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Security in MySQL  /  ...  /  Socket Peer-Credential Pluggable Authentication

6.1.10 Socket Peer-Credential Pluggable Authentication

The server-side auth_socket authentication plugin authenticates clients that connect from the local host through the Unix socket file. The plugin uses the SO_PEERCRED socket option to obtain information about the user running the client program. Thus, the plugin can be used only on systems that support the SO_PEERCRED option, such as Linux.

The source code for this plugin can be examined as a relatively simple example demonstrating how to write a loadable authentication plugin.

The following table shows the plugin and library file names. The file must be located in the directory named by the plugin_dir system variable.

Table 6.11 Plugin and Library Names for Socket Peer-Credential Authentication

Plugin or File Plugin or File Name
Server-side plugin auth_socket
Client-side plugin None, see discussion
Library file

The following sections provide installation and usage information specific to socket pluggable authentication:

For general information about pluggable authentication in MySQL, see Section 4.17, “Pluggable Authentication”.

Installing Socket Pluggable Authentication

This section describes how to install the socket authentication plugin. For general information about installing plugins, see Installing and Uninstalling Plugins.

To be usable by the server, the plugin library file must be located in the MySQL plugin directory (the directory named by the plugin_dir system variable). If necessary, configure the plugin directory location by setting the value of plugin_dir at server startup.

To load the plugin at server startup, use the --plugin-load-add option to name the library file that contains it. With this plugin-loading method, the option must be given each time the server starts. For example, put these lines in the server my.cnf file:


After modifying my.cnf, restart the server to cause the new settings to take effect.

Alternatively, to load the plugin at runtime, use this statement:


INSTALL PLUGIN loads the plugin immediately, and also registers it in the mysql.plugins system table to cause the server to load it for each subsequent normal startup without the need for --plugin-load-add.

To verify plugin installation, examine the Information Schema PLUGINS table or use the SHOW PLUGINS statement (see Obtaining Server Plugin Information). For example:

       WHERE PLUGIN_NAME LIKE '%socket%';
| auth_socket | ACTIVE        |

If the plugin fails to initialize, check the server error log for diagnostic messages.

To associate MySQL accounts with the socket plugin, see Using Socket Pluggable Authentication.

Uninstalling Socket Pluggable Authentication

The method used to uninstall the socket authentication plugin depends on how you installed it:

  • If you installed the plugin at server startup using a --plugin-load-add option, restart the server without the option.

  • If you installed the plugin at runtime using an INSTALL PLUGIN statement, it remains installed across server restarts. To uninstall it, use UNINSTALL PLUGIN:

    UNINSTALL PLUGIN auth_socket;

Using Socket Pluggable Authentication

The socket plugin checks whether the socket user name (the operating system user name) matches the MySQL user name specified by the client program to the server. If the names do not match, the plugin checks whether the socket user name matches the name specified in the authentication_string column of the mysql.user system table row. If a match is found, the plugin permits the connection. The authentication_string value can be specified using an IDENTIFIED ...AS clause with CREATE USER or ALTER USER.

Suppose that a MySQL account is created for an operating system user named valerie who is to be authenticated by the auth_socket plugin for connections from the local host through the socket file:

CREATE USER 'valerie'@'localhost' IDENTIFIED WITH auth_socket;

If a user on the local host with a login name of stefanie invokes mysql with the option --user=valerie to connect through the socket file, the server uses auth_socket to authenticate the client. The plugin determines that the --user option value (valerie) differs from the client user's name (stephanie) and refuses the connection. If a user named valerie tries the same thing, the plugin finds that the user name and the MySQL user name are both valerie and permits the connection. However, the plugin refuses the connection even for valerie if the connection is made using a different protocol, such as TCP/IP.

To permit both the valerie and stephanie operating system users to access MySQL through socket file connections that use the account, this can be done two ways:

  • Name both users at account-creation time, one following CREATE USER, and the other in the authentication string:

    CREATE USER 'valerie'@'localhost' IDENTIFIED WITH auth_socket AS 'stephanie';
  • If you have already used CREATE USER to create the account for a single user, use ALTER USER to add the second user:

    CREATE USER 'valerie'@'localhost' IDENTIFIED WITH auth_socket;
    ALTER USER 'valerie'@'localhost' IDENTIFIED WITH auth_socket AS 'stephanie';

To access the account, both valerie and stephanie specify --user=valerie at connect time.