Tutorial 2 - API usage

Note: There is a bug in 1.3.b4 and 1.4.r1300 with "flows". The example below fails when flows are encountered due to this bug. The bug has been fixed and will be released with next release of 1.3 and 1.4 which are currently being tested before a broader public release.

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package org.jnetpcap.examples.packet;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.jnetpcap.Pcap;
import org.jnetpcap.nio.JMemory;
import org.jnetpcap.packet.JFlow;
import org.jnetpcap.packet.JFlowKey;
import org.jnetpcap.packet.JFlowMap;
import org.jnetpcap.packet.JPacket;
import org.jnetpcap.packet.JPacketHandler;
import org.jnetpcap.packet.JScanner;
import org.jnetpcap.packet.PcapPacket;
import org.jnetpcap.protocol.tcpip.Http;
import org.jnetpcap.protocol.tcpip.Tcp;

 * This example demonstrates various usage scenerios for jNetPcap API. The test
 * file used in this example can be found under the "tests" directory located
 * under the root installation directory of the source package. The tests
 * directory is not normally provided with binary distribution of jnetpcap. The
 * test file contains 483 packets most of which are http or tcp segments.
 * @author Mark Bednarczyk
 * @author Sly Technologies, Inc.
public class CommonUsageExamples {

	 * Various examples
	 * @param args
	 *          none expected
	public static void main(String[] args) {

		 * Example #1 open offline capture file for reading packets.
		final String FILENAME = "tests/test-http-jpeg.pcap";
		final StringBuilder errbuf = new StringBuilder();

		final Pcap pcap = Pcap.openOffline(FILENAME, errbuf);
		if (pcap == null) {
			System.err.println(errbuf); // Error is stored in errbuf if any

		 * We have an opened the capture file now time to read packets. We use a
		 * Pcap.loop function to retrieve 10 packets from the file. We supply an
		 * annonymous handler which will receive packets as they are read from the
		 * offline file by libpcap. We parameterize it with a StringBuilder class.
		 * This allows us to pass in any type of object we need inside the our
		 * dispatch handler. For this example we are passing in the errorbuf object
		 * so we can pass back a string, if we need to. Of course in our example
		 * this is not strictly needed since our anonymous class can access errbuf
		 * object directly from the enclosing main method as that local variable is
		 * marked final allowing anonymous classes access to it.
		pcap.loop(10, new JPacketHandler<StringBuilder>() {

			 * We purposely define and allocate our working tcp header (accessor)
			 * outside the dispatch function and thus the libpcap loop, as this type
			 * of object is reusable and it would be a very big waist of time and
			 * resources to allocate it per every dispatch of a packet. We mark it
			 * final since we do not plan on allocating any other instances of Tcp.
			final Tcp tcp = new Tcp();

			 * Same thing for our http header
			final Http http = new Http();

			 * Our custom handler that will receive all the packets libpcap will
			 * dispatch to us. This handler is inside a libpcap loop and will receive
			 * exactly 10 packets as we specified on the Pcap.loop(10, ...) line
			 * above.
			 * @param packet
			 *          a packet from our capture file
			 * @param errbuf
			 *          our custom user parameter which we chose to be a StringBuilder
			 *          object, but could have chosen anything else we wanted passed
			 *          into our handler by libpcap
			public void nextPacket(JPacket packet, StringBuilder errbuf) {

				 * Here we receive 1 packet at a time from the capture file. We are
				 * going to check if we have a tcp packet and do something with tcp
				 * header. We are actually going to do this twice to show 2 different
				 * ways how we can check if a particular header exists in the packet and
				 * then get that header (peer header definition instance with memory in
				 * the packet) in 2 separate steps.
				if (packet.hasHeader(Tcp.ID)) {

					 * Now get our tcp header definition (accessor) peered with actual
					 * memory that holds the tcp header within the packet.

