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Android Reference

VpnService


android.net

Class VpnService

  • All Implemented Interfaces:
    ComponentCallbacks, ComponentCallbacks2


    public class VpnService
    extends Service
    VpnService is a base class for applications to extend and build their own VPN solutions. In general, it creates a virtual network interface, configures addresses and routing rules, and returns a file descriptor to the application. Each read from the descriptor retrieves an outgoing packet which was routed to the interface. Each write to the descriptor injects an incoming packet just like it was received from the interface. The interface is running on Internet Protocol (IP), so packets are always started with IP headers. The application then completes a VPN connection by processing and exchanging packets with the remote server over a tunnel.

    Letting applications intercept packets raises huge security concerns. A VPN application can easily break the network. Besides, two of them may conflict with each other. The system takes several actions to address these issues. Here are some key points:

    • User action is required to create a VPN connection.
    • There can be only one VPN connection running at the same time. The existing interface is deactivated when a new one is created.
    • A system-managed notification is shown during the lifetime of a VPN connection.
    • A system-managed dialog gives the information of the current VPN connection. It also provides a button to disconnect.
    • The network is restored automatically when the file descriptor is closed. It also covers the cases when a VPN application is crashed or killed by the system.

    There are two primary methods in this class: prepare(android.content.Context) and VpnService.Builder.establish(). The former deals with user action and stops the VPN connection created by another application. The latter creates a VPN interface using the parameters supplied to the VpnService.Builder. An application must call prepare(android.content.Context) to grant the right to use other methods in this class, and the right can be revoked at any time. Here are the general steps to create a VPN connection:

    1. When the user press the button to connect, call prepare(android.content.Context) and launch the returned intent.
    2. When the application becomes prepared, start the service.
    3. Create a tunnel to the remote server and negotiate the network parameters for the VPN connection.
    4. Supply those parameters to a VpnService.Builder and create a VPN interface by calling VpnService.Builder.establish().
    5. Process and exchange packets between the tunnel and the returned file descriptor.
    6. When onRevoke() is invoked, close the file descriptor and shut down the tunnel gracefully.

    Services extended this class need to be declared with appropriate permission and intent filter. Their access must be secured by android.Manifest.permission#BIND_VPN_SERVICE permission, and their intent filter must match SERVICE_INTERFACE action. Here is an example of declaring a VPN service in AndroidManifest.xml:

     <service android:name=".ExampleVpnService"
             android:permission="android.permission.BIND_VPN_SERVICE">
         <intent-filter>
             <action android:name="android.net.VpnService"/>
         </intent-filter>
     </service>
    See Also:
    VpnService.Builder
    • Field Detail

      • SERVICE_INTERFACE

        public static final String SERVICE_INTERFACE
        The action must be matched by the intent filter of this service. It also needs to require android.Manifest.permission#BIND_VPN_SERVICE permission so that other applications cannot abuse it.
        See Also:
        Constant Field Values
    • Constructor Detail

      • VpnService

        public VpnService()
    • Method Detail

      • prepare

        public static Intent prepare(Context context)
        Prepare to establish a VPN connection. This method returns null if the VPN application is already prepared. Otherwise, it returns an Intent to a system activity. The application should launch the activity using Activity.startActivityForResult(android.content.Intent, int) to get itself prepared. The activity may pop up a dialog to require user action, and the result will come back via its Activity.onActivityResult(int, int, android.content.Intent). If the result is Activity.RESULT_OK, the application becomes prepared and is granted to use other methods in this class.

        Only one application can be granted at the same time. The right is revoked when another application is granted. The application losing the right will be notified via its onRevoke(). Unless it becomes prepared again, subsequent calls to other methods in this class will fail.

        See Also:
        onRevoke()
      • protect

        public boolean protect(int socket)
        Protect a socket from VPN connections. The socket will be bound to the current default network interface, so its traffic will not be forwarded through VPN. This method is useful if some connections need to be kept outside of VPN. For example, a VPN tunnel should protect itself if its destination is covered by VPN routes. Otherwise its outgoing packets will be sent back to the VPN interface and cause an infinite loop. This method will fail if the application is not prepared or is revoked.

        The socket is NOT closed by this method.

        Returns:
        true on success.
      • protect

        public boolean protect(Socket socket)
        Convenience method to protect a Socket from VPN connections.
        Returns:
        true on success.
        See Also:
        protect(int)
      • onBind

        public IBinder onBind(Intent intent)
        Return the communication interface to the service. This method returns null on Intents other than SERVICE_INTERFACE action. Applications overriding this method must identify the intent and return the corresponding interface accordingly.
        Specified by:
        onBind in class Service
        Parameters:
        intent - The Intent that was used to bind to this service, as given to Context.bindService. Note that any extras that were included with the Intent at that point will not be seen here.
        Returns:
        Return an IBinder through which clients can call on to the service.
        See Also:
        Service.onBind(android.content.Intent)
      • onRevoke

        public void onRevoke()
        Invoked when the application is revoked. At this moment, the VPN interface is already deactivated by the system. The application should close the file descriptor and shut down gracefully. The default implementation of this method is calling Service.stopSelf().

        Calls to this method may not happen on the main thread of the process.

        See Also:
        prepare(android.content.Context)


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