Introduction to LDAP under Linux

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System Administration

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original in en Atif Ghaffar 

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Atif is a chameleon. He changes his roles, from System Administrator, to programmer, to teacher, to project manager, to whatever is required to get the job done.
Occasionally he likes to program on his laptop while watching a movie in the cinema.
Atif thinks that he owes a lot to Linux and opensource community and projects for being his teacher.
More about him can be found at his homepage

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In this Article we will explore LDAP and its implementations using our favourite OS Linux.

There is a lot of literature about LDAP out there so I wont repeat it again here, neither will I talk about advanced LDAP terms, schemas, v2 versus v3 difference etc. In fact i do not know much about them. Instead I will try to explain in very simple words about what LDAP is, what its benefits and how we can use it.

I am not an LDAP expert. In fact I am an LDAP beginner. I will basically just write here what I do with LDAP and how. I will try not to confuse you like I was for a very long time.

Question: If you are a beginner, why write an article about it?
Recently I needed help on a project from a colleague. The base of the project was LDAP. He could help me with Perl, mail servers etc but did not know anything about LDAP, in fact each time he wanted to get into LDAP, it got more and more confusing.
Since being comfortable with LDAP was the minimum requirement for the project, I gave him a crash course about LDAP in half an hour and it became suddenly all clear. The concepts were so easy, it was all in front, but just needed a bit of focus or some funny examples.

I will try to do the same in this article.

You can find many urls in the reference section to learn more about LDAP.

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What is LDAP?

LDAP stands for (Lightweight Directory Access Protocol) and is a directory service, very similar to the directories on the file system that we are used to and also similar to the telephone book that we use to lookup phone numbers, and also network directory services such as SUN's NIS (Network Information Service), DNS (Domain Name Service) , and also to the tree that you see in your ( or your neighbor's) garden etc.

LDAP is a specialized database. It is very important to remember that LDAP is not just another database. LDAP is optimized to make lookups (read data). Reads in LDAP are performed much more often than writes.

What do all these services have in common?
The all return some information when queried about a criteria.


File system directory
ls /etc
This will return all files and subdirectories in the folder /etc

ls /etc/p*
This will return all files and subdirectories that begin with p

find /usr/local/apache -name index.html
This will search the file system for a file/directory called index.html , starting from the base "/usr/local/apache"

NIS directory
ypcat passwd
This will return the username, password, userid etc from the NIS database

ypmatch atif passwd
This will return entries from the password for the user atif

DNS directory
This will return the ip address for the entry from the DNS database

nslookup -type MX
This will return only MX information from the dns database where hostname matches

LDAP directory
(we will look into them in detail below)

ldapsearch uid=aghaffar
This will return all public information about the user aghaffar
Similar to unix find / -uid aghaffar command

ldapsearch uid=aghaffar mail
This will only return mail of user aghaffar

Directory Base or Root

In each directory service we mentioned above there is always a starting point from where one can start browsing or searching. This starting point is often called "root". This is similar to the root in the tree.
Each tree has a root, then some branches and more branches and leaves and flowers etc.

Each root can further have more branches (as in the tree in your neighbor's garden), for file system for example these branches could be another subdirectory or a file.
Each of these branches can have attributes.
For example the file system tree's branches (subdirectories and files) have the following attributes. Below is a diagram that shows a file system Directory.
The attributes represents the output of the unix command ls -ld /usr
drwxr-xr-x 29 root root 749 Jun 17 23:45 /usr

Now a diagram for an LDAP Directory
We will talk about this diagram later.

Distinguished Name

Unlike the natural tree, each branch of the filesystem/LDAP/Phonebook Directory has at least one unique attribute , which helps us tell difference of one from the other.

On file systems this unique attribute is the filename along with the path. for example
/etc/passwd Here The filename passwd has to be unique within this path. Of course we can have /usr/passwd , /opt/passwd which are all unique by their fully qualified filename.
Similarly the DNS system has FQDN (Fully Qualified Domain Name) which is a unique entry (of coarse you can assign a lot of ip to the same FQDN smart boy!).

In LDAP the fully qualified name of an entry is called "dn" or Distinguished name. This name is always unique in a directory. for example my dn is "uid=aghaffar, ou=People,"
It is not possible to have another entry with the same dn, but surely we can have a dn such as "uid=aghaffar, ou=Administrators,"
This represents the example of file system entries /etc/passwd and /usr/passwd
We have a unique attribute called uid in the tree "ou=Administrators," and we have a unique attribute called uid in the tree "ou=People,".
They do not clash.

