OpenLDAP Server

The Lightweight Directory Access Protocol, or LDAP, is a protocol for querying and modifying a X.500-based directory service running over TCP/IP. The current LDAP version is LDAPv3, as defined in RFC4510, and the LDAP implementation used in Ubuntu is OpenLDAP, currently at version 2.4.25 (Oneiric).

So this protocol accesses LDAP directories. Here are some key concepts and terms:

  • A LDAP directory is a tree of data entries that is hierarchical in nature and is called the Directory Information Tree (DIT).

  • An entry consists of a set of attributes.

  • An attribute has a type (a name/description) and one or more values.

  • Every attribute must be defined in at least one objectClass.

  • Attributes and objectclasses are defined in schemas (an objectclass is actually considered as a special kind of attribute).

  • Each entry has a unique identifier: it's Distinguished Name (DN or dn). This consists of it's Relative Distinguished Name (RDN) followed by the parent entry's DN.

  • The entry's DN is not an attribute. It is not considered part of the entry itself.

The terms object, container, and node have certain connotations but they all essentially mean the same thing as entry, the technically correct term.

For example, below we have a single entry consisting of 11 attributes. It's DN is "cn=John Doe,dc=example,dc=com"; it's RDN is "cn=John Doe"; and it's parent DN is "dc=example,dc=com".

 dn: cn=John Doe,dc=example,dc=com
 cn: John Doe
 givenName: John
 sn: Doe
 telephoneNumber: +1 888 555 6789
 telephoneNumber: +1 888 555 1232
 mail: john@example.com
 manager: cn=Larry Smith,dc=example,dc=com
 objectClass: inetOrgPerson
 objectClass: organizationalPerson
 objectClass: person
 objectClass: top

The above entry is in LDIF format (LDAP Data Interchange Format). Any information that you feed into your DIT must also be in such a format. It is defined in RFC2849.

Although this guide will describe how to use it for central authentication, LDAP is good for anything that involves a large number of access requests to a mostly-read, attribute-based (name:value) backend. Examples include an address book, a list of email addresses, and a mail server's configuration.

Installation

Install the OpenLDAP server daemon and the traditional LDAP management utilities. These are found in packages slapd and ldap-utils respectively.

The installation of slapd will create a working configuration. In particular, it will create a database instance that you can use to store your data. However, the suffix (or base DN) of this instance will be determined from the domain name of the localhost. If you want something different, edit /etc/hosts and replace the domain name with one that will give you the suffix you desire. For instance, if you want a suffix of dc=example,dc=com then your file would have a line similar to this:

127.0.1.1       hostname.example.com	hostname

You can revert the change after package installation.

This guide will use a database suffix of dc=example,dc=com.

Proceed with the install:

sudo apt-get install slapd ldap-utils

Since Ubuntu 8.10 slapd is designed to be configured within slapd itself by dedicating a separate DIT for that purpose. This allows one to dynamically configure slapd without the need to restart the service. This configuration database consists of a collection of text-based LDIF files located under /etc/ldap/slapd.d. This way of working is known by several names: the slapd-config method, the RTC method (Real Time Configuration), or the cn=config method. You can still use the traditional flat-file method (slapd.conf) but it's not recommended; the functionality will be eventually phased out.

Ubuntu now uses the slapd-config method for slapd configuration and this guide reflects that.

During the install you were prompted to define administrative credentials. These are LDAP-based credentials for the rootDN of your database instance. By default, this user's DN is cn=admin,dc=example,dc=com. Also by default, there is no administrative account created for the slapd-config database and you will therefore need to authenticate externally to LDAP in order to access it. We will see how to do this later on.

Some classical schemas (cosine, nis, inetorgperson) come built-in with slapd nowadays. There is also an included "core" schema, a pre-requisite for any schema to work.

Post-install Inspection

The installation process set up 2 DITs. One for slapd-config and one for your own data (dc=example,dc=com). Let's take a look.

  • This is what the slapd-config database/DIT looks like. Recall that this database is LDIF-based and lives under /etc/ldap/slapd.d:

    
        /etc/ldap/slapd.d/
    
    	├── cn=config
    	│   ├── cn=module{0}.ldif
    	│   ├── cn=schema
    	│   │   ├── cn={0}core.ldif
    	│   │   ├── cn={1}cosine.ldif
    	│   │   ├── cn={2}nis.ldif
    	│   │   └── cn={3}inetorgperson.ldif
    	│   ├── cn=schema.ldif
    	│   ├── olcBackend={0}hdb.ldif
    	│   ├── olcDatabase={0}config.ldif
    	│   ├── olcDatabase={-1}frontend.ldif
    	│   └── olcDatabase={1}hdb.ldif
    	└── cn=config.ldif
    
    

    Do not edit the slapd-config database directly. Make changes via the LDAP protocol (utilities).

