isValidHostname function

bool isValidHostname(String hostname)

Implementation

bool isValidHostname(String hostname) {
  if (hostname.length > 255) {
    return false;
  }
  if (hostname.isEmpty) {
    return false;
  }
  if (!isValidAscii(hostname.codeUnitAt(0)) &&
          hostname.codeUnitAt(0) != 46 && // '.' (dot)
          hostname.codeUnitAt(0) != 95 // '_' (underscore)
      ) {
    return false;
  }
  // Validate hostname according to RFC
  int lastDotIndex = -1;
  int lastCharCode = -1;
  int len = hostname.length;
  for (int i = 0; i < len; i += 1) {
    int code = hostname.codeUnitAt(i);
    if (code == 46) {
      // '.'
      if (
          // Check that previous label is < 63 bytes long (64 = 63 + '.')
          i - lastDotIndex > 64 ||
              // Check that previous character was not already a '.'
              lastCharCode == 46 ||
              // Check that the previous label does not end with a '-' (dash)
              lastCharCode == 45 ||
              // Check that the previous label does not end with a '_' (underscore)
              lastCharCode == 95) {
        return false;
      }
      lastDotIndex = i;
    } else if (!isValidAscii(code) && code != 45 && code != 95) {
      // Check if there is a forbidden character in the label
      return false;
    }
    lastCharCode = code;
  }
  return (
      // Check that last label is shorter than 63 chars
      len - lastDotIndex - 1 <= 63 &&
          // Check that the last character is an allowed trailing label character.
          // Since we already checked that the char is a valid hostname character,
          // we only need to check that it's different from '-'.
          lastCharCode != 45);
}