6.3.11 Proxy Users

For proxying to occur for a given authentication plugin, these conditions must be satisfied:

The proxy mechanism permits mapping only the client user name to the proxied user name. There is no provision for mapping host names. When a connecting client matches a proxy account, the server attempts to find a match for a proxied account using the user name returned by the authentication plugin and the host name of the proxy account.

Consider the following account definitions:

When a client connects as employee_ext from the local host, MySQL uses the plugin named my_auth_plugin to perform authentication. Suppose that my_auth_plugin returns a user name of employee to the server, based on the content of 'my_auth_string' and perhaps by consulting some external authentication system. The name employee differs from employee_ext, so returning employee serves as a request to the server to treat the employee_ext client, for purposes of privilege checking, as the employee local user.

In this case, employee_ext is the proxy user and employee is the proxied user.

The server verifies that proxy authentication for employee is possible for the employee_ext user by checking whether employee_ext (the proxy user) has the PROXY privilege for employee (the proxied user). If this privilege has not been granted, an error occurs. Otherwise, employee_ext assumes the privileges of employee. The server checks statements executed during the client session by employee_ext against the privileges granted to employee. In this case, employee_ext can access tables in the employees database.

When proxying occurs, the USER() and CURRENT_USER() functions can be used to see the difference between the connecting user (the proxy user) and the account whose privileges apply during the current session (the proxied user). For the example just described, those functions return these values:

In the CREATE USER statement that creates the proxy user account, the IDENTIFIED WITH clause that names the authentication plugin is optionally followed by an AS 'auth_string' clause specifying a string that the server passes to the plugin when the user connects. If present, the string provides information that helps the plugin determine how to map the external client user name to a proxied user name. It is up to each plugin whether it requires the AS clause. If so, the format of the authentication string depends on how the plugin intends to use it. Consult the documentation for a given plugin for information about the authentication string values it accepts.

The PROXY privilege is needed to enable an external user to connect as and have the privileges of another user. To grant this privilege, use the GRANT statement. For example:

The statement creates a row in the mysql.proxies_priv grant table.

At connection time, proxy_user must represent a valid externally authenticated MySQL user, and proxied_user must represent a valid locally authenticated user. Otherwise, the connection attempt fails.

The corresponding REVOKE syntax is:

MySQL GRANT and REVOKE syntax extensions work as usual. For example:

The PROXY privilege can be granted in these cases:

The initial root account created during MySQL installation has the PROXY ... WITH GRANT OPTION privilege for ''@'', that is, for all users and all hosts. This enables root to set up proxy users, as well as to delegate to other accounts the authority to set up proxy users. For example, root can do this:

Those statements create an admin user that can manage all GRANT PROXY mappings. For example, admin can do this:

To specify that some or all users should connect using a given authentication plugin, create a “blank” MySQL account (''@''), associate it with that plugin, and let the plugin return the real authenticated user name (if different from the blank user). For example, suppose that there exists a plugin named ldap_auth that implements LDAP authentication and maps connecting users onto either a developer or manager account. To set up proxying of users onto these accounts, use the following statements:

Now assume that a client connects as follows:

shell> mysql --user=myuser --password ...
Enter password: myuser_pass

The server will not find myuser defined as a MySQL user. But because there is a blank user account (''@'') that matches the client user name and host name, the server authenticates the client against that account: The server invokes the ldap_auth authentication plugin and passes myuser and myuser_pass to it as the user name and password.

If the ldap_auth plugin finds in the LDAP directory that myuser_pass is not the correct password for myuser, authentication fails and the server rejects the connection.

If the password is correct and ldap_auth finds that myuser is a developer, it returns the user name developer to the MySQL server, rather than myuser. Returning a user name different from the client user name of myuser signals to the server that it should treat myuser as a proxy. The server verifies that ''@'' can authenticate as developer (because that account has the PROXY privilege to do so) and accepts the connection. The session proceeds with myuser having the privileges of developer, the proxied user. (These privileges should be set up by the DBA using GRANT statements, not shown.) The USER() and CURRENT_USER() functions return these values:

If the plugin instead finds in the LDAP directory that myuser is a manager, it returns manager as the user name and the session proceeds with myuser having the privileges of manager.

For simplicity, external authentication cannot be multilevel: Neither the credentials for developer nor those for manager are taken into account in the preceding example. However, they are still used if a client tries to connect and authenticate directly as the developer or manager account, which is why those accounts should be assigned passwords.

If you intend to create a default proxy user, check for other existing “match any user” accounts that take precedence over the default proxy user because they can prevent that user from working as intended.

In the preceding discussion, the default proxy user account has '' in the host part, which matches any host. If you set up a default proxy user, take care to also check whether nonproxy accounts exist with the same user part and '%' in the host part, because '%' also matches any host, but has precedence over '' by the rules that the server uses to sort account rows internally (see Section 6.2.6, “Access Control, Stage 1: Connection Verification”).

Suppose that a MySQL installation includes these two accounts:

The first account (''@'') is intended as the default proxy user, used to authenticate connections for users who do not otherwise match a more-specific account. The second account (''@'%') is an anonymous-user account, which might have been created, for example, to enable users without their own account to connect anonymously.

Both accounts have the same user part (''), which matches any user. And each account has a host part that matches any host. Nevertheless, there is a priority in account matching for connection attempts because the matching rules sort a host of '%' ahead of ''. For accounts that do not match any more-specific account, the server attempts to authenticate them against ''@'%' (the anonymous user) rather than ''@'' (the default proxy user). The result is that the default proxy account is never used.

To avoid this problem, use one of the following strategies:

Some authentication plugins implement proxy user mapping for themselves (for example, the PAM and Windows authentication plugins). Other authentication plugins do not support proxy users by default. Of these, some can request that the MySQL server itself map proxy users according to granted proxy privileges: mysql_native_password, sha256_password. If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request:

Proxy user mapping performed by the server is subject to these restrictions:

Two system variables help trace the proxy login process: