微軟dns能做cname嗎

This post will use the above question to explore DNS, dig, A records, CNAME records, and ALIAS/ANAME records from a beginner’s perspective. So let’s get started.

這篇文章將使用上述問題從初學者的角度探討DNS ， dig ， A記錄， CNAME記錄和ALIAS/ANAME記錄。 因此，讓我們開始吧。

首先，一些定義 (First, some definitions)

• Domain Name System (DNS): the overall system for converting a human memorable domain name (example.com) to an IP address (93.184.216.34). The IP address is of a server, commonly a web server, where the files needed to display a webpage are stored.

  域名系統 (DNS)：將人類難忘的域名(example.com)轉換為IP地址(93.184.216.34)的整體系統。 IP地址是服務器(通常是Web服務器)的IP地址，其中存儲了顯示網頁所需的文件。

• DNS Server (also known as a name server or nameserver): Uses DNS software to store information about domain addresses. There are several levels — those belonging to each ISP, Root (13 total worldwide), Top Level Domain (TLD, e.g. ‘.com’), and Domain level DNS Servers.

  DNS服務器 (也稱為名稱服務器或名稱服務器)：使用DNS軟件存儲有關域地址的信息。 有幾個級別-屬于每個ISP，根(全球共有13個)，頂級域(TLD，例如“ .com”)和域DNS服務器。

• Domain name: the domain (example) combined with the TLD (.com). The term ‘domain’ is often used synonymously with the domain name, though they are different. When you buy a ‘domain’ from a a registrar or reseller, you buy the rights to a specific domain name (example.com), and any subdomains you want to create (my-site.example.com, mail.example.com, etc).

  域名 ：域名(示例)與TLD(.com)組合。 術語“域”通常與域名同義使用， 盡管它們是不同的 。 當您從注冊商或轉售商處購買“域名”時，即購買了特定域名(example.com)以及您要創建的任何子域(my-site.example.com，mail.example.com，等等)。

高級查詢流程 (High level query flow)

The high-level flow of what happens when you type “example.com” into your browser can be simplified to remove the hops to the ISP, Root, and TLD DNS Servers as below:

可以簡化在瀏覽器中鍵入“ example.com”時發生的高級操作，以刪除到ISP，根和TLD DNS服務器的躍點，如下所示：

A domain typically has two or more name servers, containing records relating to the domain name (example.com).

域通常具有兩個或多個名稱服務器，其中包含與域名(example.com)有關的記錄。

Many types of records can be stored, most of which can have multiple entries per type:

可以存儲多種類型的記錄，每種類型中的大多數可以具有多個條目：

• A: Address records that map the domain name to an IP address

  A ：地址記錄將域名映射到IP地址

• CNAME: Canonical Name Record. Used to alias one domain name (or subdomain name) to another. We’ll look at this in more detail later.

  CNAME ：規范名稱記錄。 用于將一個域名(或子域名)別名為另一個。 我們將在后面詳細介紹。

• MX: Mail eXchange records that tell email delivery agents where they should deliver your email

  MX ：郵件eXchange記錄，告訴電子郵件傳遞代理他們應該在哪里傳遞您的電子郵件

• TXT: flexible Text records, for storing strings for a variety of uses

  TXT ：靈活的文本記錄，用于存儲各種用途的字符串

• SOA: singular Start of Authority record kept at the top level of the domain. Contains specific required information about the domain, for example its primary name server

  SOA ：單一的“開始授權”記錄保存在域的頂層。 包含有關域的特定必需信息，例如其主域名服務器

• NS: The name servers associated with the domain

  NS ：與域關聯的名稱服務器

When your device sends a query that reaches a name server, the server looks in the domain’s record node for an A record, and the associated stored IP address (example.com: 93.184.216.34). This is then returned to the device, to be used to send a request to the correct web server to retrieve the requested webpage or resource.

當您的設備發送查詢到達名稱服務器時，該服務器在域的記錄節點中查找A記錄以及關聯的存儲IP地址(example.com：93.184.216.34)。 然后將其返回給設備，用于將請求發送到正確的Web服務器以檢索請求的網頁或資源。

使用“挖” (Using ‘dig’)

dig (domain information groper) is a command-line tool for querying DNS servers. This command is generally used for troubleshooting, or as now to understand more about the setup of a system.

dig ( 域信息groper )是用于查詢DNS服務器的命令行工具。 該命令通常用于故障排除，或者如現在一樣，以了解有關系統設置的更多信息。

$ dig example.com results in a long response printed to the terminal, the default output detailed here, of which we are interested in the ANSWER SECTION.

