Yesterday's Windows update causes IPv4 to be default
farmer at umn.edu
Thu Nov 15 08:01:09 CET 2012
On 11/14/12 21:52 , Dale W. Carder wrote:
> So, what happens when there is a captive portal to force daily
> registration such as at hotels or for corporate guests? With this
> algorithm it would appear that ipv6 would be effectively permanently
> disabled even though the network is dual stacked.
> Or am I missing something?
How does it work?
Windows does indeed check a Microsoft site for connectivity, using the
Network Connectivity Status Indicator site. There are a few variations
of the connection checking process:
1. NCSI performs a DNS lookup on www.msftncsi.com, then requests
http://www.msftncsi.com/ncsi.txt. This file is a plain-text file and
contains only the text Microsoft NCSI.
2. NCSI sends a DNS lookup request for dns.msftncsi.com. This DNS
address should resolve to 126.96.36.199. If the address does not
match, then it is assumed that the internet connection is not
The exact sequence of when which test is run is not documented; however,
a little bit of digging around with a packet sniffing tool like
Wireshark reveals some info. It appears that on any connection, the
first thing NCSI does is requests the text file (step 1 above). NCSI
expects a 200 OK response header with the proper text returned. If the
response is never received, or if there is a redirect, then a DNS
request for dns.msftncsi.com is made. If DNS resolves properly but the
page is inaccessible, then it is assumed that there is a working
internet connection, but an in-browser authentication page is blocking
access to the file. This results in the pop-up balloon above. If DNS
resolution fails or returns the wrong address, then it is assumed that
the internet connection is completely unsuccessful, and the “no internet
access” error is shown.
So, it looks like there is a test for a captive portal in the IPv4
portion of NCSI. I seems logical that this happens before it gets to
the ipv6.msftncsi.com test, but I don't know for sure.
David Farmer Email: farmer at umn.edu
Office of Information Technology
University of Minnesota
2218 University Ave SE Phone: 1-612-626-0815
Minneapolis, MN 55414-3029 Cell: 1-612-812-9952
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