Configuring PAM Authentication and User Mapping with LDAP 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_ldap PAM module, which performs LDAP authentication. We will also set up an OpenLDAP server.

Hypothetical Requirements

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

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

Setting up the OpenLDAP Server

Before we can use LDAP authentication, we first need to set up our OpenLDAP Server. This is usually done on a server that is completely separate from the database server.

Installing the OpenLDAP Server and Client Components

On the server acting as the OpenLDAP Server, first, we need to install the OpenLDAP components.

On RHEL, CentOS, and other similar Linux distributions that use RPM packages, that would go like this:

sudo yum install openldap openldap-servers openldap-clients nss-pam-ldapd

Configuring the OpenLDAP Server

Next, let's to configure the OpenLDAP Server. The easiest way to do that is to copy the template configuration file that is included with the installation. In many installations, that will be at /usr/share/openldap-servers/DB_CONFIG.example. For example:

sudo cp /usr/share/openldap-servers/DB_CONFIG.example /var/lib/ldap/DB_CONFIG
sudo chown ldap. /var/lib/ldap/DB_CONFIG

Configuring the OpenLDAP Port

Sometimes it is useful to change the port used by OpenLDAP. For example, some cloud environments block well-known authentication services, so they block the default LDAP port.

On some systems, the port can be changed by setting SLAPD_URLS in /etc/sysconfig/slapd:

SLAPD_URLS="ldapi:/// ldap://0.0.0.0:3306/"

I used 3306 because that is the port that is usually used by mysqld, so I know that it is not blocked.

Starting the OpenLDAP Server

Next, let's start the OpenLDAP Server and configure it to start on reboot. On systemd systems, that would go like this:

sudo systemctl start slapd
sudo systemctl enable slapd

Installing the Standard LDAP objectClasses

In order to use LDAP for authentication, we also need to install some standard objectClasses, such as posixAccount and posixGroup. In LDAP, things like objectClasses are defined in LDIF files. In many installations, these specific objectClasses are defined in /etc/openldap/schema/nis.ldif. nis.ldif also depends on core.ldif and cosine.ldif. However, core.ldif is usually installed by default.

We can install them with ldapmodify:

sudo ldapmodify -a -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/cosine.ldif
sudo ldapmodify -a -Y EXTERNAL -H ldapi:/// -f /etc/openldap/schema/nis.ldif

Creating the LDAP Directory Manager User

Next, let’s create a directory manager user. We can do this by using OpenLDAP's olc configuration system to change the olcRootDN directive to the DN of the directory manager user, which means that the user will be a privileged LDAP user that is not subject to access controls. We will also set the root password for the user by changing the olcRootPW directive.

We will also set the DN suffix for our backend LDAP database by changing the olcSuffix directive.

Let’s use the slappasswd utility to generate a password hash from a clear-text password. Simply execute:

slappasswd

This utility should provide a password hash that looks kind of like this: {SSHA}AwT4jrvmokeCkbDrFAnGvzzjCMb7bvEl

OpenLDAP's olc configuration system also uses LDIF files. Now that we have the password hash, let’s create an LDIF file to create the directory manager user:

tee ~/setupDirectoryManager.ldif <<EOF
dn: olcDatabase={1}monitor,cn=config
changetype: modify
replace: olcAccess
olcAccess: {0}to * 
    by dn.base="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth" read 
    by dn.base="cn=Manager,dc=support,dc=mariadb,dc=com" read 
    by * none

dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcSuffix
olcSuffix: dc=support,dc=mariadb,dc=com

dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcRootDN
olcRootDN: cn=Manager,dc=support,dc=mariadb,dc=com

dn: olcDatabase={2}hdb,cn=config
changetype: modify
add: olcRootPW
olcRootPW: {SSHA}AwT4jrvmokeCkbDrFAnGvzzjCMb7bvEl

dn: olcDatabase={2}hdb,cn=config
changetype: modify
add: olcAccess
olcAccess: {0}to attrs=userPassword,shadowLastChange 
    by   dn="cn=Manager,dc=support,dc=mariadb,dc=com" write 
    by anonymous auth 
    by self write 
    by * none
olcAccess: {1}to dn.base="" 
    by * read
olcAccess: {2}to * 
    by dn="cn=Manager,dc=support,dc=mariadb,dc=com" write 
    by * read
EOF

Note that I am using the dc=support,dc=mariadb,dc=com domain for my directory. You can change this to whatever is relevant to you.

Now let’s run the ldif file with ldapmodify:

sudo ldapmodify -Y EXTERNAL -H ldapi:/// -f ~/setupDirectoryManager.ldif

We will use the new directory manager user to make changes to the LDAP directory after this step.

