Issue on 3.11 with TAP-driver and ip V6

Hello,

When using openvpn3 (3.11 or 3.11.2) on windows 10 LTSC 2019, TAP driver (dco not available) I'm not able to communicate when server (2.6.3) assign an IPV6.

• before starting openvpn3, net adapater is available and not connected.
• after starting openvpn3 the client connect on server, get configuration, net adapater is marked connected and an ipV6 address is set.
• nc command executed on serveur not work (nc -6 -q 1 -w 1 ) .

If I change the server configuration to use IP V4, communication (nc) is ok (nc -q 1 -w 1 )

If I use openvpn 2.6.12 with ipv6, communication (nc) is ok (nc -6 -q 1 -w 1 ) .

I don't see any relevant error message in log except a route definition issue

Mon Jun 23 16:30:22 2025 TAP ADAPTERS:
guid='{915B103D-925B-41BD-AA78-DF962D2243A1}' index=3 name='OpenVPN TAP-Windows6'

Open TAP device "OpenVPN TAP-Windows6" PATH="\\.\Global\{915B103D-925B-41BD-AA78-DF962D2243A1}.tap" SUCCEEDED
TAP-Windows Driver Version 9.27
ActionDeleteAllRoutesOnInterface iface_index=3
netsh interface ipv6 delete route fc86:56bb:3d06:7189::/64 3 store=active
Ok.

netsh interface ipv6 set address 3 fc86:56bb:3d06:7189:32a9:60e:23d:f005 store=active

netsh interface ipv6 add route fc86:56bb:3d06:7189:32a9:60e:23d:f006/64 3 fe80::8 store=active
The parameter is incorrect.


action exception: nrpt_error: cannot open NRPT base key
DNS::ActionCreate interface name=[OpenVPN TAP-Windows6] search domains=[]
DNS::ActionApply: successful
ipconfig /flushdns

Windows IP Configuration

Is existing specific limitation or is it a bug ?

Reproduce the report on Windows 10 LTSC 2019 with OpenVPN 3.11 or 3.11.2, the TAP-Windows6 adapter, and an IPv6-assigned server configuration. Start by examining the logged netsh interface ipv6 add route failure and compare the behavior with OpenVPN 2.6.12. Done means identifying whether the route error is the cause and restoring successful IPv6 connectivity with the nc -6 test.