OpenVPN Synology from using IPv4 (working fine) to IPv6 (does not connect) -> ?

[v.vanhem]

EDIT: the windows client is launched from the network of my university... If I test my config file using my mobile phone OpenVPN app in 4G it is working -> I did not pay attention but there is not IPv6 at the university so i suppose the reason I cannot connect

Hello,

So I am changing from an ISP offering public IPv4 and IPv6 to a new one which gives only a private IPv4 but well a public IPv6. I have therefore to switch to IPv6 to access my NAS externally and also to adapt my settings for my OpenVPN connection to the NAS. I have changed all my (sub-)domains DNS with a AAAA pointing to the IPv6 public address of my NAS and I can indeed access my server and NAS services externally without any problem.
However while my OpenVPN connection to the NAS was working perfectly fine using a public IPv4, it is not the case with the public IPv6 address of the NAS.

My config file which is working fine with my public IPv4 address from my previous ISP

dev tun
tls-client

# -> my IPv4 public address
remote vpn.domain.be 1194

dhcp-option DNS 10.8.0.1
pull
proto udp
script-security 2
comp-lzo
reneg-sec 0
cipher AES-256-CBC
auth SHA512
auth-user-pass
setenv CLIENT_CERT 0

<ca>
-----BEGIN CERTIFICATE-----
Private
-----END CERTIFICATE-----

</ca>
key-direction 1
<tls-auth>
#
# 2048 bit OpenVPN static key
#
-----BEGIN OpenVPN Static key V1-----
Private
-----END OpenVPN Static key V1-----

</tls-auth>

From my understanding to connect using IPv6 instead I have juste to replace my IPv4 A vpn.domain.be to my AAAA IPv6 address of the NAS.
+
using proto udp6 instead of proto udp

To be sure it was not a problem of firewall access (which was most probably not the case as all my other NAS services are working fine) I completely exposed my NAS to the outside (firewall router+NAS deactivated) during my testing.

Log with the config file using a AAAA sub-domain with the public IPv6 of the NAS ->

[Apr 9, 2025, 23:25:16] OpenVPN core 3.10.5 win x86_64 64-bit OVPN-DCO built on Dec 17 2024 12:24:32
⏎[Apr 9, 2025, 23:25:16] Frame=512/2112/512 mssfix-ctrl=1250
⏎[Apr 9, 2025, 23:25:16] EVENT: RESOLVE ⏎[Apr 9, 2025, 23:25:17] Transport Error: DNS resolve error on 'vpn.domain.be' for UDP session: asio.system error
⏎[Apr 9, 2025, 23:25:17] Client terminated, restarting in 2000 ms...

Log with the config file using the public IPv6 of the NAS in clear ->

⏎[Apr 9, 2025, 23:25:30] EVENT: DISCONNECTED ⏎[Apr 9, 2025, 23:27:17] OpenVPN core 3.10.5 win x86_64 64-bit OVPN-DCO built on Dec 17 2024 12:24:32
⏎[Apr 9, 2025, 23:27:17] Frame=512/2112/512 mssfix-ctrl=1250
⏎[Apr 9, 2025, 23:27:17] EVENT: RESOLVE ⏎[Apr 9, 2025, 23:27:17] Contacting [2a02:xxxx:xxx:xxxx:xxx:xxxx:xxxx:xxxx]:1194 via UDP
⏎[Apr 9, 2025, 23:27:17] EVENT: WAIT ⏎[Apr 9, 2025, 23:27:17] WinCommandAgent: transmitting bypass route to 2a02:xxxx:xxx:xxxx:xxx:xxxx:xxxx:xxxx
{
    "host" : "2a02:xxxx:xxx:xxxx:xxx:xxxx:xxxx:xxxx",
    "ipv6" : true
}

⏎[Apr 9, 2025, 23:27:17] Transport Error: socket_protect error (UDP)

The OpenVPN config of the NAS



Anyone has an idea of what could be the problem ?

Thank you for your help!