Creating the Structure of the Directory

Next, let's create the structure of the directory by creating parts of our tree.

tee ~/setupDirectoryStructure.ldif <<EOF
dn: dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: dcObject
objectclass: organization
o: MariaDB Support Team
dc: support

dn: cn=Manager,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: organizationalRole
cn: Manager
description: Directory Manager

dn: ou=People,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: organizationalUnit
ou: People

dn: ou=Groups,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: organizationalUnit
ou: Groups

dn: ou=System Users,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: organizationalUnit
ou: System Users
EOF

Now let’s use our new directory manager user and run the LDIF file with ldapmodify:

ldapmodify -a -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -f ~/setupDirectoryStructure.ldif

Creating the LDAP Users and Groups

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

First, let's create the the foo user:

tee ~/createFooUser.ldif <<EOF
dn: uid=foo,ou=People,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: account
objectClass: posixAccount
objectClass: shadowAccount
cn: foo
uid: foo
uidNumber: 16859
gidNumber: 100
homeDirectory: /home/foo
loginShell: /bin/bash
gecos: foo
userPassword: {crypt}x
shadowLastChange: -1
shadowMax: -1
shadowWarning: 0
EOF
ldapmodify -a -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -f ~/createFooUser.ldif

Then let's create a couple users to go into the dba group:

tee ~/createDbaUsers.ldif <<EOF
dn: uid=gmontee,ou=People,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: account
objectClass: posixAccount
objectClass: shadowAccount
cn: gmontee
uid: gmontee
uidNumber: 16860
gidNumber: 100
homeDirectory: /home/gmontee
loginShell: /bin/bash
gecos: gmontee
userPassword: {crypt}x
shadowLastChange: -1
shadowMax: -1
shadowWarning: 0

dn: uid=bstillman,ou=People,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: account
objectClass: posixAccount
objectClass: shadowAccount
cn: bstillman
uid: bstillman
uidNumber: 16861
gidNumber: 100
homeDirectory: /home/bstillman
loginShell: /bin/bash
gecos: bstillman
userPassword: {crypt}x
shadowLastChange: -1
shadowMax: -1
shadowWarning: 0
EOF
ldapmodify -a -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -f ~/createDbaUsers.ldif

Note that each of these users needs a password, so we can set it for each user with ldappasswd:

ldappasswd -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -S uid=foo,ou=People,dc=support,dc=mariadb,dc=com
ldappasswd -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -S uid=gmontee,ou=People,dc=support,dc=mariadb,dc=com
ldappasswd -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -S uid=bstillman,ou=People,dc=support,dc=mariadb,dc=com

And then let's create our dba group

tee ~/createDbaGroup.ldif <<EOF
dn: cn=dba,ou=Groups,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: posixGroup
gidNumber: 678
EOF
ldapmodify -a -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -f ~/createDbaGroup.ldif

And then let's add our two users to it:

tee ~/addUsersToDbaGroup.ldif <<EOF
dn: cn=dba,ou=Groups,dc=support,dc=mariadb,dc=com
changetype: modify
add: memberuid
memberuid: gmontee

dn: cn=dba,ou=Groups,dc=support,dc=mariadb,dc=com
changetype: modify
add: memberuid
memberuid: bstillman
EOF
ldapmodify -a -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -f ~/addUsersToDbaGroup.ldif

We also need to create LDAP 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. Instead of the People organizationalUnit, we will create them in the System Users organizationalUnit.

tee ~/createSystemUsers.ldif <<EOF
dn: uid=bar,ou=System Users,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: account
objectClass: posixAccount
objectClass: shadowAccount
cn: bar
uid: bar
uidNumber: 16862
gidNumber: 100
homeDirectory: /home/bar
loginShell: /bin/bash
gecos: bar
userPassword: {crypt}x
shadowLastChange: -1
shadowMax: -1
shadowWarning: 0

dn: uid=dba,ou=System Users,dc=support,dc=mariadb,dc=com
objectClass: top
objectClass: account
objectClass: posixAccount
objectClass: shadowAccount
cn: dba
uid: dba
uidNumber: 16863
gidNumber: 100
homeDirectory: /home/dba
loginShell: /bin/bash
gecos: dba
userPassword: {crypt}x
shadowLastChange: -1
shadowMax: -1
shadowWarning: 0
EOF
ldapmodify -a -x -D cn=Manager,dc=support,dc=mariadb,dc=com -W -f ~/createSystemUsers.ldif

Setting up the MariaDB Server

At this point, we can move onto setting up the MariaDB Server.

Installing LDAP and PAM Libraries

First, we need to make sure that the LDAP and PAM libraries are installed.

On RHEL, CentOS, and other similar Linux distributions that use RPM packages, we need to install the following packages:

sudo yum install openldap-clients nss-pam-ldapd pam pam-devel

Configuring LDAP

Next, let's configure LDAP on the system. We can use authconfig for this:

sudo authconfig --enableldap \
   --enableldapauth \
   --ldapserver="ldap://172.30.0.238:3306" \
   --ldapbasedn="dc=support,dc=mariadb,dc=com" \
   --enablemkhomedir \
   --update

Be sure to replace -–ldapserver and -–ldapbasedn with values that are relevant for your environment.

