February 8, 2015

Understanding LDAP and LDAP Authentication


If you let an IT administrator pick a persistence storage technique, he would probably choose a LDAP directory. But if you asked a system developer, then he would probably choose a RDBMS (Relational Database Management System). Why is that? I would guess because of history.

So is the two different technique so much different? They both stores data. Right. But they also have some key differences.

  • LDAP stores data in a tree (hierarchical database) and RDBMS stores data in tables with relationship between them via primary keys and foreign keys.
  • LDAP uses path (Distinguished Name, DN) to make entries unique. RDBMS uses primary keys in tables.
  • In a RDBMS you design your tables and columns. In LDAP you pick from predefined object class (RDBMS table), with predefined attributes (RDBMS column).
  • In RDBMS you define the column data types, but in LDAP everything are strings.

So by that said lets start to discover LDAP, with OpenLDAP.


openldap A package containing the libraries necessary to run the OpenLDAP server and client applications.

openldap-clients A package containing the command-line utilities for viewing and modifying directories on an LDAP server.

openldap-servers A package containing both the services and utilities to configure and run an LDAP server. This includes the Standalone LDAP Daemon, slapd.

compat-openldap A package containing the OpenLDAP compatibility libraries.

Choosing a Suffix

The LDAP suffix is the global LDAP name space or the toop root of you LDAP tree.

There are two standard approaches:

  1. The X.500 naming model, which is geographically and organizationally based.
    Example: o=Magnus K Karlsson,l=Stockholm,s=Stockholm,c=SE
  2. The Internet domain naming model, i.e. the organizations's DNS domain.
    Example: dc=magnuskkarlsson,dc=com

dc, Domain Component
dn, Distinguished Name
cn, Common Name
sn, Surname (family name/last name)
uid, User ID
o, Organization
ou, Organization Unit
l, Location
s, State
c, Country
RDN, Relative Distinuished Name
OID, Object Identifier
DIT, Directory Information Tree
LDIF, LDAP Data Interchange Format

The preferred method is the organizations's DNS domain.

Password Storing Policy

Storing password securily is importand. The default way in OpenLDAP is SSHA-1, in RHEL 6 it is SSHA-512 with 8-bit salt, see crypt(3).

   Glibc Notes
       The glibc2 version of this function supports additional encryption 

       If salt is a character string starting with the characters "$id$" 
       followed by a string terminated by "$":


       then instead of using the DES machine, id identifies the encryption 
       method used and this then determines how the rest of the password string 
       is interpreted.  The following values of id are supported:

              ID  | Method
              1   | MD5
              2a  | Blowfish (not in mainline glibc; added in some
                  | Linux distributions)
              5   | SHA-256 (since glibc 2.7)
              6   | SHA-512 (since glibc 2.7)

       So $5$salt$encrypted is an SHA-256 encoded password and $6$salt$encrypted 
       is an SHA-512 encoded one.

       "salt" stands for the up to 16 characters following "$id$" in the salt. 
       The encrypted part of the password string is the actual computed password.  
       The size of this string is fixed:

       MD5     | 22 characters
       SHA-256 | 43 characters
       SHA-512 | 86 characters

       The characters in "salt" and "encrypted" are drawn from the set 
       [a–zA–Z0–9./]. In the MD5 and SHA implementations the entire key is 
       significant (instead of only the first 8 bytes in DES).

The slappasswd tool can be given options to use crypt(3), see slappasswd(8).

       -c crypt-salt-format
              Specify the format of the salt passed to crypt(3) when generating 
              {CRYPT} passwords.  This string needs to be in sprintf(3) format 
              and may include one (and only one) %s conversion. This  conversion
              will be substituted with a string of random characters from 
              [A-Za-z0-9./]. For example, ’%.2s’ provides a two character salt 
              and ’$1$%.8s’ tells some versions of crypt(3) to use an MD5 
              algorithm and provides 8 random characters of salt. The default is 
              ’%s’, which provides 31 characters of salt.

The recommended way is to use the strongest option, i.e. SSHA-512 with 16 bit salt.


The configuration in newer OpenLDAP versions has been moved from a single configuration file (/etc/openldap/slapd.conf) to a configuration directory (/etc/openldap/slapd.d/), containing several configuration files.

For more novice users is the single configuration file more easier to understand and easier to get an overview of all configuration. Here we will use the single configuration file.

Now we are ready to configure LDAP suffix and username and password for LDAP root user.



To test we can run a simple search.

Another test can be to print all configuration.

GUI Administration Tools

Apache Directory Studio Eclipse-based LDAP tools

For Bug on RHEL 6 and CentOS 6:

See resolution https://issues.apache.org/jira/browse/DIRSTUDIO-999

Designing the Name Space

Designing your LDAP tree or Directory Information Tree (DIT) is an important thing.

Flat Name Space



  • Names do not need to change when job roles change or the organization changes.
  • Simple design avoids need to object categoratization by directory administrators.


