OpenLDAP Server

LDAP is an acronym for Lightweight Directory Access Protocol, it is a simplified version of the X.500 protocol. The directory setup in this section will be used for authentication. Nevertheless, LDAP can be used in numerous ways: authentication, shared directory (for mail clients), address book, etc.

To describe LDAP quickly, all information is stored in a tree structure. With OpenLDAP you have freedom to determine the directory arborescence (the Directory Information Tree: the DIT) yourself. We will begin with a basic tree containing two nodes below the root:

  • "People" node where your users will be stored

  • "Groups" node where your groups will be stored

Before beginning, you should determine what the root of your LDAP directory will be. By default, your tree will be determined by your Fully Qualified Domain Name (FQDN). If your domain is example.com (which we will use in this example), your root node will be dc=example,dc=com.

Instal·lació

First, install the OpenLDAP server daemon slapd and ldap-utils, a package containing LDAP management utilities:

sudo apt-get install slapd ldap-utils

The installation process will prompt you for the LDAP directory admin password and confirmation.

By default the directory suffix will match the domain name of the server. For example, if the machine's Fully Qualified Domain Name (FQDN) is ldap.example.com, the default suffix will be dc=example,dc=com. If you require a different suffix, the directory can be reconfigured using dpkg-reconfigure. Enter the following in a terminal prompt:

sudo dpkg-reconfigure slapd

You will then be taken through a menu based configuration dialog, allowing you to configure various slapd options.

Configuration

OpenLDAP uses a separate database which contains the cn=config Directory Information Tree (DIT). The cn=config DIT is used to dynamically configure the slapd daemon, allowing the modification of schema definitions, indexes, ACLs, etc without stopping the service.

The cn=config tree can be manipulated using the utilities in the ldap-utils package. For example:

  • Use ldapsearch to view the tree, entering the admin password set during installation or reconfiguration:

    ldapsearch -xLLL -b cn=config -D cn=admin,cn=config -W olcDatabase={1}hdb
    
    Enter LDAP Password: 
    dn: olcDatabase={1}hdb,cn=config
    objectClass: olcDatabaseConfig
    objectClass: olcHdbConfig
    olcDatabase: {1}hdb
    olcDbDirectory: /var/lib/ldap
    olcSuffix: dc=example,dc=com
    olcAccess: {0}to attrs=userPassword,shadowLastChange by dn="cn=admin,dc=exampl
     e,dc=com" write by anonymous auth by self write by * none
    olcAccess: {1}to dn.base="" by * read
    olcAccess: {2}to * by dn="cn=admin,dc=example,dc=com" write by * read
    olcLastMod: TRUE
    olcDbCheckpoint: 512 30
    olcDbConfig: {0}set_cachesize 0 2097152 0
    olcDbConfig: {1}set_lk_max_objects 1500
    olcDbConfig: {2}set_lk_max_locks 1500
    olcDbConfig: {3}set_lk_max_lockers 1500
    olcDbIndex: objectClass eq
    
    

    The output above is the current configuration options for the hdb backend database. Which in this case containes the dc=example,dc=com suffix.

  • Refine the search by supplying a filter, in this case only show which attributes are indexed:

    ldapsearch -xLLL -b cn=config -D cn=admin,cn=config -W olcDatabase={1}hdb olcDbIndex
    
    Enter LDAP Password: 
    dn: olcDatabase={1}hdb,cn=config
    olcDbIndex: objectClass eq
    
    
  • As an example of modifying the cn=config tree, add another attribute to the index list using ldapmodify:

    ldapmodify -x -D cn=admin,cn=config -W
    
    Enter LDAP Password:
    dn: olcDatabase={1}hdb,cn=config
    add: olcDbIndex
    olcDbIndex: entryUUID eq
    
    modifying entry "olcDatabase={1}hdb,cn=config"
    
    

    Once the modification has completed, press Ctrl+D to exit the utility.

