Small changes based on review comments

nameserver/nameserver.go

@@ -2,6 +2,7 @@ package nameserver
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net"
 	"strings"
@@ -247,7 +248,7 @@ func handleRequest(ctx context.Context, w dns.ResponseWriter, query *dns.Msg) er
 		// TODO: analyze nxdomain requests, malware could be trying DGA-domains
 		tracer.Warningf("nameserver: %s requested %s%s: %s", conn.Process(), q.FQDN, q.QType, err)
 
-		if _, ok := err.(*resolver.BlockedUpstreamError); ok {
+		if errors.Is(err, &resolver.BlockedUpstreamError{}) {
 			conn.Block(err.Error())
 		} else {
 			conn.Failed("failed to resolve: " + err.Error())