Clients can't connect using MQTT 5.0

[eisaev]

Environment

• OS: Ubuntu 16.04.6 LTS
• Erlang/OTP: 22.2.7
• EMQ: 4.0.4

Description

When I try to connect to EMQX using mosqitto client using MQTT v5, I get an error:

$ mosquitto_sub -d -V mqttv5 -h 192.168.0.1 -p 1883 -t test
Client (null) sending CONNECT
Error: A network protocol error occurred when communicating with the broker.
$ mosquitto_pub -d -V mqttv5 -h 192.168.0.1 -p 1883 -t test -m test_message
Client (null) sending CONNECT
Error: A network protocol error occurred when communicating with the broker.

When using versions 3.1 and 3.1.1, the issue is not observed:

$ mosquitto_pub -d -V mqttv311 -h 192.168.0.1 -p 1883 -t test -m test_message
Client mosq-D8n0wzeaIZCgceT2lw sending CONNECT
Client mosq-D8n0wzeaIZCgceT2lw received CONNACK (0)
Client mosq-D8n0wzeaIZCgceT2lw sending PUBLISH (d0, q0, r0, m1, 'test', ... (12 bytes))
Client mosq-D8n0wzeaIZCgceT2lw sending DISCONNECT

I use mosqitto client version 1.6.9:

$ mosquitto_pub --help | grep -m 1 version
mosquitto_pub version 1.6.9 running on libmosquitto 1.6.9.
$ mosquitto_sub --help | grep -m 1 version
mosquitto_sub version 1.6.9 running on libmosquitto 1.6.9.

The issue is reproduced on the default EMQX configuration as well.

[eisaev]

Mosquitto client throws an error because EMQX server passes the value of Maximum QoS property (0x24) equal to 2. According to the MQTT 5.0 Protocol specification, this property must be omitted when set to 2.
https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901084

Followed by a Byte with a value of either 0 or 1. It is a Protocol Error to include Maximum QoS more than once, or to have a value other than 0 or 1. If the Maximum QoS is absent, the Client uses a Maximum QoS of 2.

[eisaev]

I fixed it locally like this:

diff --git a/src/emqx_channel.erl b/src/emqx_channel.erl
index 1535924..7df3134 100644
--- a/src/emqx_channel.erl
+++ b/src/emqx_channel.erl
@@ -1150,7 +1150,7 @@ enrich_connack_caps(AckProps, #channel{conninfo   = #{proto_ver := ?MQTT_PROTO_V
       shared_subscription   := Shared,
       wildcard_subscription := Wildcard
      } = emqx_mqtt_caps:get_caps(Zone),
-    AckProps#{'Maximum-QoS'         => MaxQoS,
+    AckProps#{'Maximum-QoS' => case MaxQoS < 2 of true -> MaxQoS; false -> undefined end,
               'Retain-Available'    => flag(Retain),
               'Maximum-Packet-Size' => MaxPktSize,
               'Topic-Alias-Maximum' => MaxAlias,

[tigercl]

@eisaev Thanks for your feedback, we will fix this issue as soon as possible.

[HJianBo]

Hi @eisaev We have released v4.0.5, you can try it later