本文转载自微信公众号「小菜学编程」，作者小菜学编程。转载本文请联系小菜学编程公众号。
上一小节，我们学习了 DNS 服务器的工作原理，它们可以分为很多个角色：
- 根域名服务器
- 顶级域名服务器
- 权威域名服务器
- 递归解析器(DNS缓存服务器)
于此同时，我们还了解了域名 迭代解析 的过程。本节我们趁热打铁，安排一次实验——按步骤迭代解析域名 test.fasionchan.com ，以此加深理解。
迭代解析从 根域名服务器 开始，根服务器列表可以从 root-servers.org 上获取，也可以通过 dig 命令查询：
root@netbox [ ~ ] ➜ dig . NS
; <<>> DiG 9.16.1-Ubuntu <<>> @10.2.66.66 . NS
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 42791
;; flags: qr rd ra; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 8
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4000
;; QUESTION SECTION:
;. IN NS
;; ANSWER SECTION:
. 1700 IN NS k.root-servers.net.
. 1700 IN NS m.root-servers.net.
. 1700 IN NS l.root-servers.net.
. 1700 IN NS b.root-servers.net.
. 1700 IN NS g.root-servers.net.
. 1700 IN NS f.root-servers.net.
. 1700 IN NS d.root-servers.net.
. 1700 IN NS e.root-servers.net.
. 1700 IN NS i.root-servers.net.
. 1700 IN NS a.root-servers.net.
. 1700 IN NS h.root-servers.net.
. 1700 IN NS j.root-servers.net.
. 1700 IN NS c.root-servers.net.
;; ADDITIONAL SECTION:
k.root-servers.net. 3282 IN A 193.0.14.129
g.root-servers.net. 2845 IN A 192.112.36.4
d.root-servers.net. 118 IN A 199.7.91.13
e.root-servers.net. 494 IN A 192.203.230.10
a.root-servers.net. 1771 IN A 198.41.0.4
j.root-servers.net. 3197 IN A 192.58.128.30
c.root-servers.net. 1830 IN A 192.33.4.12
;; Query time: 14 msec
;; SERVER: 10.2.66.66#53(10.2.66.66)
;; WHEN: Thu Apr 08 09:01:17 CST 2021
;; MSG SIZE rcvd: 364
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根域名服务器总共有 13 台，编号从 A 到 M 。我们可以从中选择一台，比如 A ，它的 IP 地址是 198.41.0.4 。
接下来，我们执行 dig 命令，向根域名服务器 A 发起域名解析请求：
root@netbox [ ~ ] ➜ dig @198.41.0.4 test.fasionchan.com
; <<>> DiG 9.16.1-Ubuntu <<>> @198.41.0.4 test.fasionchan.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44843
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 27
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1472
;; QUESTION SECTION:
;test.fasionchan.com. IN A
;; AUTHORITY SECTION:
com. 172800 IN NS a.gtld-servers.net.
com. 172800 IN NS b.gtld-servers.net.
com. 172800 IN NS c.gtld-servers.net.
com. 172800 IN NS d.gtld-servers.net.
com. 172800 IN NS e.gtld-servers.net.
com. 172800 IN NS f.gtld-servers.net.
com. 172800 IN NS g.gtld-servers.net.
com. 172800 IN NS h.gtld-servers.net.
com. 172800 IN NS i.gtld-servers.net.
com. 172800 IN NS j.gtld-servers.net.
com. 172800 IN NS k.gtld-servers.net.
com. 172800 IN NS l.gtld-servers.net.
com. 172800 IN NS m.gtld-servers.net.
