dns-suite.js/dns.unpack-labels.js

(function (exports) {
'use strict';

// unpack labels with 0x20 compression pointer support

// ui8 is the ArrayBuffer of the entire packet
// ptr is the current index of the packet
// q = { byteLength: 0, cpcount: 0, labels: [], name: '' }
//
// NOTE:
// "NAME"s are terminated with 0x00
// "RDATA" is terminated by its length
exports.DNS_UNPACK_LABELS = function (ui8, ptr, q) {
  if (q.cpcount > 25) {
    throw new Error("compression pointer loop detected (over 25 pointers seems like a loop)");
  }

  var total = ptr;
  var i;

  var len;
  var label = [];

  do {
    label.length = 0;
    len = ui8[total];
    if (len === undefined){
      // RDATA is terminated by undefined, not len === 0
      q.trunc = true;
      break;
    }

    // Handle message compression pointers. See 4.1.4 of RFC1035 for details.
    // 0xc0 // 192 // parseInt('11000000', 2).toString(16)
    if (len >= 0xc0) {
      // Only the two highest bits are used to signify the pointer.
      // The remaining bits may be used to specify the address of the pointer
      // (it would seem that only 1 extra bit is actually used since the message size is 512 bytes)
      var cpptr = ((ui8[total] & 0x3f) << 8) |  ui8[total + 1];

      // We're not coming back, so if this is the first time we're following one of
      // these pointers we up the byteLength to mark the pointer as part of the label
      if (!q.cpcount) {
        q.byteLength += 2; // cp and len
      }
      q.cpcount += 1;

      // recursion
      return exports.DNS_UNPACK_LABELS(ui8, cpptr, q);
    }

    //str.length = 0; // fast empty array
    if (ui8.byteLength - total < len) {
      //console.error('-1', ui8[total - 1]);
      //console.error(' 0', ui8[total]);
      //console.error(' 1', ui8[total + 1]);
      //console.error(' 1', ui8[total + 2]);
      throw new Error(
        "Expected a string of length " + len
          + " but packet itself has " + (ui8.byteLength - total) + " bytes remaining"
      );
    }

    // Advance the pointer to after the length indicator, then read the string in.
    total += 1;
    for (i = 0; i < len; i += 1) {
      // TODO check url-allowable characters
      label.push(String.fromCharCode(ui8[total]));
      total += 1;
    }

    if (label.length) {
      q.labels.push(label.join(''));
    }

    // It is important this be done every time, so that if we run into a message compression
    // pointer later we still have a record of what was consumed before that.
    if (0 === q.cpcount) {
      q.byteLength = total - ptr;
    }
  } while (0 !== len && undefined !== len);

  //str.pop(); // remove trailing '.'

  q.name = q.labels.join('.');
  return q;
};

}('undefined' !== typeof window ? window : exports));