diff --git a/apps/emqx/src/emqx_channel.erl b/apps/emqx/src/emqx_channel.erl
index 2ffe880f3..130ec99c8 100644
--- a/apps/emqx/src/emqx_channel.erl
+++ b/apps/emqx/src/emqx_channel.erl
@@ -1551,6 +1551,8 @@ set_username(
 set_username(_ConnPkt, ClientInfo) ->
     {ok, ClientInfo}.
 
+%% The `is_bridge` bit flag in CONNECT packet (parsed as `bridge_mode`)
+%% is invented by mosquitto, named 'try_private': https://mosquitto.org/man/mosquitto-conf-5.html
 set_bridge_mode(#mqtt_packet_connect{is_bridge = true}, ClientInfo) ->
     {ok, ClientInfo#{is_bridge => true}};
 set_bridge_mode(_ConnPkt, _ClientInfo) ->
diff --git a/apps/emqx/src/emqx_frame.erl b/apps/emqx/src/emqx_frame.erl
index 0799a24ee..22170b6de 100644
--- a/apps/emqx/src/emqx_frame.erl
+++ b/apps/emqx/src/emqx_frame.erl
@@ -301,6 +301,7 @@ parse_connect2(
         proto_name = ProtoName,
         proto_ver = ProtoVer,
         %% For bridge mode, non-standard implementation
+        %% Invented by mosquitto, named 'try_private': https://mosquitto.org/man/mosquitto-conf-5.html
         is_bridge = (BridgeTag =:= 8),
         clean_start = bool(CleanStart),
         will_flag = bool(WillFlag),
@@ -772,6 +773,7 @@ serialize_variable(
         proto_name = ProtoName,
         proto_ver = ProtoVer,
         %% For bridge mode, non-standard implementation
+        %% Invented by mosquitto, named 'try_private': https://mosquitto.org/man/mosquitto-conf-5.html
         is_bridge = IsBridge,
         clean_start = CleanStart,
         will_flag = WillFlag,