net: use the extended RCode from EDNS(0) OPT resources

For a while now we support EDNS, but the current
implementation only sends the OPT resource and doesn't
do anything with the response OPT resource.

For reference the miekg/dns updates the RCode in the
header when there is a OPT resource:
https://github.com/miekg/dns/blob/48f38ebef989eedc6b57f1869ae849ccc8f5fe29/msg.go#L868-L872

Change-Id: I0a7146aed3e50654f340a3925f48612561cb85f4
GitHub-Last-Rev: adc304167e0540cb1f066f07a249d67fad89182e
GitHub-Pull-Request: golang/go#61695
Reviewed-on: https://go-review.googlesource.com/c/go/+/514835
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

2 files changed, 54 insertions, 4 deletions

diff --git a/src/net/dnsclient_unix.go b/src/net/dnsclient_unix.go
index dab5144e5d..ed32ba0280 100644
--- a/src/net/dnsclient_unix.go
+++ b/src/net/dnsclient_unix.go
@@ -205,7 +205,9 @@ func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Que
 
 // checkHeader performs basic sanity checks on the header.
 func checkHeader(p *dnsmessage.Parser, h dnsmessage.Header) error {
-	if h.RCode == dnsmessage.RCodeNameError {
+	rcode := extractExtendedRCode(*p, h)
+
+	if rcode == dnsmessage.RCodeNameError {
 		return errNoSuchHost
 	}
 
@@ -216,17 +218,17 @@ func checkHeader(p *dnsmessage.Parser, h dnsmessage.Header) error {
 
 	// libresolv continues to the next server when it receives
 	// an invalid referral response. See golang.org/issue/15434.
-	if h.RCode == dnsmessage.RCodeSuccess && !h.Authoritative && !h.RecursionAvailable && err == dnsmessage.ErrSectionDone {
+	if rcode == dnsmessage.RCodeSuccess && !h.Authoritative && !h.RecursionAvailable && err == dnsmessage.ErrSectionDone {
 		return errLameReferral
 	}
 
-	if h.RCode != dnsmessage.RCodeSuccess && h.RCode != dnsmessage.RCodeNameError {
+	if rcode != dnsmessage.RCodeSuccess && rcode != dnsmessage.RCodeNameError {
 		// None of the error codes make sense
 		// for the query we sent. If we didn't get
 		// a name error and we didn't get success,
 		// the server is behaving incorrectly or
 		// having temporary trouble.
-		if h.RCode == dnsmessage.RCodeServerFailure {
+		if rcode == dnsmessage.RCodeServerFailure {
 			return errServerTemporarilyMisbehaving
 		}
 		return errServerMisbehaving
@@ -253,6 +255,23 @@ func skipToAnswer(p *dnsmessage.Parser, qtype dnsmessage.Type) error {
 	}
 }
 
+// extractExtendedRCode extracts the extended RCode from the OPT resource (EDNS(0))
+// If an OPT record is not found, the RCode from the hdr is returned.
+func extractExtendedRCode(p dnsmessage.Parser, hdr dnsmessage.Header) dnsmessage.RCode {
+	p.SkipAllAnswers()
+	p.SkipAllAuthorities()
+	for {
+		ahdr, err := p.AdditionalHeader()
+		if err != nil {
+			return hdr.RCode
+		}
+		if ahdr.Type == dnsmessage.TypeOPT {
+			return ahdr.ExtendedRCode(hdr.RCode)
+		}
+		p.SkipAdditional()
+	}
+}
+
 // Do a lookup for a single name, which must be rooted
 // (otherwise answer will not find the answers).
 func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype dnsmessage.Type) (dnsmessage.Parser, string, error) {
diff --git a/src/net/dnsclient_unix_test.go b/src/net/dnsclient_unix_test.go
index 8d435a557f..9ae68f9a99 100644
--- a/src/net/dnsclient_unix_test.go
+++ b/src/net/dnsclient_unix_test.go
@@ -2598,3 +2598,34 @@ func TestLookupOrderFilesNoSuchHost(t *testing.T) {
 		}
 	}
 }
+
+func TestExtendedRCode(t *testing.T) {
+	fake := fakeDNSServer{
+		rh: func(_, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			fraudSuccessCode := dnsmessage.RCodeSuccess | 1<<10
+
+			var edns0Hdr dnsmessage.ResourceHeader
+			edns0Hdr.SetEDNS0(maxDNSPacketSize, fraudSuccessCode, false)
+
+			return dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    fraudSuccessCode,
+				},
+				Questions: []dnsmessage.Question{q.Questions[0]},
+				Additionals: []dnsmessage.Resource{{
+					Header: edns0Hdr,
+					Body:   &dnsmessage.OPTResource{},
+				}},
+			}, nil
+		},
+	}
+
+	r := &Resolver{PreferGo: true, Dial: fake.DialContext}
+	_, _, err := r.tryOneName(context.Background(), getSystemDNSConfig(), "go.dev.", dnsmessage.TypeA)
+	var dnsErr *DNSError
+	if !(errors.As(err, &dnsErr) && dnsErr.Err == errServerMisbehaving.Error()) {
+		t.Fatalf("r.tryOneName(): unexpected error: %v", err)
+	}
+}