  • This is what the slapd-config DIT looks like via the LDAP protocol:

    sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b cn=config dn
    
    dn: cn=config
    
    dn: cn=module{0},cn=config
    
    dn: cn=schema,cn=config
    
    dn: cn={0}core,cn=schema,cn=config
    
    dn: cn={1}cosine,cn=schema,cn=config
    
    dn: cn={2}nis,cn=schema,cn=config
    
    dn: cn={3}inetorgperson,cn=schema,cn=config
    
    dn: olcBackend={0}hdb,cn=config
    
    dn: olcDatabase={-1}frontend,cn=config
    
    dn: olcDatabase={0}config,cn=config
    
    dn: olcDatabase={1}hdb,cn=config
    
    

    Explanation of entries:

    • cn=config: global settings

    • cn=module{0},cn=config: a dynamically loaded module

    • cn=schema,cn=config: contains hard-coded system-level schema

    • cn={0}core,cn=schema,cn=config: the hard-coded core schema

    • cn={1}cosine,cn=schema,cn=config: the cosine schema

    • cn={2}nis,cn=schema,cn=config: the nis schema

    • cn={3}inetorgperson,cn=schema,cn=config: the inetorgperson schema

    • olcBackend={0}hdb,cn=config: the 'hdb' backend storage type

    • olcDatabase={-1}frontend,cn=config: frontend database, default settings for other databases

    • olcDatabase={0}config,cn=config: slapd configuration database (cn=config)

    • olcDatabase={1}hdb,cn=config: your database instance (dc=examle,dc=com)

  • This is what the dc=example,dc=com DIT looks like:

    ldapsearch -x -LLL -H ldap:/// -b dc=example,dc=com dn
    
    dn: dc=example,dc=com
    
    dn: cn=admin,dc=example,dc=com
    
    

    Explanation of entries:

    • dc=example,dc=com: base of the DIT

    • cn=admin,dc=example,dc=com: administrator (rootDN) for this DIT (set up during package install)

Modifying/Populating your Database

Let's introduce some content to our database. We will add the following:

  • a node called People (to store users)

  • a node called Groups (to store groups)

  • a group called miners

  • a user called john

Create the following LDIF file and call it add_content.ldif:

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

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

dn: cn=miners,ou=Groups,dc=example,dc=com
objectClass: posixGroup
cn: miners
gidNumber: 5000

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: 10000
gidNumber: 5000
userPassword: johnldap
gecos: John Doe
loginShell: /bin/bash
homeDirectory: /home/john

It's important that uid and gid values in your directory do not collide with local values. Use high number ranges, such as starting at 5000. By setting the uid and gid values in ldap high, you also allow for easier control of what can be done with a local user vs a ldap one. More on that later.

Add the content:

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

Enter LDAP Password: ********
adding new entry "ou=People,dc=example,dc=com"

adding new entry "ou=Groups,dc=example,dc=com"

adding new entry "cn=miners,ou=Groups,dc=example,dc=com"

adding new entry "uid=john,ou=People,dc=example,dc=com"

We can check that the information has been correctly added with the ldapsearch utility:

ldapsearch -x -LLL -b dc=example,dc=com 'uid=john' cn gidNumber

dn: uid=john,ou=People,dc=example,dc=com
cn: John Doe
gidNumber: 5000

Explanation of switches:

  • -x: "simple" binding; will not use the default SASL method

  • -LLL: disable printing extraneous information

  • uid=john: a "filter" to find the john user

  • cn gidNumber: requests certain attributes to be displayed (the default is to show all attributes)

Modifying the slapd Configuration Database

The slapd-config DIT can also be queried and modified. Here are a few examples.

  • Use ldapmodify to add an "Index" (DbIndex attribute) to your {1}hdb,cn=config database (dc=example,dc=com). Create a file, call it uid_index.ldif, with the following contents:

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

    Then issue the command:

    sudo ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f uid_index.ldif
    
    modifying entry "olcDatabase={1}hdb,cn=config"
    
    

    You can confirm the change in this way:

    sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
    cn=config '(olcDatabase={1}hdb)' olcDbIndex
    
    dn: olcDatabase={1}hdb,cn=config
    olcDbIndex: objectClass eq
    olcDbIndex: uid eq,pres,sub
    
    
  • Let's add a schema. It will first need to be converted to LDIF format. You can find unconverted schemas in addition to converted ones in the /etc/ldap/schema directory.

