MySQL :: MySQL 8.4 Reference Manual :: 8.2.19 Proxy Users (original) (raw)

8.2.19 Proxy Users

The MySQL server authenticates client connections using authentication plugins. The plugin that authenticates a given connection may request that the connecting (external) user be treated as a different user for privilege-checking purposes. This enables the external user to be a proxy for the second user; that is, to assume the privileges of the second user:

This section describes how the proxy user capability works. For general information about authentication plugins, seeSection 8.2.17, “Pluggable Authentication”. For information about specific plugins, see Section 8.4.1, “Authentication Plugins”. For information about writing authentication plugins that support proxy users, seeImplementing Proxy User Support in Authentication Plugins.

Note

One administrative benefit to be gained by proxying is that the DBA can set up a single account with a set of privileges and then enable multiple proxy users to have those privileges without having to assign the privileges individually to each of those users. As an alternative to proxy users, DBAs may find that roles provide a suitable way to map users onto specific sets of named privileges. Each user can be granted a given single role to, in effect, be granted the appropriate set of privileges. See Section 8.2.10, “Using Roles”.

Requirements for Proxy User Support

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

The proxy mechanism permits mapping only the external client user name to the proxied user name. There is no provision for mapping host names:

Simple Proxy User Example

Consider the following account definitions:

-- create proxy account
CREATE USER 'employee_ext'@'localhost'
  IDENTIFIED WITH my_auth_plugin
  AS 'my_auth_string';

-- create proxied account and grant its privileges;
-- use mysql_no_login plugin to prevent direct login
CREATE USER 'employee'@'localhost'
  IDENTIFIED WITH mysql_no_login;
GRANT ALL
  ON employees.*
  TO 'employee'@'localhost';

-- grant to proxy account the
-- PROXY privilege for proxied account
GRANT PROXY
  ON 'employee'@'localhost'
  TO 'employee_ext'@'localhost';

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

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

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

The proxied account, employee, uses themysql_no_login authentication plugin to prevent clients from using the account to log in directly. (This assumes that the plugin is installed. For instructions, seeSection 8.4.1.9, “No-Login Pluggable Authentication”.) For alternative methods of protecting proxied accounts against direct use, seePreventing Direct Login to Proxied Accounts.

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:

mysql> SELECT USER(), CURRENT_USER();
+------------------------+--------------------+
| USER()                 | CURRENT_USER()     |
+------------------------+--------------------+
| employee_ext@localhost | employee@localhost |
+------------------------+--------------------+

In the CREATE USER statement that creates the proxy user account, the IDENTIFIED WITH clause that names the proxy-supporting authentication plugin is optionally followed by an AS '_`authstring`_' 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 proxy (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.

Preventing Direct Login to Proxied Accounts

Proxied accounts generally are intended to be used only by means of proxy accounts. That is, clients connect using a proxy account, then are mapped onto and assume the privileges of the appropriate proxied user.

There are multiple ways to ensure that a proxied account cannot be used directly:

Granting and Revoking the PROXY Privilege

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 theGRANT statement. For example:

GRANT PROXY ON 'proxied_user' TO 'proxy_user';

The statement creates a row in themysql.proxies_priv grant table.

At connect time, proxyuser must represent a valid externally authenticated MySQL user, and_proxieduser_ must represent a valid locally authenticated user. Otherwise, the connection attempt fails.

The corresponding REVOKE syntax is:

REVOKE PROXY ON 'proxied_user' FROM 'proxy_user';

MySQL GRANT andREVOKE syntax extensions work as usual. Examples:

-- grant PROXY to multiple accounts
GRANT PROXY ON 'a' TO 'b', 'c', 'd';

-- revoke PROXY from multiple accounts
REVOKE PROXY ON 'a' FROM 'b', 'c', 'd';

-- grant PROXY to an account and enable the account to grant
-- PROXY to the proxied account
GRANT PROXY ON 'a' TO 'd' WITH GRANT OPTION;

-- grant PROXY to default proxy account
GRANT PROXY ON 'a' TO ''@'';

The PROXY privilege can be granted in these cases:

The initial root account created during MySQL installation has thePROXY ... WITH GRANT OPTION privilege for ''@'', that is, for all users and all hosts. This enablesroot 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:

CREATE USER 'admin'@'localhost'
  IDENTIFIED BY 'admin_password';
GRANT PROXY
  ON ''@''
  TO 'admin'@'localhost'
  WITH GRANT OPTION;

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

GRANT PROXY ON sally TO joe;

Default Proxy Users

To specify that some or all users should connect using a given authentication plugin, create a “blank” MySQL account with an empty user name and host name (''@''), associate it with that plugin, and let the plugin return the real authenticated user name (if different from the blank user). 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:

-- create default proxy account
CREATE USER ''@''
  IDENTIFIED WITH ldap_auth
  AS 'O=Oracle, OU=MySQL';

-- create proxied accounts; use
-- mysql_no_login plugin to prevent direct login
CREATE USER 'developer'@'localhost'
  IDENTIFIED WITH mysql_no_login;
CREATE USER 'manager'@'localhost'
  IDENTIFIED WITH mysql_no_login;

-- grant to default proxy account the
-- PROXY privilege for proxied accounts
GRANT PROXY
  ON 'manager'@'localhost'
  TO ''@'';
GRANT PROXY
  ON 'developer'@'localhost'
  TO ''@'';

Now assume that a client connects as follows:

$> mysql --user=myuser --password ...
Enter password: myuser_password

The server does 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_myuserpassword_ to it as the user name and password.

