Ubuntu – Ubuntu GNOME 16.10/17.04 issue with creating hot-spot using Network Manager

I hope this article can help with your problem.

How to Overcome Ubuntu 17.04 Failed to Make Wifi Hotspot

1. If Hotspot is on then turn it off first. Go to System Settings > Network

   Turn off Hotspot by clicking ON so it becomes OFF

   Click Stop Hotspot

2. Click All Settings > Software and Updates

   Make sure the settings shown are checked.

   

3. Click the Close button and let Ubuntu 17.04 update after entering the password. Wait for it to finish. Click Reload.

4. Press Ctrl+Alt+T to start the Terminal application.

5. Type the following command in Terminal:

   sudo apt install hostapd dnsmasq
   

6. Go back to the Network window and click Use as Hotspot then click Turn On.

   Hotspot is ON with the name HP-EliteBook-2760p (on my computer)

7. In order for Android devices to be able to connect to this Hotspot, there is still one more step, namely:

   Click Edit Connections

   Select Hotspot then click Edit

   Change Mode to Hotspot (provided Ad-hoc)

   Click Save

   Click Enable Wi-Fi to disable your Wi-Fi

   Click again Enable Wi-Fi to enable Wi-Fi

And…. Alhamdulillah ... now Hotspot on Ubuntu 17.04 can function normally. Android devices can also connect.

It's my understanding that when you plug in the Ethernet cable that trumps the wifi connection and the wifi connection drops automatically. In similar fashion, disconnecting the Ethernet cable auto connects to your preferred wifi (provided of course that you have the box checked to auto connect to that Wifi network on the general tab of network manager. Tested on a default install of Ubuntu 16.04.3

So if I'm understanding you correctly you just want to automatically start the hotspot service on your laptop when the Ethernet cable is connected and disable the service when the Ethernet cable is disconnected.

This should be fairly simple to do with a bash script and nmcli 'nmcli' is a powerful networknig tool that allows you to bring connections up and down at will and obtain tons of relevant information.

To find out what connections you will be scripting for simply run nmcli -t monitor| grep primary when connected via wifi and plug in your Ethernet cable. The connections you are interested in bringing up and down with nmcli will be those surrounded by 's Example: 'Wired connection 1' the logic is that when 'Connection' is connected bring up the hotspot (you may find it necessary to use the command sleep or utilize the -w switch to make nmcliin your script wait for a specified number of seconds for a command to complete before issuing the next.

Bringing a connection down is as simple as nmcli 'connection name' down and bringing one up is nmcli 'connection name' up

Note: For full control via the script you may need to actually disable the automatically connect option at least for your wifi connection in Network manager as it will attempt to auto-connect whenever it's available and can interfere with what you are attempting to accomplish. Excerpt from man nmcli:

There may be multiple connections that apply
       to a device, but only one of them can be active on that device at any
       given time. The additional connections can be used to allow quick
       switching between different networks and configurations.

So once you've determined that your Ethernet is connected you can issue the commands to bring the wifi connection down Example: nmcli connection my-wifi down and then the hotspot connection up Example: nmcli connection my-hotspot up

For more on how to automatically run scripts based on network connection see this. For more detail about 'nmcli' check the man page. And here's info on how to create a hotspot.