  • Hard to partition the directory later if needed.
  • May be hard to maintain unique DNs.

Deeper Name Space



  • Easier to delegate control to organizational units.
  • May simplify later partitioning of directory service among several servers.


  • Names to tend to change more often: job transfer, organizational changes, etc.
  • May require more work to correctly categorize entries, keep up to date.

Compromise Name Space



OpenLDAP Tool

The openldap-clients packages installes a number of tools, the most common used are:

  • ldapsearch, tool to search the directory.
  • ldapadd and ldapmodify, tool that use LDIF (LDAP Data Interchange Format) files to update the directory.
  • ldapdelete, tool to delete entry.

Common options for these tools are:

  • -H host
  • -x Use simple, not SASL binds (login method)
  • -D dn Bind using dn (username)
  • -W prompt for simple bind password


Common options for ldapsearch:

  • -b dn Base DN in tree to start search from.


User Class

User Structural Class

After we have designed and created our LDAP tree structure, it's time to create the actual user entry. But before that we need to decide which base class we want to use for our user entry.

Note that an entry must have one and only one structural object class. Each object class have a defined set of attributes, some mandatory and other optional. You can extend or add one or more addition auxiliary object classes.

The two most frequently used structural classes are:

  • account Is useful if you are only using LDAP as a NIS (Network Information Service) replacement.
  • inetOrgPerson Is best if you are also using LDAP to provide contact information.

User Auxiliary Classes

If you are planning to use the LDAP directory for RHEL authentication, you need to add the following auxiliary classes.

posixAccount represents a line from /etc/passwd.

shadowAccount represents a line from /etc/shadow.

Group Structural Class

To complete a UNIX user/authentication you also needs groups.

posixGroup represents a line from /etc/group.


Access Control Instructions, ACI

Writing ACI is out of the scoop of this blog, so comment out every ACI in /etc/openldap/slapd.conf, which will give everyone read, but restricts updates to rootdn.

Installation Apache Web Server (httpd) and LDAP authentication

Lets test our new LDAP directory, by configure LDAP authentication against httpd manual pages.

The httpd ldap module is alreaddy by default installed.

Lets configure httpd-manual authentication.

Restart httpd and test.

RHEL 6 LDAP Authentication Migrations Tools

Create LDIF export of local user and its password.

Create LDIF export of local groups.

To export and import everything.





LDAP Schemas

Default installed LDAP schemas.


objectclass ( 0.9.2342.19200300.100.4.5 NAME 'account'
        SUP top STRUCTURAL
        MUST userid
        MAY ( description $ seeAlso $ localityName $
                organizationName $ organizationalUnitName $ host )


objectclass ( NAME 'posixAccount'
        DESC 'Abstraction of an account with POSIX attributes'
        SUP top AUXILIARY
        MUST ( cn $ uid $ uidNumber $ gidNumber $ homeDirectory )
        MAY ( userPassword $ loginShell $ gecos $ description ) )

objectclass ( NAME 'shadowAccount'
        DESC 'Additional attributes for shadow passwords'
        SUP top AUXILIARY
        MUST uid
        MAY ( userPassword $ shadowLastChange $ shadowMin $
              shadowMax $ shadowWarning $ shadowInactive $
              shadowExpire $ shadowFlag $ description ) )

objectclass ( NAME 'posixGroup'
        DESC 'Abstraction of a group of accounts'
        SUP top STRUCTURAL
        MUST ( cn $ gidNumber )
        MAY ( userPassword $ memberUid $ description ) )


objectclass ( NAME 'person'
        DESC 'RFC2256: a person'
        SUP top STRUCTURAL
        MUST ( sn $ cn )
        MAY ( userPassword $ telephoneNumber $ seeAlso $ description ) )

objectclass ( NAME 'organizationalPerson'
        DESC 'RFC2256: an organizational person'
        SUP person STRUCTURAL
        MAY ( title $ x121Address $ registeredAddress $ destinationIndicator $
                preferredDeliveryMethod $ telexNumber $ teletexTerminalIdentifier $
                telephoneNumber $ internationaliSDNNumber $ 
                facsimileTelephoneNumber $ street $ postOfficeBox $ postalCode $
                postalAddress $ physicalDeliveryOfficeName $ ou $ st $ l ) )


objectclass     ( 2.16.840.1.113730.3.2.2
    NAME 'inetOrgPerson'
        DESC 'RFC2798: Internet Organizational Person'
    SUP organizationalPerson
        MAY (
                audio $ businessCategory $ carLicense $ departmentNumber $
                displayName $ employeeNumber $ employeeType $ givenName $
                homePhone $ homePostalAddress $ initials $ jpegPhoto $
                labeledURI $ mail $ manager $ mobile $ o $ pager $
                photo $ roomNumber $ secretary $ uid $ userCertificate $
                x500uniqueIdentifier $ preferredLanguage $
                userSMIMECertificate $ userPKCS12 )

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