  • ldapmodify can also read the changes from a file. Copy and paste the following into a file named uid_index.ldif:

    dn: olcDatabase={1}hdb,cn=config
    add: olcDbIndex
    olcDbIndex: uid eq,pres,sub
    

    Then execute ldapmodify:

    ldapmodify -x -D cn=admin,cn=config -W -f uid_index.ldif
    
    Enter LDAP Password: 
    modifying entry "olcDatabase={1}hdb,cn=config"
    
    

    The file method is very useful for large changes.

  • Adding additional schemas to slapd requires the schema to be converted to LDIF format. Fortunately, the slapd program can be used to automate the conversion. The following example will add the misc.schema:

    1. First, create a conversion schema_convert.conf file containing the following lines:

      include /etc/ldap/schema/core.schema
      include /etc/ldap/schema/collective.schema
      include /etc/ldap/schema/corba.schema
      include /etc/ldap/schema/cosine.schema
      include /etc/ldap/schema/duaconf.schema
      include /etc/ldap/schema/dyngroup.schema
      include /etc/ldap/schema/inetorgperson.schema
      include /etc/ldap/schema/java.schema
      include /etc/ldap/schema/misc.schema
      include /etc/ldap/schema/nis.schema
      include /etc/ldap/schema/openldap.schema
      include /etc/ldap/schema/ppolicy.schema
      
    2. Next, create a temporary directory to hold the output:

      mkdir /tmp/ldif_output
      
    3. Now using slaptest convert the schema files to LDIF:

      slaptest -f schema_convert.conf -F /tmp/ldif_output
      

      Adjust the configuration file name and temporary directory names if yours are different. Also, it may be worthwhile to keep the ldif_output directory around in case you want to add additional schemas in the future.

    4. Edit the /tmp/ldif_output/cn=config/cn=schema/cn={8}misc.ldif file, changing the following attributes:

      dn: cn=misc,cn=schema,cn=config
      ...
      cn: misc
      

      And remove the following lines from the bottom of the file:

      structuralObjectClass: olcSchemaConfig
      entryUUID: 10dae0ea-0760-102d-80d3-f9366b7f7757
      creatorsName: cn=config
      createTimestamp: 20080826021140Z
      entryCSN: 20080826021140.791425Z#000000#000#000000
      modifiersName: cn=config
      modifyTimestamp: 20080826021140Z
      
      [Nota]

      The attribute values will vary, just be sure the attributes are removed.

    5. Finally, using the ldapadd utility, add the new schema to the directory:

      ldapadd -x -D cn=admin,cn=config -W -f /tmp/ldif_output/cn\=config/cn\=schema/cn\=\{8\}misc.ldif
      

    There should now be a dn: cn={4}misc,cn=schema,cn=config entry in the cn=config tree.

Populating LDAP

The directory has been created during installation and reconfiguration, and now it is time to populate it. It will be populated with a "classical" scheme that will be compatible with address book applications and with Unix Posix accounts. Posix accounts will allow authentication to various applications, such as web applications, email Mail Transfer Agent (MTA) applications, etc.

[Nota]

For external applications to authenticate using LDAP they will each need to be specifically configured to do so. Refer to the individual application documentation for details.

LDAP directories can be populated with LDIF (LDAP Directory Interchange Format) files. Copy the following example LDIF file, naming it example.com.ldif, somewhere on your system:

dn: ou=people,dc=example,dc=com
objectClass: organizationalUnit
ou: people

dn: ou=groups,dc=example,dc=com
objectClass: organizationalUnit
ou: groups

dn: uid=john,ou=people,dc=example,dc=com
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: shadowAccount
uid: john
sn: Doe
givenName: John
cn: John Doe
displayName: John Doe
uidNumber: 1000
gidNumber: 10000
userPassword: password
gecos: John Doe
loginShell: /bin/bash
homeDirectory: /home/john
shadowExpire: -1
shadowFlag: 0
shadowWarning: 7
shadowMin: 8
shadowMax: 999999
shadowLastChange: 10877
mail: john.doe@example.com
postalCode: 31000
l: Toulouse
o: Example
mobile: +33 (0)6 xx xx xx xx
homePhone: +33 (0)5 xx xx xx xx
title: System Administrator
postalAddress: 
initials: JD

dn: cn=example,ou=groups,dc=example,dc=com
objectClass: posixGroup
cn: example
gidNumber: 10000

In this example the directory structure, a user, and a group have been setup. In other examples you might see the objectClass: top added in every entry, but that is the default behaviour so you do not have to add it explicitly.

