Knowledge Base

My application needs an IPv4-formatted IP on Linux, but ip addr shows IPv6s only. How can I communicate to/from it?

Common problems sometimes demand uncommon solutions

Occasionally, one or other (not updated/lazy) application may not support direct usage when the starting point is an IPv6 address, because they are designed to use an IPv4-formatted address. Despite this, they usually are tolerant to the IPv4 range you're using, allowing you to specify internal IPv4s (which are in the range 10.x.x.x/8, 172.16.x.x/12 - 172.32.x.x/12, or 192.168.x.x/16), as you would already have on any other purchased NAT VPS, since they don't distinguish between addresses.

The only difference between C-Servers and other providers on NAT VPS is that while others provide a IPv4 NAT internal address by default (which is not public, only private), we prefer to provide an IPv6 public address that is dual-function (also sends/recieves IPv4 requests) and then translate + internally route your request to/from the main server's IPv4. 

Due to SolusVM 2 templates and cloud-init images not predicting an alternative internal IPv4 address, and C-Servers' stance as an IPv6-first company, we don't provide by default such addresses. However, in this guide, we will show you how to have IPv4 connectivity on applications that demand an IPv4-only address. This is accomplished by a simple three-step process:

- disabling cloud-init on your VPS;
- adding the internal IPv4 address to the system;
- creating an internal socat between that IPv4 and IPv6 for full connectivity.

Packets that arrive in your VPS from IPv6 are already complete, the redirection is entirely internal at the TCP/IP socket level, and this has been a solution used for quite a while now. 

How to have native internal IPv4 connectivity on MultiVPS

1. First, let's deactivate cloud-init changes for your VPS. This is merely an example; you may want to manually edit a different area to deactivate cloud-init or even to edit directly the networking section of your Linux distro. 
   
   SSH or VNC into your server and create a file to prevent cloud-init from running on subsequent boots:
   
   sudo touch /etc/cloud/cloud-init.disabled
   
   If this does not work, you may wish to edit directly your file to avoid any interface recreation that erases what we're about to do. Since cloudinit (sometimes formatted as cloud-init) is merely a script run by SolusVM 2 at the end of each restart, you may simply do
   
   which cloudinit
   
   or
   
   which cloud-init
   
   in order to see where your script is on the disk - sometimes it's loaded from the network config file section, sometimes it's a full script on its own right. Another alternative is to check directly the Linux system log. Simply write, on your command line:
   
   journalctl
   
   and use PgDn / Page Down key on your keyboard to scroll down until the last boot log you've made on your VPS - which is exactly the screen you see during every Linux boot. Have attention to the hour/day provided on the left side of each log to track down cloud-init actions and locations.
   
   For further support you can always use Google or an AI when configuring this. 

2. Add a static IPv4 address to your network interface. This is merely an example for Enterprise Linux and Debian-based distros, kindly check the specific process needed for your chosen OS and version Edit the network configuration file (location depends on your distro):

For Ubuntu 22.04/24.04 (using Netplan):

For Enterprise Linux (RHEL/CentOS/Tencent/AlmaLinux/Rocky Linux, example for RHEL 9/AlmaLinux 9), on a normal command line via SSH or VNC, write:

nmcli con mod eno1 ipv4.addresses 192.168.1.100/24 [you can insert any other private IPv4 range, ensure subnet and IP are OK]
nmcli con mod eno1 ipv4.gateway 192.168.1.1 [from within the same private IPv4 range chosen, always the first address of it]
nmcli con mod eno1 ipv4.dns 8.8.8.8 [use any regular IPv4 DNS]
nmcli con mod eno1 ipv4.method manual [do NOT insert auto, this is a non-DHCP address]

3. Install socat if it's not already installed:

4. Create a systemd service to run socat and forward traffic from your internal IPv4 to IPv6:

Add the following content:

Replace:

• 10.0.0.10 with your desired internal IPv4 address
• [YOUR_IPV6_ADDRESS] with the public provided IPv6 address for your VPS
• PORT with a port number you want to forward

5. Enable and start the service:

6. If you need multiple ports forwarded, you can either create multiple service files or modify the socat command to handle multiple ports.

Socat works TCP/UDP, from IPv4 to IPv6, and from IPv6 to IPv4. Examples of commands are:

socat UDP4-LISTEN:10683,bind=10.0.0.10,reuseaddr,su=nobody,fork UDP6:[2a00::212:4b00:615:a1f7]:10683 » says to listen for UDP at port 10683 on the internal IPv4 connection, routes to a public IPv6 address at the given address and at the same port.

socat TCP6-LISTEN:10022,bind=[2a00::212:4b00:615:a1f7],reuseaddr,su=nobody,fork TCP4:10.0.0.10:10022 » says to listen for TCP under IPv6 at port 10022 on the given public IPv6 address, routes to IPv4 VPS internal IP stack at the same port.

Fair warning

This solution is persistent and will stay on every reboot.

However, if this does fail for some reason (e.g. updates that change systemd, socat or networking functions), you may need to create a script to run on every boot - crontab or similar scripting tools may be interesting for this - such as that the IPv4 is reinserted upon every boot on your VPS. If that is needed, it may be also a good idea to do the same to socat.

Solus VM 2 has cloud-init, which usually provides the network info upon every boot, erasing the previous information - unless a user disables it and automates the process of including an IP upon boot on their VPS (which, since it originates from userspace, never collides with cloud-init). 

If everything else goes wrong, simply use our C-Servers GoBack backup service to restore your VPS to an earlier point in time, or simply reinstall the VPS using one of our OS templates, from scratch.

One thing is for sure

This solution will fix most issues with IPv4-only applications.