    • It is not trivial to remove a schema from the slapd-config database. Practice adding schemas on a test system.

    • Before adding any schema, you should check which schemas are already installed (shown is a default, out-of-the-box output):

      sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
      cn=schema,cn=config dn
      
      dn: cn=schema,cn=config
      
      dn: cn={0}core,cn=schema,cn=config
      
      dn: cn={1}cosine,cn=schema,cn=config
      
      dn: cn={2}nis,cn=schema,cn=config
      
      dn: cn={3}inetorgperson,cn=schema,cn=config
      
      

    In the following example we'll add the CORBA schema.

    1. Create the conversion configuration file schema_convert.conf 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
      include /etc/ldap/schema/ldapns.schema
      include /etc/ldap/schema/pmi.schema
      
    2. Create the output directory ldif_output.

    3. Determine the index of the schema:

      slapcat -f schema_convert.conf -F ldif_output -n 0 | grep corba,cn=schema
      
      cn={1}corba,cn=schema,cn=config
      
      

      When slapd injests objects with the same parent DN it will create an index for that object. An index is contained within braces: {X}.

    4. Use slapcat to perform the conversion:

      slapcat -f schema_convert.conf -F ldif_output -n0 -H \
      ldap:///cn={1}corba,cn=schema,cn=config -l cn=corba.ldif
      

      The converted schema is now in cn=corba.ldif

    5. Edit cn=corba.ldif to arrive at the following attributes:

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

      Also remove the following lines from the bottom:

      structuralObjectClass: olcSchemaConfig
      entryUUID: 52109a02-66ab-1030-8be2-bbf166230478
      creatorsName: cn=config
      createTimestamp: 20110829165435Z
      entryCSN: 20110829165435.935248Z#000000#000#000000
      modifiersName: cn=config
      modifyTimestamp: 20110829165435Z
      

      Your attribute values will vary.

    6. Finally, use ldapadd to add the new schema to the slapd-config DIT:

      sudo ldapadd -Q -Y EXTERNAL -H ldapi:/// -f cn\=corba.ldif
      
      adding new entry "cn=corba,cn=schema,cn=config"
      
      
    7. Confirm currently loaded schemas:

      sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b cn=schema,cn=config dn
      
      dn: cn=schema,cn=config
      
      dn: cn={0}core,cn=schema,cn=config
      
      dn: cn={1}cosine,cn=schema,cn=config
      
      dn: cn={2}nis,cn=schema,cn=config
      
      dn: cn={3}inetorgperson,cn=schema,cn=config
      
      dn: cn={4}corba,cn=schema,cn=config
      
      

For external applications and clients to authenticate using LDAP they will each need to be specifically configured to do so. Refer to the appropriate client-side documentation for details.

Logging

Activity logging for slapd is indispensible when implementing an OpenLDAP-based solution yet it must be manually enabled after software installation. Otherwise, only rudimentary messages will appear in the logs. Logging, like any other slapd configuration, is enabled via the slapd-config database.

OpenLDAP comes with multiple logging subsystems (levels) with each one containing the lower one (additive). A good level to try is stats. The slapd-config man page has more to say on the different subsystems.

Create the file logging.ldif with the following contents:

dn: cn=config
changetype: modify
add: olcLogLevel
olcLogLevel: stats

Implement the change:

sudo ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f logging.ldif

This will produce a significant amount of logging and you will want to throttle back to a less verbose level once your system is in production. While in this verbose mode your host's syslog engine (rsyslog) may have a hard time keeping up and may drop messages:

rsyslogd-2177: imuxsock lost 228 messages from pid 2547 due to rate-limiting

You may consider a change to rsyslog's configuration. In /etc/rsyslog.conf, put:

# Disable rate limiting
# (default is 200 messages in 5 seconds; below we make the 5 become 0)
$SystemLogRateLimitInterval 0

And then restart the rsyslog daemon:

sudo service rsyslog restart

Replication

The LDAP service becomes increasingly important as more networked systems begin to depend on it. In such an environment, it is standard practice to build redundancy (high availability) into LDAP to prevent havoc should the LDAP server become unresponsive. This is done through LDAP replication.

Replication is achieved via the Syncrepl engine. This allows changes to be synchronized using a Consumer - Provider model. The specific kind of replication we will implement in this guide is a combination of the following modes: refreshAndPersist and delta-syncrepl. This has the Provider push changed entries to the Consumer as soon as they're made but, in addition, only actual changes will be sent, not entire entries.

Provider Configuration

Begin by configuring the Provider.