To add the entries to the LDAP directory use the ldapadd utility:

ldapadd -x -D cn=admin,dc=example,dc=com -W -f example.com.ldif

We can check that the content has been correctly added with the tools from the ldap-utils package. In order to execute a search of the LDAP directory:

ldapsearch -xLLL -b "dc=example,dc=com" uid=john sn givenName cn

dn: uid=john,ou=people,dc=example,dc=com
cn: John Doe
sn: Doe
givenName: John

Just a quick explanation:

  • -x: will not use SASL authentication method, which is the default.

  • -LLL: disable printing LDIF schema information.

LDAP replication

LDAP often quickly becomes a highly critical service to the network. Multiple systems will come to depend on LDAP for authentication, authorization, configuration, etc. It is a good idea to setup a redundant system through replication.

Replication is achieved using the Syncrepl engine. Syncrepl allows the directory to be synced using either a push or pull based system. In a push based configuration a “primary” server will push directory updates to “secondary” servers, while a pull based approach allows replication servers to sync on a time based interval.

The following is an example of a Multi-Master configuration. In this configuration each OpenLDAP server is configured for both push and pull replication.

  1. First, configure the server to sync the cn=config database. Copy the following to a file named syncrepl_cn-config.ldif:

    dn: cn=module{0},cn=config
    changetype: modify
    add: olcModuleLoad
    olcModuleLoad: syncprov
    
    dn: cn=config
    changetype: modify
    replace: olcServerID
    olcServerID: 1 ldap://ldap01.example.com
    olcServerID: 2 ldap://ldap02.example.com
    
    dn: olcOverlay=syncprov,olcDatabase={0}config,cn=config
    changetype: add
    objectClass: olcOverlayConfig
    objectClass: olcSyncProvConfig
    olcOverlay: syncprov
    
    dn: olcDatabase={0}config,cn=config
    changetype: modify
    add: olcSyncRepl
    olcSyncRepl: rid=001 provider=ldap://ldap01.example.com binddn="cn=admin,cn=config" bindmethod=simple
      credentials=secret searchbase="cn=config" type=refreshAndPersist
      retry="5 5 300 5" timeout=1
    olcSyncRepl: rid=002 provider=ldap://ldap02.example.com binddn="cn=admin,cn=config" bindmethod=simple
      credentials=secret searchbase="cn=config" type=refreshAndPersist
      retry="5 5 300 5" timeout=1
    -
    add: olcMirrorMode
    olcMirrorMode: TRUE
    
  2. Edit the file changing:

    • ldap://ldap01.example.com and ldap://ldap02.example.com to the hostnames of your LDAP servers.

      [Nota]

      You can have more than two LDAP servers, and when a change is made to one of them it will by synced to the rest. Be sure to increment the olcServerID for each server, and the rid for each olcSyncRepl entry.

    • And adjust credentials=secret to match your admin password.

  3. Next, add the LDIF file using the ldapmodify utility:

    ldapmodify -x -D cn=admin,cn=config -W -f syncrepl_cn-config.ldif
    
  4. Copy the syncrepl_cn-config.ldif file to the next LDAP server and repeat the ldapmodify command above.

  5. Because a new module has been added, the slapd daemon, on all replicated servers, needs to be restarted:

    sudo /etc/init.d/slapd restart
    
  6. Now that the configuration database is synced between servers, the backend database needs to be synced as well. Copy and paste the following into another LDIF file named syncrepl_backend.ldif:

    dn: olcDatabase={1}hdb,cn=config
    changetype: modify
    add: olcRootDN
    olcRootDN: cn=admin,dc=example,dc=com
    -
    add: olcSyncRepl
    olcSyncRepl: rid=003 provider=ldap://ldap01.example.com binddn="cn=admin,dc=example,dc=com" 
     bindmethod=simple credentials=secret searchbase="dc=example,dc=com" type=refreshOnly 
     interval=00:00:00:10 retry="5 5 300 5" timeout=1
    olcSyncRepl: rid=004 provider=ldap://ldap02.example.com binddn="cn=admin,dc=example,dc=com" 
     bindmethod=simple credentials=secret searchbase="dc=example,dc=com" type=refreshOnly 
     interval=00:00:00:10 retry="5 5 300 5" timeout=1
    -
    add: olcMirrorMode
    olcMirrorMode: TRUE
    
    dn: olcOverlay=syncprov,olcDatabase={1}hdb,cn=config
    changetype: add
    objectClass: olcOverlayConfig
    objectClass: olcSyncProvConfig
    olcOverlay: syncprov
    
  7. Like the previous LDIF file, edit this one changing:

    • searchbase="dc=example,dc=com" to your directory's searchbase.

    • If you use a different admin user, change binddn="cn=admin,dc=example,dc=com".

    • Also, replace credentials=secret with your admin password.

  8. Add the LDIF file:

    ldapmodify -x -D cn=admin,cn=config -W -f syncrepl_backend.ldif
    

    Because the servers' configuration is already synced there is no need to copy this LDIF file to the other servers.

The configuration and backend databases should now sycnc to the other servers. You can add additional servers using the ldapmodify utility as the need arises. See “Configuration” for details.

[Nota]

The slapd daemon will send log information to /var/log/syslog by default. So if all does not go well check there for errors and other troubleshooting information. Also, be sure that each server knows it's Fully Qualified Domain Name (FQDN). This is configured in /etc/hosts with a line similar to:

127.0.0.1	ldap01.example.com ldap01

.

Setting up ACL

Authentication requires access to the password field, that should be not accessible by default. Also, in order for users to change their own password, using passwd or other utilities, shadowLastChange needs to be accessible once a user has authenticated.

To view the Access Control List (ACL), use the ldapsearch utility:

ldapsearch -xLLL -b cn=config -D cn=admin,cn=config -W olcDatabase=hdb olcAccess
Enter LDAP Password: 
dn: olcDatabase={1}hdb,cn=config
olcAccess: {0}to attrs=userPassword,shadowLastChange by dn="cn=admin,dc=exampl
 e,dc=com" write by anonymous auth by self write by * none
olcAccess: {1}to dn.base="" by * read
olcAccess: {2}to * by dn="cn=admin,dc=example,dc=com" write by * read

TLS and SSL

When authenticating to an OpenLDAP server it is best to do so using an encrypted session. This can be accomplished using Transport Layer Security (TLS) and/or Secure Sockets Layer (SSL).

The first step in the process is to obtain or create a certificate. See “Certificates” and “Certification Authority” for details.

Once you have a certificate, key, and CA cert installed, use ldapmodify to add the new configuration options:

ldapmodify -x -D cn=admin,cn=config -W
Enter LDAP Password:
dn: cn=config
add: olcTLSCACertificateFile
olcTLSCACertificateFile: /etc/ssl/certs/cacert.pem
-
add: olcTLSCertificateFile
olcTLSCertificateFile: /etc/ssl/certs/server.crt
-
add: olcTLSCertificateKeyFile
olcTLSCertificateKeyFile: /etc/ssl/private/server.key

modifying entry "cn=config"

[Nota]

Adjust the server.crt, server.key, and cacert.pem names if yours are different.

Next, edit /etc/default/slapd uncomment the SLAPD_SERVICES option:

SLAPD_SERVICES="ldap:/// ldapi:/// ldaps:///"

Now the openldap user needs access to the certificate:

sudo adduser openldap ssl-cert
sudo chgrp ssl-cert /etc/ssl/private/server.key
[Nota]

If the /etc/ssl/private and /etc/ssl/private/server.key have different permissions, adjust the commands appropriately.

Finally, restart slapd:

sudo /etc/init.d/slapd restart

The slapd daemon should now be listening for LDAPS connections and be able to use STARTTLS during authentication.

TLS Replication

If you have setup Syncrepl between servers, it is prudent to encrypt the replication traffic using Transport Layer Security (TLS). For details on setting up replication see “LDAP replication”.

After setting up replication, and following the instructions in “TLS and SSL”, there are a couple of consequences that should be kept in mind:

  • The configuration only needs to be modified on one server.

