fix(netxlite/dns): more stricly mirror stdlib error strings (#513)

This diff attempts to modify the errors reported by our custom
resolver by matching more strings from the stdlib.

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/errorsx/classify.go

@@ -84,12 +84,18 @@ func classifyWithStringSuffix(err error) string {
 	if strings.HasSuffix(s, "TLS handshake timeout") {
 		return FailureGenericTimeoutError
 	}
-	if strings.HasSuffix(s, "no such host") {
+	if strings.HasSuffix(s, DNSNoSuchHostSuffix) {
 		// This is dns_lookup_error in MK but such error is used as a
 		// generic "hey, the lookup failed" error. Instead, this error
 		// that we return here is significantly more specific.
 		return FailureDNSNXDOMAINError
 	}
+	if strings.HasSuffix(s, DNSServerMisbehavingSuffix) {
+		return FailureDNSServerMisbehaving
+	}
+	if strings.HasSuffix(s, DNSNoAnswerSuffix) {
+		return FailureDNSNoAnswer
+	}
 	if strings.HasSuffix(s, "use of closed network connection") {
 		return FailureConnectionAlreadyClosed
 	}
@@ -223,6 +229,21 @@ func quicIsCertificateError(alert uint8) bool {
 // filters for DNS bogons MUST use this error.
 var ErrDNSBogon = errors.New("dns: detected bogon address")
 
+// These strings are same as the standard library.
+const (
+	DNSNoSuchHostSuffix        = "no such host"
+	DNSServerMisbehavingSuffix = "server misbehaving"
+	DNSNoAnswerSuffix          = "no answer from DNS server"
+)
+
+// These errors are returned by the decoder and/or the serial resolver.
+var (
+	ErrOODNSNoSuchHost  = fmt.Errorf("ooniresolver: %s", DNSNoSuchHostSuffix)
+	ErrOODNSRefused     = errors.New("ooniresolver: refused")
+	ErrOODNSMisbehaving = fmt.Errorf("ooniresolver: %s", DNSServerMisbehavingSuffix)
+	ErrOODNSNoAnswer    = fmt.Errorf("ooniresolver: %s", DNSNoAnswerSuffix)
+)
+
 // ClassifyResolverError maps an error occurred during a domain name
 // resolution to the corresponding OONI failure string.
 //
@@ -239,6 +260,11 @@ func ClassifyResolverError(err error) string {
 	if errors.Is(err, ErrDNSBogon) {
 		return FailureDNSBogonError // not in MK
 	}
+	// Implementation note: we match errors that share the same
+	// string of the stdlib in the generic classifier.
+	if errors.Is(err, ErrOODNSRefused) {
+		return FailureDNSRefusedError // not in MK
+	}
 	return ClassifyGenericError(err)
 }
 

internal/netxlite/errorsx/classify_test.go

@@ -77,6 +77,18 @@ func TestClassifyGenericError(t *testing.T) {
 		}
 	})
 
+	t.Run("for dns server misbehaving", func(t *testing.T) {
+		if ClassifyGenericError(errors.New("dns server misbehaving")) != FailureDNSServerMisbehaving {
+			t.Fatal("unexpected results")
+		}
+	})
+
+	t.Run("for no answer from DNS server", func(t *testing.T) {
+		if ClassifyGenericError(errors.New("no answer from DNS server")) != FailureDNSNoAnswer {
+			t.Fatal("unexpected results")
+		}
+	})
+
 	t.Run("for use of closed network connection", func(t *testing.T) {
 		err := errors.New("read tcp 10.0.2.15:56948->93.184.216.34:443: use of closed network connection")
 		if ClassifyGenericError(err) != FailureConnectionAlreadyClosed {
@@ -251,6 +263,12 @@ func TestClassifyResolverError(t *testing.T) {
 		}
 	})
 
+	t.Run("for refused", func(t *testing.T) {
+		if ClassifyResolverError(ErrOODNSRefused) != FailureDNSRefusedError {
+			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/errorsx/errno.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-27 14:20:26.411929 +0200 CEST m=+0.233208042
+// Generated: 2021-09-27 15:57:58.500515 +0200 CEST m=+0.304199251
 
 package errorsx
 
@@ -50,6 +50,9 @@ const (
 	//
 	FailureDNSBogonError           = "dns_bogon_error"
 	FailureDNSNXDOMAINError        = "dns_nxdomain_error"
+	FailureDNSRefusedError         = "dns_refused_error"
+	FailureDNSServerMisbehaving    = "dns_server_misbehaving"
+	FailureDNSNoAnswer             = "dns_no_answer"
 	FailureEOFError                = "eof_error"
 	FailureGenericTimeoutError     = "generic_timeout_error"
 	FailureQUICIncompatibleVersion = "quic_incompatible_version"
@@ -94,6 +97,9 @@ var failuresMap = map[string]string{
 	"wrong_protocol_type":            "wrong_protocol_type",
 	"dns_bogon_error":                "dns_bogon_error",
 	"dns_nxdomain_error":             "dns_nxdomain_error",
+	"dns_refused_error":              "dns_refused_error",
+	"dns_server_misbehaving":         "dns_server_misbehaving",
+	"dns_no_answer":                  "dns_no_answer",
 	"eof_error":                      "eof_error",
 	"generic_timeout_error":          "generic_timeout_error",
 	"quic_incompatible_version":      "quic_incompatible_version",

internal/netxlite/errorsx/errno_test.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-27 14:20:26.459041 +0200 CEST m=+0.280321626
+// Generated: 2021-09-27 15:57:58.553101 +0200 CEST m=+0.356786917
 
 package errorsx
 

internal/netxlite/errorsx/errno_unix.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-27 14:20:26.179327 +0200 CEST m=+0.000601459
+// Generated: 2021-09-27 15:57:58.197535 +0200 CEST m=+0.001212709
 
 package errorsx
 

internal/netxlite/errorsx/errno_windows.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-27 14:20:26.380511 +0200 CEST m=+0.201789292
+// Generated: 2021-09-27 15:57:58.455744 +0200 CEST m=+0.259427834
 
 package errorsx
 

internal/netxlite/errorsx/internal/generrno/main.go

@@ -93,6 +93,9 @@ var Specs = []*ErrorSpec{
 	// we must write "DNS" rather than writing "dns".
 	NewLibraryError("DNS_bogon_error"),
 	NewLibraryError("DNS_NXDOMAIN_error"),
+	NewLibraryError("DNS_refused_error"),
+	NewLibraryError("DNS_server_misbehaving"),
+	NewLibraryError("DNS_no_answer"),
 	NewLibraryError("EOF_error"),
 	NewLibraryError("generic_timeout_error"),
 	NewLibraryError("QUIC_incompatible_version"),