Lecture 194: What is a DNS?
- 课程：[NEW] AWS Certified Advanced Networking Specialty 2026
- 视频时长：06:24
- 来源：Udemy Lecture 28556373
- Slide 数量：6
- 画面切换：01:11、02:01、02:51、04:19、05:01
扫描说明
逐条读取了 Udemy 页面中的 144 条英文 Transcript cue，并按时间轴扫描全长约 06:24 的视频。依据 DNS 作用与层级、基础术语、FQDN/URL 分解、Local/Root 查询、TLD Referral、Authoritative Answer/Cache 划分为 6 个小节。截图均在 Transcript 侧栏关闭、播放器控件隐藏后复核，统一为 1364×770。
Transcript 来自 Udemy 页面自带的英文自动字幕，没有使用 ASR。下面保留页面原文，包括其中的语法或识别错误。
1. DNS 把人类可读 Hostname 翻译为服务器 IP
时间范围：00:00–01:11
总结 / Summary
中文
Domain Name System（DNS）把 www.google.com 等 Human-friendly Hostname 解析为浏览器实际连接的 IP Address，是 Internet 的基础组件。DNS 命名采用层级结构：Root 下有 .com 等 Top-level Domain，再往下是 example.com、www.example.com 与 api.example.com 等更具体名称。
English
The Domain Name System translates human-friendly hostnames such as www.google.com into the IP addresses that browsers actually connect to, making it a foundation of the Internet. DNS naming is hierarchical: the root contains top-level domains such as .com, followed by increasingly specific names such as example.com, www.example.com, and api.example.com.
Transcript
00:00 Okay, so before we talk about Route 53
00:02 we have to talk about what is a DNS?
00:05 So, this is a basic level lecture,
00:08 but at least it will help you understand how DNS works.
00:10 And this is something you've been using
00:12 behind the scenes every day
00:13 but you don't know about it exactly.
00:15 So, let's have a look.
00:16 So, a DNS is a domain name system, and what it will do
00:19 is that it will translate human friendly hostnames
00:22 into the target server IP addresses.
00:25 For example, when you type in your web browser,
00:27 www.google.com, it will in the end
00:31 give you back an IP address and this is IP address
00:35 that your web browser will be able to access
00:36 behind the scenes and get some data back from Google.
00:40 So, the DNS is the backbone of the internet.
00:42 It is a way for you to understand
00:44 how to translate these URLs, these hostnames into IPs.
00:49 So, there is a hierarchical naming structure for DNS
00:53 and the idea is that at the root
00:54 of www.google.com for example, there is the .com,
00:58 but then there is example.com which is a bit more precise.
01:01 Then, www.example.com or api.example.com.
01:06 So, all of these are going to be the hierarchy
01:09 of your domain names.
2. Registrar、Record、Zone File、Name Server、TLD 与 SLD
时间范围：01:11–02:01
总结 / Summary
中文
Domain Registrar（如 Route 53 或 GoDaddy）负责注册域名；DNS Record 有 A、AAAA、CNAME、NS 等类型；Zone File 保存某个 Zone 的记录；Name Server 解析 Query，可为 Authoritative 或 Non-authoritative。TLD 包括 .com/.org/.gov，SLD 则是 amazon.com 或 google.com 这类二级域名。
English
A domain registrar such as Route 53 or GoDaddy registers domain names. DNS records include A, AAAA, CNAME, NS, and other types; a zone file stores the records for a zone; and name servers resolve queries as authoritative or non-authoritative servers. TLDs include .com, .org, and .gov, while SLDs include names such as amazon.com and google.com.
Transcript
01:11 Next, we need to define a little bit
01:13 of terminology regarding your DNS.
01:15 So, there is a domain registrar.
01:17 This is where you're going to register your domain names
01:20 and it could be Amazon Route 53, or it could be GoDaddy
01:23 or any other domain registrar you can find online.
01:26 Then you have DNS Records and they have different types
01:28 and we'll have a look at them in detail in this section.
01:30 So it could be A, AAAA, CNAME, NS, et cetera, et cetera.
01:34 Don't worry, we'll see those in detail in this section.
01:37 A zone file which contains all the DNS records.