;; ADDITIONAL SECTION:
a.gtld-servers.net. 172800 IN A 192.5.6.30
b.gtld-servers.net. 172800 IN A 192.33.14.30
c.gtld-servers.net. 172800 IN A 192.26.92.30
d.gtld-servers.net. 172800 IN A 192.31.80.30
e.gtld-servers.net. 172800 IN A 192.12.94.30
f.gtld-servers.net. 172800 IN A 192.35.51.30
g.gtld-servers.net. 172800 IN A 192.42.93.30
h.gtld-servers.net. 172800 IN A 192.54.112.30
i.gtld-servers.net. 172800 IN A 192.43.172.30
j.gtld-servers.net. 172800 IN A 192.48.79.30
k.gtld-servers.net. 172800 IN A 192.52.178.30
l.gtld-servers.net. 172800 IN A 192.41.162.30
m.gtld-servers.net. 172800 IN A 192.55.83.30
a.gtld-servers.net. 172800 IN AAAA 2001:503:a83e::2:30
b.gtld-servers.net. 172800 IN AAAA 2001:503:231d::2:30
c.gtld-servers.net. 172800 IN AAAA 2001:503:83eb::30
d.gtld-servers.net. 172800 IN AAAA 2001:500:856e::30
e.gtld-servers.net. 172800 IN AAAA 2001:502:1ca1::30
f.gtld-servers.net. 172800 IN AAAA 2001:503:d414::30
g.gtld-servers.net. 172800 IN AAAA 2001:503:eea3::30
h.gtld-servers.net. 172800 IN AAAA 2001:502:8cc::30
i.gtld-servers.net. 172800 IN AAAA 2001:503:39c1::30
j.gtld-servers.net. 172800 IN AAAA 2001:502:7094::30
k.gtld-servers.net. 172800 IN AAAA 2001:503:d2d::30
l.gtld-servers.net. 172800 IN AAAA 2001:500:d937::30
m.gtld-servers.net. 172800 IN AAAA 2001:501:b1f9::30
;; Query time: 169 msec
;; SERVER: 198.41.0.4#53(198.41.0.4)
;; WHEN: Fri Apr 09 08:38:10 CST 2021
;; MSG SIZE rcvd: 843
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其中，@ 指定目标 DNS 服务器的地址。
从 dig 命令的输出可以看到，根域名服务器告诉我们 .com 域名应该去找 a.gtld-servers.net. 等服务器。这些服务器都是 .com 的 顶级域名服务器 ，根域名服务器把它们的地址一并告诉我们了。
我们可以从这些顶级域名服务器中选择一台，比如 a.gtld-servers.net. ，它的 IP 地址是 192.5.6.30 。然后，再次指定 dig 命令，向它发起域名解析请求：
root@netbox [ ~ ] ➜ dig @192.5.6.30 test.fasionchan.com
; <<>> DiG 9.16.1-Ubuntu <<>> @192.5.6.30 test.fasionchan.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 7836
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;test.fasionchan.com. IN A
;; AUTHORITY SECTION:
fasionchan.com. 172800 IN NS f1g1ns1.dnspod.net.
fasionchan.com. 172800 IN NS f1g1ns2.dnspod.net.
;; Query time: 212 msec
;; SERVER: 192.5.6.30#53(192.5.6.30)
;; WHEN: Fri Apr 09 08:38:58 CST 2021
;; MSG SIZE rcvd: 101
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顶级域名服务器告诉我们，fasionchan.com 这个域应该去找 f1g1ns1.dnspod.net. 或 f1g1ns2.dnspod.net. 来解析。这二者就是 fasionchan.com 的 权威域名服务器 。
但这次 .com 域名没有告诉我们 f1g1ns1.dnspod.net. 或 f1g1ns2.dnspod.net. 的 IP 地址。因此，我们必须先自己动手，将它们的域名解析成 IP 地址。
我们可以直接执行 dig 命令，一键完成解析：
root@netbox [ ~ ] ➜ dig f1g1ns1.dnspod.net.
; <<>> DiG 9.16.1-Ubuntu <<>> f1g1ns1.dnspod.net.
