Windows is quite adept at telling you if you have a properly functioning Internet connection, but how exactly does it do so? Digging into how Windows handles the problem offers insight into Windows connectivity messages.

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，它是一個由Q&a網站組成的社群驅動分組。

問題

超級使用者讀者N.Hinkle提出了以下有關Windows internet檢查和身份驗證過程的問題：

In Windows 7, the notification area networking icon will show an error indicator if there is no internet access , and the error icon goes away once there is a successful connection to the internet . Sometimes, if the WiFi connection requires an in-browser authentication step, like on many guest networks in hotels or universities, then the following pop-up bubble appears, saying as much:

How does Windows know whether or not it has a successful internet connection?

Presumably it is checking some online Microsoft service to see whether it has a successful connection, gets redirected to some other page, or doesn’t get any resp***e at all, but I haven’t seen anywhere that this process or the services used are documented. Can anybody explain how this works?

在使用現代作業系統時，這樣的檢查是我們認為理所當然的事情之一，但即使是使用者體驗中最理所當然的元素也有其內在機制。Windows如何告訴我們是否有internet連線以及是否需要登入Wi-Fi驗證門戶？

答案

超級使用者貢獻者Tobias Plutat提供了對流程的一些見解：

After some digging (the sheer number of network and Internet related services in Windows is astonishing), I think I found it. Windows Vista and 7 have a variety of Network Awareness features, one of which is the Network Connectivity Status Indicator that performs connectivity tests that in turn are used by the network systray icon. The test for internet connectivity is simple:

1. NCSI tries to load a specific page via HTTP (more precisely: a text document) and tests whether it can be retrieved.
2. If that is not successful, Windows reports “No Internet access”.

The mechani** also checks whether the domain the document is hosted on resolves to the expected IP address. So, it might also assume proper internet access if this test is successful but the document can’t be retrieved.

The reason it reports “No Internet Access” when you haven’t authenticated on a Hotspot yet lies in the way a Hotspot works. It blocks all ports besides 80 and 443 (for HTTP and HTTPS, respectively), which get redirected to the Hotspot’s authentication server and might mess with DNS requests in one way or another. Thus, NCSI can neither resolve the domain its test file is hosted on, and even if it could it wouldn’t reach the actual file because HTTP traffic is redirected to the Authentication server.

另一位撰稿人Jeff Atwood強調了Tobias參考文獻中的一些關鍵引用：

Here are the details of the connection status determination process:

The following list describes how NCSI might communicate with a Web site to determine whether a network has Internet connectivity:

1. A request for DNS name resolution of dns.msftncsi.com
2. A HTTP request for http://www.msftncsi.com/ncsi.txt returning 200 OK and the textMicrosoft NCSI

This can be disabled with a registry setting. If you set:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ NlaSvc\Parameters\Internet\EnableActiveProbing

to 0, Windows will no longer probe for internet connectivity.

Apple does something very similar in iOS to detect internet connectivity and possible hotel “login” wifi pages, etc.

雖然整個過程對終端使用者通常是100%透明的，但偶爾在透過Wi-Fi熱點的web門戶進行身份驗證之後，您會發現自己盯著NSCSI.txt檔案. 你最終是如何看到一個不起眼的文字文件而不是你試圖載入的網頁，再加上之前關於這個主題的答案，這已經不再是一個謎了。

有什麼要補充的解釋嗎？在評論中發出聲音。想從其他精通技術的Stack Exchange使用者那裡瞭解更多答案嗎？在這裡檢視完整的討論主題。