01:40 So, this is how to match these hostnames
01:42 to IPs or addresses.
01:44 Name servers is servers
01:46 that will actually resolve the DNS queries.
01:48 And we'll have a look at them as well in this section.
01:50 Top level domains which is .com, .us, .in, .gov, .org,
01:54 et cetera, et cetera.
01:55 Second level domain which is amazon.com and google.com.
01:58 So you can see there are two words in between a dot.
3. FQDN 与 URL 由不同层级和协议组成
时间范围：02:01–02:51
总结 / Summary
中文
以 http://api.www.example.com. 为例，末尾点是 DNS Root，.com 是 TLD，example.com 是 SLD，www.example.com 是 Subdomain，api.www.example.com. 是 Fully Qualified Domain Name（FQDN）。前面的 http 是 Protocol；Protocol 与完整名称等组合在一起形成 URL。
English
For http://api.www.example.com., the trailing dot is the DNS root, .com is the TLD, example.com is the SLD, www.example.com is a subdomain, and api.www.example.com. is the fully qualified domain name (FQDN). The leading http is the protocol, and the protocol plus the full name and related components form the URL.
Transcript
02:01 And so if we take a look for example
02:03 at this FQDN, so fully qualified domain name,
02:06 we have http://api.www.example.com.
02:12 Okay?
02:13 So, the last dot the end is called the root
02:15 and it is the root of all the domain names.
02:18 Then the .com,
02:20 so .com is your TLD, so that's your top level domain.
02:25 The example.com is going to be your second level domain.
02:31 Then we have www.example.com.
02:34 That's your subdomain.
02:36 api.www.example.com is your FQDN,
02:41 your fully qualified domain name.
02:44 HTTP is going to be your protocol
02:46 and all these things together is going to be your URL.
4. Local DNS 从 Root Server 获得 .com Name Server Referral
时间范围：02:51–04:19
总结 / Summary
中文
浏览器查询 example.com 时，先把请求交给由 Company 或 ISP 分配的 Local DNS Server。Cache 未命中后，Local DNS 询问 ICANN 管理的 Root DNS Server；Root 不直接返回 Web Server IP，而返回 .com TLD 的 NS 与地址，让 Resolver 沿命名层级逐步接近答案。
English
When a browser queries example.com, it first asks the local DNS server assigned by its company or ISP. On a cache miss, the resolver asks an ICANN-managed root DNS server. The root does not return the web-server IP; it returns the .com TLD name server and address, moving the resolver one level closer to the answer.
Transcript
02:51 So, now that we know a little bit of terminology,
02:53 let's have a look at how DNS works.
02:55 So we have a web server, and let's say for example
02:58 that we have an IP, it's a public IP,
03:00 could be an EC2 instance, for example.
03:02 And the public IP is 9.10.11.12
03:06 and we want to be able to access this
03:09 using the example.com domain name.
03:11 So, we're going to register this example.com domain name
03:13 on one of our servers for the DNS.
03:16 But let's see how the computer, your web browser,
03:19 can access it and can get that response.
03:22 So, your web browser is going to want to access example.com.
03:25 And to do so, it's going to ask its local DNS server.
03:28 "Hey, do you know what example.com is?"
03:30 Now, this local DNS server is usually assigned
03:33 and managed by your company or assigned
03:35 by your internet service provider dynamically.
03:38 And if the local DNS server has never seen this query before
03:41 what it's going to do is first ask the root DNS server
03:45 managed by the I-C-A-N-N, the ICANN organization
03:49 and is going to say, "Hey, do you know what's example.com?"
03:52 Which is the first server that's going to be asked.
03:55 And the Root DNS Server is going to say,
03:57 "I've never seen it, but I know .com."
04:00 So, .com is NS, so it's an NS Record Name server
04:04 and go see 1234 this public IP.
04:07 So, this is saying to local DNS,
04:08 "Hey, I don't have this answer
04:10 but I'm getting you a little bit closer to your answer
04:12 because I know the .com domain,
04:14 and the .com domain name server has this IP, 1234."
