Configure OpenLDAP

This material is work-in-progress ... Do not follow instructions here until this notice is removed. (Last edited by Dhorton on 5 May 2022.)

Lightweight Directory Access Protocol (LDAP) is often employed as an authentication mechanism, providing a common username and password across many different applications. This tutorial shows how to install and configure the OpenLDAP package on Alpine Linux.

Installing Packages

There is an Alpine package for OpenLDAP. However, simply adding the openldap apk is not enough to get you up and running. You'll also need to install a backend database and some LDAP command-line tools.

Here's how:

 apk add openldap openldap-back-mdb openldap-clients

But, before you start up the slapd service and go, there's a bit of configuration to do.

Customizing Configuration for OpenLDAP 2.3+

The Alpine OpenLDAP package can use either a configuration directory (slapd.d) or a configuration file (slapd.conf). Since OpenLDAP version 2.3, the preferred method is to use the slapd.d configuration directory. Any official OpenLDAP documentation, including their quickstart guide, will use this configuration method.

First, create the slapd.d directory with the proper ownership and permissions.

 install -m 755 -o ldap -g ldap -d /etc/openldap/slapd.d

Next, edit the slapd startup configuration to use the directory instead of the file.

1. Open up /etc/conf.d/slapd in your favorite editor
2. Comment out cfgfile="/etc/openldap/slapd.conf"
3. Uncomment cfgdir="/etc/openldap/slapd.d"

Finally, get rid of the included slapd.conf file.

 rm /etc/openldap/slapd.conf

Updating Shared Libraries File Names

Open up /etc/openldap/slapd.ldif in your favorite editor. Search for the file names ending with .la and change the extension to .so

Customizing Configuration for Your Domain

Your LDAP domain can be the same as your DNS domain or it can be completely different. Whatever you choose, be sure to use the LDAP naming convention of dc=domain,dc=tld rather than the dot separated DNS style of domain.tld

1. Edit slapd.ldif again.
2. Find the olcSuffix: keyword.
3. Change the value to match your domain
4. Find olcRootDN:
5. Change the value to match your domain

Import the Configuration

Before starting the slapd service, import the configuration into the backend database.

Use the slapadd command like this:

 slapadd -n 0 -F /etc/openldap/slapd.d -l /etc/openldap/slapd.ldif

There should be no errors, only a "Closing DB..." message.

Change ownership on the files or the slapd service will refuse to start.

 chown -R ldap:ldap /etc/openldap/slapd.d/*

Configuring the slapd Service

The pid directory is missing. We'll need to create it or the service won't start.

 install -m 755 -o ldap -g ldap -d /var/lib/openldap/run

Testing

The OpenLDAP quickstart guide uses the ldapsearch utility to test the configuration.

 ldapsearch -x -b  -s base '(objectclass=*)' namingContexts

You should see your domain.

You can also test with slapcat

 slapcat -n 0

This will dump the entire config database in LDIF format. You can also pipe to grep and specify your domain name to verify everything looks correct. When using grep, remember LDAP uses the format dc=domain,dc=com and not the more familiar domain.com.

Later, when you begin populating your LDAP database, you can use slapcat -n 1 to see your information. (Zero is the config database. Numbers above zero are user-defined databases.)

Finally, you can run netstat -tln and look for LDAP port 389 in the output.

Scripted Installation

If you want to automate the process, use the following script. Be sure to adjust the DOMAIN value to fit your needs.

 export DOMAIN="dc=contoso,dc=com"
 
 echo "Installing packages..."
 apk add openldap openldap-back-mdb openldap-clients
 
 echo "Configuring for v2.3+ style slapd.d config directory..."
 install -m 755 -o ldap -g ldap -d /etc/openldap/slapd.d
 sed -i~ \
 -e 's/^cfgfile=/#cfgfile=/' \
 -e 's/^#cfgdir=.*/cfgdir=\"\/etc\/openldap\/slapd.d\"/' \
 /etc/conf.d/slapd
 rm /etc/openldap/slapd.conf
 
 echo "Customizing for domain: ${DOMAIN}..."
 sed -i~ \
   -e 's/\.la$/.so/' \
   -e "s/dc=my-domain,dc=com/${DOMAIN}/" /etc/openldap/slapd.ldif
 
 echo "Importing configuration..."
 slapadd -n 0 -F /etc/openldap/slapd.d -l /etc/openldap/slapd.ldif
 chown -R ldap:ldap /etc/openldap/slapd.d/*
 
 echo "Configuring slapd service..."
 install -m 755 -o ldap -g ldap -d /var/lib/openldap/run
 service slapd start
 rc-update add slapd
 
 # To test, use ldapsearch:
 # ldapsearch -x -b  -s base '(objectclass=*)' namingContexts
 #
 # To wipe installation and redo:
 #   service slapd stop
 #   rm /etc/openldap/slapd.d/*
 #   rm /var/lib/openldap/openldap-data/*

Next Steps

Before you can use LDAP for authentication, you'll need to populate the database with user information. You may also want to enable secure communications with TLSSTART or LDAPS. Refer to the OpenLDAP project documentation to learn how to do this.

Reference

https://openldap.org/doc/admin26/quickstart.html