Make PackRR enforce RR data length limit

• Make rawSetRdlength return false if length would overflow
• Make PackRR assume rawSetRdlength returning false indicates an overflow

dns_test.go

@@ -463,3 +463,19 @@ func TestNoRdataUnpack(t *testing.T) {
 		t.Logf("%s\n", rr)
 	}
 }
+
+func TestRdataOverflow(t *testing.T) {
+	rr := new(RFC3597)
+	rr.Hdr.Name = "."
+	rr.Hdr.Class = ClassINET
+	rr.Hdr.Rrtype = 65280
+	rr.Rdata = hex.EncodeToString(make([]byte, 0xFFFF))
+	buf := make([]byte, 0xFFFF*2)
+	if _, err := PackRR(rr, buf, 0, nil, false); err != nil {
+		t.Fatalf("Maximum size rrdata pack failed: %v", err)
+	}
+	rr.Rdata += "00"
+	if _, err := PackRR(rr, buf, 0, nil, false); err != ErrRdata {
+		t.Fatalf("Oversize rrdata pack didn't return ErrRdata - instead: %v", err)
+	}
+}

msg.go

@@ -1306,8 +1306,10 @@ func PackRR(rr RR, msg []byte, off int, compression map[string]int, compress boo
 	if err != nil {
 		return len(msg), err
 	}
-	rawSetRdlength(msg, off, off1)
-	return off1, nil
+	if rawSetRdlength(msg, off, off1) {
+		return off1, nil
+	}
+	return off, ErrRdata
 }
 
 // UnpackRR unpacks msg[off:] into an RR.

rawmsg.go

@@ -89,7 +89,11 @@ Loop:
 		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)))
+	//so 'end' - 'off+2' is the length of the rdata
+	rdatalen := end - (off + 2)
+	if rdatalen > 0xFFFF {
+		return false
+	}
+	msg[off], msg[off+1] = packUint16(uint16(rdatalen))
 	return true
 }