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The core module of Pcap4J.
There is a newer version: 2.0.0-alpha.6
/*_##########################################################################
  _##
  _##  Copyright (C) 2012 Pcap4J.org
  _##
  _##########################################################################
*/

package org.pcap4j.util;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import org.pcap4j.packet.IpV4Packet;
import org.pcap4j.packet.IpV4Packet.IpV4Header;
import org.pcap4j.packet.Packet;
import org.pcap4j.packet.SimpleBuilder;
import org.pcap4j.packet.UnknownPacket;
import org.pcap4j.packet.factory.PacketFactories;
import org.pcap4j.packet.namednumber.IpNumber;

/**
 * @author Kaito
 * @since pcap4j 0.9.9
 */
public final class IpV4Helper {

  private static Comparator comparator = new ComparatorImpl();

  private IpV4Helper() {
    throw new AssertionError();
  }

  /**
   * @param packet packet
   * @param mtu mtu
   * @return a list containing fragmented packets.
   */
  public static List fragment(IpV4Packet packet, int mtu) {
    List list = new ArrayList();

    if (packet.length() <= mtu) {
      list.add(packet);
      return list;
    }

    IpV4Header header = packet.getHeader();
    byte[] payload = packet.getPayload().getRawData();
    int max_payload_length = mtu - header.length();
    int actual_max_payload_length =
        max_payload_length % 8 == 0
            ? max_payload_length
            : max_payload_length - max_payload_length % 8;
    int rest_length = payload.length;
    int srcPos = 0;
    while (rest_length > 0) {
      if (rest_length > max_payload_length) {
        byte[] fragmented_payload = new byte[actual_max_payload_length];
        System.arraycopy(payload, srcPos, fragmented_payload, 0, actual_max_payload_length);

        IpV4Packet.Builder b = packet.getBuilder();
        b.moreFragmentFlag(true)
            .fragmentOffset((short) (srcPos / 8))
            .payloadBuilder(new UnknownPacket.Builder().rawData(fragmented_payload))
            .correctChecksumAtBuild(true)
            .correctLengthAtBuild(true);
        list.add(b.build());

        rest_length -= fragmented_payload.length;
        srcPos += fragmented_payload.length;
      } else {
        byte[] fragmented_payload = new byte[rest_length];
        System.arraycopy(payload, srcPos, fragmented_payload, 0, rest_length);

        IpV4Packet.Builder b = packet.getBuilder();
        b.moreFragmentFlag(false)
            .fragmentOffset((short) (srcPos / 8))
            .payloadBuilder(new UnknownPacket.Builder().rawData(fragmented_payload))
            .correctChecksumAtBuild(true)
            .correctLengthAtBuild(true);
        list.add(b.build());

        break;
      }
    }

    return list;
  }

  /**
   * @param list list
   * @return a defragmented packet.
   */
  public static IpV4Packet defragment(List list) {
    Collections.sort(list, comparator);

    IpV4Header lastPacketHeader = list.get(list.size() - 1).getHeader();
    int payloadLength =
        lastPacketHeader.getFragmentOffset() * 8
            + lastPacketHeader.getTotalLengthAsInt()
            - lastPacketHeader.getIhl() * 4;
    if (payloadLength <= 0) {
      throw new IllegalArgumentException("Can't defragment: " + list);
    }

    final byte[] defragmentedPayload = new byte[payloadLength];
    int destPos = 0;
    try {
      for (IpV4Packet p : list) {
        byte[] rawPayload = p.getPayload().getRawData();
        System.arraycopy(rawPayload, 0, defragmentedPayload, destPos, rawPayload.length);
        destPos += rawPayload.length;
      }
    } catch (NullPointerException e) {
      throw new IllegalArgumentException("Can't defragment: " + list);
    } catch (ArrayStoreException e) {
      throw new IllegalArgumentException("Can't defragment: " + list);
    } catch (IndexOutOfBoundsException e) {
      throw new IllegalArgumentException("Can't defragment: " + list);
    }

    IpV4Packet.Builder b = list.get(0).getBuilder();

    b.moreFragmentFlag(false)
        .fragmentOffset((short) 0)
        .payloadBuilder(
            new SimpleBuilder(
                PacketFactories.getFactory(Packet.class, IpNumber.class)
                    .newInstance(
                        defragmentedPayload,
                        0,
                        defragmentedPayload.length,
                        list.get(0).getHeader().getProtocol())))
        .correctChecksumAtBuild(true)
        .correctLengthAtBuild(true);

    return b.build();
  }

  private static final class ComparatorImpl implements Comparator {

    @Override
    public int compare(IpV4Packet p1, IpV4Packet p2) {
      return p1.getHeader().getFragmentOffset() - p2.getHeader().getFragmentOffset();
    }
  }
}