How to switch back networking to /etc/network/interfaces on Ubuntu 20.04 Focal Fossa Linux

This article will explain how to switch back networking from NetPlan/CloudInit on Ubuntu 20.04 Focal Fossa Linux to yet now already obsolete networking managed via /etc/network/interfaces.

In this tutorial you will learn:

How to revert to eth0..n network naming convention
How to install ifupdown
How to remove CloudInit
How to enable networking daemon

WARNING
Switching back from NetPlan/CloudInit to the now obsolete networking daemon is not supported nor recommended as you might end up with a broken system.

How to switch back networking to /etc/network/interfaces on Ubuntu 20.04 Focal Fossa Linux

Software Requirements and Conventions Used

Software Requirements and Linux Command Line Conventions
Category	Requirements, Conventions or Software Version Used
System	Installed Ubuntu 20.04 or upgraded Ubuntu 20.04 Focal Fossa
Software	N/A
Other	Privileged access to your Linux system as root or via the `sudo` command.
Conventions	# – requires given linux commands to be executed with root privileges either directly as a root user or by use of `sudo` command $ – requires given linux commands to be executed as a regular non-privileged user

How to switch back networking to /etc/network/interfaces on Ubuntu 20.04 step by step instructions

First step is to install tools to configure network interfaces
```
$ sudo apt update
$ sudo apt install ifupdown net-tools
```
Next, change from current enp0s3 to old network interfaces naming convention eth0. To do so with administrative privileges edit the /etc/default/grub file anf change the following line:
```
FROM:
GRUB_CMDLINE_LINUX=""
TO:
GRUB_CMDLINE_LINUX="net.ifnames=0 biosdevname=0"
```
Edit Grub boot to change to old network interfaces naming convention eg. eth0

Once ready update Grub with:
```
$ sudo update-grub
```
Reboot your system:
```
$ sudo reboot
```
As root or any administrative user edit the /etc/network/interfaces file and set eth0 network interface to obtain the IP address from DHCP:
```
source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp
```
Check this article if you need to set your network interface to a static IP address.
Restart eth0 interface:
```
$ sudo ifdown --force eth0
$ sudo ifup eth0
```
NOTE
Network restart via /etc/init.d/networking is not functional. To restart your network use the ifdown and ifup commands as shown above.

At this stage you should have your eth0 configured. Use ifconfig command to check the networking interface configuration:

ifconfig 
eth0: flags=4163  mtu 1500
        inet 192.168.1.28  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::2dc0:208f:6d8b:f8fc  prefixlen 64  scopeid 0x20 	
        ether 08:00:27:a7:75:ad  txqueuelen 1000  (Ethernet)
        RX packets 22820  bytes 33504917 (33.5 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 3446  bytes 282861 (282.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 409  bytes 34213 (34.2 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 409  bytes 34213 (34.2 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

First disable and stop Configure DNS resolution to eg. 8.8.8.8 nameserver:

$ sudo unlink /etc/resolv.conf
$ sudo echo nameserver 8.8.8.8 >> /etc/resolv.conf

Let’s perform soul cleanup. Remove cloud init package:

$ sudo dpkg -P cloud-init
$ sudo rm -fr /etc/cloud/

Disable and stop systemd-resolved service:

$ sudo systemctl disable --now systemd-resolved

Software Requirements and Conventions Used

How to switch back networking to /etc/network/interfaces on Ubuntu 20.04 step by step instructions

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