IN NS ns2.baidu.com. baidu.com. 86400 IN NS ns4.baidu.com. baidu.com. 86400 IN NS dns.baidu.com. baidu.com. 86400 IN NS ns7.baidu.com. ;; Received 245 bytes from 202.108.22.220#53(dns.baidu.com) in 129 ms从上面的输出可以看出，DNS是一个分布式的、阶层式的架构，先访问根服务器(.)，根据根服务器返回的结果，再访问下一层DNS服务器，以此类推，直至找到目标主机的IP地址为止。
原文：http://blog.csdn.net/dbanote/article/details/19118017