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Percona Software for MySQL Documentation

PAM authentication plugin

Percona PAM Authentication Plugin is a free and Open Source implementation of the MySQL’s authentication plugin. This plugin acts as a mediator between the MySQL server, the MySQL client, and the PAM stack. The server plugin requests authentication from the PAM stack, forwards any requests and messages from the PAM stack over the wire to the client (in cleartext) and reads back any replies for the PAM stack.

PAM plugin uses dialog as its client side plugin. Dialog plugin can be loaded to any client application that uses libperconaserverclient/libmysqlclient library.

Here are some of the benefits that Percona dialog plugin offers over the default one:

  • It correctly recognizes whether PAM wants input to be echoed or not, while the default one always echoes the input on the user’s console.

  • It can use the password which is passed to MySQL client via “-p” parameter.

  • Dialog client installation bug has been fixed.

Percona offers two versions of this plugin:

  • Full PAM plugin called auth_pam. This plugin uses dialog.so. It fully supports the PAM protocol with arbitrary communication between client and server.

  • Oracle-compatible PAM called auth_pam_compat. This plugin uses mysql_clear_password which is a part of Oracle MySQL client. It also has some limitations, such as, it supports only one password input. You must use -p option in order to pass the password to auth_pam_compat.

These two versions of plugins are physically different. To choose which one you want used, you must use IDENTIFIED WITH ‘auth_pam’ for auth_pam, and IDENTIFIED WITH ‘auth_pam_compat’ for auth_pam_compat.

Version specific information

Implemented in Percona Server for MySQL 8.0.12-1: The feature was ported from Percona Server for MySQL 5.7.

A plugin may not be supported in later releases of MySQL or Percona Server for MySQL since version changes may introduce incompatible changes.

Installation

This plugin requires manual installation because it isn’t installed by default.

mysql> INSTALL PLUGIN auth_pam SONAME 'auth_pam.so';

After the plugin has been installed it should be present in the plugins list. To check if the plugin has been correctly installed and active

mysql> SHOW PLUGINS;
Expected output 
...
| auth_pam                       | ACTIVE   | AUTHENTICATION     | auth_pam.so | GPL     |

Configuration

In order to use the plugin, authentication method should be configured. Simple setup can be to use the standard UNIX authentication method (pam_unix).

Note

To use pam_unix, mysql will need to be added to the shadow group in order to have enough privileges to read the /etc/shadow.

A sample /etc/pam.d/mysqld file:

auth       required     pam_unix.so
account    required     pam_unix.so

For added information in the system log, you can expand it to be:

auth       required     pam_warn.so
auth       required     pam_unix.so audit
account    required     pam_unix.so audit

Creating a user

After the PAM plugin has been configured, users can be created with the PAM plugin as authentication method

mysql> CREATE USER 'newuser'@'localhost' IDENTIFIED WITH auth_pam;

This will create a user newuser that can connect from localhost who will be authenticated using the PAM plugin. If the pam_unix method is being used user will need to exist on the system.

Supplementary groups support

Percona Server for MySQL has implemented PAM plugin support for supplementary groups. Supplementary or secondary groups are extra groups a specific user is member of. For example user joe might be a member of groups: joe (his primary group) and secondary groups developers and dba. A complete list of groups and users belonging to them can be checked with cat /etc/group command.

This feature enables using secondary groups in the mapping part of the authentication string, like “mysql, developers=joe, dba=mark”. Previously only primary groups could have been specified there. If user is a member of both developers and dba, PAM plugin will map it to the joe because developers matches first.

Known issues

Default mysql stack size is not enough to handle pam_encryptfs module. The workaround is to increase the MySQL stack size by setting the thread-stack variable to at least 512KB or by increasing the old value by 256KB.

PAM authentication can fail with mysqld: pam_unix(mysqld:account): Fork failed: Cannot allocate memory error in the /var/log/secure even when there is enough memory available. Current workaround is to set vm.overcommit_memory to 1:

echo 1 /proc/sys/vm/overcommit_memory

and by adding the vm.overcommit_memory = 1 to /etc/sysctl.conf to make the change permanent after reboot. Authentication of internal (i.e. non PAM) accounts continues to work fine when mysqld reaches this memory utilization level. NOTE: Setting the vm.overcommit_memory to 1 will cause kernel to perform no memory overcommit handling which could increase the potential for memory overload and invoking of OOM killer.

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Last update: 2023-09-27