$ dig example.com導致打印到終端的響應很長，這是此處詳細介紹的默認輸出 ，我們對ANSWER SECTION感興趣。

;; ANSWER SECTION: example.com. 72703 IN A 93.184.216.34

And there we go, we can see that example.com returns an A record of 93.184.216.34. Sometimes domains will have more than one A record, if more than one web server can provide the information needed.

93.184.216.34 ，我們可以看到example.com返回A記錄93.184.216.34 。 如果多個Web服務器可以提供所需的信息，則有時域將具有多個A記錄。

There’s more! If we try out some other examples, we can soon see that another common record appears: CNAME.

還有更多！ 如果我們嘗試其他一些示例，我們很快就會看到另一個通用記錄出現了： CNAME 。

$ dig www.skyscanner.net:

$ dig www.skyscanner.net ：

;; ANSWER SECTION: www.skyscanner.net. 169 IN CNAME www.skyscanner.net.edgekey.net. www.skyscanner.net.edgekey.net. 5639 IN CNAME e11316.a.akamaiedge.net. e11316.a.akamaiedge.net. 20 IN A 23.217.6.192www.skyscanner.net.edgekey.net. 5639 IN CNAME e11316.a.akamaiedge.net.e11316.a.akamaiedge.net. 20 IN A 23.217.6.192

Using the +short flag allows us to clearly see the path formed:

使用+short標志可以使我們清楚地看到形成的路徑：

$ dig www.skyscanner.net +short

$ dig www.skyscanner.net +short

www.skyscanner.net.edgekey.net. e11316.a.akamaiedge.net. 23.217.6.192

CNAME (CNAME)

A CNAME record allows a domain name to be used as an alias for another canonical (true) domain.

CNAME記錄允許將域名用作另一個規范(true)域的別名。

When the DNS server returns a CNAME record, it will not return that to the client. Rather it will again look up the returned domain name, and in turn return the A record’s IP address. This chain can continue many CNAME levels deep, but then suffers minor performance hits from multiple lookups before caching takes place.

DNS服務器返回CNAME記錄時，不會將其返回給客戶端。 而是它將再次查找返回的域名，然后返回A記錄的IP地址。 該鏈可以深入許多CNAME級別，但是在進行緩存之前，多次查找會對性能造成輕微影響。

A simple example of this could be if you have a server where you keep all your photos. You may normally access it through photos.example.com. However, you might also want it to allow access via photographs.example.com. One way to make this possible is to add a CNAME record that points photographs to photos. This means that when someone visits photographs.example.com they would be given the same content as photos.example.com.

一個簡單的例子就是如果您有一臺服務器來保存所有照片。 您通常可以通過photos.example.com訪問它。 但是，您可能還希望它允許通過photographs.example.com進行訪問。 使之成為可能的一種方式是添加CNAME記錄是點photographs到photos 。 這意味著，當有人訪問photographs.example.com他們將被給予相同的內容photos.example.com 。

Using the query $ dig photographs.example.com we would see:

使用查詢$ dig photographs.example.com我們將看到：

photographs.example.com IN CNAME photos.example.com photos.example.com IN A xx.xxx.x.xxx

It’s important to note that the CNAME is that piece to the right hand side. The left hand side is the alias name, or label.

請務必注意， CNAME在右側。 左側是別名或標簽。

Another common use is for the www subdomain. Having purchased example.com you likely also want users who type in www.example.com to see the same content.

另一個常見用途是www子域。 購買了example.com您可能還希望輸入www.example.com用戶看到相同的內容。

It is worth noting here that example.com can be called the apex, root, or naked domain name.

在這里值得注意的是example.com可以稱為頂點，根或裸域名。

One option would be to set up another A record, pointing to the same IP address as for example.com. This is completely valid, and is what the real example.com does, but it does not scale well. What happens if you need to update the IP address that example.com points to? You would also need to update it for the www subdomain, and any others you may use.

一種選擇是設置另一個A記錄，指向與example.com相同的IP地址。 這是完全有效的，并且是真正的example.com所做的，但是擴展性不好。 如果您需要更新example.com指向的IP地址，該怎么辦？ 您還需要針對www子域以及您可能使用的其他任何域進行更新。

If a CNAME record was used to alias www.example.com to point to example.com then only the root domain would have to be updated, as all other nodes point to it.

