Install and configure LDAP

Installing slapd (the Stand-alone LDAP Daemon) creates a minimal working configuration with a top level entry, and an administrator’s Distinguished Name (DN).

In particular, it creates 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 host. If you want something different, you can change it right after the installation (before it contains any useful data).

Note: This guide will use a database suffix of dc=example,dc=com. You can change this to match your particular setup.

Install slapd

You can install the server and the main command line utilities with the following command:

sudo apt install slapd ldap-utils

Change the instance suffix (optional)

If you want to change your Directory Information Tree (DIT) suffix, now would be a good time since changing it discards your existing one. To change the suffix, run the following command:

sudo dpkg-reconfigure slapd

To switch your DIT suffix to dc=example,dc=com, for example, so you can follow this guide more closely, answer example.com when asked about the DNS domain name.

Throughout this guide we will issue many commands with the LDAP utilities. To save some typing, we can configure the OpenLDAP libraries with certain defaults in /etc/ldap/ldap.conf (adjust these entries for your server name and directory suffix):

BASE dc=example,dc=com
URI ldap://ldap01.example.com

Configuration options

slapd is designed to be configured within the service itself by dedicating a separate DIT for that purpose. This allows for dynamic configuration of slapd without needing to restart the service or edit config files. This configuration database consists of a collection of text-based LDIF files located under /etc/ldap/slapd.d, but these should never be edited directly. This way of working is known by several names: the “slapd-config” method, the “Real Time Configuration (RTC)” method, or the “cn=config” method. You can still use the traditional flat-file method (slapd.conf) but that will not be covered in this guide.

Right after installation, you will get two databases, or suffixes: one for your data, which is based on your host’s domain (dc=example,dc=com), and one for your configuration, with its root at cn=config. To change the data on each we need different credentials and access methods:

  • dc=example,dc=com The administrative user for this suffix is cn=admin,dc=example,dc=com and its password is the one selected during the installation of the slapd package.

  • cn=config The configuration of slapd itself is stored under this suffix. Changes to it can be made by the special DN gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth. This is how the local system’s root user (uid=0/gid=0) is seen by the directory when using SASL EXTERNAL authentication through the ldapi:/// transport via the /run/slapd/ldapi Unix socket. Essentially what this means is that only the local root user can update the cn=config database. More details later.

Example slapd-config DIT

This is what the slapd-config DIT looks like via the LDAP protocol (listing only the DNs):

$ 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: olcDatabase={-1}frontend,cn=config
dn: olcDatabase={0}config,cn=config
dn: olcDatabase={1}mdb,cn=config

Where the entries mean the following:

  • 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 Network Information Services (NIS) schema

  • cn={3}inetorgperson,cn=schema,cn=config: The InetOrgPerson schema

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

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

  • olcDatabase={1}mdb,cn=config: Your database instance (dc=example,dc=com)

Example dc=example,dc=com DIT

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

Where the entries mean the following:

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

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

Notice how we used two different authentication mechanisms:

  • -x This is called a “simple bind”, and is essentially a plain text authentication. Since no Bind DN was provided (via -D), this became an anonymous bind. Without -x, the default is to use a Simple Authentication Security Layer (SASL) bind.

  • -Y EXTERNAL This is using a SASL bind (no -x was provided), and further specifying the EXTERNAL type. Together with -H ldapi:///, this uses a local UNIX socket connection.

In both cases we only got the results that the server access-control lists (ACLs) allowed us to see, based on who we are. A very handy tool to verify the authentication is ldapwhoami, which can be used as follows:

$ ldapwhoami -x

anonymous

$ ldapwhoami -x -D cn=admin,dc=example,dc=com -W

Enter LDAP Password:
dn:cn=admin,dc=example,dc=com

When you use simple bind (-x) and specify a Bind DN with -D as your authentication DN, the server will look for a userPassword attribute in the entry, and use that to verify the credentials. In this particular case above, we used the database Root DN entry, i.e., the actual administrator, and that is a special case whose password is set in the configuration when the package is installed.

Note: A simple bind without some sort of transport security mechanism is clear text, meaning the credentials are transmitted in the clear. You should add Transport Layer Security (TLS) support to your OpenLDAP server as soon as possible.