LDAP Servers

There are many LDAP servers available on the market today and most will work on Linux.
For this article we will use openLDAP.

Why I chose openLDAP? Why should you choose openLDAP?

open LDAP is available from You can either download source package and build it yourself or see if you Linux distribution already has this package. If it does then simply spare yourself the effort and install the pre-compiled version.
I have tested openLDAP on SuSE6.x and RedHat6.x.

Building the LDAP directory

In this section we will setup an LDAP server step by step.
Steps to take:

Download and install openLDAP

As mentioned before, either download the sources from and install it according to the documentation or install a pre-compiled package(installing of the packages or a howto on compiling applications is out of the scope of this Article)

Configure the LDAP server

For this example, I will build and LDAP directory for
You can change names etc to reflect your site. to setup the main server to have to edit the files slapd.conf and ldap.conf in your favorite editor.
On my servers this file is in /etc/openldap. Yours could be in /usr/local/etc/openldap or else where depending on your Linux Distribution or compile time configuration if you build openldap yourself.

######### /etc/openldap/slapd.conf ###################################
# the following are defined by default in my suse 6.4 linux distribution
# We will talk more about them in part II or part III of this article
# Perhaps by then I will know what they mean :)
include		/etc/openldap/
include		/etc/openldap/slapd.oc.conf
schemacheck	off

pidfile		/var/run/
argsfile	/var/run/slapd.args

# ldbm database definitions

# this defined the kind of database to use. keep the default ldbm
database	ldbm

# suffix or directory root. This is the top node in your LDAP directory
suffix		""

# this is where the ldap dbs will be kept
directory	/var/lib/ldap

# the distinguished name of the directory manager
rootdn		"cn=Manager,"

# its bad idea to keep the ldap manager password in clear text but we
# will use it in the beinning to get used to LDAP
rootpw		secret    

# Thats all. for now.

Edit your /etc/openldap/ldap.conf
This file is on the ldap client (we will use the same machine as client and server. They could be same or separate)
This file is often read by ldap utilities such as ldapdelete , ldapadd, etc

# LDAP Defaults

# See ldap.conf(5) for details
# This file should be world readable.

# this defines the ldapserver. you can use hostname or ip address

# this is the directory root we want to use to start searching from.
# we will use the top node in our configuration
# it need not be the top level node in the directory, for example
# we can use base = ou=users,
# if we do that then all our searched will start from that branch 
of the tree base
# thats all
Now start the ldap server.
If you are using the pre-installed openldap server from SuSE, then you can start it by
/etc/rc.d/ldap start
On RedHat, I believe it is
/etc/rc.d/init.d/ldap start
If you have compiled and installed by yourself and used the defaults then you may start
/usr/local/libexec/slapd & or where ever you installed it. Just find slapd and run it

Adding data to the newly installed LDAP server

At this point you have an ldap server running, ready to be filled up with information.
The standard way to fill information in the ldap server is to create an LDIF (LDAP Directory Interchange Format)* file. You can read man ldif to find more information about ldif.
Very simply, ldif is the textual representation for ldap entries. These entries are human readable and interchangeable between two different LDAP servers running from different vendors, using different database back-ends or running on different operating systems.
* Yes Yet another format. I wonder why XML is not used instead of LDIF

So without further ado lets create this ldif file. Some things to remember.

  • Every record/entry in the ldif file is separated with a blank line.
  • Spaces in values are very important "Atif Ghaffar" is not the same as "Atif Ghaffar "
    The ldif file.
    objectclass: top
    objectclass: organization
    dn: ou=editors,
    ou: editors
    objectclass: organizationalUnit
    dn: uid=aghaffar, ou=editors,
    uid: aghafar
    cn: Atif Ghaffar
    sn: Ghaffar
    givenname: Atif
    objectclass: person
    userpassword: {CRYPT}yIvSBWSuLs2N2
    ou: editors
    dn: uid=mkempe, ou=editors,
    uid: mkempe
    cn: Magnus Kempe 
    sn: Kempe
    givenname: Magnus
    objectclass: person
    userpassword: clearpass
    preferredlanguage: fr
    ou: editors
    So now we have to add this information in the ldap directory. Use the command line program called ldapadd

    ldapadd -D "cn=Manager," -w secret <
    This command will use "cn=Manager," as the manager's dn(distinguish name) and secret as password and read information from the file and insert it into the ldap directory.
    If every thing went all right you are now ready to make queries on your ldap directory, or if it didn't went all right then you are probably ready to flood my mailbox ;)

    For my mail server's sake I hope that everything went all right.

