Presence Publisher

Presence Publisher is a simple app that regularly publishes to a configurable MQTT topic whenever connected to a given Wi-Fi network or in proximity to a Bluetooth beacon. This can be used to integrate the presence of your phone in home automation.

Several networks and beacons can be configured at once, and the message to be sent can be configured for each of them.

If your MQTT server is available on the internet, you can also choose to send an 'offline' message whenever you are not connected to any of the configured Wi-Fi networks and not in range of any configured beacon.

As an additional feature, you can send the battery level of your device whenever a condition is met, so that you can recharge it before it turns off.

The app uses the built-in Android alarm manager, so notifications are sent even if the phone is in stand-by.

For details on which data this app processes and how it does so, please have a look at the privacy policy.

TLS with self-signed certificates

The app uses the default Android CA trust store for checking the server certificate validity. You can simply add your certificate via:

• Android 4.0 - 7.1:
  • Settings → Security → Install from SD card
• Android 8.0 - 9.0:
  • Settings → Security & location → Encryption & credentials → Install from SD card
• Android 10:
  • Security → Encryption & credentials → Install from SD card
• Android 11+:
  • Security → Encryption & credentials → Install a certificate → CA Certificate

Please note you need to have your certificate in DER format to be able to import it correctly. You can check this using:

openssl x509 -inform der -in server.crt -text

This should show you the correct certificate information. If you instead see something like unable to load certificate, the certificate is most likely formatted in PEM format and needs to be converted, e.g like this:

openssl x509 -inform pem -outform der -in server.crt -out server_der.crt

Alternatively, you can use the KeyStore Explorer to do the conversion:

• Create a new KeyStore → choose PKCS #12
• Tools → Import Trusted Certificates → open your certificate file and give it some alias
• Right-click the entry → Export → Export Certificate → select export format X.509 and uncheck PEM
• afterwards, you can close the keystore without saving it

Client certificates

The Android keychain will only allow you to import a PKCS#12 keystore. If you have created a client certificate along the lines of https://mosquitto.org/man/mosquitto-tls-7.html, you will need to combine the certificate and key file together like this:

openssl pkcs12 -inkey client.key -in client.crt -export -out client.pfx

If you do not need your client certificate to be signed by a root certificate, because you plan to add it directly to the trusted certificates of your MQTT broker, you can also use the KeyStore Explorer to generate your client certificate:

• Create a new KeyStore → choose PKCS #12
• Tools → Generate Key Pair → choose one of Android's supported algorithms → configure properties of the public certificate → set a password for the private key
• File → Save → use the same password as above for the keystore

Make sure your PKCS#12 keystore file has the .pfx extension, otherwise Android will not recognize it.

To be able to select this client certificate in Presence Publisher, you first need to add it to the Android Keystore. This works similar to the process for the server certificate:

• Android 4.0 - 7.1:
  • Settings → Security → Install from SD card
• Android 8.0 - 9.0:
  • Settings → Security & location → Encryption & credentials → Install from SD card
• Android 10:
  • Security → Encryption & credentials → Install from SD card
• Android 11+:
  • Security → Encryption & credentials → Install a certificate → VPN & app user certificate

After you have imported your client certificate, you will be able to choose it from the app.

Permissions

• ACCESS_BACKGROUND_LOCATION: on Android 10+, necessary to retrieve SSID of connected Wi-Fi while running in background
• ACCESS_FINE_LOCATION: necessary to discover beacons; on Android 9+, necessary to retrieve SSID of connected Wi-Fi
• ACCESS_NETWORK_STATE: necessary to register network change listener
• ACCESS_WIFI_STATE: necessary to retrieve SSID of connected Wi-Fi
• BLUETOOTH: necessary to communicate with beacons
• BLUETOOTH_ADMIN: necessary to discover beacons
• INTERNET: only necessary if your MQTT server is not running locally
• RECEIVE_BOOT_COMPLETED: necessary to start service on start-up
• REQUEST_IGNORE_BATTERY_OPTIMIZATIONS: on Android 6+, necessary to request disabling battery optimization
• WRITE_EXTERNAL_STORAGE: only necessary if you want to export log files in Android 4.0 - 4.3