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6244
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;f1g1ns1.dnspod.net. IN A
;; ANSWER SECTION:
f1g1ns1.dnspod.net. 15 IN A 58.247.212.36
f1g1ns1.dnspod.net. 15 IN A 61.151.180.44
f1g1ns1.dnspod.net. 15 IN A 129.211.176.187
f1g1ns1.dnspod.net. 15 IN A 162.14.25.230
f1g1ns1.dnspod.net. 15 IN A 183.192.164.118
;; Query time: 13 msec
;; SERVER: 192.168.65.1#53(192.168.65.1)
;; WHEN: Fri Apr 09 08:39:38 CST 2021
;; MSG SIZE rcvd: 116
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如果 dig 命令未指定 DNS 服务器，它默认会到本地的 递归解析器 ，也就是 DNS缓存服务器 上去查询。DNS缓存服务器会帮我们做迭代解析，再把结果告诉我们。
如果亲不厌其烦，可以自己进行迭代解析。具体步骤也是类似的，附在本文后半部分。
得到权威域名服务器的 IP 地址后，我们就可以向它发起解析请求：
root@netbox [ ~ ] ➜ dig @58.247.212.36 test.fasionchan.com
; <<>> DiG 9.16.1-Ubuntu <<>> @58.247.212.36 test.fasionchan.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 42646
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;test.fasionchan.com. IN A
;; ANSWER SECTION:
test.fasionchan.com. 600 IN A 10.0.0.1
;; AUTHORITY SECTION:
fasionchan.com. 600 IN NS f1g1ns2.dnspod.net.
fasionchan.com. 600 IN NS f1g1ns1.dnspod.net.
;; Query time: 57 msec
;; SERVER: 58.247.212.36#53(58.247.212.36)
;; WHEN: Fri Apr 09 08:55:47 CST 2021
;; MSG SIZE rcvd: 128
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瞧，我们成功解析域名 test.fasionchan.com ，得到它对应的 IP 地址 10.0.0.1 !
迭代解析 f1g1ns1.dnspod.net.
如果不想通过递归解析器，可以自行执行迭代解析，查询 f1g1ns1.dnspod.net. 的 IP 地址。
首先，向 根域名服务器 发起解析请求：
root@netbox [ ~ ] ➜ dig @198.41.0.4 f1g1ns1.dnspod.net.
; <<>> DiG 9.16.1-Ubuntu <<>> @198.41.0.4 f1g1ns1.dnspod.net.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49636
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 27
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1472
;; QUESTION SECTION:
;f1g1ns1.dnspod.net. IN A
;; AUTHORITY SECTION:
net. 172800 IN NS a.gtld-servers.net.
net. 172800 IN NS b.gtld-servers.net.
net. 172800 IN NS c.gtld-servers.net.
net. 172800 IN NS d.gtld-servers.net.
net. 172800 IN NS e.gtld-servers.net.
net. 172800 IN NS f.gtld-servers.net.
net. 172800 IN NS g.gtld-servers.net.
net. 172800 IN NS h.gtld-servers.net.
net. 172800 IN NS i.gtld-servers.net.
net. 172800 IN NS j.gtld-servers.net.
net. 172800 IN NS k.gtld-servers.net.
net. 172800 IN NS l.gtld-servers.net.
net. 172800 IN NS m.gtld-servers.net.
;; ADDITIONAL SECTION:
a.gtld-servers.net. 172800 IN A 192.5.6.30
b.gtld-servers.net. 172800 IN A 192.33.14.30
c.gtld-servers.net. 172800 IN A 192.26.92.30
d.gtld-servers.net. 172800 IN A 192.31.80.30
e.gtld-servers.net. 172800 IN A 192.12.94.30
f.gtld-servers.net. 172800 IN A 192.35.51.30
g.gtld-servers.net. 172800 IN A 192.42.93.30
h.gtld-servers.net. 172800 IN A 192.54.112.30
i.gtld-servers.net. 172800 IN A 192.43.172.30
j.gtld-servers.net. 172800 IN A 192.48.79.30
k.gtld-servers.net. 172800 IN A 192.52.178.30
l.gtld-servers.net. 172800 IN A 192.41.162.30
m.gtld-servers.net. 172800 IN A 192.55.83.30
a.gtld-servers.net. 172800 IN AAAA 2001:503:a83e::2:30
b.gtld-servers.net. 172800 IN AAAA 2001:503:231d::2:30
c.gtld-servers.net. 172800 IN AAAA 2001:503:83eb::30
d.gtld-servers.net. 172800 IN AAAA 2001:500:856e::30
e.gtld-servers.net. 172800 IN AAAA 2001:502:1ca1::30
f.gtld-servers.net. 172800 IN AAAA 2001:503:d414::30
g.gtld-servers.net. 172800 IN AAAA 2001:503:eea3::30
h.gtld-servers.net. 172800 IN AAAA 2001:502:8cc::30
i.gtld-servers.net. 172800 IN AAAA 2001:503:39c1::30
j.gtld-servers.net. 172800 IN AAAA 2001:502:7094::30
k.gtld-servers.net. 172800 IN AAAA 2001:503:d2d::30
l.gtld-servers.net. 172800 IN AAAA 2001:500:d937::30
m.gtld-servers.net. 172800 IN AAAA 2001:501:b1f9::30
;; Query time: 172 msec
;; SERVER: 198.41.0.4#53(198.41.0.4)
;; WHEN: Fri Apr 09 08:40:49 CST 2021
;; MSG SIZE rcvd: 840
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根域名服务器告诉我们，.net 应该去找 a.gtld-servers.net. 等服务器，服务器地址附在后面的附加节。
我们选择 a.gtld-servers.net. 这台 顶级域名服务器 ，它的地址是 192.5.6.30 。然后向它发起解析请求：
root@netbox [ ~ ] ➜ dig @192.5.6.30 f1g1ns1.dnspod.net.
