Fix DNS request context marshaling

nameserver/nsutil/nsutil.go

@@ -2,6 +2,7 @@ package nsutil
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"strings"
@@ -45,6 +46,11 @@ func (rf ResponderFunc) ReplyWithDNS(ctx context.Context, request *dns.Msg) *dns
 	return rf(ctx, request)
 }
 
+// MarshalJSON disables JSON marshaling for ResponderFunc.
+func (rf ResponderFunc) MarshalJSON() ([]byte, error) {
+	return json.Marshal(nil)
+}
+
 // BlockIP is a ResponderFunc than replies with either 0.0.0.17 or ::17 for
 // each A or AAAA question respectively. If there is no A or AAAA question, it
 // defaults to replying with NXDomain.