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Pro Tip for Proxmox Users: Need to Add Multiple IP Addresses for a Single NIC in LXC?

A Note About Proxmox and LXC (Linux Containers)

For those unfamiliar with LXC and Proxmox, LXC is similar to a virtual machine that runs a guest OS (Windows, Linux, Mac, Android, etc.) but the container part of LXC excludes the core part of the OS and simply provides networking and storage inside a container. Unlike virtual machines, applications inside a container can access resources on a host system directly. Proxmox is a hypervisor for running virtual machines and Linux containers (LXC) in a server hardware.

IP Addresses and Subnetting

Do you need to have multiple IP addresses assigned to a network interface inside an LXC container? In terms of networking, a single NIC can have multiple IP addresses. This is useful if you want to run a single server with multiple websites that have their own IP address. Here's what I mean:

172.20.31.0/23

A small HTML file with a list of websites hosted by the web server.

172.20.31.1/23

A web application running Adminer, a lightweight alternative to phpMyAdmin.

172.20.31.2/23

A development version of my website that mirrors a production version. Anyone who visits my site sees my production version of my website. Once I test the changes I made in the development website, I push the changes up to the production website.

172.20.31.3/23

A custom-built web application for taking notes. Any notes written in HTML gets shown up in the web browser.

172.20.31.4/23

A development version of the note-taking web application taken from 172.20.31.3.

A note for those new to networking: pay attention to the subnet. A slash 23 subnet can start with 172.20.30.1 and ends at 172.20.31.254. Both 172.20.30.255 and 172.20.31.0 are both valid IP addresses. 172.20.30.0 is a network address and 172.20.31.255 is a broadcast address. Cisco has an article about IP addressing and subnetting in order to help you understand how subnetting works. With that out of the way, let's get into configuring a Linux container running in a Proxmox server.

How I Initially Configure the Network Interface Inside a Linux Container?

When you create a new Linux container, you get to the Networking tab and filled in the IP address and default gateway. Once you start the container, the output of /etc/network/interfaces is as follows:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

Now you want to add additional IP addresses to a single network interface:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

iface eth0 inet static
        address 172.20.31.1/23

iface eth0 inet static
        address 172.20.31.2/23

iface eth0 inet static
        address 172.20.31.3/23

iface eth0 inet static
        address 172.20.31.4/23

You saved the file and you did a restart of networking service:

systemctl restart networking

What happens if you reboot? The output will be as follows:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

iface eth0 inet static
        address 172.20.31.0/23
        gateway 172.20.30.1

If I did not check out the /etc/network/interfaces file, I would execute the ip a command that lists IP addresses for all interfaces and saw something similar to this:

gpadmin-local@webservers:~$ ip a
1: lo:  mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0@if111:  mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 7e:ef:18:b4:a4:b9 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 172.20.31.0/23 brd 172.20.31.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::7cef:18ff:feb4:a4b9/64 scope link 
       valid_lft forever preferred_lft forever

Solution

Turns out I have to remove the IP address and gateway from the network configuration. With the Linux container selected (in my case: webservers), I went into Network, double-click the eth0 row containing an IP address, tab over to IPv4/CIDR edit box and removed both the IP address/subnet mask and gateway. Once I've done that, I clicked OK, made configuration changes in /etc/network/interfaces, and rebooted the container. In order to verify full networking functionality, I rebooted the Linux container, logged back into the container, and once I execute ip a command, all the IP addresses are preserved.

Oh, and I almost forgot: this network configuration can also apply to Ubuntu, AlmaLinux, Rocky Linux, OpenSUSE, and just about any other Linux distributions.

Notes For Those Who Use a Screen Reader

For anyone using a screen reader such as JAWS (Windows), NVDA (Windows), VoiceOver (Mac), or Orca (Linux), I'm not sure how anyone can access a virtual machine within a Proxmox web interface. And I'm not even sure if screen readers can read the text within the Linux container console; however, I am able to copy text from the console. The only difference is I have to highlight using a mouse, right-click in the console, and select copy. Doing Control+C sends a termination signal or displays ^C in a terminal. Doing Control+V once does nothing, but executing Control=V again shows a ^V in a console, so pasting text in a Proxmox console does nothing for those who do not want or cannot use a mouse.

Summary

The bottom line is this: in the networking section of the Proxmox web interface, leave the IP address and default gateway blank. Once you get into the console, edit the /etc.network/interfaces and add your IP addresses manually.

Happy Computing!

Article published: 2022-06-21 20:22

Categories: The World of Computers, Networking,