  1. Create an LDIF file with the following contents and name it provider_sync.ldif:

    # Add indexes to the frontend db.
    dn: olcDatabase={1}hdb,cn=config
    changetype: modify
    add: olcDbIndex
    olcDbIndex: entryCSN eq
    -
    add: olcDbIndex
    olcDbIndex: entryUUID eq
    
    #Load the syncprov and accesslog modules.
    dn: cn=module{0},cn=config
    changetype: modify
    add: olcModuleLoad
    olcModuleLoad: syncprov
    -
    add: olcModuleLoad
    olcModuleLoad: accesslog
    
    # Accesslog database definitions
    dn: olcDatabase={2}hdb,cn=config
    objectClass: olcDatabaseConfig
    objectClass: olcHdbConfig
    olcDatabase: {2}hdb
    olcDbDirectory: /var/lib/ldap/accesslog
    olcSuffix: cn=accesslog
    olcRootDN: cn=admin,dc=example,dc=com
    olcDbIndex: default eq
    olcDbIndex: entryCSN,objectClass,reqEnd,reqResult,reqStart
    
    # Accesslog db syncprov.
    dn: olcOverlay=syncprov,olcDatabase={2}hdb,cn=config
    changetype: add
    objectClass: olcOverlayConfig
    objectClass: olcSyncProvConfig
    olcOverlay: syncprov
    olcSpNoPresent: TRUE
    olcSpReloadHint: TRUE
    
    # syncrepl Provider for primary db
    dn: olcOverlay=syncprov,olcDatabase={1}hdb,cn=config
    changetype: add
    objectClass: olcOverlayConfig
    objectClass: olcSyncProvConfig
    olcOverlay: syncprov
    olcSpNoPresent: TRUE
    
    # accesslog overlay definitions for primary db
    dn: olcOverlay=accesslog,olcDatabase={1}hdb,cn=config
    objectClass: olcOverlayConfig
    objectClass: olcAccessLogConfig
    olcOverlay: accesslog
    olcAccessLogDB: cn=accesslog
    olcAccessLogOps: writes
    olcAccessLogSuccess: TRUE
    # scan the accesslog DB every day, and purge entries older than 7 days
    olcAccessLogPurge: 07+00:00 01+00:00
    

    Change the rootDN in the LDIF file to match the one you have for your directory.

  2. The apparmor profile for slapd will need to be adjusted for the accesslog database location. Edit /etc/apparmor.d/local/usr.sbin.slapd by adding the following:

    /var/lib/ldap/accesslog/ r,
    /var/lib/ldap/accesslog/** rwk,
    

    Create a directory, set up a databse config file, and reload the apparmor profile:

    sudo -u openldap mkdir /var/lib/ldap/accesslog
    sudo -u openldap cp /var/lib/ldap/DB_CONFIG /var/lib/ldap/accesslog
    sudo service apparmor reload
    
  3. Add the new content and, due to the apparmor change, restart the daemon:

    sudo ldapadd -Q -Y EXTERNAL -H ldapi:/// -f provider_sync.ldif
    sudo service slapd restart
    

The Provider is now configured.

Consumer Configuration

And now configure the Consumer.

  1. Install the software by going through Installation. Make sure the slapd-config databse is identical to the Provider's. In particular, make sure schemas and the databse suffix are the same.

  2. Create an LDIF file with the following contents and name it consumer_sync.ldif:

    dn: cn=module{0},cn=config
    changetype: modify
    add: olcModuleLoad
    olcModuleLoad: syncprov
    
    dn: olcDatabase={1}hdb,cn=config
    changetype: modify
    add: olcDbIndex
    olcDbIndex: entryUUID eq
    -
    add: olcSyncRepl
    olcSyncRepl: rid=0 provider=ldap://ldap01.example.com bindmethod=simple binddn="cn=admin,dc=example,dc=com" 
     credentials=secret searchbase="dc=example,dc=com" logbase="cn=accesslog" 
     logfilter="(&(objectClass=auditWriteObject)(reqResult=0))" schemachecking=on 
     type=refreshAndPersist retry="60 +" syncdata=accesslog
    -
    add: olcUpdateRef
    olcUpdateRef: ldap://ldap01.example.com
    

    Ensure the following attributes have the correct values:

    • provider (Provider server's hostname -- ldap01.example.com in this example -- or IP address)

    • binddn (the admin DN you're using)

    • credentials (the admin DN password you're using)

    • searchbase (the database suffix you're using)

    • olcUpdateRef (Provider server's hostname or IP address)

    • rid (Replica ID, an unique 3-digit that identifies the replica. Each consumer should have at least one rid)

  3. Add the new content:

    sudo ldapadd -Q -Y EXTERNAL -H ldapi:/// -f consumer_sync.ldif
    

You're done. The two databases (suffix: dc=example,dc=com) should now be synchronizing.

