Centralized authentication using OpenLDAP

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This guide introduces the basics of LDAP and shows you how to setup OpenLDAP for authentication purposes between a group of Gentoo boxes.

Getting Started with OpenLDAP

What is LDAP?

LDAP stands for Lightweight Directory Access Protocol . Based on X.500 it encompasses most of its primary functions, but lacks the more esoteric functions that X.500 has. Now what is this X.500 and why is there an LDAP?

X.500 is a model for Directory Services in the OSI concept. It contains namespace definitions and the protocols for querying and updating the directory. However, X.500 has been found to be overkill in many situations. Enter LDAP. Like X.500 it provides a data/namespace model for the directory and a protocol too. However, LDAP is designed to run directly over the TCP/IP stack. See LDAP as a slim-down version of X.500.

I don't get it. What is a directory?

A directory is a specialized database designed for frequent queries but infrequent updates. Unlike general databases they don't contain transaction support or roll-back functionality. Directories are easily replicated to increase availability and reliability. When directories are replicated, temporary inconsistencies are allowed as long as they get synchronised eventually.

How is information structured?

All information inside a directory is structured hierarchically. Even more, if you want to enter data inside a directory, the directory must know how to store this data inside a tree. Lets take a look at a fictional company and an Internet-like tree:

CodeOrganisational structure for GenFic, a Fictional Gentoo company

dc:         com
dc:        genfic         ## (Organisation)
          /      \
ou:   People   servers    ## (Organisational Units)
      /    \     ..
uid: ..   John            ## (OU-specific data)

Since you don't feed data to the database in this ascii-art like manner, every node of such a tree must be defined. To name such nodes, LDAP uses a naming scheme. Most LDAP distributions (including OpenLDAP) already contain quite a number of predefined (and general approved) schemes, such as the inetorgperson, a frequently used scheme to define users.

Interested users are encouraged to read the OpenLDAP Admin Guide .

So... What's the Use?

LDAP can be used for various things. This document focuses on centralised user management, keeping all user accounts in a single LDAP location (which doesn't mean that it's housed on a single server, LDAP supports high availability and redundancy), yet other goals can be achieved using LDAP as well.

  • Public Key Infrastructure
  • Shared Calendar
  • Shared Addressbook
  • Storage for DHCP, DNS, ...
  • System Class Configuration Directives (keeping track of several server configurations)
  • ...

Configuring OpenLDAP

Initial Configuration

In this document we use the genfic.com address as an example. You will ofcourse have to change this. However, make sure that the top node is an official top level domain (net, com, cc, be, ...).

Let's first emerge OpenLDAP:

root # emerge --ask openldap

Now generate an encrypted password we'll use later on:

root # slappasswd
New password: my-password
Re-enter new password: my-password

Now edit the LDAP Server config at /etc/openldap/slapd.conf . Below we'll give a sample configuration file to get things started. For a more detailed analysis of the configuration file, we suggest that you work through the OpenLDAP Administrator's Guide.


include	/etc/openldap/schema/core.schema
include /etc/openldap/schema/cosine.schema
include /etc/openldap/schema/inetorgperson.schema
include /etc/openldap/schema/nis.schema
include	/etc/openldap/schema/misc.schema
pidfile /var/run/openldap/slapd.pid
argsfile /var/run/openldap/slapd.args
serverID 0 ## Used in case of replication
loglevel 0
## ## Access Controls
access to dn.base="" by * read
access to dn.base="cn=Subschema" by * read
access to *
  by self write
  by users read
  by anonymous read
## ## Database definition
database hdb
suffix "dc=genfic,dc=com"
checkpoint 32 30
rootdn "cn=Manager,dc=genfic,dc=com"
rootpw "{SSHA}EzP6I82DZRnW+ou6lyiXHGxSpSOw2XO4" ## # See earlier slappasswd command
directory "/var/lib/openldap-ldbm"
index objectClass eq
## ## Synchronisation (pull from other LDAP server)
syncrepl rid=000
  retry="5 5 300 +"
index entryCSN eq
index entryUUID eq
mirrormode TRUE
overlay syncprov
syncprov-checkpoint 100 10

