Setting up a fallback hotspot script in case of Wi-Fi disconnection

I’m currently preparing a conference in London entitled “Rust and Robotics.” For this occasion, I need to bring some of my robots for live demonstrations. The issue: I don’t have access to Wi-Fi ahead of the event to configure auto-network connection.

Imagine you have a robot, controllable remotely via a self-hosted interface (like Hermes-Studio), or a computer accessible via SSH. As long as you’re on a known network (home, office), everything works fine. But what happens if the network goes down? Or if you have to move your robot to an unknown place, without any network coverage? And what if you don’t have physical access to the computer, or the robot has neither screen nor control peripherals? You’re stuck.

That’s the problem we’re going to address: a script will check the connection every minute. In case of disconnection, it will automatically spawn a Wi-Fi hotspot, allowing you to reconnect to the robot — either to control it, or to configure a new network.

1. If the robot is connected to the network, I must be able to connect to it via SSH using ssh user@machine.local.
2. If the connection is lost, a hotspot named HotspotFallback must start automatically.
3. Once connected to this hotspot, I must be able to access the robot via ssh user@machine.local or ssh user@192.168.42.1.
4. If Wi-Fi comes back, the hotspot must stop and the robot must reconnect to the initial network.
5. I must be able to manually switch between Wi-Fi and hotspot.

We’ll use NetworkManager (nmcli) to manage the connection, not hostapd like in older tutorials. For DNS resolution in hotspot mode, we’ll also use dnsmasq.

Prerequisites

1. Enable SSH connection

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   sudo apt update
   sudo apt instal openssh-server

2. Check connection

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   hostname                  # Displays machine name  
   whoami                    # Displays user name  
   ssh user@machine.local    # SSH connection test

3. Secure with firewall (UFW)

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   sudo apt install ufw
   sudo ufw allow ssh

1. Install dependencies

   sudo apt install iw dnsmasq network-manager

2. Create the script

   nano ~/hotspot_switcher.sh

3. Copy the script content from:
   https://github.com/dclause/hotspot-fallback/blob/develop/hotspot_switcher.sh

4. Configure script variables
   • WIFI_INTERFACE → obtained via ip link | grep wl
   • WIFI_ID → exact name of the main Wi-Fi network to connect to
   • HOTSPOT_SSID et HOTSPOT_PASSWORD → hotspot SSID and password
   • HOTSPOT_CON_NAME → hotspot connection name
   • HOTSPOT_IP → fixed IP in hotspot mode
     

5. Make the script executable

   chmod +x ~/hotspot_switcher.sh

Activate hotspot:

sudo ~/hotspot_switcher.sh start

Return to Wi-Fi:

sudo ~/hotspot_switcher.sh stop

1. Create a SystemD service

   sudo nano /etc/systemd/system/wifi-hotspot-check.service

2. Edit content (WARNING: update the absolute path to your script)

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   [Unit]
   Description=Checks main Wi-Fi connection  
    
   [Service]
   Type=oneshot
   ExecStart=/home/user/hotspot_switcher.sh check

3. Start the service

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   sudo systemctl daemon-reexec
   sudo systemctl daemon-reload
   sudo systemctl start wifi-hotspot-check.service

1. Create the timer file

   sudo nano /etc/systemd/system/wifi-hotspot-check.timer

2. Edit content

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   [Unit]
   Description=Triggers Wi-Fi check every 60s  
    
   [Timer]
   OnBootSec=1min
   OnUnitActiveSec=1min
   Persistent=true
    
   [Install]
   WantedBy=timers.target

3. Start the timer

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   sudo systemctl daemon-reexec
   sudo systemctl daemon-reload
   sudo systemctl start wifi-hotspot-check.timer

Without a router, you must manage DNS resolution yourself. This is what allows you in Hotspot mode to use the machine name instead of the IP for your SSH connection: ssh user@machine.local

However, in Hotspot mode, we no longer benefit from the DNS service of our home box or network provider. Our robot or remote computer must resolve DNS aliases itself, so we need to configure this:

1. Create the dnsmasq config

   sudo nano /etc/dnsmasq.conf

2. Edit the config (WARNING: update the network interface)

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   interface=wlp2s0
   dhcp-range=192.168.42.100,192.168.42.200,12h
   dhcp-option=3,192.168.42.1
   dhcp-option=6,8.8.8.8,8.8.4.4
   server=8.8.8.8
   server=8.8.4.4

3. Reboot the machine

   sudo reboot

Your machine is now capable of automatically switching to hotspot mode if the Wi-Fi connection is lost. This guarantees remote recovery via SSH.