fix(netxlite): map servfail error (#728)

This error occurred for example when querying kazemjalali.com
in websteps measurements run from Iran.

This error is relatively uncommon, but it still makes sense to
create a specific mapping rule for it.

Originally: bassosimone/websteps-illustrated@4269e82

See ooni/probe#2096

internal/netxlite/classify.go

@@ -248,11 +248,17 @@ const (
 // unexported errors used by the Go standard library.
 var (
 	ErrOODNSNoSuchHost  = fmt.Errorf("ooniresolver: %s", DNSNoSuchHostSuffix)
-	ErrOODNSRefused     = errors.New("ooniresolver: refused")
 	ErrOODNSMisbehaving = fmt.Errorf("ooniresolver: %s", DNSServerMisbehavingSuffix)
 	ErrOODNSNoAnswer    = fmt.Errorf("ooniresolver: %s", DNSNoAnswerSuffix)
 )
 
+// These errors are not part of the Go standard library but we can
+// return them in our custom resolvers.
+var (
+	ErrOODNSRefused  = errors.New("ooniresolver: refused")
+	ErrOODNSServfail = errors.New("ooniresolver: servfail")
+)
+
 // classifyResolverError maps DNS resolution errors to
 // OONI failure strings.
 //
@@ -278,6 +284,9 @@ func classifyResolverError(err error) string {
 	if errors.Is(err, ErrOODNSRefused) {
 		return FailureDNSRefusedError // not in MK
 	}
+	if errors.Is(err, ErrOODNSServfail) {
+		return FailureDNSServfailError
+	}
 	return classifyGenericError(err)
 }
 

internal/netxlite/classify_test.go

@@ -269,6 +269,12 @@ func TestClassifyResolverError(t *testing.T) {
 		}
 	})
 
+	t.Run("for servfail", func(t *testing.T) {
+		if classifyResolverError(ErrOODNSServfail) != FailureDNSServfailError {
+			t.Fatal("unexpected result")
+		}
+	})
+
 	t.Run("for another kind of error", func(t *testing.T) {
 		if classifyResolverError(io.EOF) != FailureEOFError {
 			t.Fatal("unexpected result")

internal/netxlite/dnsdecoder.go

@@ -22,6 +22,8 @@ func (d *DNSDecoderMiekg) parseReply(data []byte) (*dns.Msg, error) {
 		return nil, ErrOODNSNoSuchHost
 	case dns.RcodeRefused:
 		return nil, ErrOODNSRefused
+	case dns.RcodeServerFailure:
+		return nil, ErrOODNSServfail
 	default:
 		return nil, ErrOODNSMisbehaving
 	}

internal/netxlite/dnsdecoder_test.go

@@ -47,6 +47,18 @@ func TestDNSDecoder(t *testing.T) {
 			}
 		})
 
+		t.Run("Servfail", func(t *testing.T) {
+			d := &DNSDecoderMiekg{}
+			data, err := d.DecodeLookupHost(
+				dns.TypeA, dnsGenReplyWithError(t, dns.TypeA, dns.RcodeServerFailure))
+			if !errors.Is(err, ErrOODNSServfail) {
+				t.Fatal("not the error we expected", err)
+			}
+			if data != nil {
+				t.Fatal("expected nil data here")
+			}
+		})
+
 		t.Run("no address", func(t *testing.T) {
 			d := &DNSDecoderMiekg{}
 			data, err := d.DecodeLookupHost(dns.TypeA, dnsGenLookupHostReplySuccess(t, dns.TypeA))

internal/netxlite/dnsencoder_test.go

@@ -8,7 +8,6 @@ import (
 )
 
 func TestDNSEncoder(t *testing.T) {
-
 	t.Run("encode A", func(t *testing.T) {
 		e := &DNSEncoderMiekg{}
 		data, err := e.Encode("x.org", dns.TypeA, false)

internal/netxlite/internal/generrno/main.go

@@ -158,6 +158,7 @@ var Specs = []*ErrorSpec{
 	NewLibraryError("DNS_refused_error"),
 	NewLibraryError("DNS_server_misbehaving"),
 	NewLibraryError("DNS_no_answer"),
+	NewLibraryError("DNS_servfail_error"),
 	NewLibraryError("EOF_error"),
 	NewLibraryError("generic_timeout_error"),
 	NewLibraryError("QUIC_incompatible_version"),