Air-Gap Server: The Four Rust Servers

The air-gap server (airgap.local) is one Raspberry Pi that runs the full DroneEngage backend. It does not need the internet. Vehicles, the WebClient and MAVLink ground stations connect to it on the local network.

The backend has four servers. Each server is a small Rust program with one job and one config file.

The Four Servers

Server	Program	Job
Authenticator	andruavauth	Checks the email and access code of each user and unit. Tells the client which Communication Server to use. Has an admin web page.
Communication Server	andruavmainserver	Keeps a WebSocket open to each unit and each WebClient. Routes all messages: telemetry, commands, video signaling and missions.
Storage Server	de_storage_server	Keeps tasks, missions and news in a SQLite database. Keeps messages for units that are offline, and sends them when the unit comes back. Sends news to all WebClients. Has an admin dashboard.
UDP Relay	droneengage-udp-proxy	Relays MAVLink between a vehicle and QGroundControl or Mission Planner. This server is optional.

Note

The earlier backend was written in Node.js and had only two servers. The UDP proxy was inside the Communication Server, and there was no Storage Server. The new Rust servers keep the same wire protocol, so units and the WebClient work without changes.

How a Client Connects

1. When the Communication Server starts, it connects to the Authenticator and registers.

2. The unit or the WebClient sends its email and access code to the Authenticator.

3. The Authenticator gets a temporary login key from the Communication Server.

4. The Authenticator sends the server address, the port and the key to the client.

5. The client opens a secure WebSocket to the Communication Server with this key.

Ports

These are the default ports. You can change each port in the config file of its server.

Server	Port	Used for	Who connects
Authenticator	19408	Login API (HTTPS)	Units, WebClient
Authenticator	8089	Admin web page	You, from a browser
Authenticator	19001	S2S link	Communication Server (local only)
Communication Server	9966	Messages (WSS)	Units, WebClient
Storage Server	9000	S2S link (WSS)	Communication Server (local only)
Storage Server	9001	Dashboard	You, from a browser
UDP Relay	9100	Control link (WSS)	Communication Server
UDP Relay	40000–40999	MAVLink (UDP)	de_mavlink, QGroundControl, Mission Planner
WebClient web site	8001	WebClient page (HTTPS)	Browsers
Map server	88	Offline map tiles	WebClient
Wiki	8080	This wiki and GUIDME.md (HTTPS)	Everybody

Tip

S2S means server to server. These links are inside the Pi. Each server proves who it is before the other server accepts it. Two methods are available: Ed25519 keys, or a shared secret (HMAC). The shared secret is the easier method for a single Pi.

Config File Rules

All four servers use the same config rules:

• Each server reads server.config from its own folder.

• Put your own changes in server.config.local in the same folder. The server reads it after server.config, and its values replace the values with the same name. An update does not overwrite your .local file.

• You can write comments in the config files with // and /* */.

• To use a different file, start the server with --config=<file>.

• For a password, write $$HASH$$('my-password'). At the next start the server changes it to a bcrypt hash and saves the hash in the file.

Example — server.config.local for the Communication Server:

{
    // the IP or name that phones and laptops use to reach this Pi
    "public_host" : "192.168.1.161"
}

Settings You Must Change

The default files work on a test bench. Before you use the Pi in the field, change these settings:

Server	Setting	Why
Authenticator	single_account_user_name, single_account_access_code	The default account is public in this wiki.
Authenticator	admin_password, session_secret	They protect the admin web page.
Storage Server	admin_password, session_secret	They protect the dashboard.
Communication Server	public_host	Phones and laptops must reach this name or IP.
UDP Relay	public_host	QGroundControl must reach this name or IP.
All S2S links	s2s_shared_secret.key	Use the same random secret on all servers (see below).

Shared Secret for the S2S Links

The Storage Server and the UDP Relay can use a shared secret to accept the Communication Server.

1. Start the Storage Server one time. It makes s2s_shared_secret.key with a random value.

2. Copy this file to the folders of the Communication Server and the UDP Relay.

3. Set the file mode to 600:

   chmod 600 s2s_shared_secret.key
   

The secret is never in server.config. You can also give it in the de_s2s_shared_secret environment variable.

Warning

The Communication Server does not make this file. If the file is missing or different, the Communication Server cannot connect to the Storage Server or the UDP Relay.

Build and Start

Each server is a Rust project. Build each one on the Pi:

cargo build --release

Start the servers in this order:

1. Authenticator

2. Storage Server

3. UDP Relay

4. Communication Server

The Communication Server connects to the other three, and it tries again if one of them is not ready.

Start each server from its own folder, because the server reads its config and SSL files from the current folder.

Example — a systemd service for the UDP Relay:

[Unit]
Description=DroneEngage UDP relay server
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
User=droneengage
WorkingDirectory=/opt/droneengage/udp_proxy
ExecStart=/opt/droneengage/udp_proxy/droneengage-udp-proxy
Restart=on-failure
RestartSec=3
LimitNOFILE=65536

[Install]
WantedBy=multi-user.target

Make one service for each server: de_auth, de_storage, de_udp_relay and de_comm. Then enable them:

sudo systemctl daemon-reload
sudo systemctl enable --now de_auth de_storage de_udp_relay de_comm

To see the log of a server:

journalctl -u de_comm -f

Check That It Works

1. Open the admin page of the Authenticator: https://airgap.local:8089.

2. Log in. Open Servers.

3. Make sure that the Communication Server card shows Online.

4. Open the WebClient: https://airgap.local:8001.

5. Log in with the account of the Authenticator.

6. Start a unit. Make sure that it shows on the WebClient map.

Important

Each phone and laptop must trust the root certificate of the Pi. Without it, the browser and Andruav refuse the secure connection. See Air-Gap RPI-4 with Access Point for the steps.

Performance on a Raspberry Pi 4

We tested the earlier Node.js server and the new Rust server on the same Raspberry Pi 4 (4 cores, 1 GB RAM). A laptop flooded the server with messages for 8 seconds in each test.

Test	Node.js delivered	Rust delivered	Rust CPU
3 drones + 1 GCS, broadcast	1,906 msg/s	75,648 msg/s	3.3 cores
7 drones + 2 GCS, broadcast	555 msg/s	37,417 msg/s	3.2 cores
12 pairs, point to point	1,904 msg/s	24,331 msg/s	3.4 cores

Node.js uses only one core. The Rust server uses all four cores of the Pi. This is a stress test. A real vehicle sends far fewer messages, so the numbers show how much spare capacity one small Pi has.

Source Code

• Authenticator: droneengage_authenticator

• Communication Server: droneengage_server

• Storage Server: droneengage_storage_server

• UDP Relay: droneengage_udp_proxy

• Shared library: droneengage_server_common