16. Using TLS

OpenLDAP clients and servers are capable of using the Transport Layer Security (TLS) framework to provide integrity and confidentiality protections and to support LDAP authentication using the SASL EXTERNAL mechanism. TLS is defined in RFC4346.

Note: For generating certificates, please reference http://www.openldap.org/faq/data/cache/185.html

16.1. TLS Certificates

TLS uses X.509 certificates to carry client and server identities. All servers are required to have valid certificates, whereas client certificates are optional. Clients must have a valid certificate in order to authenticate via SASL EXTERNAL. For more information on creating and managing certificates, see the OpenSSL or GnuTLS documentation, depending on which TLS implementation libraries you are using.

16.1.1. Server Certificates

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. Additional alias names and wildcards may be present in the subjectAltName certificate extension. More details on server certificate names are in RFC4513.

16.1.2. Client Certificates

The DN of a client certificate can be used directly as an authentication DN. Since X.509 is a part of the X.500 standard and LDAP is also based on X.500, both use the same DN formats and generally the DN in a user's X.509 certificate should be identical to the DN of their LDAP entry. However, sometimes the DNs may not be exactly the same, and so the mapping facility described in Mapping Authentication Identities can be applied to these DNs as well.

16.2. TLS Configuration

After obtaining the required certificates, a number of options must be configured on both the client and the server to enable TLS and make use of the certificates. At a minimum, the clients must be configured with the name of the file containing all of the Certificate Authority (CA) certificates it will trust. The server must be configured with the CA certificates and also its own server certificate and private key.

Typically a single CA will have issued the server certificate and all of the trusted client certificates, so the server only needs to trust that one signing CA. However, a client may wish to connect to a variety of secure servers managed by different organizations, with server certificates generated by many different CAs. As such, a client is likely to need a list of many different trusted CAs in its configuration.

16.2.1. Server Configuration

The configuration directives for slapd belong in the global directives section of slapd.conf(5).

16.2.1.1. TLSCACertificateFile <filename>

This directive specifies the PEM-format file containing certificates for the CA's that slapd will trust. The certificate for the CA that signed the server certificate must be included among these certificates. If the signing CA was not a top-level (root) CA, certificates for the entire sequence of CA's from the signing CA to the top-level CA should be present. Multiple certificates are simply appended to the file; the order is not significant.

16.2.1.2. TLSCACertificatePath <path>

This directive specifies the path of a directory that contains individual CA certificates in separate files. In addition, this directory must be specially managed using the OpenSSL c_rehash utility. When using this feature, the OpenSSL library will attempt to locate certificate files based on a hash of their name and serial number. The c_rehash utility is used to generate symbolic links with the hashed names that point to the actual certificate files. As such, this option can only be used with a filesystem that actually supports symbolic links. In general, it is simpler to use the TLSCACertificateFile directive instead.

16.2.1.3. TLSCertificateFile <filename>

This directive specifies the file that contains the slapd server certificate. Certificates are generally public information and require no special protection.

16.2.1.4. TLSCertificateKeyFile <filename>

This directive specifies the file that contains the private key that matches the certificate stored in the TLSCertificateFile file. Private keys themselves are sensitive data and are usually password encrypted for protection. However, the current implementation doesn't support encrypted keys so the key must not be encrypted and the file itself must be protected carefully.

16.2.1.5. TLSCipherSuite <cipher-suite-spec>

This directive configures what ciphers will be accepted and the preference order. <cipher-suite-spec> should be a cipher specification for OpenSSL. You can use the command

        openssl ciphers -v ALL

to obtain a verbose list of available cipher specifications.

Besides the individual cipher names, the specifiers HIGH, MEDIUM, LOW, EXPORT, and EXPORT40 may be helpful, along with TLSv1, SSLv3, and SSLv2.

To obtain the list of ciphers in GnuTLS use:

        gnutls-cli -l

16.2.1.6. TLSRandFile <filename>

This directive specifies the file to obtain random bits from when /dev/urandom is not available. If the system provides /dev/urandom then this option is not needed, otherwise a source of random data must be configured. Some systems (e.g. Linux) provide /dev/urandom by default, while others (e.g. Solaris) require the installation of a patch to provide it, and others may not support it at all. In the latter case, EGD or PRNGD should be installed, and this directive should specify the name of the EGD/PRNGD socket. The environment variable RANDFILE can also be used to specify the filename. Also, in the absence of these options, the .rnd file in the slapd user's home directory may be used if it exists. To use the .rnd file, just create the file and copy a few hundred bytes of arbitrary data into the file. The file is only used to provide a seed for the pseudo-random number generator, and it doesn't need very much data to work.

This directive is ignored with GnuTLS.

16.2.1.7. TLSDHParamFile <filename>

This directive specifies the file that contains parameters for Diffie-Hellman ephemeral key exchange. This is required in order to use DHE-based cipher suites, including all DSA-based suites (i.e. TLSCertificateKeyFile points to a DSA key), and RSA when the 'key encipherment' key usage is not specified in the certificate. Parameters can be generated using the following command

        openssl dhparam [-dsaparam] -out <filename> <numbits> or
        certtool --generate-dh-params --bits <numbits> --outfile <filename>

16.2.1.8. TLSECName <name>

This directive specifies the curve to use for Elliptic Curve Diffie-Hellman ephemeral key exchange. This is required in order to use ECDHE-based cipher suites in OpenSSL. The names of supported curves may be shown using the following command

        openssl ecparam -list_curves

This directive is not used for GnuTLS. For GnuTLS the curves may be specified in the ciphersuite.

16.2.1.9. TLSVerifyClient { never | allow | try | demand }

This directive specifies what checks to perform on client certificates in an incoming TLS session, if any. This option is set to never by default, in which case the server never asks the client for a certificate. With a setting of allow the server will ask for a client certificate; if none is provided the session proceeds normally. If a certificate is provided but the server is unable to verify it, the certificate is ignored and the session proceeds normally, as if no certificate had been provided. With a setting of try the certificate is requested, and if none is provided, the session proceeds normally. If a certificate is provided and it cannot be verified, the session is immediately terminated. With a setting of demand the certificate is requested and a valid certificate must be provided, otherwise the session is immediately terminated.

Note: The server must request a client certificate in order to use the SASL EXTERNAL authentication mechanism with a TLS session. As such, a non-default TLSVerifyClient setting must be configured before SASL EXTERNAL authentication may be attempted, and the SASL EXTERNAL mechanism will only be offered to the client if a valid client certificate was received.

16.2.2. Client Configuration

Most of the client configuration directives parallel the server directives. The names of the directives are different, and they go into ldap.conf(5) instead of slapd.conf(5), but their functionality is mostly the same. Also, while most of these options may be configured on a system-wide basis, they may all be overridden by individual users in their .ldaprc files.

The LDAP Start TLS operation is used in LDAP to initiate TLS negotiation. All OpenLDAP command line tools support a -Z and -ZZ flag to indicate whether a Start TLS operation is to be issued. The latter flag indicates that the tool is to cease processing if TLS cannot be started while the former allows the command to continue.

In LDAPv2 environments, TLS is normally started using the LDAP Secure URI scheme (ldaps://) instead of the normal LDAP URI scheme (ldap://). OpenLDAP command line tools allow either scheme to used with the -H flag and with the URI ldap.conf(5) option.