fix: distinguish timeout from negative DNS responses (closes #35)

internal/resolver/iterative.go

@@ -480,7 +480,6 @@ type queryState struct {
 	gotNXDomain bool
 	gotSERVFAIL bool
 	gotTimeout  bool
-	gotError    bool
 	hasRecords  bool
 }
 
@@ -518,11 +517,8 @@ func (r *Resolver) querySingleType(
 ) {
 	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
 	if err != nil {
-		var netErr net.Error
+		if isTimeout(err) {
-		if errors.As(err, &netErr) && netErr.Timeout() {
 			state.gotTimeout = true
-		} else {
-			state.gotError = true
 		}
 
 		return
@@ -562,6 +558,16 @@ func collectAnswerRecords(
 	}
 }
 
+// isTimeout checks whether an error is a network timeout.
+func isTimeout(err error) bool {
+	var netErr net.Error
+	if errors.As(err, &netErr) {
+		return netErr.Timeout()
+	}
+
+	return false
+}
+
 func classifyResponse(resp *NameserverResponse, state queryState) {
 	switch {
 	case state.gotNXDomain && !state.hasRecords:
@@ -569,22 +575,14 @@ func classifyResponse(resp *NameserverResponse, state queryState) {
 	case state.gotTimeout && !state.hasRecords:
 		resp.Status = StatusTimeout
 		resp.Error = "all queries timed out"
-	case (state.gotError || state.gotSERVFAIL) && !state.hasRecords:
+	case state.gotSERVFAIL && !state.hasRecords:
 		resp.Status = StatusError
-		resp.Error = errorMessageForState(state)
+		resp.Error = "server returned SERVFAIL"
 	case !state.hasRecords && !state.gotNXDomain:
 		resp.Status = StatusNoData
 	}
 }
 
-func errorMessageForState(state queryState) string {
-	if state.gotSERVFAIL {
-		return "server returned SERVFAIL"
-	}
-
-	return "query failed due to non-timeout error"
-}
-
 // extractRecordValue formats a DNS RR value as a string.
 func extractRecordValue(rr dns.RR) string {
 	switch r := rr.(type) {