Testing

Once replication starts, you can monitor it by running

ldapsearch -z1 -LLLQY EXTERNAL -H ldapi:/// -s base contextCSN

dn: dc=example,dc=com
contextCSN: 20120201193408.178454Z#000000#000#000000

on both the provider and the consumer. Once the output (20120201193408.178454Z#000000#000#000000 in the above example) for both machines match, you have replication. Every time a change is done in the provider, this value will change and so should the one in the consumer(s).

If your connection is slow and/or your ldap database large, it might take a while for the consumer's contextCSN match the provider's. But, you will know it is progressing since the consumer's contextCSN will be steadly increasing.

If the consumer's contextCSN is missing or does not match the provider, you should stop and figure out the issue before continuing. Try checking the slapd (syslog) and the auth log files in the provider to see if the consumer's authentication requests were successful or its requests to retrieve data (they look like a lot of ldapsearch statements) return no errors.

To test if it worked simply query, on the Consumer, the DNs in the database:

sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b dc=example,dc=com dn

You should see the user 'john' and the group 'miners' as well as the nodes 'People' and 'Groups'.

Access Control

The management of what type of access (read, write, etc) users should be granted to resources is known as access control. The configuration directives involved are called access control lists or ACL.

When we installed the slapd package various ACL were set up automatically. We will look at a few important consequences of those defaults and, in so doing, we'll get an idea of how ACLs work and how they're configured.

To get the effective ACL for an LDAP query we need to look at the ACL entries of the database being queried as well as those of the special frontend database instance. The ACLs belonging to the latter act as defaults in case those of the former do not match. The frontend database is the second to be consulted and the ACL to be applied is the first to match ("first match wins") among these 2 ACL sources. The following commands will give, respectively, the ACLs of the hdb database ("dc=example,dc=com") and those of the frontend database:

sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcDatabase={1}hdb)' olcAccess

dn: olcDatabase={1}hdb,cn=config
olcAccess: {0}to attrs=userPassword,shadowLastChange by self write by anonymous
              auth by dn="cn=admin,dc=example,dc=com" write by * none
olcAccess: {1}to dn.base="" by * read
olcAccess: {2}to * by self write by dn="cn=admin,dc=example,dc=com" write by *
  read

The rootDN always has full rights to it's database. Including it in an ACL does provide an explicit configuration but it also causes slapd to incur a performance penalty.

sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcDatabase={-1}frontend)' olcAccess

dn: olcDatabase={-1}frontend,cn=config
olcAccess: {0}to * by dn.exact=gidNumber=0+uidNumber=0,cn=peercred,
              cn=external,cn=auth manage by * break
olcAccess: {1}to dn.exact="" by * read
olcAccess: {2}to dn.base="cn=Subschema" by * read

The very first ACL is crucial:

olcAccess: {0}to attrs=userPassword,shadowLastChange by self write by anonymous
              auth by dn="cn=admin,dc=example,dc=com" write by * none

This can be represented differently for easier digestion:

to attrs=userPassword
	by self write
	by anonymous auth
	by dn="cn=admin,dc=example,dc=com" write
	by * none

to attrs=shadowLastChange
	by self write
	by anonymous auth
	by dn="cn=admin,dc=example,dc=com" write
	by * none

This compound ACL (there are 2) enforces the following:

  • Anonymous 'auth' access is provided to the userPassword attribute for the initial connection to occur. Perhaps counter-intuitively, 'by anonymous auth' is needed even when anonymous access to the DIT is unwanted. Once the remote end is connected, howerver, authentication can occur (see next point).

  • Authentication can happen because all users have 'read' (due to 'by self write') access to the userPassword attribute.

  • The userPassword attribute is otherwise unaccessible by all other users, with the exception of the rootDN, who has complete access to it.

  • In order for users to change their own password, using passwd or other utilities, the shadowLastChange attribute needs to be accessible once a user has authenticated.

This DIT can be searched anonymously because of 'by * read' in this ACL:

to *
	by self write
	by dn="cn=admin,dc=example,dc=com" write
	by * read

If this is unwanted then you need to change the ACLs. To force authentication during a bind request you can alternatively (or in combination with the modified ACL) use the 'olcRequire: authc' directive.

