LDAP + Kerberos

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LDAP and Kerberos together make for a great combination. Kerberos is used to manage credentials securely (authentication) while LDAP is used for determining criteria about the accounts, such as what they're allowed access to (authorization) and other account metadata. Most other LDAP setups involve in storing passwords in the LDAP directory itself using the userPassword attribute. While this is ok for a basic setup, one can do much better with just a little effort.

Overview

This guide is intended as a Debian-focused update to excellent guides, such as http://aput.net/~jheiss/krbldap/howto.html . Many of the workarounds have been fixed in recent releases of Debian, however there are still a few places one can easily get snagged.

The goal of this document is to create a Single Sign-On (SSO) system without using NIS or passwords stored in LDAP. This includes a client setup which can successfully use Kerberos for authentication and LDAP for authorization. A number of common clients are shown, such as a standard shell login and Apache2 integration.

With LDAP comes many solutions to very similar problems. Many people use LDAP due to an existing Active Directory setup so certain tools need to be used to deal with its quirks. As this starts from scratch, it can be done as simply as possible. By far the simplest way to integrate Kerberos + LDAP together on one system is to use PAM (authentication) and NSS (authorization).

This document was originally written based on experience with Debian/etch and Debian/lenny. Please update accordingly.

Kerberos server

There are plenty of guides for setting up a Kerberos server on Debian. Once you have a KDC set up with a test principal, come back to this document.

Kerberos client

Install the following packages:

The Kerberos client setup is pretty straightforward using the krb5-config package's config. For libpam-krb5, you will have to modify your /etc/pam.d/common-* files according to /usr/share/doc/libpam-krb5/README.Debian.

If you're having trouble, check the following:

LDAP Server

See ["LDAP/OpenLDAPSetup"] to get your server set up.

Getting your feet wet with LDAP

A very helpful tool for getting one's feet wet with LDAP is phpldapadmin. While it's certainly no replacement for a proper account management system, it lets you create accounts and tinker with attributes while conforming to the schemas.

LDAP data structure

Starting an LDAP server from scratch can be a bit daunting, as it starts out as a blank, unstructured slate. One tool to help get started is the migrationtools package. If one is familiar with /etc/passwd and /etc/group, one can see how that then translates to the world of LDAP using /usr/share/migrationtools/migrate_passwd.pl and friends. See ["LDAP/MigrationTools"] for more information. These tools create posixGroup and posixAccount objects which the NSS module below is familiar with.

The common hierarchical structure of ou=Users,dc=example,dc=com and ou=Groups,dc=example,dc=com seems to work quite well for most software out-of-the-box.

PAM

Instead of using LDAP PAM as described in ["LDAP/PAM"], set up PAM to authenticate using Kerberos. Install libpam-krb5 and then proceed to /usr/share/doc/libpam-krb5/README.Debian which has great directions to get going. If your Kerberos environment was properly set up above, then you should have logins working nicely.

NSS

See ["LDAP/NSS"] to get started. There is nothing special you need to do beyond ensure that you properly configure /etc/libnss-ldap.conf.

Apache2

Basic authentication over SSL

Basic + SSL is a quick way to set up restricted access to websites. It's not the best, usability wise (this is usually the fault of most browsers for not exposing identity management at all), but it works well enough for most internal-use cases.

<Location /restrictedpath>
    AuthPAM_Enabled on
    AuthType basic
    AuthName "Example.com Restricted Login"
    AuthGROUP_Enabled on
    require valid-user
</Location>

Other options are to use libapache2-mod-auth-kerb or libapache2-mod-authn-sasl, but neither of those provide group information from LDAP. It might be possible to pull that in using the authnz_ldap that comes with Apache2, but that module seems quite intent on performing the authentication phase, which is in our case is supposed to be handled by Kerberos, not LDAP.