Add RawSetRdlength back in

rawmsg.go

@@ -12,3 +12,41 @@ func RawSetId(msg []byte, off int, id uint16) bool {
 	msg[off], msg[off+1] = packUint16(id)
 	return true
 }
+
+// RawSetRdLength set the rdlength
+func RawSetRdLength(msg []byte, off, end int) bool {
+	// We are at the start of the header, walk the
+	// domainname (might be compressed), and set the
+	// length
+Loop:
+	for {
+		if off > len(msg) {
+			return false
+		}
+		c := int(msg[off])
+		off++
+		switch c & 0xC00 {
+		case 0x00:
+			if c == 0x00 {
+				// End of the domainname
+				break Loop
+			}
+
+		case 0xC0:
+			// pointer, next byte included, ends domainnames
+			off++
+			break Loop
+		}
+	}
+	// The domainname has been seen, we at the start
+	// of the fixed part in the header
+	// type is 2 bytes, class is 2 bytes, ttl 4 and then 2 bytes for the length
+	off += 2 + 2 + 4
+	if off+1 > len(msg) {
+		return false
+	}
+        //off+1 is the end of the header, 'end' is the end of the rr
+        //so 'end' - 'off+2' is the lenght of the rdata
+	msg[off], msg[off+1] = packUint16(uint16(end - (off+2)))
+	return true
+}