As previously mentioned, there is no administrative account created for the slapd-config database. There is, however, a SASL identity that is granted full access to it. It represents the localhost's superuser (root/sudo). Here it is:

dn.exact=gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth 

The following command will display the ACLs of the slapd-config database:

sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcDatabase={0}config)' olcAccess

dn: olcDatabase={0}config,cn=config
olcAccess: {0}to * by dn.exact=gidNumber=0+uidNumber=0,cn=peercred,
              cn=external,cn=auth manage by * break

Since this is a SASL identity we need to use a SASL mechanism when invoking the LDAP utility in question and and we have seen it plenty of times in this guide. It is the EXTERNAL mechanism. See the previous command for an example. Note that:

  1. You must use sudo to become the root identity in order for the ACL to match.

  2. The EXTERNAL mechanism works via IPC (UNIX domain sockets). This means you must use the ldapi URI format.

A succinct way to get all the ACLs is like this:

sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcAccess=*)' olcAccess olcSuffix

There is much to say on the topic of access control. See the man page for slapd.access.

TLS

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).

Here, we will be our own Certificate Authority and then create and sign our LDAP server certificate as that CA. Since slapd is compiled using the gnutls library, we will use the certtool utility to complete these tasks.

  1. Install the gnutls-bin and ssl-cert packages:

    sudo apt-get install gnutls-bin ssl-cert
    
  2. Create a private key for the Certificate Authority:

    sudo sh -c "certtool --generate-privkey > /etc/ssl/private/cakey.pem"
    
  3. Create the template/file /etc/ssl/ca.info to define the CA:

    cn = Example Company
    ca
    cert_signing_key
    
  4. Create the self-signed CA certificate:

    sudo certtool --generate-self-signed \
    --load-privkey /etc/ssl/private/cakey.pem \ 
    --template /etc/ssl/ca.info \
    --outfile /etc/ssl/certs/cacert.pem
    
  5. Make a private key for the server:

    sudo certtool --generate-privkey \
    --bits 1024 \
    --outfile /etc/ssl/private/ldap01_slapd_key.pem
    

    Replace ldap01 in the filename with your server's hostname. Naming the certificate and key for the host and service that will be using them will help keep things clear.

  6. Create the /etc/ssl/ldap01.info info file containing:

    organization = Example Company
    cn = ldap01.example.com
    tls_www_server
    encryption_key
    signing_key
    expiration_days = 3650
    

    The above certificate is good for 10 years. Adjust accordingly.

  7. Create the server's certificate:

    sudo certtool --generate-certificate \
    --load-privkey /etc/ssl/private/ldap01_slapd_key.pem \
    --load-ca-certificate /etc/ssl/certs/cacert.pem \
    --load-ca-privkey /etc/ssl/private/cakey.pem \
    --template /etc/ssl/ldap01.info \
    --outfile /etc/ssl/certs/ldap01_slapd_cert.pem
    

Create the file certinfo.ldif with the following contents (adjust accordingly, our example assumes we created certs using https://www.cacert.org):

dn: cn=config
add: olcTLSCACertificateFile
olcTLSCACertificateFile: /etc/ssl/certs/cacert.pem
-
add: olcTLSCertificateFile
olcTLSCertificateFile: /etc/ssl/certs/ldap01_slapd_cert.pem
-
add: olcTLSCertificateKeyFile
olcTLSCertificateKeyFile: /etc/ssl/private/ldap01_slapd_key.pem

Use the ldapmodify command to tell slapd about our TLS work via the slapd-config database:

sudo ldapmodify -Y EXTERNAL -H ldapi:/// -f /etc/ssl/certinfo.ldif

Contratry to popular belief, you do not need ldaps:// in /etc/default/slapd in order to use encryption. You should have just:

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

LDAP over TLS/SSL (ldaps://) is deprecated in favour of StartTLS. The latter refers to an existing LDAP session (listening on TCP port 389) becoming protected by TLS/SSL whereas LDAPS, like HTTPS, is a distinct encrypted-from-the-start protocol that operates over TCP port 636.

Tighten up ownership and permissions:

sudo adduser openldap ssl-cert
sudo chgrp ssl-cert /etc/ssl/private/ldap01_slapd_key.pem
sudo chmod g+r /etc/ssl/private/ldap01_slapd_key.pem
sudo chmod o-r /etc/ssl/private/ldap01_slapd_key.pem

Restart OpenLDAP:

sudo service slapd restart

Check your host's logs (/var/log/syslog) to see if the server has started properly.

Replication and TLS

If you have set up replication between servers, it is common practice to encrypt (StartTLS) the replication traffic to prevent evesdropping. This is distinct from using encryption with authentication as we did above. In this section we will build on that TLS-authentication work.

