- https://service.avm.de/help/en/FRITZ-Box-7590/019/hilfe_vpn_oeffentliche_ipv4_adresse (in English)
- https://service.avm.de/help/de/FRITZ-Box-Fon-WLAN-7490/019/hilfe_vpn_oeffentliche_ipv4_adresse (in German)
- https://en.avm.de/service/fritzbox/fritzbox-7590/knowledge-base/publication/show/200_FRITZ-VPN-cannot-establish-a-VPN-connection-to-FRITZ-Box/ (in English)
- https://avm.de/service/fritzbox/fritzbox-7360/wissensdatenbank/publication/show/200_FRITZ-Fernzugang-kann-keine-VPN-Verbindung-zur-FRITZ-Box-herstellen/ (in German)
- https://en.wikipedia.org/wiki/IPsec
Fo connecting to a FRITZ!Box via VPN à la Cisco IPsec the remote server (the FRITZ!Box) must have an IPv4 address (only) – if it also has an IPv6 address, connecting won’t work out. It took me quite some time this afternoon to figure this out.
If the DNS resolution also delivers an IPv6 address, that seems to confuse the VPN client.
2020-06-15 : “IPv6 + IPv4” (“Use native IPv6 connection” + “Establish IPv4 connection via DS Lite”) does not seem to be an obstacle defeating a VPN connection. I have no idea, why it did not work out before.