Example SASL EXTERNAL

Here are the SASL EXTERNAL examples:

$ ldapwhoami -Y EXTERNAL -H ldapi:/// -Q

dn:gidNumber=1000+uidNumber=1000,cn=peercred,cn=external,cn=auth

$ sudo ldapwhoami -Y EXTERNAL -H ldapi:/// -Q

dn:gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth

When using SASL EXTERNAL via the ldapi:/// transport, the Bind DN becomes a combination of the uid and gid of the connecting user, followed by the suffix cn=peercred,cn=external,cn=auth. The server ACLs know about this, and grant the local root user complete write access to cn=config via the SASL mechanism.

Populate the directory

Let’s introduce some content to our directory. We will add the following:

  • A node called People, to store users

    • A user called john

  • A node called Groups, to store groups

    • A group called miners

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: {CRYPT}x
gecos: John Doe
loginShell: /bin/bash
homeDirectory: /home/john

Note: It’s important that uid and gid values in your directory do not collide with local values. You can use high number ranges, such as starting at 5000 or even higher.

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. For example, let’s search for the “john” entry, and request the cn and gidnumber attributes:

$ 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

Here we used an LDAP “filter”: (uid=john). LDAP filters are very flexible and can become complex. For example, to list the group names of which john is a member, we could use the filter:

(&(objectClass=posixGroup)(memberUid=john))

That is a logical “AND” between two attributes. Filters are very important in LDAP and mastering their syntax is extremely helpful. They are used for simple queries like this, but can also select what content is to be replicated to a secondary server, or even in complex ACLs. The full specification is defined in RFC 4515.

Notice we set the userPassword field for the “john” entry to the cryptic value {CRYPT}x. This essentially is an invalid password, because no hashing will produce just x. It’s a common pattern when adding a user entry without a default password. To change the password to something valid, you can now use ldappasswd:

$ ldappasswd -x -D cn=admin,dc=example,dc=com -W -S uid=john,ou=people,dc=example,dc=com

New password:
Re-enter new password:
Enter LDAP Password:

Note: Remember that simple binds are insecure and you should add TLS support to your server as soon as possible!

Change the configuration

The slapd-config DIT can also be queried and modified. Here are some common operations.

Add an index

Use ldapmodify to add an “Index” to your {1}mdb,cn=config database definition (for dc=example,dc=com). Create a file called uid_index.ldif, and add the following contents:

dn: olcDatabase={1}mdb,cn=config
add: olcDbIndex
olcDbIndex: mail eq,sub

Then issue the command:

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

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

You can confirm the change in this way:

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

dn: olcDatabase={1}mdb,cn=config
olcDbIndex: objectClass eq
olcDbIndex: cn,uid eq
olcDbIndex: uidNumber,gidNumber eq
olcDbIndex: member,memberUid eq
olcDbIndex: mail eq,sub

Change the RootDN password:

First, run slappasswd to get the hash for the new password you want:

$ slappasswd

New password:
Re-enter new password:
{SSHA}VKrYMxlSKhONGRpC6rnASKNmXG2xHXFo

Now prepare a changerootpw.ldif file with this content:

dn: olcDatabase={1}mdb,cn=config
changetype: modify
replace: olcRootPW
olcRootPW: {SSHA}VKrYMxlSKhONGRpC6rnASKNmXG2xHXFo

Finally, run the ldapmodify command:

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

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

We still have the actual cn=admin,dc=example,dc=com DN in the dc=example,dc=com database, so let’s change that too. Since this is a regular entry in this database suffix, we can use ldappasswd:

$ ldappasswd -x -D cn=admin,dc=example,dc=com -W -S

New password:
Re-enter new password:
Enter LDAP Password:  <-- current password, about to be changed

Add a schema

Schemas can only be added to cn=config if they are in LDIF format. If not, they will first have to be converted. You can find unconverted schemas in addition to converted ones in the /etc/ldap/schema directory.

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

In the following example we’ll add one of the pre-installed policy schemas in /etc/ldap/schema/. The pre-installed schemas exists in both converted (.ldif) and native (.schema) formats, so we don’t have to convert them and can use ldapadd directly:

$ sudo ldapadd -Q -Y EXTERNAL -H ldapi:/// -f /etc/ldap/schema/corba.ldif

adding new entry "cn=corba,cn=schema,cn=config"

If the schema you want to add does not exist in LDIF format, a nice conversion tool that can be used is provided in the schema2ldif package.

Logging

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

OpenLDAP comes with multiple logging levels, with each level 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.

Example logging with the stats level

Create the file logging.ldif with the following contents:

dn: cn=config
changetype: modify
replace: 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 revert 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 like this:

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 systemctl restart syslog.service

Next steps

Now that you have successfully installed LDAP, you may want to set up users and groups, or find out more about access control.