The assumption here is that you have set up replication between Provider and Consumer according to Replication and have configured TLS for authentication on the Provider by following TLS.

As previously stated, the objective (for us) with replication is high availablity for the LDAP service. Since we have TLS for authentication on the Provider we will require the same on the Consumer. In addition to this, however, we want to encrypt replication traffic. What remains to be done is to create a key and certificate for the Consumer and then configure accordingly. We will generate the key/certificate on the Provider, to avoid having to create another CA certificate, and then transfer the necessary material over to the Consumer.

  1. On the Provider,

    Create a holding directory (which will be used for the eventual transfer) and then the Consumer's private key:

    mkdir ldap02-ssl
    cd ldap02-ssl
    sudo certtool --generate-privkey \
    --bits 1024 \
    --outfile ldap02_slapd_key.pem
    

    Create an info file, ldap02.info, for the Consumer server, adjusting it's values accordingly:

    organization = Example Company
    cn = ldap02.example.com
    tls_www_server
    encryption_key
    signing_key
    expiration_days = 3650
    

    Create the Consumer's certificate:

    sudo certtool --generate-certificate \
    --load-privkey ldap02_slapd_key.pem \
    --load-ca-certificate /etc/ssl/certs/cacert.pem \
    --load-ca-privkey /etc/ssl/private/cakey.pem \
    --template ldap02.info \
    --outfile ldap02_slapd_cert.pem
    

    Get a copy of the CA certificate:

    cp /etc/ssl/certs/cacert.pem .
    

    We're done. Now transfer the ldap02-ssl directory to the Consumer. Here we use scp (adjust accordingly):

    cd ..
    scp -r ldap02-ssl user@consumer:
    
  2. On the Consumer,

    Configure TLS authentication:

    sudo apt-get install ssl-cert
    sudo adduser openldap ssl-cert
    sudo cp ldap02_slapd_cert.pem cacert.pem /etc/ssl/certs
    sudo cp ldap02_slapd_key.pem /etc/ssl/private
    sudo chgrp ssl-cert /etc/ssl/private/ldap02_slapd_key.pem
    sudo chmod g+r /etc/ssl/private/ldap02_slapd_key.pem
    sudo chmod o-r /etc/ssl/private/ldap02_slapd_key.pem
    

    Create the file /etc/ssl/certinfo.ldif with the following contents (adjust accordingly):

    dn: cn=config
    add: olcTLSCACertificateFile
    olcTLSCACertificateFile: /etc/ssl/certs/cacert.pem
    -
    add: olcTLSCertificateFile
    olcTLSCertificateFile: /etc/ssl/certs/ldap02_slapd_cert.pem
    -
    add: olcTLSCertificateKeyFile
    olcTLSCertificateKeyFile: /etc/ssl/private/ldap02_slapd_key.pem
    

    Configure the slapd-config database:

    sudo ldapmodify -Y EXTERNAL -H ldapi:/// -f certinfo.ldif
    

    Configure /etc/default/slapd as on the Provider (SLAPD_SERVICES).

  3. On the Consumer,

    Configure TLS for Consumer-side replication. Modify the existing olcSyncrepl attribute by tacking on some TLS options. In so doing, we will see, for the first time, how to change an attribute's value(s).

    Create the file consumer_sync_tls.ldif with the following contents:

    dn: olcDatabase={1}hdb,cn=config
    replace: olcSyncRepl
    olcSyncRepl: rid=0 provider=ldap://ldap01.example.com bindmethod=simple
     binddn="cn=admin,dc=example,dc=com" credentials=secret searchbase="dc=example,dc=com"
     logbase="cn=accesslog" logfilter="(&(objectClass=auditWriteObject)(reqResult=0))"
     schemachecking=on type=refreshAndPersist retry="60 +" syncdata=accesslog
     starttls=critical tls_reqcert=demand
    

    The extra options specify, respectively, that the consumer must use StartTLS and that the CA certificate is required to verify the Provider's identity. Also note the LDIF syntax for changing the values of an attribute ('replace').

