How to share wired network connection in 18.04

The graphical program that shows the feature you need is

nm-connection-editor

You can launch it from a terminal window or with Alt+F2

Higlight an adapter and hit the cog

To anyone who comes here and is still lost on what to do, I found a tutorial that explains it here.

Basic rundown

First run nm-connection-editor in your terminal.

Next, you will see a screen that lists a bunch of different network devices.

Click on your wired connection and then click on the cog at the bottom of the window.

A window will open giving you a bunch of options to edit the connection. Go to the IPV4 settings tab and change the method to share to other computers. Finally hit save.

You should be all set to go! It's important to note that you may have to unplug and replug your network cable or reset the connection in order for it to work.

Using nm-connection-editor set the usb-ethernet connections under IPv4 Settings to Shared to other computers. Leave the internet ethernet connection alone.

For context, I wanted to give a USB ethernet (eth1) gadget (Pi Zero) access to the internet by sharing the PCI ethernet (eth0) internet connection. However, the only way I could get it working was to set eth1 to Shared ..., the total opposite of what I expected the setup to be.