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EMQX authentication (1)-MQTT connection authentication based on MySQL

EMQX Team EMQX Team Mar 26, 2020


Security is a challenge for almost all projects, especially for IoT projects. Since the widespread of IoT application, there have been too many security incidents in the IoT industry.

As the factual standard for IoT communication protocols, MQTT maintains a high level of security and provides a multi-level security design:

  • Transport layer: MQTT is based on TCP / IP protocol, and SSL/TLS is used for encrypted transmission on the transport layer:
    • SSL/TLS is used to encrypt communication data to prevent middleman attacks;
    • The client certificate is used as device identity certificate to verify the validity of the device.
  • Application layer :MQTT's security features are used for protection:
    • MQTT protocol supports user name and password for client identity verification;
    • MQTT Broker implements access control for topics (Topic ACL).

MQTT Safety specifications are fully supported by EMQ, and the built-in safety functions can be used out-of-the-box without programming, which can quickly eliminate security risks in projects. This series will focus on various levels of safety specifications, and introduce how to enable relevant functions through the configuration of EMQX to finally achieve the corresponding safety protection.

Introduction to emqx-auth-mysql

emqx_auth_mysql is an MQTT authentication/access control plug-in based on the MySQL database. It realizes connection authentication and access control of the terminal by checking whether the username and password accessed by each terminal are consistent with the information stored in the user-specified MySQL database. Its functional logic is as follows:


In this article, only the authentication function is introduced. ACL function will be introduced in the following article.

Authentication principle

When the device is connected, EMQX will execute the query statement according to the configuration and compare whether the value of the password field in the query result is the same as the value of current requesting client's password with salt processing and encryption. The verification process is as follows:

  • There must be password and salt fields in the query result set. You can use the AS syntax setting such as SELECT *,pwd as password FROM mqtt_user
  • Salt can be specified for each client in the database. EMQX generates a ciphertext based on the client's incoming password and the salt information returned through SQL.
  • If the result set is empty or the comparison results are not equal, the authentication fails

Create a database

You can use any client you like to create the corresponding database. The command-line client of MySQL is used here. Open the MySQL console, as shown below, create an authentication database named emqx, and switch to the emqx database.

mysql> create database emqx;
Query OK, 1 row affected (0.00 sec)

mysql> use emqx;
Database changed

Create a table

The suggested table structure is as follows, where:

  • username is the user name specified when the client connects
  • password_hash is the ciphertext encrypted with salt
  • salt is an encrypted string
  • is_superuser means whether it is a super user, which is used to control the ACL. The default is 0; when set to 1, it is a super user, which can skip the ACL check

The data table fields do not need to be completely consistent with the following. They can be set according to business needs and specified through the auth_query configuration item in the emqx_auth_mysql.conf configuration file.

CREATE TABLE `mqtt_user` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `username` varchar(100) DEFAULT NULL,
  `password_hash` varchar(255) DEFAULT NULL,
  `salt` varchar(40) DEFAULT NULL,
  `is_superuser` tinyint(1) DEFAULT 0,
  `created` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `mqtt_username` (`username`)

After the creation is successful, the table structure is as follows:

mysql> desc mqtt_user;
| Field         | Type             | Null | Key | Default | Extra          |
| id            | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| username      | varchar(100)     | YES  | UNI | NULL    |                |
| password_hash | varchar(255)     | YES  |     | NULL    |                |
| salt          | varchar(40)      | YES  |     | NULL    |                |
| is_superuser  | tinyint(1)       | YES  |     | 0       |                |
| created       | datetime         | YES  |     | NULL    |                |
6 rows in set (0.01 sec)

Prepare authentication data

In this article, the password of sample data is test_password and the encrypted salt is secret. That is, the password used by the client for connection is test_password.

In the configuration file auth.mysql.password_hash of EMQX, salt is just an identifier, which indicates the concatenation relationship between salt and the plaintext of the password.

  • If auth.mysql.password_hash = md5,salt is used, then the MD5 algorithm is used by EMQX to encrypt test_passwordsecret string
  • If auth.mysql.password_hash = salt,md5 is used, then the MD5 algorithm is used by EMQX to encrypt secrettest_password string

In this article, the first configuration method is used, inserting the obtained MD5 ciphertext into the table mqtt_user. Readers can use online MD5 tool or write a program to encode the password.

MD5("test_passwordsecret") -> a904b2d1d2b2f73de384955022964595
mysql> INSERT INTO mqtt_user(username,password_hash,salt) VALUES('test_username', 'a904b2d1d2b2f73de384955022964595', 'secret');

Query OK, 1 row affected (0.00 sec)

mysql> select * from mqtt_user;
| id | username       | password_hash                    | salt   | is_superuser | created |
|  3 | test_username1 | a904b2d1d2b2f73de384955022964595 | secret |            0 | NULL    |
1 row in set (0.00 sec)

Enable authentication

Modify plug in configuration and enable plugin

Modify etc/plugins/emqx_auth_mysql.conf. The effective configuration after the modification is shown below. The remaining ACL related configuration items can be commented:

## Change to the server address where the actual mysql is located
auth.mysql.server = localhost:3306

## Change to the emqx database created above
auth.mysql.database = emqx

## Connection authentication query
auth.mysql.auth_query = SELECT password_hash AS password, salt FROM mqtt_user WHERE username = '%u'

## Encryption Algorithm plain | md5 | sha | sha256 | bcrypt
## Salt encryption algorithm
auth.mysql.password_hash = md5,salt

## No salt encryption algorithm, just write the algorithm name
# auth.mysql.password_hash = md5

After the modification, use Dashboard or the command line to restart the plug in to apply the configuration. The command line for restarting example is as follows:

emqx_ctl plugins reload emqx_auth_mysql

Disable anonymous authentication

EMQX enabled anonymous authentication by default. Once the authentication function is enabled, the device can connect normally even when the database does not query the data, and the connection will be refused only when the data is queried and the password is incorrect.

Open the etc/emqx.conf configuration file and disable anonymous authentication:

## Value: true | false
allow_anonymous = false

Restart emqx to complete the configuration application.


Once ready, only devices that have passed authentication verification can successfully connect to EMQX:

  1. Connect with the correct username and password, subscribe to the topic, and the connection will succeed:
$ mosquitto_sub -p 1883 -u test_username -P test_password -t 'topic' -d
Client mosqsub|5228-wivwiv-mac sending CONNECT
Client mosqsub|5228-wivwiv-mac received CONNACK
Client mosqsub|5228-wivwiv-mac sending SUBSCRIBE (Mid: 1, Topic: topic, QoS: 0)
Client mosqsub|4119-zh
ouzibode received SUBACK
Subscribed (mid: 1): 0
  1. Connect with the wrong username or password, subscribe to the topic, and the connection will fail:
$ mosquitto_sub -p 1883 -u test_username -P test_password -t 'topic' -d
Client mosqsub/61879-wivwiv-ma sending CONNECT
Client mosqsub/61879-wivwiv-ma received CONNACK
Connection Refused: not authorised.


After understanding the principle of EMQX MySQL authentication, readers can use MySQL to expand related applications. Welcome to the EMQX security series, and this series will explain the security issues related to EMQX and IoT to help you build an IoT project with high-level security.

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