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DNS域名解析

• DNS （Domain Name System）简单来讲是将域名 www.baidu.com 解析为IP地址 14.215.177.38 的服务。
• 当我们发送一次网络请求，比如 www.baidu.com，浏览器首先需要通过DNS查询，获得域名对应的IP，才能将请求发送到www.baidu.com的服务器。

DNS查询流程

• 优先查询本地DNS缓存
• 其次查找最近的DNS服务器，一般为你的运营商（ISP）服务器
• 当ISP DNS服务器也查询不到时，将由其发起DNS递归搜索。通常，DNS服务器将在缓存中具有.com名称服务器的名称，因此不需要命中根名称服务器。

使用 dig domain a + trace 命令，查看详细的DNS查询过程。

test@ubuntu:~$ dig www.baidu.com a +trace; <<>> DiG 9.10.3-P4-Ubuntu <<>> www.baidu.com a +trace;; global options: +cmd.			5	IN	NS	e.root-servers.net..			5	IN	NS	h.root-servers.net..			5	IN	NS	i.root-servers.net..			5	IN	NS	d.root-servers.net..			5	IN	NS	a.root-servers.net..			5	IN	NS	m.root-servers.net..			5	IN	NS	k.root-servers.net..			5	IN	NS	b.root-servers.net..			5	IN	NS	g.root-servers.net..			5	IN	NS	l.root-servers.net..			5	IN	NS	c.root-servers.net..			5	IN	NS	f.root-servers.net..			5	IN	NS	j.root-servers.net.;; Received 239 bytes from 127.0.1.1#53(127.0.1.1) in 16 mscom.			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.com.			86400	IN	DS	30909 8 2 E2D3C916F6DEEAC73294E8268FB5885044A833FC5459588F4A9184CF C41A5766com.			86400	IN	RRSIG	DS 8 1 86400 20190123170000 20190110160000 16749 . IdZfBu4jr35ks1jNLlCthD6F+kFRsj/J8ON+jr0XtlykK/Rm0WZL1k01 91Lxjkxa74mVD5daaeo94+WQsVm14HoqgZ6S/AOfPwPzRwgQZ2eDXW/e iRnqkJmqvJfkBCDmz5yf/o5eqHPZLjt3pm/XrITtOanYVLQI4G6gnkvE QuSV0WD2MjbwI7Y4pQJVyf3evr5WNDHc5iE3E+z3aW1y18MsrEl+f5xv LmHb79AOM+CJvKA263EbzNEp/NmiZPlW4WduOXUVh0KIAJwMvIZoDLrn QdK1THHNoeskFuWAEi5zmtXmjJ3oie29xp+zkK3zGqVRWpD0+6AjSjFa kiaziw==;; Received 1173 bytes from 198.97.190.53#53(h.root-servers.net) in 225 msbaidu.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.CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN NSEC3 1 1 0 - CK0Q1GIN43N1ARRC9OSM6QPQR81H5M9A NS SOA RRSIG DNSKEY NSEC3PARAMCK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN RRSIG NSEC3 8 2 86400 20190115054344 20190108043344 37490 com. jK6jpwTMUEgKeuQjhYhK2I1v0ioYeX0pkKhNrbrB2sNfjrGhMc7v7g5s KEa5lkxs+TUtXnVt0oUiDA2cLOIQ7pufMKr2ujrlkrGlz9qWw6+IDnUd O1QC5nsHa/vshT35e5lVPoKxiRREEG+OFy7ECGNshnhjH95VmW0pJUSA jlE=HPVV2B5N85O7HJJRB7690IB5UVF9O9UA.com. 86400 IN NSEC3 1 1 0 - HPVVN3Q5E5GOQP2QFE2LEM4SVB9C0SJ6 NS DS RRSIGHPVV2B5N85O7HJJRB7690IB5UVF9O9UA.com. 86400 IN RRSIG NSEC3 8 2 86400 20190117060915 20190110045915 37490 com. C4/8BrgGI3NDrcUjWijcBiHyqQqQPSCRwIR7XIflVj3IDgIsLrgryKsc +JDw/8YzFYTce/X8RF+U8wH9B2GKczWT/DWujen6F8Frqvow8LXaXdUt MUipk0JwMShaKfdBVUBtz9RFinUVWcytc9QZX+Ajs7Ayl+G7VRZEE9fl +RA=;; Received 697 bytes from 192.12.94.30#53(e.gtld-servers.net) in 126 mswww.baidu.com.		1200	IN	CNAME	www.a.shifen.com.a.shifen.com.		1200	IN	NS	ns2.a.shifen.com.a.shifen.com.		1200	IN	NS	ns5.a.shifen.com.a.shifen.com.		1200	IN	NS	ns3.a.shifen.com.a.shifen.com.		1200	IN	NS	ns1.a.shifen.com.a.shifen.com.		1200	IN	NS	ns4.a.shifen.com.;; Received 239 bytes from 14.215.178.80#53(ns4.baidu.com) in 10 ms

参考文献

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