If the ldap_auth plugin finds in the LDAP directory that myuserpassword 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 treatmyuser as a proxy. The server verifies that''@'' can authenticate asdeveloper (because ''@'' has the PROXY privilege to do so) and accepts the connection. The session proceeds withmyuser having the privileges of thedeveloper proxied user. (These privileges should be set up by the DBA usingGRANT statements, not shown.) TheUSER() andCURRENT_USER() functions return these values:

mysql> SELECT USER(), CURRENT_USER();
+------------------+---------------------+
| USER()           | CURRENT_USER()      |
+------------------+---------------------+
| myuser@localhost | developer@localhost |
+------------------+---------------------+

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

mysql> SELECT USER(), CURRENT_USER();
+------------------+-------------------+
| USER()           | CURRENT_USER()    |
+------------------+-------------------+
| myuser@localhost | manager@localhost |
+------------------+-------------------+

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 thedeveloper or manager account, which is why those proxied accounts should be protected against direct login (seePreventing Direct Login to Proxied Accounts).

Default Proxy User and Anonymous User Conflicts

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 (seeSection 8.2.6, “Access Control, Stage 1: Connection Verification”).

Suppose that a MySQL installation includes these two accounts:

-- create default proxy account
CREATE USER ''@''
  IDENTIFIED WITH some_plugin
  AS 'some_auth_string';
-- create anonymous account
CREATE USER ''@'%'
  IDENTIFIED BY 'anon_user_password';

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). As a result, the default proxy account is never used.

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

CREATE USER ''@'localhost'  
  IDENTIFIED WITH some_plugin  
  AS 'some_auth_string';

In addition, modify any GRANT PROXY statements to name ''@'localhost' rather than ''@'' as the proxy user.
Be aware that this strategy prevents anonymous-user connections from localhost.

-- create proxy user for local connections  
CREATE USER ''@'localhost'  
  IDENTIFIED WITH some_plugin  
  AS 'some_auth_string';  
-- create proxy user for remote connections  
CREATE USER ''@'%'  
  IDENTIFIED WITH some_plugin  
  AS 'some_auth_string';

Create the proxied users: 

-- create proxied user for local connections  
CREATE USER 'developer'@'localhost'  
  IDENTIFIED WITH mysql_no_login;  
-- create proxied user for remote connections  
CREATE USER 'developer'@'%'  
  IDENTIFIED WITH mysql_no_login;

Grant to each proxy account thePROXY privilege for the corresponding proxied account: 

GRANT PROXY  
  ON 'developer'@'localhost'  
  TO ''@'localhost';  
GRANT PROXY  
  ON 'developer'@'%'  
  TO ''@'%';

Finally, grant appropriate privileges to the local and remote proxied users (not shown).
Assume that thesome_plugin/'_`someauthstring`_' combination causes some_plugin to map the client user name to developer. Local connections match the ''@'localhost' proxy user, which maps to the'developer'@'localhost' proxied user. Remote connections match the ''@'%' proxy user, which maps to the 'developer'@'%' proxied user.

Server Support for Proxy User Mapping

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 (deprecated), sha256_password (deprecated). If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request:

For example, to enable all the preceding capabilities, start the server with these lines in the my.cnf file:

[mysqld]
check_proxy_users=ON
mysql_native_password_proxy_users=ON
sha256_password_proxy_users=ON

Assuming that the relevant system variables have been enabled, create the proxy user as usual using CREATE USER, then grant it thePROXY privilege to a single other account to be treated as the proxied user. When the server receives a successful connection request for the proxy user, it finds that the user has the PROXY privilege and uses it to determine the proper proxied user.

-- create proxy account
CREATE USER 'proxy_user'@'localhost'
  IDENTIFIED WITH mysql_native_password
  BY 'password';

-- create proxied account and grant its privileges;
-- use mysql_no_login plugin to prevent direct login
CREATE USER 'proxied_user'@'localhost'
  IDENTIFIED WITH mysql_no_login;
-- grant privileges to proxied account
GRANT ...
  ON ...
  TO 'proxied_user'@'localhost';

-- grant to proxy account the
-- PROXY privilege for proxied account
GRANT PROXY
  ON 'proxied_user'@'localhost'
  TO 'proxy_user'@'localhost';

To use the proxy account, connect to the server using its name and password:

$> mysql -u proxy_user -p
Enter password: (enter proxy_user password here)

Authentication succeeds, the server finds thatproxy_user has thePROXY privilege forproxied_user, and the session proceeds withproxy_user having the privileges ofproxied_user.

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

Proxy User System Variables

Two system variables help trace the proxy login process:

mysql> SELECT @@proxy_user;  
+--------------+  
| @@proxy_user |  
+--------------+  
| ''@''        |  
+--------------+