diff --git a/include/compat_af_isdn.h b/include/compat_af_isdn.h
new file mode 100644
index 0000000..56cbfb3
--- /dev/null
+++ b/include/compat_af_isdn.h
@@ -0,0 +1,39 @@
+#ifdef MISDN_OLD_AF_COMPATIBILITY
+#undef AF_ISDN
+#undef PF_ISDN
+
+extern	int	AF_ISDN;
+#define PF_ISDN	AF_ISDN
+
+int	AF_ISDN;
+
+#endif
+
+extern void init_af_isdn(void);
+
+#ifdef AF_COMPATIBILITY_FUNC
+#ifdef MISDN_OLD_AF_COMPATIBILITY
+void init_af_isdn(void)
+{
+	int	s;
+
+	/* test for new value */
+	AF_ISDN = 34;
+	s = socket(AF_ISDN, SOCK_RAW, ISDN_P_BASE);
+	if (s >= 0) {
+		close(s);
+		return;
+	}
+	AF_ISDN = 27;
+	s = socket(AF_ISDN, SOCK_RAW, ISDN_P_BASE);
+	if (s >= 0) {
+		close(s);
+		return;
+	}
+}
+#else
+void init_af_isdn(void)
+{
+}
+#endif
+#endif