How to configure VPN programmatically on Android?

Configuring a VPN programmatically on Android primarily involves using the VpnService class, which was introduced in Android 4.0 (API Level 14). VpnService provides a mechanism to allow applications to create and manage Virtual Private Network (VPN) connections without requiring root access. Below, I will detail the process through several steps.

Step 1: Add Necessary Permissions

First, add necessary permissions in the project's AndroidManifest.xml file. The VPN service requires the INTERNET and BIND_VPN_SERVICE permissions:

xml
<uses-permission android:name="android.permission.INTERNET"/>
<service android:name=".MyVpnService"
         android:permission="android.permission.BIND_VPN_SERVICE">
    <intent-filter>
        <action android:name="android.net.VpnService"/>
    </intent-filter>
</service>

Step 2: Create a VpnService Service

Create a class that extends VpnService, such as MyVpnService. In this class, implement the setup and management of the VPN connection, which typically involves configuring parameters like server address, routing, and DNS.

java
public class MyVpnService extends VpnService {
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // Configure and establish the VPN connection here
        return START_STICKY;
    }
}

Step 3: Configure the VPN Connection

In the VpnService implementation, use the VpnService.Builder class to configure the VPN connection. Set parameters such as server address, DNS server, and routing:

java
Builder builder = new Builder();
builder.addAddress("192.168.1.1", 24);
builder.addRoute("0.0.0.0", 0);
builder.addDnsServer("8.8.8.8");
builder.setSession("MyVPNService");
ParcelFileDescriptor vpnInterface = builder.establish();

Step 4: Handle the VPN Connection

Once the establish() method is called, the system begins establishing the VPN connection. Within your service, handle connection setup, monitor its status, and close the connection appropriately when necessary.

Step 5: Request User Authorization

Due to security requirements, Android mandates explicit user authorization for the VPN connection. This is typically achieved by launching a system VPN authorization dialog:

java
Intent intent = VpnService.prepare(getApplicationContext());
if (intent != null) {
    startActivityForResult(intent, 0);
} else {
    // Already authorized; directly connect to the VPN
    startService(new Intent(this, MyVpnService.class));
}

Example Use Case

Suppose I need to connect to the company's VPN at vpn.example.com, routing all traffic through the VPN server and using Google's public DNS. I will configure my VPN service according to the above steps, ensuring correct server address, routing, and DNS settings are applied.

This outlines the fundamental process for programmatically configuring a VPN on Android. In practical applications, additional considerations may include exception handling and retry mechanisms for failed connections.