  • The path names for the certificate and key must be the same on all servers.

So on each replicated server: install a certificate, edit /etc/default/slapd, and restart slapd.

Once TLS has been setup on each server, modify the cn=config replication by entering the following in a terminal:

ldapmodify -x -D cn=admin,cn=config -W
Enter LDAP Password: 
dn: olcDatabase={0}config,cn=config
replace: olcSyncrepl
olcSyncrepl: {0}rid=001 provider=ldap://ldap01.example.com binddn="cn=admin,cn
 =config" bindmethod=simple credentials=secret searchbase="cn=config" type=refre
 shAndPersist retry="5 5 300 5" timeout=1 starttls=yes
olcSyncrepl: {1}rid=002 provider=ldap://ldap02.example.com binddn="cn=admin,cn
 =config" bindmethod=simple credentials=secret searchbase="cn=config" type=refre
 shAndPersist retry="5 5 300 5" timeout=1 starttls=yes

modifying entry "olcDatabase={0}config,cn=config"

Now adjust the backend database replication:

ldapmodify -x -D cn=admin,cn=config -W
Enter LDAP Password:
dn: olcDatabase={1}hdb,cn=config
replace: olcSyncrepl
olcSyncrepl: {0}rid=003 provider=ldap://ldap01.example.com binddn="cn=admin,dc=example,dc=
 com" bindmethod=simple credentials=secret searchbase="dc=example,dc=com" type=r
 efreshOnly interval=00:00:00:10 retry="5 5 300 5" timeout=1 starttls=yes
olcSyncrepl: {1}rid=004 provider=ldap://ldap02.example.com binddn="cn=admin,dc=example,dc=
 com" bindmethod=simple credentials=secret searchbase="dc=example,dc=com" type=r
 efreshOnly interval=00:00:00:10 retry="5 5 300 5" timeout=1 starttls=yes

modifying entry "olcDatabase={1}hdb,cn=config"

If the LDAP server hostname does not match the Fully Qualified Domain Name (FQDN) in the certificate, you may have to edit /etc/ldap/ldap.conf and add the following TLS options:

TLS_CERT /etc/ssl/certs/server.crt
TLS_KEY /etc/ssl/private/server.key
TLS_CACERT /etc/ssl/certs/cacert.pem

Finally, restart slapd on each of the servers:

sudo /etc/init.d/slapd restart

LDAP Authentication

Once you have a working LDAP server, the auth-client-config and libnss-ldap packages take the pain out of configuring an Ubuntu client to authenticate using LDAP. To install the packages from, a terminal prompt enter:

sudo apt-get install libnss-ldap

During the install a menu dialog will ask you connection details about your LDAP server.

If you make a mistake when entering your information you can execute the dialog again using:

sudo dpkg-reconfigure ldap-auth-config

The results of the dialog can be seen in /etc/ldap.conf. If your server requires options not covered in the menu edit this file accordingly.

Now that libnss-ldap is configured enable the auth-client-config LDAP profile by entering:

sudo auth-client-config -t nss -p lac_ldap
  • -t: only modifies /etc/nsswitch.conf.

  • -p: name of the profile to enable, disable, etc.

  • lac_ldap: the auth-client-config profile that is part of the ldap-auth-config package.

Using the pam-auth-update utility, configure the system to use LDAP for authentication:

sudo pam-auth-update

From the pam-auth-update menu, choose LDAP and any other authentication mechanisms you need.

You should now be able to login using user credentials stored in the LDAP directory.

[Nota]

If you are going to use LDAP to store Samba users you will need to configure the server to authenticate using LDAP. See “Samba and LDAP” for details.

User and Group Management

The ldap-utils package comes with multiple utilities to manage the directory, but the long string of options needed, can make them a burden to use. The ldapscripts package contains configurable scripts to easily manage LDAP users and groups.