Installing the pam_user_map PAM Module

The following steps apply to MariaDB Server in versions 10.2.32.7, 10.3.23., 10.4.13.6, 10.5.2 and earlier. In later releases, the pam_user_map PAM module is now included in the base install.

Next, let's install the pam_user_map PAM module.

Before the module can be compiled from source, we may need to install some dependencies.

On RHEL, CentOS, and other similar Linux distributions that use RPM packages, we need to install gcc and pam-devel:

sudo yum install gcc pam-devel

On Debian, Ubuntu, and other similar Linux distributions that use DEB packages, we need to install gcc and libpam0g-dev:

sudo apt-get install gcc libpam0g-dev

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 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.

Configuring PAM to Allow Only LDAP Authentication

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

auth required pam_ldap.so
auth required pam_user_map.so
account required pam_ldap.so

Configuring PAM to Allow LDAP and Local Unix Authentication

If we want to allow authentication from LDAP users and from local Unix users through pam_unix, while giving priority to the local users, then we could do this instead:

auth [success=1 new_authtok_reqd=1 default=ignore] pam_unix.so audit
auth required pam_ldap.so try_first_pass
auth required pam_user_map.so
account sufficient pam_unix.so audit
account required pam_ldap.so

Configuring the pam_unix PAM Module

If you also want to allow authentication from local Unix users, 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.

Creating MariaDB Users

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

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

That means that we need to create a bar user:

CREATE USER 'bar'@'%' IDENTIFIED BY 'strongpassword';
GRANT ALL PRIVILEGES ON *.* TO 'bar'@'%' ;

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

That means that we need to create a dba user:

CREATE USER 'dba'@'%' IDENTIFIED BY 'strongpassword';
GRANT ALL PRIVILEGES ON *.* TO 'dba'@'%' ;

And then to allow for the user and group mapping, we need to create an anonymous user that authenticates with the pam authentication plugin that is also able to PROXY as the bar and dba users. 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 'bar'@'%' TO ''@'%';
GRANT PROXY ON 'dba'@'%' TO ''@'%';

Testing our Configuration

Next, let's test out our configuration by verifying that mapping is occurring. We can verify this by logging in as each of our users and comparing the return value of USER(), which is the original user name and the return value of CURRENT_USER(), which is the authenticated user name.

Testing LDAP Authentication

First, let's test out our foo user:

$ mysql -u foo -h 172.30.0.198
[mariadb] Password:
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 134
Server version: 10.3.10-MariaDB MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> SELECT USER(), CURRENT_USER();
+------------------------------------------------+----------------+
| USER()                                         | CURRENT_USER() |
+------------------------------------------------+----------------+
| [email protected] | bar@%          |
+------------------------------------------------+----------------+
1 row in set (0.000 sec)

We can verify that our foo LDAP user was properly mapped to the bar MariaDB user by looking at the return value of CURRENT_USER().

Then let's test out our gmontee user in the dba group:

$ mysql -u gmontee -h 172.30.0.198
[mariadb] Password:
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 135
Server version: 10.3.10-MariaDB MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> SELECT USER(), CURRENT_USER();
+----------------------------------------------------+----------------+
| USER()                                             | CURRENT_USER() |
+----------------------------------------------------+----------------+
| [email protected] | dba@%          |
+----------------------------------------------------+----------------+
1 row in set (0.000 sec)

And then let's test out our bstillman user in the dba group:

$ mysql -u bstillman -h 172.30.0.198
[mariadb] Password:
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 136
Server version: 10.3.10-MariaDB MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> SELECT USER(), CURRENT_USER();
+------------------------------------------------------+----------------+
| USER()                                               | CURRENT_USER() |
+------------------------------------------------------+----------------+
| [email protected] | dba@%          |
+------------------------------------------------------+----------------+
1 row in set (0.000 sec)

We can verify that our gmontee and bstillman LDAP users in the dba LDAP group were properly mapped to the dba MariaDB user by looking at the return values of CURRENT_USER().

Testing Local Unix Authentication

If you chose the option that also allowed local Unix authentication, then let's test that out. Let's create a Unix user and give the user a password real quick:

sudo useradd alice
sudo passwd alice

And let's also map this user to dba:

@dba:dba
foo: bar
alice: dba

And we know that the existing anonymous user already has the PROXY privilege granted to the dba user, so this should just work without any other configuration. Let's test it out:

$ mysql -u alice -h 172.30.0.198
[mariadb] Password:
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 141
Server version: 10.3.10-MariaDB MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> SELECT USER(), CURRENT_USER();
+--------------------------------------------------+----------------+
| USER()                                           | CURRENT_USER() |
+--------------------------------------------------+----------------+
| [email protected] | dba@%          |
+--------------------------------------------------+----------------+
1 row in set (0.000 sec)

We can verify that our alice Unix user was properly mapped to the dba MariaDB user by looking at the return values of CURRENT_USER().