DNSRecord = function(name, type, cl, opt_ttl, opt_data) {
  this.name = name;
  this.type = type;
  this.cl = cl;

  this.isQD = (arguments.length == 3);
  if (!this.isQD) {
    this.ttl = opt_ttl;
    this.data_ = opt_data;
  }
}
DNSRecord is a record inside a DNS packet; e.g. a QUESTION, or an ANSWER,
AUTHORITY, or ADDITIONAL record. Note that QUESTION records are special,
and do not have ttl or data.