content: package com.aaronjwood.portauthority.network;
  
  import java.io.Closeable;
  import java.io.IOException;
  import java.net.DatagramPacket;
  import java.net.DatagramSocket;
  import java.net.InetAddress;
  import java.net.SocketException;
  
  public class NetBIOSResolver implements Closeable, Resolver {
      private static final int NETBIOS_UDP_PORT = 137;
      private static final byte[] REQUEST_DATA = {(byte) 0xA2, 0x48, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0x43, 0x4b, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x00, 0x00, 0x21, 0x00, 0x01};
  
      private static final int RESPONSE_TYPE_POS = 47;
      private static final byte RESPONSE_TYPE_NBSTAT = 33;
      private static final int RESPONSE_BASE_LEN = 57;
      private static final int RESPONSE_NAME_LEN = 15;
      private static final int RESPONSE_NAME_BLOCK_LEN = 18;
  
      private static final int GROUP_NAME_FLAG = 128;
      private static final int NAME_TYPE_DOMAIN = 0x00;
      private static final int NAME_TYPE_MESSENGER = 0x03;
  
      DatagramSocket socket = new DatagramSocket();
  
      public NetBIOSResolver(int timeout) throws SocketException {
          socket.setSoTimeout(timeout);
      }
  
      public String[] resolve(InetAddress ip) throws IOException {
          socket.send(new DatagramPacket(REQUEST_DATA, REQUEST_DATA.length, ip, NETBIOS_UDP_PORT));
  
          byte[] response = new byte[1024];
          DatagramPacket responsePacket = new DatagramPacket(response, response.length);
          socket.receive(responsePacket);
  
          if (responsePacket.getLength() < RESPONSE_BASE_LEN || response[RESPONSE_TYPE_POS] != RESPONSE_TYPE_NBSTAT) {
              return null; // response was too short - no names returned
          }
  
          int nameCount = response[RESPONSE_BASE_LEN - 1] & 0xFF;
          if (responsePacket.getLength() < RESPONSE_BASE_LEN + RESPONSE_NAME_BLOCK_LEN * nameCount) {
              return null; // data was truncated or something is wrong
          }
  
          return extractNames(response, nameCount);
      }
  
      static String[] extractNames(byte[] response, int nameCount) {
          String computerName = nameCount > 0 ? name(response, 0) : null;
  
          String groupName = null;
          for (int i = 1; i < nameCount; i++) {
              if (nameType(response, i) == NAME_TYPE_DOMAIN && (nameFlag(response, i) & GROUP_NAME_FLAG) > 0) {
                  groupName = name(response, i);
                  break;
              }
          }
  
          String userName = null;
          for (int i = nameCount - 1; i > 0; i--) {
              if (nameType(response, i) == NAME_TYPE_MESSENGER) {
                  userName = name(response, i);
                  break;
              }
          }
  
          String macAddress = String.format("%02X-%02X-%02X-%02X-%02X-%02X",
                  nameByte(response, nameCount, 0), nameByte(response, nameCount, 1),
                  nameByte(response, nameCount, 2), nameByte(response, nameCount, 3),
                  nameByte(response, nameCount, 4), nameByte(response, nameCount, 5));
  
          return new String[]{computerName, userName, groupName, macAddress};
      }
  
      private static String name(byte[] response, int i) {
          // as we have no idea in which encoding are the received names,
          // assume that local default encoding matches the remote one (they are on the same LAN most probably)
          return new String(response, RESPONSE_BASE_LEN + RESPONSE_NAME_BLOCK_LEN * i, RESPONSE_NAME_LEN).trim();
      }
  
      private static int nameByte(byte[] response, int i, int n) {
          return response[RESPONSE_BASE_LEN + RESPONSE_NAME_BLOCK_LEN * i + n] & 0xFF;
      }
  
      private static int nameFlag(byte[] response, int i) {
          return response[RESPONSE_BASE_LEN + RESPONSE_NAME_BLOCK_LEN * i + RESPONSE_NAME_LEN + 1] & 0xFF +
                  (response[RESPONSE_BASE_LEN + RESPONSE_NAME_BLOCK_LEN * i + RESPONSE_NAME_LEN + 2] & 0xFF) * 0xFF;
      }
  
      private static int nameType(byte[] response, int i) {
          return response[RESPONSE_BASE_LEN + RESPONSE_NAME_BLOCK_LEN * i + RESPONSE_NAME_LEN] & 0xFF;
      }
  
      public void close() {
          socket.close();
      }
  }
  