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The Windows 7 network icon down in the system tray (sorry, taskbar notification area) shows your connectivity status - but more than just whether the connection is on or off, it also apparently can tell if your Internet connection is down.

Case in point: my Internet connection went down, and the icon changed to this:

Windows 7 network connection icon

Note the little yellow exclamation point overlay. Windows put this icon there just after my Internet connection went down - less than a minute I would say, although I didn't time it.

It's worth mentioning that my local LAN was still up and running fine.

My question is... how did Windows know this?

Is it monitoring network traffic and watching for failures? Did the router (a modern uPnP router) somehow tell Windows that the WAN connection had gone down? Does Windows periodically ping certain servers to test Internet connectivity?

(p.s. I mention Windows 7 because that's what I have and I don't recall if Vista had the same behavior as I didn't have it long enough to find out. I do remember that Windows XP and earlier did not have this "feature.")

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marked as duplicate by Heptite, Kevin Panko, Tog, Dave, studiohack Apr 30 at 22:04

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1 Answer 1

up vote 11 down vote accepted

This behavior is activated by setting the DWORD value EnableActiveProbing in

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

to the value 1, and respectively 0 to disable it.

On certain events like login or connection problems it checks DNS with nslookup dns.msftncsi.com and verifies that it returns 131.107.255.255, then to test your HTTP it tries to download ncsi.txt from msftncsi.com. It should contain Microsoft NCSI, which stands for Network Connectivity Status Indicator...

More information can be read in the Microsoft Technical Library.

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2  
I wish my android phone would do something similar to determine if the wifi connection really works, or if it's just connected but there is some stupid website spoofing dns until some checkbox is clicked. –  Jerry Apr 11 '11 at 23:39
    
Not sure if it's just Gingerbread but on my Nexus S, wifi/data indicators turn green when it could connect to the Google servers. Otherwise, the wifi/data indicators are white. –  Belmin Fernandez Apr 12 '11 at 22:06

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