跟读练习: Everything You Need to Know About DNS: Crash Course System Design #4 - 通过YouTube学习英语口语
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DNS or domain name system is the backbone of the internet,
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DNS or domain name system is the backbone of the internet,
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but few know exactly how it works.
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In this video, we'll learn all about the system design of DNS.
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Let's dive right in.
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DNS is the internet's directory.
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It translates human readable domain names such as google.com to machine readable IP addresses.
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DNS is a little confusing because there are different types of DNS server in the DNS hierarchy,
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each serving a different purpose.
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When a browser makes a DNS query,
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it's asking a DNS resolver.
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Now this DNS resolver could be from our ISP or from popular DNS providers like Cloudflare's 1.1.1.1 or Google's 8.8.8.8.
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If the DNS resolver does not have the answer in its cache,
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it finds the right authoritative nameserver and asks it.
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The authoritative nameserver is the one that holds the answer.
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When we update a domain's DNS records as a site owner,
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we are updating its authoritative nameserver.
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How does the DNS resolver find the authoritative nameserver?
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This is where the system of DNS gets interesting.
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There are three main levels of authoritative DNS servers.
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There are the root name servers,
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the top level domain or TLD name servers,
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and the authoritative name servers for the domains.
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The root name servers store the IP addresses of the TLD name servers.
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There are 13 logical root name servers.
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Each root name server has a single IP address assigned to it.
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There are actually many physical servers behind each IP address.
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Through the magic of Anycast,
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we get routed to the one closest to us.
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The TLD name servers store the IP addresses of the authoritative name servers for all the domains under them.
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There are many types of TLD names.
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We are all familiar with .com, .org, and .edu.
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There are also country code TLDs like .de and .uk.
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There are many others.
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The authoritative nameservers for a domain provide well,
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authoritative answers to DNS queries.
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When we register a domain,
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the registrar hosts the authoritative nameservers by default,
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but we can change them to others.
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Cloud providers like AWS and Cloudflare run robust authoritative nameservers.
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This hierarchical design makes DNS highly decentralized and robust.
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Now let's walk through the life of a typical DNS query.
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The user types google.com into the browser.
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The browser first checks its cache.
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If it has no answer,
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it makes an operating system call to try to get the answer.
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The operating system call would most likely have its own cache too.
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If the answer isn't there,
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it reaches out to the DNS resolver.
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The DNS resolver first checks its cache.
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If it is not there or if the answer has expired,
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it asks the root name server.
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The root name server responds with the list of the .com TLD name servers.
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Note that since .com is such a common TLD,
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the resolver most likely already caches the IP addresses for those .com TLD name servers.
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The DNS resolver then reaches out to the .com TLD name server and the .com TLD name server
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returns the authoritative name servers for google.com.
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And finally, the DNS resolver reaches out to google.com's authoritative name server,
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and it returns the IP addresses of google.com.
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Now the DNS resolver then returns the IP addresses to the operating system,
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and the operating system returns them to the browser.
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Finally, let's go over some gotchas when updating DNS records for a live high traffic production system.
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DNS propagation is slow because there is a TTL on each DNS record.
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And some of the default TTLs are pretty long.
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Also, not every DNS resolver is a good citizen.
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There are some buggy ones out there that don't honor the TTL.
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To mitigate the risk, there are two practical steps to take.
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First, reduce the TTL for the record that we want to change to something very short,
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say 60 seconds, well in advance before the update actually happens.
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This gives ample time for all the DNS servers to receive the shortened TTL,
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which would allow the actual record update to take effect based on the new shortened TTL.
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Second, leave the server running on the old IP addresses for a while.
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Only decommission the server when traffic dies down to an acceptable level.
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Because some DNS resolvers don't honor the TTL,
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this could take a bit of time and patience.
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This concludes our video on DNS.
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We hope you have a better understanding of how its hierarchical design makes it decentralized and robust.
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Remember, DNS is what translates domain names to IP addresses,
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making it a critical component of the Internet backbone.
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You like our videos, you may like our system design newsletter as well.
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It covers topics and trends in large-scale system design.
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为什么要通过这个视频练习口语？
通过观看和模仿这段关于DNS的视频，您可以在实际语境中练习英语口语。视频内容涵盖了互联网的重要组成部分，使您在掌握技术术语的同时，提升了语言能力。您不仅仅是在学习单词，还在理解如何将人类可读的域名转换为机器可读的IP地址。这个过程会帮助您在雅思口语练习中，流利地表述有关技术话题，以增强您的自信心和表达能力。
语法与表达在语境中的应用
- “DNS is the backbone of the internet” - 这句话展示了一个常见的主谓结构，强调了DNS的重要性。学习这种结构可以帮助您在描述系统或概念时更加清晰。
- “When a browser makes a DNS query” - 这种条件句的使用非常普遍，能帮助您在复杂的句子中保持逻辑的清晰性。
- “This hierarchical design makes DNS highly decentralized” - 通过理解名词短语和形容词的搭配，您可以更好地描述事物的特点和功能。
在这个视频中，快速对比不同类型的DNS服务器的句子也能激发您在讨论技术时使用更复杂的结构。
常见的发音陷阱
在视频中，有些词汇的发音可能会让学习者感到困惑。例如，“authoritative”这个词的重音位置常常会被忽视，它应当强调在第三个音节上。此外，“resolver”的发音对于一些非母语学习者也可能具有挑战性。为了克服这些发音难点，建议您重复视频中的句子，并注意语音的变化和语调，这对提高您的听力和口语都会有很大帮助。
结合这些技巧与“看YouTube学英语”的方法，您可以创造一个理想的学习氛围，利用shadowspeaks和shadowing site等技巧，推动您的英语口语能力迈上新台阶。通过持续的练习，您将能够更自如地参与到技术性的对话中，使您在雅思口语练习过程中脱颖而出。
什么是跟读法？
跟读法 (Shadowing) 是一种有科学依据的语言学习技巧，最初开发用于专业口译员的培训，并由多语言者Alexander Arguelles博士普及。这个方法简单而强大：您在听英语母语原声的同时立即大声重复——就像是一个延迟1-2秒紧跟说话者的影子。与被动听力或语法练习不同，跟读法强迫您的大脑和口腔肌肉同时处理并模仿真实的讲话模式。研究表明它能显着提高发音准确性，语调，节奏，连读，听力理解和口语流利度——使其成为雅思口语备考和真实英语交流最有效的方法之一。