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.

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问题

超级用户读者N.Hinkle提出了以下有关Windows internet检查和身份验证过程的问题：

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?

在使用现代操作系统时，这样的检查是我们认为理所当然的事情之一，但即使是用户体验中最理所当然的元素也有其内在机制。Windows如何告诉我们是否有internet连接以及是否需要登录Wi-Fi验证门户？

答案

超级用户贡献者Tobias Plutat提供了对流程的一些见解：

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强调了Tobias参考文献中的一些关键引用：

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.

虽然整个过程对最终用户通常是100%透明的，但偶尔在通过Wi-Fi热点的web门户进行身份验证之后，您会发现自己盯着NSCSI.txt文件. 你最终是如何看到一个不起眼的文本文档而不是你试图加载的网页，再加上之前关于这个主题的答案，这已经不再是一个谜了。

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