RawRdlength seems to work;

msg.go

@@ -880,7 +880,7 @@ func packRR(rr RR, msg []byte, off int, compression map[string]int, compress boo
 	if !ok {
 		return len(msg), false
 	}
-	RawSetRdLength(msg, off, off1)
+	RawSetRdlength(msg, off, off1)
 	return off1, true
 }
 
@@ -892,7 +892,14 @@ func unpackRR(msg []byte, off int) (rr RR, off1 int, ok bool) {
 	if off, ok = unpackStruct(&h, msg, off); !ok {
 		return nil, len(msg), false
 	}
+        rrtype, rdlength, off4 := RawTypeRdlength(msg, off0)
+        if rrtype == 0 {
+                return nil, len(msg), false
+        }
 	end := off + int(h.Rdlength)
+        println(h.Rdlength, "me l", rdlength)
+        println(h.Rrtype, "me t", rrtype)
+        println(end, "me tl", off4+int(rdlength))
 
 	// make an rr of that type and re-unpack.
 	// again inefficient but doesn't need to be fast. TODO speed

rawmsg.go

@@ -14,13 +14,12 @@ func RawSetId(msg []byte, off int, id uint16) bool {
 	return true
 }
 
-// RawSetRdLength sets the rdlength in the header of
+// RawSetRdlength sets the rdlength in the header of
 // the RR. The offset 'off' must be positioned at the
-// start of the header of the RR.
+// start of the header of the RR, 'end' must be the
-func RawSetRdLength(msg []byte, off, end int) bool {
+// end of the RR.
-	// We are at the start of the header, walk the
+func RawSetRdlength(msg []byte, off, end int) bool {
-	// domainname (might be compressed), and set the
+	// We are at the start of the header, walk the domainname (might be compressed)
-	// length
 Loop:
 	for {
 		if off > len(msg) {
@@ -35,21 +34,52 @@ Loop:
 				break Loop
 			}
 
+		case 0xC0:
+			// pointer, next byte included, ends domainname
+			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
+}
+
+// RawSetRdlength return the type and length as found
+// in the RR header.
+func RawTypeRdlength(msg []byte, off int) (uint16, uint16, int) {
+Loop:
+	for {
+		if off > len(msg) {
+			return 0, 0, off
+		}
+		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
+	off--
-	// of the fixed part in the header
+	if off+8 > len(msg) {
-	// type is 2 bytes, class is 2 bytes, ttl 4 and then 2 bytes for the length
+		return 0, 0, off
-	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
+	t, off := unpackUint16(msg, off)   // type
-        //so 'end' - 'off+2' is the lenght of the rdata
+	l, off := unpackUint16(msg, off+6) // length
-	msg[off], msg[off+1] = packUint16(uint16(end - (off+2)))
+	return t, l, off
-	return true
 }