To automatically authenticate Tailscale on startup in Ubuntu, follow these steps:

1. Generate a reusable auth key from the Tailscale admin console at https://login.tailscale.com/admin/authkeys. Make sure to select the appropriate options for your use case (e.g., whether the node is ephemeral or not).

2. On your Ubuntu machine, run the following commands to set up the auth key and create a systemd service:

# Create a directory for the auth key
sudo mkdir -p /etc/tailscale

# Store the auth key in a file with secure permissions
# Replace 'tskey-auth-abc123' with the actual auth key you generated.
echo "tskey-auth-abc123" | sudo tee /etc/tailscale/authkey
sudo chmod 600 /etc/tailscale/authkey

# Create the systemd service file
sudo tee /etc/systemd/system/tailscale-up.service <<EOF
[Unit]
Description=Tailscale Up
After=tailscaled.service network-online.target
Requires=tailscaled.service
Wants=network-online.target

[Service]
ExecStart=/usr/bin/tailscale up ––auth–key file:/etc/tailscale/authkey ––accept–routes
Restart=on–failure
RestartSec=5s
StartLimitInterval=60s
StartLimitBurst=3

[Install]
WantedBy=multi-user.target
EOF

# Enable and start the new service
sudo systemctl daemon-reload
sudo systemctl enable tailscale-up.service
sudo systemctl start tailscale-up.service

3. Verify that the service is running correctly:

sudo systemctl status tailscale-up.service
tailscale status

4. If you need to update the auth key in the future, simply update the /etc/tailscale/authkey file with the new key and restart the service:

sudo systemctl restart tailscale-up.service

This setup will ensure that your machine automatically authenticates and connects to the Tailscale network on boot. Make sure to adjust the flags in the tailscale up command (e.g., --accept-routes, --accept-dns) based on your specific requirements.

If you encounter any issues, you can check the logs for the service with:

sudo journalctl -u tailscale-up.service

This should help you diagnose any problems with the authentication process.