如果使用CNAME記錄為www.example.com別名以指向example.com則只需更新根域，其他所有節點都指向該域。

CNAME限制 (CNAME limitations)

At the time when the DNS standards were written, some rules were set out to govern their use. RFC 1912 and RFC 2181 set out that:

在制定DNS標準時，制定了一些規則來管理其使用。 RFC 1912和RFC 2181規定：

• SOA and NS records are mandatory to be present at the root domain

  根域中必須存在SOA和NS記錄

• CNAME records can only exist as single records and can not be combined with any other resource record ( DNSSEC SIG, NXT, and KEY RR records excepted)

  CNAME記錄只能作為單個記錄存在，不能與任何其他資源記錄(DNSSEC SIG ， NXT和KEY RR記錄除外)結合使用

This excludes a CNAME being used on the root domain, as the two rules would contradict each other.

這不包括在根域上使用的CNAME ，因為這兩個規則會相互矛盾。

What’s important here is that this is a contractual limitation, not a technical one. It is possible to use a CNAME at the root, but it can result in unexpected errors, as it is breaking the expected contract of behavior.

這里重要的是，這是合同限制，而不是技術限制。 可以在根目錄使用CNAME ，但由于它破壞了預期的行為約定，因此可能導致意外錯誤。

An example of this is told by Cloudflare, describing problems they encountered with Microsoft Exchange mail servers after having used a CNAME on their root domain:

Cloudflare講述了一個這樣的示例，描述了他們在根域上使用CNAME之后在Microsoft Exchange郵件服務器上遇到的問題：

Domains generally designate the servers that handle their email through what’s known as a MX Record. The problem was that Exchange servers … could pick up the CNAME at the root record and then not properly respect the CNAME set at the MX record. You can’t really blame Exchange. They were operating under the assumptions laid out by the DNS specification.

域通常指定通過所謂的MX記錄來處理其電子郵件的服務器。 問題是Exchange服務器…可能會在根記錄中選擇CNAME，然后不能正確遵守MX記錄中設置的CNAME。 您不能真正責怪Exchange。 它們是在DNS規范提出的假設下運行的。

Here you see the downside that can appear in several server softwares or libraries. Because a standard is in place for a CNAME to be the only record at a node, no other records are looked for. All other records will be silently ignored, without warning or error messages. Even if an MX record was set to receive email, the MX will be ignored as if it doesn’t exist because the CNAME is evaluated first. The same is true if there were an A record: the CNAME would take precedence and the A record would not be read.

在這里，您會看到可能出現在幾種服務器軟件或庫中的缺點。 由于已經建立了將CNAME作為節點上唯一記錄的標準，因此不會尋找其他記錄。 所有其他記錄將被靜默忽略，而不會發出警告或錯誤消息。 即使將MX記錄設置為接收電子郵件，該MX也將被忽略，因為它不存在，因為會首先評估CNAME 。 如果有一條A記錄，也是這樣： CNAME優先，并且A記錄不會被讀取。

現代互聯網 (The modern internet)

So why is this a problem? Why would you ever want to use a CNAME for your root domain anyway? Surely that is the end of the path when looking for the IP address of the web server hosting your content?

那么為什么這是一個問題呢？ 您為何仍要為根域使用CNAME ？ 當尋找托管您內容的Web服務器的IP地址時，路徑肯定結束了嗎？

In the modern internet landscape, that is no longer the case. The world is very different from when the DNS standards were written.

在現代互聯網環境中，情況已不再如此。 與編寫DNS標準時的世界大不相同。

You may choose to use a Platform as a Service (PaaS) provider like Heroku and store content on their web servers. You control the content, but not the infrastructure, and the PaaS provider does the heavy lifting of the network maintenance. They typically provide you with a URL (my-app.herokuapp.com) that is a subdomain of their root domain, and you can view the IP addresses for the web server(s) your content is on. But these are entirely under the PaaS provider’s control, and will change without warning.

您可以選擇使用像Heroku這樣的平臺即服務(PaaS)提供程序，并將內容存儲在其Web服務器上。 您可以控制內容，但不能控制基礎結構，而PaaS提供程序可以極大地減輕網絡維護的負擔。 他們通常為您提供URL( my-app.herokuapp.com )，該URL是其根域的子域，并且您可以查看內容所在的Web服務器的IP地址。 但是這些完全在PaaS提供商的控制之下，并且會隨時更改而不會發出警告。

The scale and frequency of backend changes made by the PaaS provider can make it hard to maintain your root domain A record pointing at a single IP address. Ideally you would wish to do this:

PaaS提供商進行的后端更改的規模和頻率可能使維護指向單個IP地址的根域A記錄變得困難。 理想情況下，您希望這樣做：

example.com IN CNAME my-app.herokuapp.com.www.example.com IN CNAME my-app.herokuapp.com.example.com IN CNAME my-app.herokuapp.com. www.example.com IN CNAME my-app.herokuapp.com.

to allow Heroku (or your chosen host provider) to manage updating the A record that the CNAME points to without any changes made on your side. However, as we now know, this breaks the DNS specification, so is a very bad idea.