5. .com TLD Server 再指向 example.com Authoritative Server
时间范围：04:19–05:01
总结 / Summary
中文
Local DNS 接着询问 IANA 管理的 .com TLD DNS Server。TLD Server 仍不保存 example.com 的最终 A Record，而是返回负责 example.com 的 NS 与 Public IP（示例 5.6.7.8）。Resolver 因此知道应向哪个 SLD/Authoritative DNS Server 发出最后一次查询。
English
The local resolver next asks the .com TLD DNS server managed by IANA. The TLD server still does not store the final A record for example.com; it returns the name server responsible for example.com and its public address (5.6.7.8 in the example). The resolver now knows which SLD or authoritative server to query last.
Transcript
04:19 So the local DNS server saying, "Okay, good.
04:21 Now I'm going to ask the top level domain."
04:24 So the .com domain server at 1234.
04:27 "I'm going to ask for the answer of my query."
04:30 So, this is another domain managed by I-A-N-A, the IANA
04:34 and the example.com, okay, is going to be asked again
04:38 to this DNS server.
04:40 So, do you know about example.com?
04:42 And the DNS server is gonna say,
04:43 "Hey, I do know about example.com.
04:46 I don't have the answer to your query right away.
04:49 I don't know which record it is,
04:50 but there is a server called example.com that I know about
04:54 which is at 5.6.7.8, that's a public IP
04:57 that you should ask the answer to your question.
6. Authoritative Server 返回 A Record，Resolver 缓存并答复客户端
时间范围：05:01–06:24
总结 / Summary
中文
最终，Local DNS 询问由 Registrar/Route 53 等运营的 example.com Authoritative Server，获得 A Record 9.10.11.12。Resolver 把结果放入 Cache，以便后续相同 Query 快速返回，再把 IP 交给 Browser；浏览器随后直接用该地址连接 Web Server。
English
Finally, the local resolver asks the example.com authoritative server operated by the registrar or Route 53 and receives the A record 9.10.11.12. The resolver caches the result for faster repeated queries and returns the IP to the browser, which then connects directly to the web server.
Transcript
05:01 So, local DNS server is then going go to our final server
05:05 which is the second-level domain DNS server
05:08 and this is a server that is going to be managed
05:11 by your domain registrar.
05:12 So it could be, for example, Amazon Route 53 and so on.
05:16 So the DNS server is gonna say,
05:17 "Hey, do you know about example.com?"
05:19 And the DNS server will have an entry for example.com.
05:22 And so it will say,
05:23 "Hey, yes, of course I know example.com."
05:25 And it turns out that example.com,
05:27 I know that it's an A record
05:29 and that the result of it is the IP 9.10.11.12.
05:34 So, the DNS server now knows the answer
05:36 by recursively asking DNS servers
05:39 and finding the most specific one.
05:42 And then it says, "Okay, hey, yes.
05:43 I'm going to cache that answer right away
05:46 because I want to be able to,
05:47 if someone is asking me again for example.com
05:49 I want to get it right away, give them the answer."
05:52 So, it's going to send back the answer into your browser
05:56 and your web browser now has the answer
05:58 and using this IP address
06:00 then is going to be able to access your web server.
06:03 So, this is how DNS works.
06:04 So, you've been using DNS behind the scenes
06:07 all along all your life.
06:08 For example, when you access www.google.com
06:10 you're using DNS or any website.
06:12 But now we see how the DNS queries work.
06:15 So, this is just some background knowledge
06:16 because now we're going to go into Route 53
06:18 and learn how to manage a DNS server on our own.
06:21 So I hope you liked it
06:22 and I will see you in the next lecture.
整讲总结 / Overall Summary
中文
DNS 是分层命名与解析系统，把 Hostname 转换为 IP。Resolver 在 Cache Miss 时依次经过 Root、TLD 与 Authoritative Name Server：Root 返回 TLD Referral，TLD 返回域的 NS，Authoritative Server 返回最终 Record；Local DNS 缓存答案后交给客户端。
English
DNS is a hierarchical naming and resolution system that converts hostnames into IP addresses. On a cache miss, a resolver walks from root to TLD to authoritative name server: the root returns a TLD referral, the TLD returns the domain's name server, and the authoritative server returns the final record. The local DNS caches the answer and returns it to the client.
返回顶部 ↑