Next we edit the LDAP Client configuration file:

root # nano -w /etc/openldap/ldap.conf
## (Add the following...)
BASE         dc=genfic, dc=com
URI          ldap://ldap.genfic.com:389/ ldap://ldap1.genfic.com:389/ ldap://ldap2.genfic.com:389/

Now edit /etc/conf.d/slapd and set the following OPTS line:


OPTS="-h 'ldaps:// ldap:// ldapi://%2fvar%2frun%2fopenldap%2fslapd.sock'"

Finally, create the /var/lib/openldap-ldbm structure:

root # mkdir -p /var/lib/openldap-ldbm
root #
chown ldap:ldap /var/lib/openldap-ldbm
root #
chmod 700 /var/lib/openldap-ldbm

Start slapd:

root # /etc/init.d/slapd start

You can test with the following command:

user $ ldapsearch -x -D "cn=Manager,dc=genfic,dc=com" -W

If you receive an error, try adding -d 255 to increase the verbosity and solve the issue you have.


If you need high availability

If your environment requires high availability, then you need to setup replication of changes across multiple LDAP systems. Replication within OpenLDAP is, in this guide, set up using a specific replication account ( ldapreader ) which has read rights on the primary LDAP server and which pulls in changes from the primary LDAP server to the secundary.

This setup is then mirrored, allowing the secundary LDAP server to act as a primary. Thanks to OpenLDAP's internal structure, changes are not re-applied if they are already in the LDAP structure.

Setting Up Replication

To setup replication, first setup a second OpenLDAP server, similarly as above. However take care that, in the configuration file,

  • the sync replication provider is pointing to the other system
  • the serverID of each OpenLDAP system is different

Next, create the synchronisation account. We will create an LDIF file (the format used as data input for LDAP servers) and add it to each LDAP server:

user $ slappasswd -s myreaderpassword
user $ cat ldapreader.ldif
dn: cn=ldapreader,dc=genfic,dc=com
userPassword: {SSHA}XvbdAv6rdskp9HgFaFL9YhGkJH3HSkiM
objectClass: organizationalRole
objectClass: simpleSecurityObject
cn: ldapreader
description: LDAP reader used for synchronization
user $ ldapadd -x -W -D "cn=Manager,dc=genfic,dc=com" -f ldapreader.ldif
Password: ## enter the administrative password

Client Configuration

Migrate existing data to ldap

Configuring OpenLDAP for centralized administration and management of common Linux/Unix items isn't easy, but thanks to some tools and scripts available on the Internet, migrating a system from a single-system administrative point-of-view towards an OpenLDAP-based, centralized managed system isn't hard either.

Go to http://www.padl.com/OSS/MigrationTools.html and fetch the scripts there. You'll need the migration tools and the make_master.sh script.

Next, extract the tools and copy the make_master.sh script inside the extracted location:

root # mktemp -d
root # cd /tmp/tmp.zchomocO3Q
root #
tar xvzf /path/to/MigrationTools.tgz
root #
mv /path/to/make_master.sh MigrationTools-47
root #
cd MigrationTools-47</pre>

The next step now is to migrate the information of your system to OpenLDAP. The make_master.sh script will do this for you, after you have provided it with the information regarding your LDAP structure and environment.

At the time of writing, the tools require the following input:

Input Description Example
LDAP BaseDN The base location (root) of your tree dc=genfic,dc=com
Mail domain Domain used in e-mail addresses genfic.com
Mail host FQDN of your mail server infrastructure smtp.genfic.com
LDAP Root DN Administrative account information for your LDAP structure cn=Manager,dc=genfic,dc=com
LDAP Root Password Password for the administrative account, cfr earlier slappasswd command

The tool will also ask you which accounts and settings you want to migrate.

