When the MySQL client connects to the server, the handshake process mainly includes three steps: 1. The server sends an initialization message, including the version number, thread ID and random salt; 2. The client generates an encrypted response and sends it; 3. After the server verifies the information, decides whether to allow the connection. Common reasons for handshake failure include mismatch in passwords or authentication plugins (such as caching_sha2_password and mysql_native_password), SSL configuration errors, and network or firewall problems. Troubleshooting methods include checking MySQL error log, testing connection commands, and querying user authentication plug-in settings. Protocol version differences may lead to compatibility issues, and solutions include updating client drivers, adjusting server configurations, and clarifying connection parameters. You can check the Initial Handshake package and client response package through tools such as Wireshark to debug connection problems.

MySQL's client-server protocol and handshake process are the basis for understanding the MySQL connection mechanism. Simply put, when you connect to a MySQL server with a client (such as MySQL Workbench, command line tools, or applications), there is actually a complete communication process behind it, and the handshake is the beginning of this process.

Let’s talk about how this process works from several common perspectives and what we need to pay attention to.

Basic process of handshake

When a MySQL client connects to the server, it first goes through a handshake process. This process mainly includes the following steps:

• The server sends an initial handshake package : When the client establishes a TCP connection, the server will send an initialization message (Initial Handshake Packet), which contains information such as version number, thread ID, random salt value, etc.
• Client responds to authentication information : The client generates an encrypted response based on the received information, combined with the user name, password and salt and sends it to the server.
• The server verifies the identity and returns the result : the server checks whether the authentication information sent by the client is correct. If it is passed, the connection will be allowed to continue; otherwise the connection will be disconnected.

The entire handshake process determines whether the user can successfully log into the database, and also affects the security and efficiency of the connection.

Common reasons for handshake failure and troubleshooting methods

Handshake failure is a common connection problem, especially after a remote connection or configuration change. Here are a few typical reasons:

• Password error or encryption method mismatch : MySQL 8.0 uses the caching_sha2_password authentication plug-in by default, while some old clients may only support mysql_native_password . If you encounter errors like "Authentication plugin not supported", you may need to adjust the authentication method of the user.

• SSL configuration problem : Some clients require SSL encrypted connections by default, and if the server does not enable SSL or the certificate configuration is incorrect, it will cause the handshake to fail.

• Network or firewall restrictions : Although it is not a protocol-level problem, network failure or port blockage can also be manifested as a handshake failure, which usually prompts "Connection timeout" or "No connection could be made".

Troubleshooting suggestions:

• Check MySQL logs, especially the error logs, if there are any related records
• Test connection using mysql -h host -u user -p
• Check user permissions and authentication plugin settings (can use SELECT plugin FROM mysql.user WHERE User='xxx' )

Protocol version and compatibility issues

MySQL's client-server protocol is also evolving with version updates. The protocols may vary between different versions, especially when major versions are upgraded (such as from 5.7 to 8.0). These changes may lead to compatibility issues.

For example:

• 8.0 introduces new authentication methods and default character sets (utf8mb4)
• Some old drivers or ORM frameworks may not be able to handle new protocol features if they are not updated.

The solution is usually:

• Update client driver to the latest version
• Adjust server configuration to be compatible with old clients (such as modifying the default authentication plug-in)
• Clearly specify connection parameters, such as character set, SSL mode, etc.

How to view packets for the handshake process?

If you want to get a deeper look at the details of the handshake, you can use a packet grabbing tool (such as Wireshark) to observe the actual communication content. The specific operations are as follows:

• Start packet capture when the client initiates a connection
• Filter out the ports used by MySQL (default is 3306)
• View two key data packets in the handshake stage: one is the Initial Handshake sent by the server, and the other is the client's response packet

By analyzing these data packets, you can see specific protocol version, authentication method, salt value and other information. This is very useful for debugging complex connection problems.

Basically that's it. Mastering the handshake process of MySQL can help you better understand the connection mechanism, troubleshoot connection failures, and provide basic support for optimizing application connection performance.

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