Add support for extended RCODEs and detect bad RCODEs

edns.go

@@ -122,6 +122,16 @@ func (rr *OPT) SetVersion(v uint8) {
 	rr.Hdr.Ttl = rr.Hdr.Ttl&0xFF00FFFF | (uint32(v) << 16)
 }
 
+// ExtRcode returns the EDNS extended RCODE field (the upper 8 bits of the RCODE).
+func (rr *OPT) ExtRcode() uint8 {
+	return uint8((rr.Hdr.Ttl & 0xFF000000) >> 24)
+}
+
+// SetExtRcode sets the EDNS extended RCODE field.
+func (rr *OPT) SetExtRcode(v uint8) {
+	rr.Hdr.Ttl = rr.Hdr.Ttl&0x00FFFFFF | (uint32(v) << 24)
+}
+
 // UDPSize returns the UDP buffer size.
 func (rr *OPT) UDPSize() uint16 {
 	return rr.Hdr.Class

edns_test.go

@@ -31,4 +31,18 @@ func TestOPTTtl(t *testing.T) {
 	if e.Hdr.Ttl != oldTtl {
 		t.Fail()
 	}
+
+	if e.ExtRcode() != 0 {
+		t.Fail()
+	}
+
+	e.SetExtRcode(42)
+	if e.ExtRcode() != 42 {
+		t.Fail()
+	}
+
+	e.SetExtRcode(0)
+	if e.Hdr.Ttl != oldTtl {
+		t.Fail()
+	}
 }

msg.go

@@ -12,6 +12,7 @@ import (
 	"encoding/base32"
 	"encoding/base64"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"math/rand"
 	"net"
@@ -1452,6 +1453,19 @@ func (dns *Msg) PackBuffer(buf []byte) (msg []byte, err error) {
 		compression = make(map[string]int) // Compression pointer mappings
 	}
 
+	if dns.Rcode < 0 || dns.Rcode > 0xFFF {
+		return nil, errors.New("Invalid RCODE")
+	}
+	if dns.Rcode > 0xF {
+		// Regular RCODE field is 4 bits
+		opt := dns.IsEdns0()
+		if opt == nil {
+			return nil, errors.New("RCODE >= 16 but no EDNS record")
+		}
+		opt.SetExtRcode(uint8(dns.Rcode >> 4))
+		dns.Rcode &= 0xF
+	}
+
 	// Convert convenient Msg into wire-like Header.
 	dh.Id = dns.Id
 	dh.Bits = uint16(dns.Opcode)<<11 | uint16(dns.Rcode)