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Chapter 45. LDAP and Transport Layer Security

Gavin Henry

Suretec Systems Limited, UK

July 8, 2005

Table of Contents

Introduction
Configuring
Generating the Certificate Authority
Generating the Server Certificate
Installing the Certificates
Testing
Troubleshooting

Introduction

Up until now, we have discussed the straightforward configuration of OpenLDAP™, with some advanced features such as ACLs. This does not however, deal with the fact that the network transmissions are still in plain text. This is where Transport Layer Security (TLS) comes in.

OpenLDAP™ clients and servers are capable of using the Transport Layer Security (TLS) framework to provide integrity and confidentiality protections in accordance with RFC 2830; Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security.

TLS uses X.509 certificates. All servers are required to have valid certificates, whereas client certificates are optional. We will only be discussing server certificates.

Tip

The DN of a server certificate must use the CN attribute to name the server, and the CN must carry the server's fully qualified domain name (FQDN). Additional alias names and wildcards may be present in the subjectAltName certificate extension. More details on server certificate names are in RFC2830.

We will discuss this more in the next sections.

Configuring

Now on to the good bit.

Generating the Certificate Authority

In order to create the relevant certificates, we need to become our own Certificate Authority (CA). [8] This is necessary, so we can sign the server certificate.

We will be using the OpenSSL [9] software for this, which is included with every great Linux® distribution.

TLS is used for many types of servers, but the instructions[10] presented here, are tailored for OpenLDAP.

Note

The Common Name (CN), in the following example, MUST be the fully qualified domain name (FQDN) of your ldap server.

First we need to generate the CA:


root#  mkdir myCA

Move into that directory:


root#  cd myCA

Now generate the CA:[11]


root#  /usr/share/ssl/misc/CA.pl -newca
CA certificate filename (or enter to create)
  
Making CA certificate ...
Generating a 1024 bit RSA private key
.......................++++++
.............................++++++
writing new private key to './demoCA/private/cakey.pem'
Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:AU
State or Province Name (full name) [Some-State]:NSW
Locality Name (eg, city) []:Sydney
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Abmas
Organizational Unit Name (eg, section) []:IT
Common Name (eg, YOUR name) []:ldap.abmas.biz
Email Address []:support@abmas.biz

There are some things to note here.

  1. You MUST remember the password, as we will need it to sign the server certificate..

  2. The Common Name (CN), MUST be the fully qualified domain name (FQDN) of your ldap server.

Generating the Server Certificate

Now we need to generate the server certificate:


root#  openssl req -new -nodes -keyout newreq.pem -out newreq.pem
Generating a 1024 bit RSA private key
.............++++++
........................................................++++++
writing new private key to 'newreq.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:AU
State or Province Name (full name) [Some-State]:NSW
Locality Name (eg, city) []:Sydney
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Abmas
Organizational Unit Name (eg, section) []:IT
Common Name (eg, YOUR name) []:ldap.abmas.biz
Email Address []:support@abmas.biz
  
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

Again, there are some things to note here.

  1. You should NOT enter a password.

  2. The Common Name (CN), MUST be the fully qualified domain name (FQDN) of your ldap server.

Now we sign the certificate with the new CA:


root#  /usr/share/ssl/misc/CA.pl -sign
Using configuration from /etc/ssl/openssl.cnf
Enter pass phrase for ./demoCA/private/cakey.pem:
Check that the request matches the signature
Signature ok
Certificate Details:
Serial Number: 1 (0x1)
Validity
	Not Before: Mar  6 18:22:26 2005 EDT
	Not After : Mar  6 18:22:26 2006 EDT
Subject:
	countryName               = AU
	stateOrProvinceName       = NSW
	localityName              = Sydney
	organizationName          = Abmas
	organizationalUnitName    = IT
	commonName                = ldap.abmas.biz
	emailAddress              = support@abmas.biz
X509v3 extensions:
	X509v3 Basic Constraints:
	    CA:FALSE
	Netscape Comment:
	    OpenSSL Generated Certificate
	X509v3 Subject Key Identifier:
	    F7:84:87:25:C4:E8:46:6D:0F:47:27:91:F0:16:E0:86:6A:EE:A3:CE
	X509v3 Authority Key Identifier:
	    keyid:27:44:63:3A:CB:09:DC:B1:FF:32:CC:93:23:A4:F1:B4:D5:F0:7E:CC
	    DirName:/C=AU/ST=NSW/L=Sydney/O=Abmas/OU=IT/
						CN=ldap.abmas.biz/emailAddress=support@abmas.biz
	    serial:00

Certificate is to be certified until Mar  6 18:22:26 2006 EDT (365 days)
Sign the certificate? [y/n]:y


1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated
Signed certificate is in newcert.pem

That completes the server certificate generation.

