Capture ping packets throws exception (too many instances)

Question

I have written a code snippet to cpature ping using JPCAP. the code I have written is given below :

while (true) {
    try {
    PacketCapture m_pcap;
    m_pcap = new PacketCapture();
    m_pcap.open("\\Device\\NPF_{007262BD-....-7EE83D72EBEA}",true);//param 1 is actual device ID
    m_pcap.setFilter("proto ICMP", true);
    pktlistener a = new pktlistener(); //handles the packet
    m_pcap.addPacketListener(a);
    System.out.println("going to sleep");
    Thread.sleep(1 * 1000);// Waiting for 1 second before ending capture
    System.out.println("woken up");
    m_pcap.removePacketListener(a);
    m_pcap.endCapture();
    m_pcap.close();
    a = null;
    m_pcap = null;
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Now the above code starts a new capture after every one second. The problem with the above is that after 10 runs of the loop, it throws the exception :

 Exception in thread "Thread-6" java.lang.Error: Too many instances, exceeds 10
    at net.sourceforge.jpcap.capture.PacketCapture.<init>(PacketCapture.java:51)

Q1. How do i prevent this. I need to start a new PacketCapture every second.

Q2. Is there any other simpler way to capture ping messages recived on a system through java?

Answer 1

You cannot use the constructor of PacketCapture more than ten times. This behavior is hardcoded as the constructor looks like this:

/**
 * Create a new packet capture instance.
 */
public PacketCapture() {
    if (nextInstance >= INSTANCE_MAX) {
        throw new Error("Too many instances, exceeds " + INSTANCE_MAX);
    }

    instanceNum = nextInstance++;
}

To capture ping requests, you should try the following code

public class Main {

    public static void main(String[] args) throws CaptureDeviceLookupException {
        Capture cap = new Capture();
        cap.doCapture();
    }
}

class PingListener implements PacketListener {

    @Override
    public void packetArrived(Packet packet) {
        try {
            // only ICMP packages
            if (packet instanceof ICMPPacket) {
                ICMPPacket tcpPacket = (ICMPPacket) packet;
                int data = tcpPacket.getMessageCode();
                // only echo request packages
                if (data == ICMPMessages.ECHO) {
                    // print source and destination.
                    String srcHost = tcpPacket.getSourceAddress();
                    String dstHost = tcpPacket.getDestinationAddress();
                    System.out.println("Ping from: " + srcHost + " to " + dstHost);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

class Capture {
    public void doCapture() {
        // create capture instance
        PacketCapture capture = new PacketCapture();
        // add listener that handles incomming and outgoing packages
        PingListener pingListener = new PingListener();
        capture.addPacketListener(pingListener);
        // m_pcap.setFilter("filter here or in handler", true);

        try {
            capture.open("\\Device\\NPF_{...}", true); // connect capture to device
            while (true) {
                capture.capture(1); // capture one package
            }
        } catch (Exception e) {
            e.printStackTrace(); // exception during capture or handling of
                                 // packages
        } finally {
            // technically never reached as the loop goes on forever.
            // if loop terminates after a while then:

            // remove listener
            capture.removePacketListener(pingListener);
            // end capture (only necessary, if PacketCapture still waits for
            // other packages)
            capture.endCapture();
            // close connection to capture device
            capture.close();
        }
    }
}

I think there is a misunderstanding of the class PacketCapture. It does not actually capture one package and is then discarded. It opens a connection to the device you want to capture packages of and then starts listening for as long as you hold that connection. You then start capturing n packages by calling capture.capture(n). For each package arriving while "capture" blocks your program, the listener is called.

Alternatively you can drop the while-loop and use capture.capture(-1). This will block your program forever until you close the capture from another device.