Configuring PAM Authentication and User Mapping with Unix Authentication

In this article, we will walk through the configuration of PAM authentication using the PAM authentication plugin and user and group mapping with the pam_user_map PAM module. The primary authentication will be handled by the pam_unix PAM module, which standard Unix password authentication.

Hypothetical Requirements

In this walkthrough, we are going to assume the following hypothetical requirements:

• The PAM user foo should be mapped to the MariaDB user bar. (foo: bar)
• Any PAM user in the PAM group dba should be mapped to the MariaDB user dba. (@dba: dba)

Creating Our Unix Users and Groups

Let's go ahead and create the Unix users and groups that we are using for this hypothetical scenario.

First, let's create the the foo user and a couple users to go into the dba group. Note that each of these users needs a password.

sudo useradd foo
sudo passwd foo
sudo useradd alice
sudo passwd alice
sudo useradd bob
sudo passwd bob

And then let's create our dba group and add our two users to it:

sudo groupadd dba
sudo usermod -a -G dba alice 
sudo usermod -a -G dba bob 

We also need to create PAM users with the same name as the bar and dba MariaDB users. See here to read more about why. No one will be logging in as these users, so they do not need passwords.

sudo useradd bar
sudo useradd dba -g dba

Installing the pam_user_map PAM Module

Next, let's install the pam_user_map PAM module.

On RHEL/CentOS, we need to install gcc and pam-devel:

sudo yum install gcc pam-devel

And then we can build and install the library with the following:

wget https://raw.githubusercontent.com/MariaDB/server/10.4/plugin/auth_pam/mapper/pam_user_map.c 
gcc pam_user_map.c -shared -lpam -fPIC -o pam_user_map.so 
sudo install --mode=0755 pam_user_map.so /lib64/security/ 

Configuring the pam_user_map PAM Module

Next, let's configure the pam_user_map PAM module based on our hypothetical requirements.

The configuration file for the pam_user_map PAM module is /etc/security/user_map.conf. Based on our hypothetical requirements, ours would look like:

foo: bar
@dba:dba

Installing the PAM Authentication Plugin

Next, let's install the PAM authentication plugin.

Log into the MariaDB Server and execute the following:

INSTALL PLUGIN pam SONAME 'auth_pam';

Configuring the PAM Service

Next, let's configure the PAM service. We will call our service mariadb, so our PAM service configuration file will be located at /etc/pam.d/mariadb on most systems.

Since we are only doing Unix authentication with the pam_unix PAM module and group mapping with the pam_user_map PAM module, our configuration file would look like this:

auth required pam_unix.so audit
auth required pam_user_map.so
account required pam_unix.so audit

Configuring the pam_unix PAM Module

The pam_unix PAM module adds some additional configuration steps on a lot of systems. We basically have to give the user that runs mysqld access to /etc/shadow.

If the mysql user is running mysqld, then we can do that by executing the following:

sudo groupadd shadow
sudo usermod -a -G shadow mysql
sudo chown root:shadow /etc/shadow
sudo chmod g+r /etc/shadow

The server needs to be restarted for this change to take affect.

Configuring SELinux

If SELinux is enabled on a system, then we also have to disable it. This is usually done by modifying /etc/selinux/config and then rebooting.

Creating MariaDB Users

Next, let's create the MariaDB users. Remember that our PAM service is called mariadb.

Next, let's create the MariaDB users for the user mapping: foo: bar

That means that we need to create a bar user that authenticates with PAM:

CREATE USER 'bar'@'%' IDENTIFIED VIA pam USING 'mariadb';

And then let's create the MariaDB users for the group mapping: @dba: dba

That means that we need to create a dba user:

CREATE USER 'dba'@'%' ;

And we need to create an anonymous user that is able to PROXY as the dba user. Before we can create the proxy user, we might need to clean up some defaults:

DELETE FROM mysql.db WHERE User='' AND Host='%';
FLUSH PRIVILEGES;

And then let's create the anonymous proxy user:

CREATE USER ''@'%' IDENTIFIED VIA pam USING 'mariadb';
GRANT PROXY ON 'dba'@'%' TO ''@'%';

Testing our Configuration

Next, let's test out our configuration.

First, let's test out our foo: bar user mapping: