diff --git a/filter.go b/filter.go
new file mode 100644
index 0000000..d178966
--- /dev/null
+++ b/filter.go
@@ -0,0 +1,12 @@
+package dynamiclistener
+
+func OnlyAllow(str string) func(...string) []string {
+	return func(s2 ...string) []string {
+		for _, s := range s2 {
+			if s == str {
+				return []string{s}
+			}
+		}
+		return nil
+	}
+}