Strip OPT from response when client lacked EDNS0; add tests; show EDE in capture

client/internal/dns/upstream.go

@@ -267,8 +267,13 @@ func (u *upstreamResolverBase) queryUpstream(parentCtx context.Context, w dns.Re
 	// Advertise EDNS0 so the upstream may include Extended DNS Errors
 	// (RFC 8914) in failure responses; we use those to short-circuit
 	// failover for definitive answers like DNSSEC validation failures.
-	if r.IsEdns0() == nil {
-		r.SetEdns0(currentMTU-ipUDPHeaderSize, false)
+	// Operate on a copy so the inbound request is unchanged: a client that
+	// did not advertise EDNS0 must not see an OPT in the response.
+	hadEdns := r.IsEdns0() != nil
+	reqUp := r
+	if !hadEdns {
+		reqUp = r.Copy()
+		reqUp.SetEdns0(upstreamUDPSize(), false)
 	}
 
 	var startTime time.Time
@@ -278,7 +283,7 @@ func (u *upstreamResolverBase) queryUpstream(parentCtx context.Context, w dns.Re
 		defer cancel()
 		ctx, upstreamProto = contextWithupstreamProtocolResult(ctx)
 		startTime = time.Now()
-		rm, t, err = u.upstreamClient.exchange(ctx, upstream.String(), r)
+		rm, t, err = u.upstreamClient.exchange(ctx, upstream.String(), reqUp)
 	}()
 
 	if err != nil {
@@ -292,16 +297,47 @@ func (u *upstreamResolverBase) queryUpstream(parentCtx context.Context, w dns.Re
 	if rm.Rcode == dns.RcodeServerFailure || rm.Rcode == dns.RcodeRefused {
 		if code, ok := nonRetryableEDE(rm); ok {
 			resutil.SetMeta(w, "ede", edeName(code))
+			if !hadEdns {
+				stripOPT(rm)
+			}
 			u.writeSuccessResponse(w, rm, upstream, r.Question[0].Name, t, upstreamProto, logger)
 			return nil
 		}
 		return &upstreamFailure{upstream: upstream, reason: dns.RcodeToString[rm.Rcode]}
 	}
 
+	if !hadEdns {
+		stripOPT(rm)
+	}
 	u.writeSuccessResponse(w, rm, upstream, r.Question[0].Name, t, upstreamProto, logger)
 	return nil
 }
 
+// upstreamUDPSize returns the EDNS0 UDP buffer size we advertise to upstreams,
+// derived from the tunnel MTU and bounded against underflow.
+func upstreamUDPSize() uint16 {
+	if currentMTU > ipUDPHeaderSize {
+		return currentMTU - ipUDPHeaderSize
+	}
+	return dns.MinMsgSize
+}
+
+// stripOPT removes any OPT pseudo-RRs from the response's Extra section so
+// the response complies with RFC 6891 when the client did not advertise EDNS0.
+func stripOPT(rm *dns.Msg) {
+	if len(rm.Extra) == 0 {
+		return
+	}
+	out := rm.Extra[:0]
+	for _, rr := range rm.Extra {
+		if _, ok := rr.(*dns.OPT); ok {
+			continue
+		}
+		out = append(out, rr)
+	}
+	rm.Extra = out
+}
+
 func (u *upstreamResolverBase) handleUpstreamError(err error, upstream netip.AddrPort, startTime time.Time) *upstreamFailure {
 	if !errors.Is(err, context.DeadlineExceeded) && !isTimeout(err) {
 		return &upstreamFailure{upstream: upstream, reason: err.Error()}

