How to transparently tunnel a port from IPv4 to a remote IPv6 device?

Question

At home, I am connected with an IPv6 address and additionally, my provider provides a NAT-like setup through which I receive a public IPv4 address that I share with other customers (the reason being obviously that we are running low on IPv4 addresses).

As a result, I cannot reach my devices at home (e.g. my VPN gateway) when I am on an IPv4 network. However, I do have a server that has both an IPv4 and an IPv6 address. Thus, it should be possible to reach my home devices if I go through my server.

Here is what I have in mind so far: Since on IPv6, each device gets their own IP, my server at home gets a static IPv6 IP. My remote server already has both static IPv4 and IPv6.

When I now want to reach my OpenVPN server at home, previously I would open port 1194 in my router and NAT would pass connections along to the server there. In my new scenario, I want to connect on port 1194 (or a different, doesn't matter) on my remote server and it should take that connection and tunnel it to my home server (seeing as they both have IPv6).

Graphically, this would mean:

Mobile Device (IPv4) --> Remote Server (IPv4+IPv6) --> Home Server (IPv6)

But this should only happen on select ports (or is there even a more clever way than to select by port?).

My question is, how do I achieve this setup?

At which level should this operate? If I want to do it by port, I obviously have to forward the packet at the TCP/UDP layer. My first idea would be iptables, but can iptables forward a packet to a remote IP? Or is there other software that can? Or should I create a tunnel between the two servers and then forward this locally? How would I go about that?

Solution

I finally found a solution with Forwarding IPv4 Ports to IPv6-only Hosts which basically uses socat:

socat TCP4-LISTEN:22,fork,su=nobody TCP6:[2a01:198:79d:1::8]:22

My solution is basically the same except I use a hostname except for a static IP there. Take care not to use square brackes with a hostname as it will then interpret it as an IP.

Comments

[Michael Hampton]

Have you considered using an IPv6 tunnel broker for your laptop?

[javex]

@MichaelHampton thanks for the suggestion, but I much rather would like to have a way that does not need changes on the device as I also intend to make connection from my phone (android) and likely other devices. The setup should be as simple as before from the devices perspective.