If you need to set up port forwarding from one IPv4 port to another IPv6 port, we will use socat.

It is assumed that IPv6 is already configured and working. You can check this with the commands ping6 <domain> in Linux or ping -6 <domain> in Windows.

In rare cases, ping may work, but actual traffic does not flow. You can further verify connectivity using the command curl --ipv6 -k -I https://ipv6.google.com for both Linux and Windows (if curl is not available in cmd, open PowerShell).

Suppose we have a task to forward port 443 from IPv4 to port 4445 on IPv6.

1. Installing socat

apt install socat

2. Port Forwarding

socat TCP6-LISTEN:4445,fork,reuseaddr TCP4:127.0.0.1:443

Ensure that port 443 is accessible locally, or change it to an external IP.

telnet 127.0.0.1 443

2.1 Verification

telnet <ipv6> 4445

3. Creating a Service

Now, let’s create a service file so that the forwarding resumes automatically after a reboot without needing to run it manually. We’ll also ensure that the nginx service starts before our command.

nano /etc/systemd/system/socat-portforward.service

[Unit]
Description=Socat IPv6 to IPv4 Port Forwarding
After=network.target
After=nginx.service
Requires=nginx.service

[Service]
ExecStart=/usr/bin/socat TCP6-LISTEN:4445,fork,reuseaddr TCP4:127.0.0.1:443
Restart=always
RestartSec=3
User=root
Group=root
KillMode=process

[Install]
WantedBy=multi-user.target

3.1 Stopping the Service

Press Ctrl + C or, if you ran it with &:

killall socat

3.2 Starting as a Service

systemctl daemon-reload
systemctl enable --now socat-portforward
systemctl status socat-portforward

3.3 Startup Sequence

We added lines to the [Unit] section to ensure the correct startup sequence:

After=nginx.service
Requires=nginx.service

Done! This command can be used to configure port forwarding in various setups.