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I am connecting to a VPN using OpenConnect 8.03 on Windows 10. The profile of the VPN only sets a route for the subnet of the VPN. All other IPv4-traffic is still routed via my normal connection. My home setup is dual stacked with IPv4 and IPv6.

If I am not connected to the VPN, IPv6 is preferred over IPv4 as it should be. However, as soon as I connect to the VPN, IPv4 is preferred over IPv6, even for traffic outside the VPN. route print shows no difference in the metrics for IPv4 or IPv6 after connecting to the tunnel, except of course it adds a low metric for the single /16 IPv4 subnet of the VPN. Also netsh interface ipv6 show prefixpolicies shows no differences. In general, IPv6 still works (confirmed by connecting to IPv6-only hosts or when forcing IPv6).

When I connect to the VPN using OpenConnect on Linux instead of Windows 10, IPv6 is correctly preferred over IPv4.

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Apparently, when connected to the IPv4-only VPN, DNS-lookups are done via the IPv4-only connection. In the default configuration, Windows will then only query for A-records, regardless if other interfaces provide IPv6-connectivity.

It is possible to force Windows to query for AAAA-records also on IPv4-only connections. For this, the following registry key needs to be added:

HKLM\System\CurrentControlSet\Services\Dnscache\Parameters\

DWORD "AddrConfigControl" = "0"

If the value doesn't exist, it needs to be created. No reboot is necessary. The setting will take effect more or less immediately.

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