diff --git a/include/osmocom/mgcp/mgcp_common.h b/include/osmocom/mgcp/mgcp_common.h
index 75d5a37..a1bbb19 100644
--- a/include/osmocom/mgcp/mgcp_common.h
+++ b/include/osmocom/mgcp/mgcp_common.h
@@ -50,6 +50,7 @@
 };
 
 #define MGCP_X_OSMO_IGN_HEADER "X-Osmo-IGN:"
+#define MGCP_X_OSMO_OSMUX_HEADER "X-Osmux:"
 
 /* Values should be bitwise-OR-able */
 enum mgcp_x_osmo_ign {