cdn简介
由于用户访问源站业务有性能瓶颈,通过cdn技术把源站的内容缓存到多个节点。用户向源站域名发起请求时,请求会被调度至最接近用户的服务节点,直接由服务节点直接快速响应,有效降低用户访问延迟,提升可用性。
了解cdn需要先了解一下知识点:
dns解析原理
CNAME作用
dns解析流程
说再多也不如实际应用,让我们从例子中了解dns解析流程
$ nslookup www.moehot.com Server: 10.0.0.1Address: 10.0.0.1#53Non-authoritative answer:Name: www.moehot.comAddress: 139.196.3.197
nslookup 是dns解析的命令,server 10.0.0.1是公司内部的dns服务器的ip. 139.196.3.197是请求域名的ip。
详细的流程使用dig命令查看
$ dig +trace www.moehot.com ; <<>> DiG 9.9.4-RedHat-9.9.4-72.el7 <<>> +trace www.moehot.com;; global options: +cmd. 518382 IN NS a.root-servers.net.. 518382 IN NS b.root-servers.net.. 518382 IN NS c.root-servers.net.. 518382 IN NS d.root-servers.net.. 518382 IN NS e.root-servers.net.. 518382 IN NS g.root-servers.net.. 518382 IN NS h.root-servers.net.. 518382 IN NS i.root-servers.net.. 518382 IN NS j.root-servers.net.. 518382 IN NS k.root-servers.net.. 518382 IN NS l.root-servers.net.. 518382 IN NS m.root-servers.net.. 518382 IN NS root1.sc163.net.. 518382 IN NS root2.sc163.net.. 518382 IN NS b.root-servers.net.. 518382 IN NS c.root-servers.net.. 518382 IN NS d.root-servers.net.. 518382 IN NS e.root-servers.net.. 518382 IN NS g.root-servers.net.. 518382 IN NS h.root-servers.net.. 518382 IN NS i.root-servers.net.. 518382 IN NS j.root-servers.net.. 518382 IN NS k.root-servers.net.. 518382 IN NS l.root-servers.net.. 518382 IN NS m.root-servers.net.. 518382 IN NS root1.sc163.net.. 518382 IN NS root2.sc163.net.. 518382 IN NS a.root-servers.net.;; Received 631 bytes from 10.0.0.1#53(10.0.0.1) in 54 ms com. 172800 IN NS i.gtld-servers.net.com. 172800 IN NS a.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 g.gtld-servers.net.com. 172800 IN NS h.gtld-servers.net.com. 172800 IN NS d.gtld-servers.net.com. 172800 IN NS j.gtld-servers.net.com. 172800 IN NS f.gtld-servers.net.com. 172800 IN NS c.gtld-servers.net.com. 172800 IN NS m.gtld-servers.net.com. 172800 IN NS b.gtld-servers.net.com. 86400 IN DS 30909 8 2 E2D3C916F6DEEAC73294E8268FB5885044A833FC5459588F4A9184CF C41A5766 com. 86400 IN RRSIG DS 8 1 86400 20190224210000 20190211200000 16749 . owDDrAEA9JJUhzbHNnVNjPpqTqeJrWMScHU4Wskrp9gnkBKI8+1PeBfP 6uIUeVT+0w/Vcfh03p7mw0w5DS0ub8wQUfBTfY37s1MhE45fWueTyUGt l9aMEsma0UHZV1TL2njmDhgr7koN935hhjiSv26wymuBx5CpOvF2y7ql ih4JGsWPnHLrsfnqjq9njmP/3RQnpXrJDP/+reb5DTpdqLEf9TM/OHbt 1UpkG6rMUx4Jz53pIksji/CUfbYPiuH8FrUnVJ5thn0o+o2ZPpQqSisA 9C9dYDl1o75CDC3FShp56EjfeGmm6ncY8Kz5Irer1HUjOjiudN4cCjWU NnljsQ==;; Received 1174 bytes from 202.12.27.33#53(m.root-servers.net) in 4386 ms moehot.com. 172800 IN NS dns9.hichina.com.moehot.com. 172800 IN NS dns10.hichina.com.CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN NSEC3 1 1 0 - CK0Q1GIN43N1ARRC9OSM6QPQR81H5M9A NS SOA RRSIG DNSKEY NSEC3PARAM CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 86400 IN RRSIG NSEC3 8 2 86400 20190216054237 20190209043237 16883 com. ZpsxE9/jbJ0Ues0CJFIz9X5DvvsTgUlhLjpRIrQcXbo9z+avYuhL5O7J MxzY0KbxK2hGWK2q5zesaB+9ye1v7LuEVn8OzHtyjxGxjr7sXqWiU+0W 3lFK8q2EdNOulZJrmJ/nSZGNZ3aXRpeFWLD2LYs1Eipw5D6KmBAWAy/w xvE=HMIQOH986C2QQFJRPQJH18GTCL9922SH.com. 86400 IN NSEC3 1 1 0 - HMISVQBER3LFS6UELQ3DN9VT6QI4LL28 NS DS RRSIG HMIQOH986C2QQFJRPQJH18GTCL9922SH.com. 86400 IN RRSIG NSEC3 8 2 86400 20190218055540 20190211044540 16883 com. R3GJVt5SSPC7MQUJTs6h0OC9qQnj2WZU2Y/ec+xbj39/Wuo/TwunvKYz NIr20VkkhMOeX/s8cfA3XheNMBdwufxLZNYTt3ptBySZdmiRKYFi/9HG jUfEcBF2tVVE/ae5qzftgIm7O+AUYxkwWMKBL4+XC3A3sV8I0Fv2fd+Y iDk=;; Received 887 bytes from 192.42.93.30#53(g.gtld-servers.net) in 152 ms www.moehot.com. 600 IN A 139.196.3.197;; Received 59 bytes from 140.205.41.26#53(dns10.hichina.com) in 36 ms
让我们来一步步分析解析流程:
首先会查看本地缓存查询是否有对应的域名解析ip
没有的情况下,向配置的dns服务器发起请求(10.0.0.1为公司内部dns服务器ip)
Received 631 bytes from 10.0.0.1#53(10.0.0.1) in 54 ms -- 10.0.0.1会返回根域名服务器的地址(根域名服务器有多个,所以返回很多域名)
Received 1174 bytes from 202.12.27.33#53(m.root-servers.net) -- 本机向根域名发起解析请求,根域名服务器返回.com顶级域名服务器地址
Received 887 bytes from 192.42.93.30#53(g.gtld-servers.net) -- 本机向com.域名服务器发起请求,返回moehot.com.域名服务器地址
最后向moehot.com.域名发起请求得到www.moehot.com. 的ip地址
CNAME作用
CNAME 即:别名记录。这种记录允许您将多个名字映射到另外一个域名。通常用于同时提供WWW和MAIL服务的计算机。例如,有一台计算机名为“host.mydomain.com”(A记录)。它同时提供WWW和MAIL服务,为了便于用户访问服务。可以为该计算机设置两个别名(CNAME):WWW和MAIL。这两个别名的全称就http://www.mydomain.com/和“mail.mydomain.com”。实际上他们都指向“host.mydomain.com”。
在cdn中实现原理是:给源站域名添加CNMAE,别名为加速节点的域名。当用户向源站发起请求时,dns服务器解析源站域名时会发现有CNMAE记录,这时dns服务器会向CNAME域名发起请求,请求会被调度至加速节点的域名。
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