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Reference: Cisco: Internetworking Basics
When a user logs in to a Linux system, the username and password combination must be verified, or authenticated, as a valid and active user. A lot of times the information needed to authenticate the user is located on the local system through entries in the /etc/passwd and /etc/shadow files. Another option is to allow the system to defer user authentication to a user database on a remote system like, for example, an LDAP directory. This is a popular option as it allows administrators to centralize username and password information.
In this guide, I will present the steps required to configure authentication for a Linux client through LDAP using Pluggable Authentication Modules (PAM) and Name Service Switch (NSS). The Linux client will be configured to access user information stored in an LDAP directory database such as username, UID number, GID number, home directory, login shell, and other user information that can be used to authenticate to the client system.
An LDAP client machine named ldaptest.idevelopment.info will be configured in this guide to authenticate users through an LDAP server named ldapsrv.idevelopment.info. The example used in this tutorial is based on a clean installation of OpenLDAP Software on the CentOS 5 platform. This tutorial will also work for Red Hat Enterprise Linux 5 and Oracle Linux 5. The LDAP directory used in this guide has been initialized with a base DN of dc=idevelopment,dc=info and organization units People, Group, and Hosts. Obviously, the name of your LDAP server and the base DN will differ and the examples presented in this guide will need to be modified accordingly for you environment.
Refer to the following three tutorials on how to install OpenLDAP Software, initialize the LDAP directory, and then import OS users and groups into the LDAP directory on the server (ldapsrv.idevelopment.info in this guide) on the CentOS 5 platform.
Installing the OpenLDAP client software will be done using the yum package manager. Yum is available on any of the Red Hat Enterprise Linux distributions including its clones like CentOS.
Log in to the client machine as root and install the OpenLDAP Software client packages from the yum repository. This will update any previously installed release of the OpenLDAP Software packages.
The following table summarizes the OpenLDAP Software packages installed in the above step.
|openldap||Contains all configuration files, libraries, and documentation for OpenLDAP.|
|openldap-clients||Contains the client programs needed for accessing and modifying LDAP directories.|
|nss_ldap||Contains access client software for using LDAP as a method of user authentication for Linux.|
|authconfig||An interface for configuring system authentication resources including basic LDAP, Kerberos 5, and SMB (authentication) client configuration.|
After installing OpenLDAP Software on the client machine, the next step is to modify the necessary configuration files to customize the client for LDAP authentication. While in some cases this can be performed by manually editing the associated configuration files, the recommended method is to use an interface that automates the process. Manually editing the configuration files used for authentication imposes the risk of losing those changes since any manual modifications will be reset any time the the configuration wizard is run.
CentOS provides two interfaces to configure client system authentication:
For users with access to an X server, system-config-authentication is a GUI application that comes recommended as it provides the most ease and flexibility. authconfig, on the other hand, is a command-line tools but does produce equivalent client configuration files. In fact, system-config-authentication runs the authconfig command-line utility behind the scenes when updating the client configuration files.
The system-config-authentication script provides a graphical utility used to configure client authentication. This method can be used if you are running an X server or have X11 forwarding set up through an SSH connection.
You can also access the Authentication Configuration application from the GNOME desktop toolbar using System / Administration / Authentication.
This will bring up the Authentication Configuration window.
Figure 1: Authentication Configuration Window
Click the check-box next to Enable LDAP Support under the User Information tab which instructs the system to retrieve user information via LDAP.
Figure 2: Enable LDAP Support for User Information
This will enable the Configure LDAP button to the right of that option. Click on that button to enter your LDAP server settings. Here you enter the LDAP search base DN and the URL address of your LDAP server. If the LDAP server is on the local machine, it is safe to accept the default IP address 127.0.0.1. If the LDAP server is on another machine (which is the case in this guide), enter the IP address or hostname of that server. When using a hostname, make certain it can be resolved with relying on LDAP itself (if your LDAP server is configured to resolve hostnames to IP addresses).
You can also indicate that the LDAP server is using TLS encryption for connections. TLS encryption falls outside the scope of this article and will not be covered.
Figure 3: LDAP Server Settings
Click [OK] on the LDAP Settings dialog.