Installing the Certificates

Now we need to copy the certificates to the right configuration directories, rename them at the same time (for convenience), change the ownership and finally the permissions:


root#  cp demoCA/cacert.pem /etc/openldap/
root#  cp newcert.pem /etc/openldap/servercrt.pem
root#  cp newreq.pem /etc/openldap/serverkey.pem
root#  chown ldap.ldap /etc/openldap/*.pem
root#  chmod 640 /etc/openldap/cacert.pem;
root#  chmod 600 /etc/openldap/serverkey.pem

Now we just need to add these locations to slapd.conf, anywhere before the database declaration as shown here:


TLSCertificateFile /etc/openldap/servercrt.pem
TLSCertificateKeyFile /etc/openldap/serverkey.pem
TLSCACertificateFile /etc/openldap/cacert.pem

Here is the declaration and ldap.conf: ldap.conf


TLS_CACERT /etc/openldap/cacert.pem

That's all there is to it. Now on to the section called “Testing”

Testing

This is the easy part. Restart the server:


root#  /etc/init.d/ldap restart
Stopping slapd:                                            [  OK  ]
Checking configuration files for slapd: config file testing succeeded
Starting slapd:                                            [  OK  ]

Then, using ldapsearch, test an anonymous search with the -ZZ[12] option:


root#  ldapsearch -x -b "dc=ldap,dc=abmas,dc=biz" \
        -H 'ldap://ldap.abmas.biz:389' -ZZ

Your results should be the same as before you restarted the server, for example:


root#  ldapsearch -x -b "dc=ldap,dc=abmas,dc=biz" \
    -H 'ldap://ldap.abmas.biz:389' -ZZ

# extended LDIF
#
# LDAPv3
# base <> with scope sub
# filter: (objectclass=*)
# requesting: ALL
#

# abmas.biz
dn: dc=ldap,dc=abmas,dc=biz
objectClass: dcObject
objectClass: organization
o: Abmas
dc: abmas

# Manager, ldap.abmas.biz
dn: cn=Manager,dc=ldap,dc=abmas,dc=biz
objectClass: organizationalRole
cn: Manager

# ABMAS, abmas.biz
dn: sambaDomainName=ABMAS,dc=ldap,dc=abmas,dc=biz
sambaDomainName: ABMAS
sambaSID: S-1-5-21-238355452-1056757430-1592208922
sambaAlgorithmicRidBase: 1000
objectClass: sambaDomain
sambaNextUserRid: 67109862
sambaNextGroupRid: 67109863

If you have any problems, please read the section called “Troubleshooting”

Troubleshooting

The most common error when configuring TLS, as I have already mentioned numerous times, is that the Common Name (CN) you entered in the section called “Generating the Server Certificate” is NOT the Fully Qualified Domain Name (FQDN) of your ldap server.

Other errors could be that you have a typo somewhere in your ldapsearch command, or that your have the wrong permissions on the servercrt.pem and cacert.pem files. They should be set with chmod 640, as per the section called “Installing the Certificates”.

For anything else, it's best to read through your ldap logfile or join the OpenLDAP mailing list.



[8] We could however, get our generated server certificate signed by proper CAs, like Thawte and VeriSign, which you pay for, or the free ones, via CAcert

[9] The downside to making our own CA, is that the certificate is not automatically recognized by clients, like the commercial ones are.

[10] For information straight from the horse's mouth, please visit http://www.openssl.org/docs/HOWTO/; the main OpenSSL site.

[11] Your CA.pl or CA.sh might not be in the same location as mine is, you can find it by using the locate command, i.e., locate CA.pl. If the command complains about the database being too old, run updatedb as root to update it.

[12] See man ldapsearch