允許Heroku(或您選擇的托管服務提供商)管理CNAME指向的A記錄的更新，而無需您做任何更改。 但是，正如我們現在所知，這違反了DNS規范，所以這是一個非常糟糕的主意。

It is possible to simply implement a 301/302 redirect from example.com to www.example.com. However, that instruction takes place either on the web server (so still having the problem of needing to use a fixed A record in DNS to point to that web server), or a custom DNS provider redirect (that suffers complications with HTTPS).

可以簡單地實現從example.com到www.example.com.的301/302重定向www.example.com. 但是，該指令發生在Web服務器上(因此仍然存在需要在DNS中使用固定的A記錄指向該Web服務器的問題)或自定義DNS提供程序重定向( 使用HTTPS會帶來麻煩 )。

This also has the side effect of changing the domain that you see in the URL bar, which you may not want. This method is intended for when your website has permanently moved, or when you’re trying to preserve SEO rankings, rather than solving our problem of pointing to a complex changing backend in a scaleable way.

這也具有更改您可能不希望在URL欄中看到的域的副作用。 此方法適用于您的網站永久移動或試圖保留SEO排名的情況 ，而不是解決我們的問題，即以可伸縮的方式指向復雜的不斷變化的后端。

解決方案 (The solution)

Several DNS providers have now developed custom solutions to work around this problem, including:

現在，一些DNS提供商已經開發了解決此問題的自定義解決方案，包括：

• ALIAS at DNSimple

  DNSimple的ALIAS

• ANAME at DNS Made Easy

  DNS輕松實現ANAME

• ANAME at easyDNS

  easyDNS的ANAME

• CNAME (virtual) at CloudFlare

  CloudFlare的CNAME (虛擬)

These are all virtual record types that provide CNAME like behaviour, with none of the downsides. The exact implementation can differ, but at a high level when the DNS server sees one of these virtual record types, it acts as a DNS resolver. It follows the chain created by the alias until it resolves at an A record (or records) and returns these A records to the DNS server. This ‘flattens’ the CNAME chain into the A record(s) returned, and is indistinguishable to the sent query. The query sees only a pure A record, which doesn’t break the DNS specification, and doesn’t have any of the disadvantages of a CNAME.

這些都是提供CNAME行為的虛擬記錄類型，沒有缺點。 確切的實現可能有所不同，但是在較高級別上，當DNS服務器看到這些虛擬記錄類型之一時，它將充當DNS解析器。 它遵循別名創建的鏈，直到解析為一個A記錄(或多個記錄)并將這些A記錄返回到DNS服務器。 這會將CNAME鏈“拉平”到返回的A記錄中，與發送的查詢沒有區別。 該查詢僅看到純A記錄，該記錄沒有違反DNS規范，并且沒有CNAME任何缺點。

These virtual records can sit alongside other records at the root without any fear of unintended behaviours. Depending on the provider’s method of DNS resolution when following the CNAME chain, they may also have performance benefits from caching previous lookups.

這些虛擬記錄可以與其他記錄一起位于根目錄，而無需擔心意外行為。 根據遵循CNAME鏈時提供商的DNS解析方法的不同，它們還可以通過緩存以前的查詢而獲得性能上的好處。

For a DNSimple setup, we would then configure as below. This solution has all the advantages of domain name aliasing, and none of the risks of using it at root level.

對于DNSimple設置，我們將進行如下配置。 此解決方案具有域名別名的所有優點，并且沒有在根級別使用它的風險。

example.com IN ALIAS my-app.herokuapp.com.www.example.com IN CNAME my-app.herokuapp.com.

Thanks for reading! ?

謝謝閱讀！ ？

As always, open to any corrections or additional points.

與往常一樣，可以進行任何更正或其他要點。

翻譯自: https://www.freecodecamp.org/news/why-cant-a-domain-s-root-be-a-cname-8cbab38e5f5c/

微軟dns能做cname嗎

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