Configuring PAM

First, we will configure PAM to allow LDAP authorization. Install sys-auth/pam_ldap so that PAM supports LDAP authorization, and sys-auth/nss_ldap so that your system can cope with LDAP servers for additional information (used by nsswitch.conf ).

root # emerge --ask pam_ldap nss_ldap

Now add the following lines in the right places to /etc/pam.d/system-auth :


## # Note: only add them. Don't kill stuff already in there or your box won't let you login again!
auth       sufficient   pam_ldap.so use_first_pass
account    sufficient   pam_ldap.so
password   sufficient   pam_ldap.so use_authtok use_first_pass
session    optional     pam_ldap.so
## # Example file:
auth       required     pam_env.so
auth       sufficient   pam_unix.so try_first_pass likeauth nullok
auth       sufficient   pam_ldap.so use_first_pass
auth       required     pam_deny.so
account    sufficient   pam_ldap.so
account    required     pam_unix.so
password   required     pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 try_first_pass retry=3
password   sufficient   pam_unix.so try_first_pass use_authtok nullok md5 shadow
password   sufficient   pam_ldap.so use_authtok use_first_pass
password   required     pam_deny.so
session    required     pam_limits.so
session    required     pam_unix.so
session    optional     pam_ldap.so

Now change /etc/ldap.conf to read:


## #host
## #base dc=padl,dc=com
suffix          "dc=genfic,dc=com"
## #rootbinddn uid=root,ou=People,dc=genfic,dc=com
bind_policy soft
bind_timelimit 2
ldap_version 3
nss_base_group ou=Group,dc=genfic,dc=com
nss_base_hosts ou=Hosts,dc=genfic,dc=com
nss_base_passwd ou=People,dc=genfic,dc=com
nss_base_shadow ou=People,dc=genfic,dc=com
pam_filter objectclass=posixAccount
pam_login_attribute uid
pam_member_attribute memberuid
pam_password exop
scope one
timelimit 2
uri ldap://ldap.genfic.com/ ldap://ldap1.genfic.com ldap://ldap2.genfic.com

Next, copy over the (OpenLDAP) ldap.conf file from the server to the client so the clients are aware of the LDAP environment:

root # scp ldap-server:/etc/openldap/ldap.conf /etc/openldap

Finally, configure your clients so that they check the LDAP for system accounts:


passwd:         files ldap
group:          files ldap
shadow:         files ldap

If you noticed one of the lines you pasted into your /etc/ldap.conf was commented out (the rootbinddn line): you don't need it unless you want to change a user's password as superuser. In this case you need to echo the root password to /etc/ldap.secret in plaintext. This isDANGEROUSand should be chmoded to 600. What you might want to do is keep that file blank and when you need to change someones password thats both in the ldap and /etc/passwd , put the pass in there for 10 seconds while changing the users password and remove it when done.

LDAP Server Security Settings

OpenLDAP permissions

If we take a look at /etc/openldap/slapd.conf you'll see that you can specify the ACLs (permissions if you like) of what data users can read and/or write:


access to *
  by dn="uid=root,ou=People,dc=genfic,dc=com" write
  by users read
  by anonymous auth
access to attrs=userPassword,gecos,description,loginShell
  by self write

This gives you access to everything a user should be able to change. If it's your information, then you got write access to it; if it's another user their information then you can read it; anonymous people can send a login/pass to get logged in. There are four levels, ranking them from lowest to greatest: auth search read write .

The next ACL is a bit more secure as it blocks normal users to read other people their shadowed password:


access to attrs="userPassword"
  by dn="uid=root,ou=People,dc=genfic,dc=com" write
  by dn="uid=John,ou=People,dc=genfic,dc=com" write
  by anonymous auth
  by self write
  by * none
access to *
  by dn="uid=root,ou=People,dc=genfic,dc=com" write
  by dn="uid=John,ou=People,dc=genfic,dc=com" write
  by * search

This example gives root and John access to read/write/search for everything in the the tree below dc=genfic,dc=com . This also lets users change their own userPassword's. As for the ending statement everyone else just has a search ability meaning they can fill in a search filter, but can't read the search results. Now you can have multiple acls but the rule of the thumb is it processes from bottom up, so your toplevel should be the most restrictive ones.

Working with OpenLDAP

Maintaining the directory

You can start using the directory to authenticate users in apache/proftpd/qmail/samba. You can manage it with phpldapadmin, diradm, jxplorer, or lat, which provide easy management interfaces.


We would like to thank Matt Heler for lending us his box for the purpose of this guide. Thanks also go to the cool guys in #ldap @ irc.freenode.net


We would like to thank the following authors and editors for their contributions to this guide:

  • Benjamin Coles
  • swift
  • Brandon Hale
  • Benny Chuang
  • jokey
  • nightmorph