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PAM Authentication Plugin

This software documentation is (C)2009-2018 Percona LLC and/or its affiliates and is distributed under the Creative Commons Attribution-ShareAlike 2.0 Generic license.

Pluggable Authentication Modules (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. The dialog plugin can be loaded to any client application that uses the libperconaserverclient/libmysqlclient library.

Some of the benefits of the 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 through the -p parameter.
  • Dialog client installation bug has been fixed.
  • This plugin works on MySQL and Percona Server.

There are 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 the -p option to pass the password to auth_pam_compat.

To choose which plugin version you want to use, use the IDENTIFIED WITH ‘auth_pam’ for auth_pam, and IDENTIFIED WITH ‘auth_pam_compat’ for auth_pam_compat.

Installation

To manually deploy the plugin, run the command below:

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, run the command below:

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

Configuration

To use the plugin, configure the authentication method. A simple setup would be to use the standard UNIX authentication method (pam_unix).

Note

To use pam_unix, mysql must be added to the shadow group, 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 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, you can create users with the PAM plugin as the 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, the user must exist on the system.

Supplementary groups support

The plugin supports 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 the 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, the PAM plugin will map it to joe, as developers matches first.

Known issues

The default mysql stack size is not enough to handle the pam_ecryptfs module. A 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 /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 (that is, 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 can increase the potential for memory overload and invoking of OOM killer.

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