To install the package, from a terminal enter:

sudo apt-get install ldapscripts

Next, edit the config file /etc/ldapscripts/ldapscripts.conf uncommenting and changing the following to match your environment:

SERVER=localhost
BINDDN='cn=admin,dc=example,dc=com'
BINDPWDFILE="/etc/ldapscripts/ldapscripts.passwd"
SUFFIX='dc=example,dc=com'
GSUFFIX='ou=Groups'
USUFFIX='ou=People'
MSUFFIX='ou=Computers'
GIDSTART=10000
UIDSTART=10000
MIDSTART=10000

Now, create the ldapscripts.passwd file to allow authenticated access to the directory:

sudo sh -c "echo -n 'secret' > /etc/ldapscripts/ldapscripts.passwd"
sudo chmod 400 /etc/ldapscripts/ldapscripts.passwd
[Nota]

Replace “secret” with the actual password for your LDAP admin user.

The ldapscripts are now ready to help manage your directory. The following are some examples of how to use the scripts:

  • Create a new user:

    sudo ldapadduser george example
    

    This will create a user with uid george and set the user's primary group (gid) to example

  • Change a user's password:

    sudo ldapsetpasswd george
    Changing password for user uid=george,ou=People,dc=example,dc=com
    New Password: 
    New Password (verify): 
    
  • Delete a user:

    sudo ldapdeleteuser george
    
  • Add a group:

    sudo ldapaddgroup qa
    
  • Delete a group:

    sudo ldapdeletegroup qa
    
  • Add a user to a group:

    sudo ldapaddusertogroup george qa
    

    You should now see a memberUid attribute for the qa group with a value of george.

  • Remove a user from a group:

    sudo ldapdeleteuserfromgroup george qa
    

    The memberUid attribute should now be removed from the qa group.

  • The ldapmodifyuser script allows you to add, remove, or replace a user's attributes. The script uses the same syntax as the ldapmodify utility. For example:

    sudo ldapmodifyuser george
    # About to modify the following entry :
    dn: uid=george,ou=People,dc=example,dc=com
    objectClass: account
    objectClass: posixAccount
    cn: george
    uid: george
    uidNumber: 1001
    gidNumber: 1001
    homeDirectory: /home/george
    loginShell: /bin/bash
    gecos: george
    description: User account
    userPassword:: e1NTSEF9eXFsTFcyWlhwWkF1eGUybVdFWHZKRzJVMjFTSG9vcHk=
    
    # Enter your modifications here, end with CTRL-D.
    dn: uid=george,ou=People,dc=example,dc=com
    replace: gecos
    gecos: George Carlin
    

    The user's gecos should now be “George Carlin”.

  • Another great feature of ldapscripts, is the template system. Templates allow you to customize the attributes of user, group, and machine objectes. For example, to enable the user template edit /etc/ldapscripts/ldapscripts.conf changing:

    UTEMPLATE="/etc/ldapscripts/ldapadduser.template"
    

    There are sample templates in the /etc/ldapscripts directory. Copy or rename the ldapadduser.template.sample file to /etc/ldapscripts/ldapadduser.template:

    sudo cp /etc/ldapscripts/ldapadduser.template.sample /etc/ldapscripts/ldapadduser.template
    

    Edit the new template to add the desired attributes. The following will create new user's as with an objectClass of inetOrgPerson:

    dn: uid=<user>,<usuffix>,<suffix>
    objectClass: inetOrgPerson
    objectClass: posixAccount
    cn: <user>
    sn: <ask>
    uid: <user>
    uidNumber: <uid>
    gidNumber: <gid>
    homeDirectory: <home>
    loginShell: <shell>
    gecos: <user>
    description: User account
    title: Employee
    

    Notice the <ask> option used for the cn value. Using <ask> will configure ldapadduser to prompt you for the attribute value during user creation.

There are more useful scripts in the package, to see a full list enter: dpkg -L ldapscripts | grep bin

Resources

  • For more information see OpenLDAP Home Page

  • Though starting to show it's age, a great source for in depth LDAP information is O'Reilly's LDAP System Administration

  • Packt's Mastering OpenLDAP is a great reference covering newer versions of OpenLDAP.

  • For more information on auth-client-config see the man page: man auth-client-config.

  • For more details regarding the ldapscripts package see the man pages: man ldapscripts, man ldapadduser, man ldapaddgroup, etc.