client/internal/dns/upstream_test.go

@@ -770,3 +770,81 @@ func TestExchangeWithFallback_TCPTruncatesToClientSize(t *testing.T) {
 	assert.Less(t, len(rm2.Answer), 20, "small EDNS0 client should get fewer records")
 	assert.True(t, rm2.Truncated, "response should be truncated for small buffer client")
 }
+
+func msgWithEDE(rcode int, codes ...uint16) *dns.Msg {
+	m := new(dns.Msg)
+	m.Response = true
+	m.Rcode = rcode
+	if len(codes) == 0 {
+		return m
+	}
+	opt := &dns.OPT{Hdr: dns.RR_Header{Name: ".", Rrtype: dns.TypeOPT}}
+	opt.SetUDPSize(dns.MinMsgSize)
+	for _, c := range codes {
+		opt.Option = append(opt.Option, &dns.EDNS0_EDE{InfoCode: c})
+	}
+	m.Extra = append(m.Extra, opt)
+	return m
+}
+
+func TestNonRetryableEDE(t *testing.T) {
+	tests := []struct {
+		name     string
+		msg      *dns.Msg
+		wantOK   bool
+		wantCode uint16
+	}{
+		{name: "no edns0", msg: msgWithEDE(dns.RcodeServerFailure)},
+		{
+			name: "opt without ede",
+			msg: func() *dns.Msg {
+				m := msgWithEDE(dns.RcodeServerFailure)
+				opt := &dns.OPT{Hdr: dns.RR_Header{Name: ".", Rrtype: dns.TypeOPT}}
+				opt.Option = append(opt.Option, &dns.EDNS0_NSID{Code: dns.EDNS0NSID})
+				m.Extra = []dns.RR{opt}
+				return m
+			}(),
+		},
+		{name: "ede dnsbogus", msg: msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeDNSBogus), wantOK: true, wantCode: dns.ExtendedErrorCodeDNSBogus},
+		{name: "ede signature expired", msg: msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeSignatureExpired), wantOK: true, wantCode: dns.ExtendedErrorCodeSignatureExpired},
+		{name: "ede blocked", msg: msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeBlocked), wantOK: true, wantCode: dns.ExtendedErrorCodeBlocked},
+		{name: "ede prohibited", msg: msgWithEDE(dns.RcodeRefused, dns.ExtendedErrorCodeProhibited), wantOK: true, wantCode: dns.ExtendedErrorCodeProhibited},
+		{name: "ede cached error retryable", msg: msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeCachedError)},
+		{name: "ede network error retryable", msg: msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeNetworkError)},
+		{name: "ede not ready retryable", msg: msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeNotReady)},
+		{
+			name:     "first non-retryable wins",
+			msg:      msgWithEDE(dns.RcodeServerFailure, dns.ExtendedErrorCodeNetworkError, dns.ExtendedErrorCodeDNSBogus),
+			wantOK:   true,
+			wantCode: dns.ExtendedErrorCodeDNSBogus,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			code, ok := nonRetryableEDE(tc.msg)
+			assert.Equal(t, tc.wantOK, ok, "ok should match")
+			if tc.wantOK {
+				assert.Equal(t, tc.wantCode, code, "code should match")
+			}
+		})
+	}
+}
+
+func TestEDEName(t *testing.T) {
+	assert.Equal(t, "DNSSEC Bogus", edeName(dns.ExtendedErrorCodeDNSBogus))
+	assert.Equal(t, "Signature Expired", edeName(dns.ExtendedErrorCodeSignatureExpired))
+	assert.Equal(t, "EDE 9999", edeName(9999), "unknown code falls back to numeric")
+}
+
+func TestStripOPT(t *testing.T) {
+	rm := &dns.Msg{
+		Extra: []dns.RR{
+			&dns.OPT{Hdr: dns.RR_Header{Name: ".", Rrtype: dns.TypeOPT}},
+			&dns.A{Hdr: dns.RR_Header{Name: "x.", Rrtype: dns.TypeA}, A: net.IPv4(1, 2, 3, 4)},
+		},
+	}
+	stripOPT(rm)
+	assert.Len(t, rm.Extra, 1, "OPT should be removed, A kept")
+	_, isOPT := rm.Extra[0].(*dns.OPT)
+	assert.False(t, isOPT, "remaining record must not be OPT")
+}