I may have found something that doesn't work well with the exposed host setup: vpn over ipv6 tunnel.
The issue affect my mobile phone since the Deutsche Telekom network has IPV6 enabled (cool!)
Basically, it works the first time after booting. After the first disconnection of the ISP router in front of mine, dyndns will eventually be updated but crucially no network event is generated so neither the tunnel nor openvpn are restarted.
Maybe one could emit such an event when dyndns finally notices the change in the external IP but I am not sure if that is possible.
Is my understanding of the problem correct?
EDIT: actually, every external service that is dualstack (think youtube, but also ZDF) will first try the tunnel and then switch to ipv4 after timeout.... I guess I'll schedule a reboot early in the morning and be on my way, if there are no "cleaner" solutions.
NOPE: even with this trick, after a while the tunnel still gets stuck. Even with a ping every minute as keepalive...