How to write packets from vpn tunnel to pcap file

Question

I used the Android's ToyVpnClient to set up a tunnel and intercept incoming & outgoing packets. I want to write these packets from this tunnel, which is stored in a ByteBuffer, to a pcap file. I've looked at jpcap and jnetpcap, but they seem to only support creating the pcap file from the packets that were captured by using these libraries to listen to the device's network interfaces in the first place.

Is there any api available that would help me create the pcap file from the tunnel's packet data stored in the ByteBuffer? If I were to create my own pcap writer, how would i go about parsing this packet from the tunnel and put it into the pcap format? (I've looked but haven't found any examples)

Here's the relevant code sample from the ToyVpnClient:

    ...
    DatagramChannel tunnel = DatagramChannel.open();
    // Connect to the server.
    tunnel.connect(server);

    // Packets to be sent are queued in this input stream.
    FileInputStream in = new FileInputStream(mInterface.getFileDescriptor());

    // Packets received need to be written to this output stream.
    FileOutputStream out = new FileOutputStream(mInterface.getFileDescriptor());

    // Allocate the buffer for a single packet.
    ByteBuffer packet = ByteBuffer.allocate(32767);

    // keep forwarding packets till something goes wrong.
    while (true) {
        // Read the outgoing packet from the input stream.
        int length = in.read(packet.array());
        if (length > 0) {
        // Write the outgoing packet to the tunnel.
        packet.limit(length);
        tunnel.write(packet);

        //How to write to pcap file here?

        packet.clear();

        }
    ...
}

Thank you

user1631605 user1631605 · Accepted Answer · 2013-05-30T17:39:49

For anyone looking for similar solution, I got it working using the netutils library's PCapFileWriter and modifying it to include a fake ethernet header for each packet (http://code.google.com/p/netutils/)

How to write packets from vpn tunnel to pcap file

1 Answers