; <<>> DiG 9.16.1-Ubuntu <<>> @192.5.6.30 f1g1ns1.dnspod.net.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 53457
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 7
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;f1g1ns1.dnspod.net. IN A
;; AUTHORITY SECTION:
dnspod.net. 172800 IN NS v6ns3.dnsv2.net.
dnspod.net. 172800 IN NS v6ns4.dnsv2.com.
;; ADDITIONAL SECTION:
v6ns3.dnsv2.net. 172800 IN A 129.211.176.248
v6ns3.dnsv2.net. 172800 IN A 162.14.24.245
v6ns3.dnsv2.net. 172800 IN A 183.192.164.118
v6ns3.dnsv2.net. 172800 IN A 223.166.151.17
v6ns3.dnsv2.net. 172800 IN AAAA 2402:4e00:1430:1102:0:9136:2b2f:bf6b
v6ns3.dnsv2.net. 172800 IN A 61.129.8.141
;; Query time: 213 msec
;; SERVER: 192.5.6.30#53(192.5.6.30)
;; WHEN: Fri Apr 09 08:41:27 CST 2021
;; MSG SIZE rcvd: 210
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顶级域名服务器告诉我们，dnspod.net. 这个域应该去找 v6ns3.dnsv2.net. 或者 v6ns4.dnsv2.com. 。这两者就是所谓的 权威域名服务器 。这次，顶级域名服务器已经在附加节中，附上了它们的 IP 地址。
我们从中挑选一个，比如 129.211.176.248 ，继续发起解析请求：
root@netbox [ ~ ] ➜ dig @129.211.176.248 f1g1ns1.dnspod.net.
; <<>> DiG 9.16.1-Ubuntu <<>> @129.211.176.248 f1g1ns1.dnspod.net.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23542
;; flags: qr aa rd; QUERY: 1, ANSWER: 5, AUTHORITY: 2, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;f1g1ns1.dnspod.net. IN A
;; ANSWER SECTION:
f1g1ns1.dnspod.net. 172800 IN A 183.192.164.118
f1g1ns1.dnspod.net. 172800 IN A 162.14.25.230
f1g1ns1.dnspod.net. 172800 IN A 58.247.212.36
f1g1ns1.dnspod.net. 172800 IN A 61.151.180.44
f1g1ns1.dnspod.net. 172800 IN A 129.211.176.187
;; AUTHORITY SECTION:
dnspod.net. 86400 IN NS v6ns3.dnsv2.net.
dnspod.net. 86400 IN NS v6ns4.dnsv2.com.
;; Query time: 34 msec
;; SERVER: 129.211.176.248#53(129.211.176.248)
;; WHEN: Fri Apr 09 08:42:07 CST 2021
;; MSG SIZE rcvd: 185
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最终，我们成功解析到 f1g1ns1.dnspod.net. 对应的 IP 地址，总共有 5 个。
扩展阅读
dig (https://linux.die.net/man/1/dig)