					System.out.printf("tcp.dst_port=%d%n", tcp.destination());
					System.out.printf("tcp.src_port=%d%n", tcp.source());
					System.out.printf("tcp.ack=%x%n", tcp.ack());


				 * An easier way of checking if header exists and peering with memory
				 * can be done using a conveniece method JPacket.hasHeader(? extends
				 * JHeader). This method performs both operations at once returning a
				 * boolean true or false. True means that header exists in the packet
				 * and our tcp header difinition object is peered or false if the header
				 * doesn't exist and no peering was performed.
				if (packet.hasHeader(tcp)) {
					System.out.printf("tcp header::%s%n", tcp.toString());

				 * A typical and common approach to getting headers from a packet is to
				 * chain them as a condition for the if statement. If we need to work
				 * with both tcp and http headers, for example, we place both of them on
				 * the command line.
				if (packet.hasHeader(tcp) && packet.hasHeader(http)) {
					 * Now we are guarranteed to have both tcp and http header peered. If
					 * the packet only contained tcp segment even though tcp may have http
					 * port number, it still won't show up here since headers appear right
					 * at the beginning of http session.

					System.out.printf("http header::%s%n", http);

					 * jNetPcap keeps track of frame numbers for us. The number is simply
					 * incremented with every packet scanned.


				System.out.printf("frame #%d%n", packet.getFrameNumber());

		}, errbuf);

		 * Now that we have captured our 10 packets, lets use Pcap.nextEx to get the
		 * next 5 packets. We will also reset the frame number back to 0 just so we
		 * can see how its done. Each scanner keeps track of its own frame numbers,
		 * so we want to get the default one, for this thread, and change it there.

		final PcapPacket packet = new PcapPacket(JMemory.POINTER);
		final Tcp tcp = new Tcp();

		for (int i = 0; i < 5; i++) {

			if (packet.hasHeader(tcp)) {
				System.out.printf("#%d seq=%08X%n", packet.getFrameNumber(), tcp.seq());

		 * Each packet scanned, also has a flow key associated with it. The flow key
		 * is generated based on the headers in each packet and stored with packet
		 * state. We can use the flow key to uniquely identify packets belonging to
		 * the same stream of packets between end host systems. We will keep a map
		 * of various flows with packets in it.
		final Map<JFlowKey, JFlow> flows = new HashMap<JFlowKey, JFlow>();

		for (int i = 0; i < 50; i++) {
			final JFlowKey key = packet.getState().getFlowKey();

			 * A hashmap uses the equals method to determine if a key is already
			 * present in the map or not and to retrieve values. jNetPcap provides us
			 * with a special object called a JFlow which keeps a list of packets part
			 * of that flow. We can add new packets to a flow and later we can get a
			 * list of those packets. So first we check if a flow for a given key
			 * already exists. All packets part of the same flow will have the same
			 * key.
			JFlow flow = flows.get(key);
			if (flow == null) {
				flows.put(key, flow = new JFlow(key));

			 * Now that we know for sure we have a flow this packet belongs to, we can
			 * add this packet to this flow. Before we can actuall add a packet to a
			 * queue for later processing, we must first make a copy of the packet to
			 * a new object. We can only process each libpcap packet immediately
			 * before any other calls or nextEx or another iteration of a loop. The
			 * packets are delivered to us without copies so what we are working with
			 * is the data within libpcap buffer. If we want to preserve a packet
			 * beyond this point, we have to make a copy of the packet and its decoded
			 * state and then we can keep the packet around for as long as its needed.
			 * There is a convenience PcapPacket constructor that does a copy of
			 * everything needed for us.
			flow.add(new PcapPacket(packet));

		 * Now that we added 50 packets to various flows maintained by the flows
		 * Map, we can now access those flows and the packet within it. The packets
		 * are now grouped into flows.

