diff --git a/libraries/GCS_MAVLink/GCS.h b/libraries/GCS_MAVLink/GCS.h
index 459add9b05..652dd6fea0 100644
--- a/libraries/GCS_MAVLink/GCS.h
+++ b/libraries/GCS_MAVLink/GCS.h
@@ -129,6 +129,7 @@ public:
     AP_HAL::UARTDriver *get_uart() { return _port; }
 
     virtual uint8_t sysid_my_gcs() const = 0;
+    virtual bool sysid_enforce() const { return false; }
 
     static const struct AP_Param::GroupInfo        var_info[];
 
@@ -278,7 +279,7 @@ protected:
 
     // overridable method to check for packet acceptance. Allows for
     // enforcement of GCS sysid
-    virtual bool accept_packet(const mavlink_status_t &status, mavlink_message_t &msg) { return true; }
+    bool accept_packet(const mavlink_status_t &status, mavlink_message_t &msg);
     virtual AP_Rally *get_rally() const = 0;
     virtual AP_AdvancedFailsafe *get_advanced_failsafe() const { return nullptr; };
     virtual AP_VisualOdom *get_visual_odom() const { return nullptr; }
diff --git a/libraries/GCS_MAVLink/GCS_Common.cpp b/libraries/GCS_MAVLink/GCS_Common.cpp
index 4171f09cab..3a566ef8e7 100644
--- a/libraries/GCS_MAVLink/GCS_Common.cpp
+++ b/libraries/GCS_MAVLink/GCS_Common.cpp
@@ -3945,6 +3945,24 @@ uint32_t GCS_MAVLINK::correct_offboard_timestamp_usec_to_ms(uint64_t offboard_us
     return corrected_us / 1000U;
 }
 
+/*
+  return true if we will accept this packet. Used to implement SYSID_ENFORCE
+ */
+bool GCS_MAVLINK::accept_packet(const mavlink_status_t &status,
+                                mavlink_message_t &msg)
+{
+    if (!sysid_enforce()) {
+      return true;
+    }
+    if (msg.msgid == MAVLINK_MSG_ID_RADIO ||
+        msg.msgid == MAVLINK_MSG_ID_RADIO_STATUS) {
+        return true;
+    }
+
+    return (msg.sysid == sysid_my_gcs());
+}
+
+
 GCS &gcs()
 {
     return *GCS::instance();