Next, click on the Authentication tab. This is where you specify that your client machine should use LDAP for authentication and not just user information. Just like with the User Information tab, click the check-box next to Enable LDAP Support which will allow the client authentication through LDAP.
Figure 4: Enable LDAP Support for Authentication
Like before, the Configure LDAP button is enabled after clicking on the check-box. In this case you don't need to enter the LDAP server settings a second time since the information is carried over from the previous step. Even though the information will be identical to that entered on the User Information tab, it is still recommended to verify the settings are correct.
Figure 5: LDAP Server Settings
Finally, click on the Options tab. This is where you can specify any additional authentication options. For this example, select:
Use Shadow Passwords
Local authentication is sufficient for local users
Password hashing algorithm: [SHA256]
Create home directories on the first login
Figure 6: Additional Authentication Options
Cache User Information Enables the name service cache daemon (nscd) and configures it to start on boot. The nscd daemon is used to cache user information. Note that it is required the nscd package be installed for this option to work. This option is commonly used in medium to large organizations when the LDAP server is receiving a significant number of requests. In a small organization (like the one described in this guide), this option is not necessary.
Use Shadow Passwords This option provides the system with the ability to use the /etc/shadow file to store passwords. Keep this option enabled in order to authenticate system and local accounts. Whether this option is selected or not, all LDAP passwords will be stored in the LDAP directory database. Shadow passwords are enabled by default during installation and are highly recommended to increase the security of the system.
Password Hashing Algorithm For the LDAP server used in this guide, I set the password-hash global directive in the /etc/openldap/slapd.conf file which will override this client setting. Therefore the setting specified here in system-config-authentication will not be used by the client. The directive set on the LDAP server configures the hash (or hashes) to be used in generation of user passwords stored in the userPassword attribute during processing of LDAP Password Modify Extended Operations (RFC 3062). This allows the directory server to handle hashing instead of the client.
Local authorization is sufficient for local users When this option is enabled, the system will not check authorization from network services (such as LDAP or Kerberos) for user accounts maintained in its /etc/passwd file. Although this isn't strictly necessary for local accounts to work, I always select it to save on potential network traffic to the directory server.
Authenticate system accounts by network services Select this option to authenticate system accounts to LDAP. This is useful in order to centralize those logins; however, it can cause conflicts if this option is selected and there are users with the same name in LDAP. It is not required that this option be selected in order to perform the method of LDAP authentication described in this guide.
Check access.conf during account authorization When selected, this option uses entries in the /etc/security/access.conf file to determine whether a login will be accepted or refused. The file specifies (user/group, host), (user/group, network/netmask) or (user/group, tty) combinations along with a permissions field for that table entry to determine whether the login will be accepted or refused. It is not required that this option be selected in order to perform the method of LDAP authentication described in this guide.
Create home directories on the first login When enabled, the user's home directory is automatically created when they log in if it does not already exist. This is an extremely useful option that I always select in order to save time from having to manually create every user's home directory.
After setting all of your options, click [OK] to close the window. The system-config-authentication application will configure client authentication to use LDAP.
The other method for configuring client authentication in CentOS is the authconfig command. If you don't have a GUI available or prefer the command-line like I do, this is the option to use. As mentioned earlier, the authconfig command-line utility is executed behind the scenes when using the system-config-authentication GUI.
The following authconfig command will perform the same actions and generate the same configuration files that was done using the system-config-authentication GUI in the previous method.
After configuring the LDAP client (using either system-config-authentication or authconfig), verify the the configuration of the client using the authconfig --test command.
Although authentication for the client was configured in the previous step (using either system-config-authentication or authconfig), there is one last setting that will need to be fixed manually. system-config-authentication and authconfig do not set up proper handling of password hashing in /etc/ldap.conf. Open this file and search for an un-commented line with either of the two directives:
This setting needs to be changed so that PAM will not perform a hash on passwords and allow LDAP to handle the hashing. Comment that line out by putting '#' at the beginning and add the following entry in its place:
The above setting will tell the client to use an external operation in LDAP. In this guide, LDAP will use the SSHA algorithm I specified in /etc/slapd.conf on the LDAP server.
