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I was trying to find a IPv6 tunnel after a site needed it, and was about to change the default Teredo config when I realized it was set to Microsoft's servers.

I later ran netsh int teredo set state client.

I visited a site to check if I have IPv6 connectivity, and it turned out I suddenly was able to access an IPv6 only website.

I thought it would be interesting to share this with you.

This is the output of netsh int teredo show state:

Teredo Parameters
---------------------------------------------
Type                    : client
Server Name             : win10.ipv6.microsoft.com.
Client Refresh Interval : 30 seconds
Client Port             : unspecified
State                   : qualified
Client Type             : teredo client
Network                 : unmanaged
NAT                     : restricted (port)
NAT Special Behaviour   : UPNP: Yes, PortPreserving: No
Local Mapping           : 192.168.1.5:62997
External NAT Mapping    : 91.142.61.174:14153

I later tried this on other versions of Windows (below Windows 10), and the "Server Name" didn't have win10 at the beginning. I also had IPv6 connectivity without having to signup for any services (with the default config).

I later tried finding Microsoft's documentation on this service, I wasn't able to find much. I searched Microsoft's site more and found this.

What Microsoft mentioned about the service was:

The Teredo client in Windows XP SP2, Windows XP SP1 with the Advanced Networking Pack for Windows XP, Windows Server 2003 with Service Pack 1, Windows Vista, and Windows Server "Longhorn" automatically attempts to determine the IPv4 addresses of Teredo servers by resolving the name teredo.ipv6.microsoft.com. Alternately, you can use the netsh interface ipv6 set teredo servername= command to configure the IPv4 address of a Teredo server.

Also this article was published in January 01, 2003 and updated in January 15, 2007. So this service must have been available for a long time.

The website I used to test IPv6 connectivity was IPv6-test.com.

What do you think, could this ruin user's privacy and/or is this service safe to use?

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Safety and privacy are probably ok if you leave The firewall enabled. The problem with Teredo is that it's so very unreliable in terms of stability and performance.

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