# HiveMQ MQTT 5 Client Not Connecting to MQTT 3 Broker **URL:** \ **Category:** HiveMQ Client Library\ **Created:** [December 4, 2023, 1:25pm UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537 "2023-12-04T13:25:00Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![hussainbadshah](https://avatars.discourse-cdn.com/v4/letter/h/a698b9/32.png) [@hussainbadshah](https://community.hivemq.com/u/hussainbadshah)\ **Post date:** [December 4, 2023, 1:25pm UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/1 "2023-12-04T13:25:00Z")
I am creating HiveMQ MQTT 5 async client which I am trying to connect to the MQTT broker v3 but my client is not connecting to the broker and below error messages are what I see in my HiveMQ MQTT broker’s **event.log** file - **2023-12-04 13:23:10,099 - Client ID: UNKNOWN, IP:172.17.0.1 was disconnected. reason: Incorrect CONNECT client id length.** Can somebody please confirm if HiveMQ MQTT 5 client library is compatible with MQTT 3 and 3.1.1 brokers? ---
**Author:** ![hivemq-support](https://yyz1.discourse-cdn.com/flex035/user_avatar/community.hivemq.com/hivemq-support/32/357_2.png) [@hivemq-support](https://community.hivemq.com/u/hivemq-support)\ **Post date:** [December 5, 2023, 9:06am UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/2 "2023-12-05T09:06:42Z")
Hi hussainbadshah, Can somebody please confirm if HiveMQ MQTT 5 client library is compatible with HiveMQ V3 brokers? > (Correction) > HiveMQ (Broker) version 3 supports MQTT 3.1 **and** MQTT 3.1.1 **and not** MQTT 5. > HiveMQ (Broker) version 4 supports MQTT 3.1 **and** MQTT 3.1.1 **and** MQTT 5. Incorrect CONNECT client id length \> This error signifies that the client id length used might not be the correct specification for MQTT 3.x broker. From official specification [MQTT Version 3.1.1](https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc385349242) > The Server MUST allow ClientIds which are between **1 and 23** UTF-8 encoded bytes in length, and that contain only the characters. “0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ” [MQTT-3.1.3-5]. In case of HiveMQ, you can use either the default length or increase it as follows [No limits for MQTT clients identifiers](https://www.hivemq.com/article/supporting-mqtt-client-identifiers-23-characters/) > By default, HiveMQ uses the standard MQTT limit of 23 characters to assure MQTT specification compatibility. If you are sure you want to change this behaviour, change the property _global.maxClientIdLength_ in the _configuration.properties_ file of your HiveMQ installation to a number you like to use. Hope this helps. ---
**Author:** ![hussainbadshah](https://avatars.discourse-cdn.com/v4/letter/h/a698b9/32.png) [@hussainbadshah](https://community.hivemq.com/u/hussainbadshah)\ **Post date:** [December 6, 2023, 8:48am UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/3 "2023-12-06T08:48:48Z")
@hivemq-support Thank you for the explanation with necessary details but I’m still getting the same error even after configuring the clientID = “client1” which is just 7 characters. Below are the details: - Docker image for HiveMQ MQTT 3 broker: **hivemq/hivemq3:3.4.7** - The default “config.xml” that comes with this image: \ ``` 1883 0.0.0.0 8080 0.0.0.0 ``` What can I do or check to debug this issue further? ---
**Author:** ![michael\_w](https://yyz1.discourse-cdn.com/flex035/user_avatar/community.hivemq.com/michael_w/32/200_2.png) [@michael\_w](https://community.hivemq.com/u/michael_w)\ **Post date:** [December 6, 2023, 6:06pm UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/4 "2023-12-06T18:06:12Z")
Hi @hussainbadshah, HiveMQ 3 only knows MQTT 3 / 3.1.1. You will need to use HiveMQ 4 for MQTT 5 support. So you will not be able to connect any MQTT 5 client to HiveMQ 3. Also note that HiveMQ 3 is no longer supported. Greetings, Michael from the HiveMQ team ---
**Author:** ![hussainbadshah](https://avatars.discourse-cdn.com/v4/letter/h/a698b9/32.png) [@hussainbadshah](https://community.hivemq.com/u/hussainbadshah)\ **Post date:** [December 7, 2023, 5:14am UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/5 "2023-12-07T05:14:13Z")
Hi @michael_w if you check the following website: [Implementing MQTT in Java](https://www.hivemq.com/blog/implementing-mqtt-in-java/#:~:text=Using%20the%20HiveMQ%20MQTT%20Client%20Library%20for%20Java&text=This%20library%20is%20a%20popular,API%20flavours%20and%20backpressure%20support). Under the heading: " Using the HiveMQ MQTT Client Library for Java" it clearly states that the HiveMQ MQTT Java Client support both MQTT 5.0 and MQTT 3 protocols. Based on the information collected from the official docs of HiveMQ MQTT Client library only, we proceeded with HiveMQ MQTT 5 Client, thinking it supports both 3 and 3.1.1 brokers as well. ---
**Author:** ![sauroter](https://yyz1.discourse-cdn.com/flex035/user_avatar/community.hivemq.com/sauroter/32/18_2.png) [@sauroter](https://community.hivemq.com/u/sauroter)\ **Post date:** [December 7, 2023, 6:05am UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/6 "2023-12-07T06:05:34Z")
Hi @hussainbadshah, > Based on the information collected from the official docs of HiveMQ MQTT Client library only, we proceeded with HiveMQ MQTT 5 Client, thinking it supports both 3 and 3.1.1 brokers as well. The HiveMQ Java clients supports MQTT 5 and MQTT 3.1.1 as stated in the [README.md](https://github.com/hivemq/hivemq-mqtt-client/blob/master/README.md) first sentence. > MQTT 5.0 and 3.1.1 compatible and feature-rich high-performance Java client library with different API flavours and backpressure support. HiveMQ (Broker) version 3 supports MQTT 3.1 **and** MQTT 3.1.1 **and not** MQTT 5. HiveMQ (Broker) version 4 supports MQTT 3.1 **and** MQTT 3.1.1 **and** MQTT 5. If you now like to connect with the HiveMQ Java client to a HiveMQ 3 broker that does not support MQTT 5 you will need to tell the client to create an MQTT 3 connection. (As the CONNECT packet is slightly different and the HiveMQ 3 Broker does not know how to handle an MQTT 5 style CONNECT packet.) To do this you create the HiveMQ Java client in the MQTT 3 flavor as documented [here](https://hivemq.github.io/hivemq-mqtt-client/docs/client-configuration/): ```auto Mqtt3Client client = Mqtt3Client.builder() .identifier(UUID.randomUUID().toString()) .serverHost("") .build(); ``` I hope this helps All the best Georg ---
**Author:** ![hussainbadshah](https://avatars.discourse-cdn.com/v4/letter/h/a698b9/32.png) [@hussainbadshah](https://community.hivemq.com/u/hussainbadshah)\ **Post date:** [December 7, 2023, 7:12am UTC](https://community.hivemq.com/t/hivemq-mqtt-5-client-not-connecting-to-mqtt-3-broker/2537/7 "2023-12-07T07:12:17Z")
Thank you for all the information. Warm Regards Hussain