The test performed in this section will verify the client and server configuration at one time. This test assumes the LDAP directory already contains users and groups that can authenticate through LDAP. Please refer to the following guide on how to load user and group information into an LDAP directory:
Use the ldapsearch utility to search through the LDAP directory tree to return user and group entries. You will be prompted to enter the password for the rootdn user you specified when installing the LDAP server.
Notice when running ldapsearch in the previous example that I didn't need to specify the host for the LDAP server with -h command-line parameter. This is because the client machine I ran the example from was configured with system-wide defaults in the /etc/openldap/ldap.conf file to specify the LDAP server URI:
We can actually take this example one step further and leave out the rootdn user (and password) given my LDAP server is configured with a global read ACL that grants anonymous access:
Log in as a directory user to the client machine configured in this guide to test authenticate through LDAP. The example client machine should not have any local user accounts or groups defined with the same name as those being authenticated through the LDAP directory.
Notice that jhunter was able to authenticate through LDAP and log in to the machine. Since this was the first time logging in as the jhunter user, the home directory was automatically created. Also notice that the jhunter user account and the associated groups are not listed in /etc/passwd and /etc/group on the local system. This account was authenticated through LDAP and uses the values from the LDAP server for the account.
This section covers several administration tasks that can be performed on users and groups in an LDAP directory.
The following example demonstrates how to create a new user account and group in a directory that can authenticate to a client machine through LDAP. The new user account will be named ahunter with a primary group of ahunter. In addition, the new ahunter user account will be added to two already existing groups in the directory named dba and oper.
The directory operations will be performed by creating or modifying a template record in the LDAP Data Interchange Format, or LDIF, file format. A separate LDIF file will be created for the new user account and the new group. The appropriate LDAP client command will then be run to either add, modify, or remove records in the directory using the contents found in the LDIF files.
Start by creating an LDAP record for the new user's group. The LDIF record below demonstrates how to create a new group in LDAP with custom changes like the group name and group ID highlighted in blue.
Before creating a new user account, you must first generate a hash of the user's password. The hashed version of the password will be used in the next section as the userPassword attribute value for the new user. There are several methods to generate the hash; however, I prefer use /usr/sbin/slappasswd.
Using the same method for creating the new group, create or modify a template record in the LDIF file format for the new user. Again, the non-static information for the new user is highlighted in blue. Remember to use the hash generated in the previous step for the userPassword attribute value as well as the correct user ID number (uidNumber) and group ID number (gidNumber) for the user's primary group.
Verify ahunter was added to directory.
Next, modify the existing dba and oper group in the directory to add ahunter as a memberUid. Create an LDIF file named modify-oraclegroup.ldif using the following template then run ldapmodify to modify the directory records. It may be helpful to use the verbose option (-v) here to verify the actions performed by ldapmodify.
Verify ahunter was added to the dba and oper group.
Finally, log in as the new directory user to the client machine configured in this guide to authenticate through the LDAP server. The client machine should not have any local user accounts or groups defined with the same name as those being authenticated through the LDAP directory.
The LDAP directory server used in this example is configured with a change user password ACL which allows directory users to change their password.
Use ldapdelete to delete an entry from the LDAP directory.
The following example deletes the ahunter user and his primary group (also ahunter). This example also demonstrates how to remove ahunter as a memberUid of the dba and oper secondary groups.
Jeffrey Hunter is an Oracle Certified Professional, Java Development Certified Professional, Author, and an Oracle ACE. Jeff currently works as a Senior Database Administrator for The DBA Zone, Inc. located in Pittsburgh, Pennsylvania. His work includes advanced performance tuning, Java and PL/SQL programming, developing high availability solutions, capacity planning, database security, and physical / logical database design in a UNIX / Linux server environment. Jeff's other interests include mathematical encryption theory, tutoring advanced mathematics, programming language processors (compilers and interpreters) in Java and C, LDAP, writing web-based database administration tools, and of course Linux. He has been a Sr. Database Administrator and Software Engineer for over 20 years and maintains his own website site at: http://www.iDevelopment.info. Jeff graduated from Stanislaus State University in Turlock, California, with a Bachelor's degree in Computer Science and Mathematics.
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