    Implement these changes:

    sudo ldapmodify -Y EXTERNAL -H ldapi:/// -f consumer_sync_tls.ldif
    

    And restart slapd:

    sudo service slapd restart
    
  4. On the Provider,

    Check to see that a TLS session has been established. In /var/log/syslog, providing you have 'conns'-level logging set up, you should see messages similar to:

    slapd[3620]: conn=1047 fd=20 ACCEPT from IP=10.153.107.229:57922 (IP=0.0.0.0:389)
    slapd[3620]: conn=1047 op=0 EXT oid=1.3.6.1.4.1.1466.20037
    slapd[3620]: conn=1047 op=0 STARTTLS
    slapd[3620]: conn=1047 op=0 RESULT oid= err=0 text=
    slapd[3620]: conn=1047 fd=20 TLS established tls_ssf=128 ssf=128
    slapd[3620]: conn=1047 op=1 BIND dn="cn=admin,dc=example,dc=com" method=128
    slapd[3620]: conn=1047 op=1 BIND dn="cn=admin,dc=example,dc=com" mech=SIMPLE ssf=0
    slapd[3620]: conn=1047 op=1 RESULT tag=97 err=0 text
    

LDAP Authentication

Once you have a working LDAP server, you will need to install libraries on the client that will know how and when to contact it. On Ubuntu, this has been traditionally accomplishd by installing the libnss-ldap package. This package will bring in other tools that will assist you in the configuration step. Install this package now:

sudo apt-get install libnss-ldap

You will be prompted for details of your LDAP server. If you make a mistake you can try 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 configure the LDAP profile for NSS:

sudo auth-client-config -t nss -p lac_ldap

Configure the system to use LDAP for authentication:

sudo pam-auth-update

From the menu, choose LDAP and any other authentication mechanisms you need.

You should now be able to log in using LDAP-based credentials.

LDAP clients will need to refer to multiple servers if replication is in use. In /etc/ldap.conf you would have something like:

uri ldap://ldap01.example.com ldap://ldap02.example.com

The request will time out and the Consumer (ldap02) will attempt to be reached if the Provider (ldap01) becomes unresponsive.

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

An alternative to the libnss-ldap package is the libnss-ldapd package. This, however, will bring in the nscd package which is problably not wanted. Simply remove it afterwards.

User and Group Management

The ldap-utils package comes with enough utilities to manage the directory but the long string of options needed can make them a burden to use. The ldapscripts package contains wrapper scripts to these utilities that some people find easier to use.

Install the package:

sudo apt-get install ldapscripts

Then edit the file /etc/ldapscripts/ldapscripts.conf to arrive at something similar to the following:

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 rootDN access to the directory:

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

Replace “secret” with the actual password for your database's rootDN user.

The scripts are now ready to help manage your directory. Here are some examples of how to use them:

  • 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”.

  • A nice 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 /usr/share/doc/ldapscripts/examples/ldapadduser.template.sample \
    /etc/ldapscripts/ldapadduser.template
    

    Edit the new template to add the desired attributes. The following will create new users 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 sn attribute. This will make ldapadduser prompt you for it's value.

There are utilities in the package that were not covered here. Here is a complete list:

Backup and Restore

Now we have ldap running just the way we want, it is time to ensure we can save all of our work and restore it as needed.

What we need is a way to backup the ldap database(s), specifically the backend (cn=config) and frontend (dc=example,dc=com). If we are going to backup those databases into, say, /export/backup, we could use slapcat as shown in the following script, called /usr/local/bin/ldapbackup:

#!/bin/bash

BACKUP_PATH=/export/backup
SLAPCAT=/usr/sbin/slapcat

nice ${SLAPCAT} -n 0 > ${BACKUP_PATH}/config.ldif
nice ${SLAPCAT} -n 1 > ${BACKUP_PATH}/example.com.ldif
nice ${SLAPCAT} -n 2 > ${BACKUP_PATH}/access.ldif
chmod 640 ${BACKUP_PATH}/*.ldif

These files are uncompressed text files containing everything in your ldap databases including the tree layout, usernames, and every password. So, you might want to consider making /export/backup an encrypted partition and even having the script encrypt those files as it creates them. Ideally you should do both, but that depends on your security requirements.

Then, it is just a matter of having a cron script to run this program as often as we feel comfortable with. For many, once a day suffices. For others, more often is required. Here is an example of a cron script called /etc/cron.d/ldapbackup that is run every night at 22:45h:

MAILTO=backup-emails@domain.com
45 22 * * *  root    /usr/local/bin/ldapbackup

Now the files are created, they should be copied to a backup server.

Assuming we did a fresh reinstall of ldap, the restore process could be something like this:

sudo service slapd stop
sudo mkdir /var/lib/ldap/accesslog
sudo slapadd -F /etc/ldap/slapd.d -n 0 -l /export/backup/config.ldif
sudo slapadd -F /etc/ldap/slapd.d -n 1 -l /export/backup/domain.com.ldif
sudo slapadd -F /etc/ldap/slapd.d -n 2 -l /export/backup/access.ldif
sudo chown -R openldap:openldap /etc/ldap/slapd.d/
sudo chown -R openldap:openldap /var/lib/ldap/
sudo service slapd start

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