    Before we continue, lets examine this ldif file line by line.

    1. this line defined the dn for our top level entry. This will be the root of the directory tree.
      This is necessary to define.
    2. This line we define the o (organization) and give it a value ""
    3. This line defined the object class of this object. we define top
    4. here we defined the type of this object (This is an organization object)
    5. The blank line separator
    6. dn for the group editors (this is the branch make for linuxfocus editors)
      like this branch we can have more branches for different purposes. For example one branch for hosts and their data, one branch for site mirrors etc, etc, etc.
    7. here is explicitly define the attribute ou (organizational unit) to editors.
      These attributes serve as searchable items for example you want to find all users that are editors you might search for "show all dn where ou=editors". If we don't define this here, this entry/record will not match.
    8. here we define the objectclass (organizationalUnit)
    9. The blank line separator
    10. dn for user aghaffar in goup editors
    11. the uid (make sure this is unique) of the user
    12. The cn (common name) of the user. For example I prefer to write my name "Firstname Lastname", some might prefer "Lastname Firstname".
    13. sn: Surname
    14. givenname (Firstname)
    15. objectclass (Person)
    16. userpassword (this is a crypted password). The string {CRYPT} states that this password is encrypted with the crypt algorithm. The rest after is the crypted password "yIvSBWSuLs2N" which is "test" crypted with salt "yI".
    17. this line defines an email address on which I can receive emails.
    18. ou. this defines that I am in the organizationalUnit editors.
    19. The blank line separator
    20. Once again. dn for another entry. username mkempe, ou editors
    21. common name /display name
    22. Surname
    23. Firstname
    24. objectclass
    25. userpassword: Note here we are using a password in clear text. You can use for different users what ever scheme you like. This is definable per entry and not per a database etc. so one user might have cleartext, the other may crypt with CRYPT, another pay prefer SHA etc.
    26. emailstring where the user can accept mail. (This generally is for the mailserver)
    27. maildrop: Again this is for the mail server. This defines the actual location of the users mailbox. In this example, this server recieves email for address "", the mailserver will then check with the server. "Is there is mailbox which accepts email for "". The ldap server will return the maildrop attribute value(s). The mail server then sends the mail to that mailbox. more on this some other time. I am building an ISP infrastructure where I am heavily using LDAP to manager virtual domains and virtually all information about domains and their users etc. If you are interested in that stuff, drop me an Email.
    28. preferredlanguage. Here we have an extra attribute that tells us the user's preferred language. We can use this information of other preferences information that can be stored centrally in the LDAP to provide this user better services. For example, this users always gets redirected to the French version of the pages directly.
      Also note that the previous record don't have the attributes (preferredlanguage, maildrop etc). This is also one of the strengths of LDAP. There is not fixed structure like in a database table. You may have one entry with only 3 attributes while the other may have 30.
    29. ou. the unit editors

    Querying the ldap database

    Lets find all data about user mkempe
    ldapsearch uid=mkempe

    Lets find the dn for all editors

    ldapsearch '(&(objectclass=person)(ou=editors))' dn

    For more examples , please read man pages for ldapsearch

    Benefits of LDAP

    So what are the benefits of moving to LDAP.

    LDAP is an open standard. Most new applications that you will use will be able to lookup to ldap database for informations. Even Windows 2000 uses LDAP for its directory services.
    Centralizing all information in one place has enormous benefits, Single point of administration, less error prone, less duplicate data everywhere and the hassle to backing it up.

    Sample LDAP implementations

    I can be a complete dork from Compaq and try to sell you LDAP by saying that you can use it as a "contacts manager". But I do not work for Compaq. So I will try to give you some more interesting use of LDAP.

    User accounts in a central place.
    You may want to use an ldap tree to manage your users, their passwords and much more information than you can store in the flat /etc/passwd file. This information can be used for Microsoft Windows / Unix /Mac users.
    Idea You may optionally tweak /etc/pam.d/login etc so that authentication is actually done from LDAP information instead of shadow file or nis etc.
    Idea You may write a small web interface for users to change their unix password without loggin in the system since the password information is in LDAP and not on the system. You will need to use pam_ldap for this. See resources for url for pam_ldap.

