使用dig时，没有没有明确指定DNS服务器地址，会尝试/etc/resolv.conf中所有的地址。如若没有找到可用的服务器，那就把请求丢到本地。
如若，执行dig没有任何的参数、选项，就打印root信息。
[view@tri ~]$ dig ; <<>> DiG 9.9.4-RedHat-9.9.4-38.el7_3.3 <<>> ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50874 ;; flags: qr rd ra; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 27 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ;; QUESTION SECTION: ;. IN NS ;; ANSWER SECTION: . 242485 IN NS b.root-servers.net. . 242485 IN NS e.root-servers.net. . 242485 IN NS f.root-servers.net. . 242485 IN NS m.root-servers.net. . 242485 IN NS j.root-servers.net. . 242485 IN NS g.root-servers.net. . 242485 IN NS l.root-servers.net. . 242485 IN NS i.root-servers.net. . 242485 IN NS a.root-servers.net. . 242485 IN NS c.root-servers.net. . 242485 IN NS k.root-servers.net. . 242485 IN NS h.root-servers.net. . 242485 IN NS d.root-servers.net. ;; ADDITIONAL SECTION: a.root-servers.net. 103972 IN A 198.41.0.4 b.root-servers.net. 169601 IN A 192.228.79.201 c.root-servers.net. 449868 IN A 192.33.4.12 d.root-servers.net. 226569 IN A 199.7.91.13 e.root-servers.net. 449861 IN A 192.203.230.10 f.root-servers.net. 198658 IN A 192.5.5.241 g.root-servers.net. 542870 IN A 192.112.36.4 h.root-servers.net. 118213 IN A 198.97.190.53 i.root-servers.net. 398452 IN A 192.36.148.17 j.root-servers.net. 542823 IN A 192.58.128.30 k.root-servers.net. 182751 IN A 193.0.14.129 l.root-servers.net. 434357 IN A 199.7.83.42 m.root-servers.net. 27576 IN A 202.12.27.33 a.root-servers.net. 446498 IN AAAA 2001:503:ba3e::2:30 b.root-servers.net. 240907 IN AAAA 2001:500:200::b c.root-servers.net. 447697 IN AAAA 2001:500:2::c d.root-servers.net. 170020 IN AAAA 2001:500:2d::d e.root-servers.net. 330907 IN AAAA 2001:500:a8::e f.root-servers.net. 14498 IN AAAA 2001:500:2f::f g.root-servers.net. 528596 IN AAAA 2001:500:12::d0d h.root-servers.net. 528596 IN AAAA 2001:500:1::53 i.root-servers.net. 14498 IN AAAA 2001:7fe::53 j.root-servers.net. 528597 IN AAAA 2001:503:c27::2:30 k.root-servers.net. 170020 IN AAAA 2001:7fd::1 l.root-servers.net. 170020 IN AAAA 2001:500:9f::42 m.root-servers.net. 14498 IN AAAA 2001:dc3::35 ;; Query time: 1 msec ;; SERVER: 100.100.2.136#53(100.100.2.136) ;; WHEN: Wed Aug 16 10:45:59 CST 2017 ;; MSG SIZE rcvd: 811
