Don't use DualMode sockets in environments where ipv6 is disabled/unavailable

[mastahg]

Describe the feature you'd like supported

I have a situation where end users with unreliable ipv6 setups are being impacted and disabling ipv6 at either the .net runtime level or having the end user disable ipv6 systemwide is the easiest method to remedy the situation. These users are then unable to utilize http3/quic due to the use of dualmode sockets

Proposed solution

Don't use DualMode sockets in environments where ipv6 is disabled/unavailable

Additional context

This was originally raised on the .net runtime github here dotnet/runtime#113100

[anrossi]

Which OS are you using?

[mastahg]

Windows, with around half on windows 10 and half on windows 11.

[anrossi]

Just making sure I understand the full issue here: you disable IPv6 support in the windows OS, and expect MsQuic to just use IPv4 only, but what actually happen is you can't connect to any IPv4 or IPv6 address using HTTP/3?

[mastahg]

That is correct. I was unable to replicate it being an issue disabling it from the system side personally but at least one user encountered it. I was able to reproduce the issue by enabling the flag System.Net.DisableIPv6

There has been some progress on the original ticket introduced in the runtime repo.

[nibanks]

I'm not sure there is anything we can really do. We require dual mode sockets.

[nibanks]

At this point, we don't have enough justification to make a change here. We strongly discourage anyone from disabling IPv6. In fact, large amounts of work have been going on to make things work exclusively over IPv6. That being said, if you propose a PR, we will review it and decide further if we want to accept at that point.