VPN server behind a NAT without port forwarding

Question

I installed the SoftEther VPN server on a raspberry pi which is connected via LAN behind a NAT/Firewall. The client should be an Android as well as an iOS device and should access the server from the internet. I tried L2TP as well as an OpenVPN connection but I couldn't connect the Android/iOS device with the VPN server if they were outside the LAN.

The thing is, the router shouldn't be modified for this, so no port-forwarding allowed but if I understood correctly, the udp hole punching method should take care of this. VPN Azure is also no option due to the lack of an iOS and Android client.

Does anyone have an idea why the udp hole punching method isn't working (it is activated) or does anybody know a different method how to achieve this?

In short, I want a little box which I connect to a router either (via LAN or WIFI, doesn't matter) and I can connect to it with an Android or iOS device.

Best regards

ztl · Answer

I think this is not possible. For UDP hole punching you need one more server to be involved. What do you mean with 'it is activated'?
For solutions like L2TP it is necessary that not only UDP packets are routed properly, e.g. you also need IP protocol 47 (GRE) to pass the firewall. NAT makes things more complicated.
If you want to connect to a device behind NAT/Firewall without making any forwarding rules, the device itself has to initiate the connection.
There are probably several ways to do this, it depends on what do you want to do with your box.

VPN server behind a NAT without port forwarding

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