		for (JFlow flow : flows.values()) {

			 * Flows can be bi-directional. That is packets going between host A and B
			 * would be considered in forward-direction, while packets between host B
			 * and A can be considered reserverse direction. Although both forward and
			 * reverse are going in the opposite directions, jnetpcap flows consider
			 * them the same flows. You have 3 types of accessors for retrieving
			 * packets from a flow. JFlow.getForward, JFlow.getReverse or
			 * JFlow.getAll. JFlow.getAll gets a list of packets, no matter which
			 * direction they are going, while the other 2 accessors only get the
			 * packets that are going in the specified direction.
			if (flow.isReversable()) {
				 * We can get directional flow packets, but only if the flow is
				 * reversable. Not all flows are reversable and this is determined by
				 * the header types. If a flow is not reversable, flow.getReverse will
				 * return empty list, which is something we don't want to have to
				 * process.

				List<JPacket> forward = flow.getForward();
				for (JPacket p : forward) {
					System.out.printf("%d, ", p.getFrameNumber());

				List<JPacket> reverse = flow.getReverse();
				for (JPacket p : reverse) {
					System.out.printf("%d, ", p.getFrameNumber());
			} else {

				 * Otherwise we have to get All the packets and there is no
				 * forward/reverse direction associated with the packets. Here is how we
				 * can do this a little more compactly.
				for (JPacket p : flow.getAll()) {
					System.out.printf("%d, ", p.getFrameNumber());

		 * We still haven't read all the packets from our offline file. Here is an
		 * easier way to retrieve all the packets while grouping them into flows.
		 * jNetPcap provides a neat little class that does all of the above work for
		 * us. Its called JFlowMap, not only that it implements a JPacketHandler
		 * interface suitable for usage with Pcap.loop or Pcap.dispatch calls and it
		 * will add all packets received into appropriate flows.
		JFlowMap superFlowMap = new JFlowMap();

		 * So lets finish this file off, and read the remaining packets into our new
		 * superFlowMap and do a pretty print of all the flows it finds. The 3rd
		 * argument to Pcap.loop is unused so we just set it to null.
		 * Pcap.LOOP_INFINITE flag tells the Pcap.loop method to read all the
		 * packets until the end of file. Since we already read some packets, this
		 * will read remaining packets from the current position in the file until
		 * the end.
		pcap.loop(Pcap.LOOP_INFINITE, superFlowMap, null);

		System.out.printf("superFlowMap::%s%n", superFlowMap);

		 * Now we have read the remaining packets and we no longer need to keep the
		 * pcap file open.


Lastly here is the output from the last printf statement that prints the contents of the JFlowmap:

superFlowMap::total packet count=418
total flow count=18
flow[0] -> Tcp fw/rev/tot pkts=[9/10/19],
flow[1] -> Tcp fw/rev/tot pkts=[9/11/20],
flow[2] -> Tcp fw/rev/tot pkts=[1/0/1],
flow[3] -> Tcp fw/rev/tot pkts=[6/6/12],
flow[4] -> Tcp fw/rev/tot pkts=[6/6/12],
flow[5] -> Tcp fw/rev/tot pkts=[5/5/10],
flow[6] -> Tcp fw/rev/tot pkts=[6/6/12],
flow[7] -> Tcp fw/rev/tot pkts=[5/5/10],
flow[8] -> Ip4 tot pkts=[18],
flow[9] -> Tcp fw/rev/tot pkts=[1/1/2],
flow[10] -> Tcp fw/rev/tot pkts=[74/135/209],
flow[11] -> Tcp fw/rev/tot pkts=[9/6/15],
flow[12] -> Tcp fw/rev/tot pkts=[8/6/14],
flow[13] -> Tcp fw/rev/tot pkts=[8/6/14],
flow[14] -> Tcp fw/rev/tot pkts=[6/6/12],
flow[15] -> Tcp fw/rev/tot pkts=[6/6/12],
flow[16] -> Tcp fw/rev/tot pkts=[6/6/12],
flow[17] -> Tcp fw/rev/tot pkts=[8/6/14],

Its much easier to just read all the packets into a JFlowMap if no special processing is needed.