fix: only set gotTimeout for actual timeout errors

Previously, querySingleType set gotTimeout=true for any queryDNS error,
misclassifying connection refused, network unreachable, etc. as timeouts.

Now uses errors.As(err, &net.Error) with Timeout() check to distinguish
real timeouts from other network errors. Non-timeout errors are tracked
via a new gotError field and classified as StatusError.

internal/resolver/iterative.go

@@ -480,6 +480,7 @@ type queryState struct {
 	gotNXDomain bool
 	gotSERVFAIL bool
 	gotTimeout  bool
+	gotError    bool
 	hasRecords  bool
 }
 
@@ -517,8 +518,11 @@ func (r *Resolver) querySingleType(
 ) {
 	msg, err := r.queryDNS(ctx, nsIP, hostname, qtype)
 	if err != nil {
-		if isTimeout(err) {
+		var netErr net.Error
+		if errors.As(err, &netErr) && netErr.Timeout() {
 			state.gotTimeout = true
+		} else {
+			state.gotError = true
 		}
 
 		return
@@ -558,16 +562,6 @@ 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:
@@ -575,14 +569,22 @@ func classifyResponse(resp *NameserverResponse, state queryState) {
 	case state.gotTimeout && !state.hasRecords:
 		resp.Status = StatusTimeout
 		resp.Error = "all queries timed out"
-	case state.gotSERVFAIL && !state.hasRecords:
+	case (state.gotError || state.gotSERVFAIL) && !state.hasRecords:
 		resp.Status = StatusError
-		resp.Error = "server returned SERVFAIL"
+		resp.Error = errorMessageForState(state)
 	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) {