Use iptables to forward ipv6 to ipv4?

Solution 1:

IPtables cannot currently do this, so you need a userspace process to proxy the connections. socat is a suitable tool for this:

socat TCP6-LISTEN:1234,fork TCP4:1.2.3.4:1234

Solution 2:

As noted in the comments on your question, NAT64 is far from being ready, even 3 years later.

You could, however, try 6tunnel, as suggested by puzzlement.

Fortunately, it is present in the Debian and Ubuntu repositories, so you can install it very easily using sudo apt-get install 6tunnel. If you are using another system, you'll have to build it from source.

Building from source really isn't hard, and is just a matter of running some commands (as root):

git clone https://github.com/wojtekka/6tunnel && cd 6tunnel && ./autogen.sh && make && make install

Here's its syntax, simplified:

6tunnel [-4|-6] [-l local-host] original-port destination-host destination-port

The [-4|-6] is optional and lets you specify whether you'll bind (listen) on IPv4 or IPv6 (respectively).
-l is also optional. It lets you choose on which address (IPv4 or IPv6) you want to bind.
The original port is the port on which you'll bind.
The destination host is where you'll forward the traffic to. It can be anywhere: localhost, or somewhere else on your network or on the Internet.
The destination port is the port on the destination host which will receive your forwarded traffic.

For example, if you want to allow an IPv4-only server, listening on port 1337, to be accessed over IPv6, use:

6tunnel -6 1337 localhost 1337

The above command will listen on port 1337 on IPv6, and forward the traffic to port 1337 on the same machine via IPv4. It will then run in the background, so you don't have to worry about it.

Actually, you should set up a cron job to make sure that it is still running. 6tunnel provides an example for your convenience! Running it on boot shouldn't be a bad idea either.

For more documentation, run 6tunnel -h or man 6tunnel.

Solution 3:

Recent versions of xinetd can also listen on IPv6 and then forward the connection to an IPv4 address.

A sample configuration which listens for IPv6 connections on port 3389 and forwards them to port 3389 of an internal IPv4 address:

service rdp_port_forward
{
    flags           = IPv6
    disable         = no
    type            = UNLISTED
    socket_type     = stream
    protocol        = tcp
    user            = nobody
    wait            = no
    redirect        = 10.187.20.42 3389
    port            = 3389
}

This may be useful in more restricted environments since xinetd is likely to be installed with your base system or available in approved vendor repositories.

Solution 4:

I wanted to comment on Leo Lams answer and upvote, but I do not have enough reputation. First of all: Thank you very much Leo Lam!

For anyone coming along this thread: My ISP changed my connection from IPv4 to IPv6 with Dual Stack Lite, which means I do not have my own IPv4 address anymore. This was an issue as I want to access my IP Camera from anyhwere which does not support IPv6. To resolve this I tried the following:

Enabling IPv6 Forward for port 99 in my Router to my Ubuntu machine.
Ubuntu machine (home network): sudo 6tunnel -6 99 192.168.178.35 80
vServer Debian with static IPv4 and IPv6 address: sudo 6tunnel -4 99 IPV6PREFIXROUTER:IPV6INTERFACEIDUUBUNTUMACHINE 99
vServer Debian: Allow incoming tcp connections on port 99 in iptables

The ipv6 prefix was denoted in my router and the interface id was mapped in the IPv6 forwarding process.

Now I can access the camera from anywhere using the vServer domain example.com:99 .. Perfect! I plan on either getting an raspberry which will fulfill this job or a banana pi m3 for other tasks aswell.

Solution 5:

More for the benefit of people finding this page than the OP necessarily (I came here looking for a solution to IPv6 connectivity for an IPv4 (Twisted) application), one possibility is the application 6tunnel, listens on IPv6 and forwards requests to another interface and port.