2. Configuring LDAP

LDAP stands for Lightweight Directory Access Protocol and is a subset of the X.500 Directory Access Protocol. Its most recent specifications are in RFC4510 and friends. Essentially it is a database that expects to be read from more often than it is written to.

The LDAP server OpenLDAP will be used in the examples in this document; while the principles here should be generally applicable to many different servers, most of the concrete administration is OpenLDAP-specific. There are several server versions in ports, for example net/openldap24-server. Client servers will need the corresponding net/openldap24-client libraries.

There are (basically) two areas of the LDAP service which need configuration. The first is setting up a server to receive connections properly, and the second is adding entries to the server's directory so that FreeBSD tools know how to interact with it.

2.1. Setting Up the Server for Connections

Note:

This section is specific to OpenLDAP. If you are using another server, you will need to consult that server's documentation.

2.1.1. Installing OpenLDAP

First, install OpenLDAP:

Example 1. Installing OpenLDAP
# cd /usr/ports/net/openldap24-server
# make install clean

This installs the slapd and slurpd binaries, along with the required OpenLDAP libraries.

2.1.2. Configuring OpenLDAP

Next we must configure OpenLDAP.

You will want to require encryption in your connections to the LDAP server; otherwise your users' passwords will be transferred in plain text, which is considered insecure. The tools we will be using support two very similar kinds of encryption, SSL and TLS.

TLS stands for Transportation Layer Security. Services that employ TLS tend to connect on the same ports as the same services without TLS; thus an SMTP server which supports TLS will listen for connections on port 25, and an LDAP server will listen on 389.

SSL stands for Secure Sockets Layer, and services that implement SSL do not listen on the same ports as their non-SSL counterparts. Thus SMTPS listens on port 465 (not 25), HTTPS listens on 443, and LDAPS on 636.

The reason SSL uses a different port than TLS is because a TLS connection begins as plain text, and switches to encrypted traffic after the STARTTLS directive. SSL connections are encrypted from the beginning. Other than that there are no substantial differences between the two.

Note:

We will adjust OpenLDAP to use TLS, as SSL is considered deprecated.

Once OpenLDAP is installed via ports, the following configuration parameters in /usr/local/etc/openldap/slapd.conf will enable TLS:

security ssf=128

TLSCertificateFile /path/to/your/cert.crt
TLSCertificateKeyFile /path/to/your/cert.key
TLSCACertificateFile /path/to/your/cacert.crt

Here, ssf=128 tells OpenLDAP to require 128-bit encryption for all connections, both search and update. This parameter may be configured based on the security needs of your site, but rarely you need to weaken it, as most LDAP client libraries support strong encryption.

The cert.crt, cert.key, and cacert.crt files are necessary for clients to authenticate you as the valid LDAP server. If you simply want a server that runs, you can create a self-signed certificate with OpenSSL:

Example 2. Generating an RSA Key
% openssl genrsa -out cert.key 1024
Generating RSA private key, 1024 bit long modulus
....................++++++
...++++++
e is 65537 (0x10001)
% openssl req -new -key cert.key -out cert.csr

At this point you should be prompted for some values. You may enter whatever values you like; however, it is important the Common Name value be the fully qualified domain name of the OpenLDAP server. In our case, and the examples here, the server is server.example.org. Incorrectly setting this value will cause clients to fail when making connections. This can the cause of great frustration, so ensure that you follow these steps closely.

Finally, the certificate signing request needs to be signed:

Example 3. Self-signing the Certificate
% openssl x509 -req -in cert.csr -days 365 -signkey cert.key -out cert.crt
Signature ok
subject=/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd
Getting Private key

This will create a self-signed certificate that can be used for the directives in slapd.conf, where cert.crt and cacert.crt are the same file. If you are going to use many OpenLDAP servers (for replication via slurpd) you will want to see Appendix B, OpenSSL Certificates for LDAP to generate a CA key and use it to sign individual server certificates.

Once this is done, put the following in /etc/rc.conf:

slapd_enable="YES"

Then run /usr/local/etc/rc.d/slapd start. This should start OpenLDAP. Confirm that it is listening on 389 with

% sockstat -4 -p 389
ldap     slapd      3261  7  tcp4   *:389                 *:*

2.1.3. Configuring the Client

Install the net/openldap24-client port for the OpenLDAP libraries. The client machines will always have OpenLDAP libraries since that is all security/pam_ldap and net/nss_ldap support, at least for the moment.

The configuration file for the OpenLDAP libraries is /usr/local/etc/openldap/ldap.conf. Edit this file to contain the following values:

base dc=example,dc=org
uri ldap://server.example.org/
ssl start_tls
tls_cacert /path/to/your/cacert.crt

Note:

It is important that your clients have access to cacert.crt, otherwise they will not be able to connect.

Note:

There are two files called ldap.conf. The first is this file, which is for the OpenLDAP libraries and defines how to talk to the server. The second is /usr/local/etc/ldap.conf, and is for pam_ldap.

At this point you should be able to run ldapsearch -Z on the client machine; -Z means use TLS. If you encounter an error, then something is configured wrong; most likely it is your certificates. Use openssl(1)'s s_client and s_server to ensure you have them configured and signed properly.

2.2. Entries in the Database

Authentication against an LDAP directory is generally accomplished by attempting to bind to the directory as the connecting user. This is done by establishing a simple bind on the directory with the user name supplied. If there is an entry with the uid equal to the user name and that entry's userPassword attribute matches the password supplied, then the bind is successful.

The first thing we have to do is figure out is where in the directory our users will live.

The base entry for our database is dc=example,dc=org. The default location for users that most clients seem to expect is something like ou=people,base, so that is what will be used here. However keep in mind that this is configurable.

So the ldif entry for the people organizational unit will look like:

dn: ou=people,dc=example,dc=org
objectClass: top
objectClass: organizationalUnit
ou: people

All users will be created as subentries of this organizational unit.

Some thought might be given to the object class your users will belong to. Most tools by default will use people, which is fine if you simply want to provide entries against which to authenticate. However, if you are going to store user information in the LDAP database as well, you will probably want to use inetOrgPerson, which has many useful attributes. In either case, the relevant schemas need to be loaded in slapd.conf.

For this example we will use the person object class. If you are using inetOrgPerson, the steps are basically identical, except that the sn attribute is required.

To add a user testuser, the ldif would be:

dn: uid=tuser,ou=people,dc=example,dc=org
objectClass: person
objectClass: posixAccount
objectClass: shadowAccount
objectClass: top
uidNumber: 10000
gidNumber: 10000
homeDirectory: /home/tuser
loginShell: /bin/csh
uid: tuser
cn: tuser

I start my LDAP users' UIDs at 10000 to avoid collisions with system accounts; you can configure whatever number you wish here, as long as it is less than 65536.

We also need group entries. They are as configurable as user entries, but we will use the defaults below:

dn: ou=groups,dc=example,dc=org
objectClass: top
objectClass: organizationalUnit
ou: groups

dn: cn=tuser,ou=groups,dc=example,dc=org
objectClass: posixGroup
objectClass: top
gidNumber: 10000
cn: tuser

To enter these into your database, you can use slapadd or ldapadd on a file containing these entries. Alternatively, you can use sysutils/ldapvi.

The ldapsearch utility on the client machine should now return these entries. If it does, your database is properly configured to be used as an LDAP authentication server.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.