code: public String getHostByAddr(byte [] addr) throws UnknownHostException {
  	Name name = ReverseMap.fromAddress(InetAddress.getByAddress(addr));
  	Record [] records = new Lookup(name, Type.PTR).run();
  	if (records == null)
  		throw new UnknownHostException();
  	return ((PTRRecord) records[0]).getTarget().toString();
  }
docs: Performs a reverse DNS lookup.
  @param addr The ip address to lookup.
  @return The host name found for the ip address.