    Single source of sign-on != single sign-on.
    A lot of LDAP vendors try to sell by saying if you implement LDAP you can have a single sign-on solution.
    Its only half true. Single Sign on is totally a different animal and a big buzz word among IT Managers these days.
    Single sign on example is:

    Of course you can use LDAP for the source of user's information, but the magic of managing sessions between different programs is called the "Single Sign on solution" and absolutely has nothing to do with LDAP.
    It can be done with LDAP, NIS, NT Domain controller accounts, databases,flat files.

    Idea You may want to give users email account on your servers but don't want to create unix accounts. No problem. I am using a combination of LDAP+Postfix MailServer+ Cyrus IMAP/POP Server to manage thousands of users and none of them have a system account.

    Idea You may want to centralize preferences for different applications. For example, Netscape preferences, bookmarks etc can be stored in LDAP, and the user can move from one machine to another, receiving his/her preferences from the LDAP server. The user may move from Windows NT Netscape to Linux/Solaris/Macintosh Netscape and still can use the same information. (Sorry Microsoft... I know thats too bad for you)

    Scenario I hate to fill out my informations again and again both at the web and on paper. I don't know why people want to know again and again my age, date of birth, address in government offices when I have already given it to them once. In my last company, we had to setup enormous forms for Help-Desk which were basically 75% the same. (Firstname, Lastname, Title, PostBox, Floor, ManagerName, Department). Instead of pissing off your users and risk being beaten up one dark night by strangers who look very familiar, you may just want to ask your users only the necessary information. For example, ask them for their userid and get the rest of the information from LDAP, then ask them just the missing part.

    Idea For example if my phone is not working, the only new information I have for you is "my phone is not working".

    A user would be so much less pissed off with the intranet if :

  • The user type the url for the problems form page (Or click a link or choose from my bookmarks)
  • The page challenges the user with the username/password box. (only once per session. Later visits to this page or other protected areas will remember the user informations)
  • The page has 2 form elements 1) textarea where the problem report can be filled, 2)submit button.

    Write articles: Teaching/Learning

    I hope that with this article, I could help you in some way or feed your hungry brain with some 10011001001001.
    You see, you don't have to be an expert to share your knowledge with the community. Neither do you have to be a professor or a teacher to write tutorials. Every little information can help users.
    Personally, I find it easy to learn if I try to teach someone about what I already know. You may want to try this technique.

    Fortunately I see these days a lot of people talking about Linux, I see a lot of them trying it out too, some give up immediately while some give it a chance for a while. These users may have absolutely no experience of unix commands and concept, while some of them maybe stuck with one problem for a while
    If you have faced these problems freshly, then you can be the best teacher for the this batch,

    If you don't think that you have any material to write about and still want to help the Linux community, you may want to volunteer to translate these articles in your language.


    PAM LDAP Module
    The pam_ldap module provides the means for Solaris and Linux workstations to authenticate against LDAP directories, and to change their passwords in the directory.
    The OpenLDAP Project is a collaborative effort to develop a robust, commercial-grade, fully featured, and open source LDAP suite of applications and development tools. The project is managed by a worldwide community of volunteers that use the Internet to communicate, plan, and develop the OpenLDAP Suite and its related documentation.
    Java based excellent LDAP Browser/Editor
    The LDAP Browser/Editor provides a user-friendly interface to LDAP directories with tightly integrated browsing and editing capabilities. It is entirely written in Java with the help of the JFC (SwingSet) and JNDI class libraries. It connects to LDAP v2 and v3 servers.
    kldap is a LDAP client (browser) for the KDE. You can browse the tree similar to the Novell Administrator.
    Lot of LDAP related tools at Freshmeat
    LDAP Implementation Cookbook, from IBM
    You can view this book online or order a Hard copy
    ypldapd - nis 2 ldap gateway
    Ypldapd is a Network Information Service (NIS) server which uses LDAP as its information source. Using ypldapd, existing NIS clients can transparently use LDAP to resolve user, group and host information. Enterprises can thus realize the benefits of LDAP, such as its distribution and scalability, without upgrading clients. Used in conjunction with LDAP server technologies such as Netscape's Directory Server, ypldapd can provide single sign on to both NT and UNIX clients.
    Introduction to the SLAPD and SLURPD Administrator's Guide
    A must read. Lot of information about LDAP protocol and ldap servers
    An LDAP RaodMap & FAQ
    A tutorial aid to navigating various LDAP and X.500 Directory Services resources on the Internet
    Lightweight Directory Access Protocol