9. Security Considerations

OpenLDAP Software is designed to run in a wide variety of computing environments from tightly-controlled closed networks to the global Internet. Hence, OpenLDAP Software provides many different security mechanisms. This chapter describes these mechanisms and discusses security considerations for using OpenLDAP Software.

9.1. Network Security

9.1.1. Selective Listening

By default, slapd(8) will listen on both the IPv4 and IPv6 "any" addresses. It is often desirable to have slapd listen on select address/port pairs. For example, listening only on the IPv4 address will disallow remote access to the directory server. E.g.:

        slapd -h ldap://

While the server can be configured to listen on a particular interface address, this doesn't necessarily restrict access to the server to only those networks accessible via that interface. To selective restrict remote access, it is recommend that an IP Firewall be used to restrict access.

See Command-line Options and slapd(8) for more information.

9.1.2. IP Firewall

IP firewall capabilities of the server system can be used to restrict access based upon the client's IP address and/or network interface used to communicate with the client.

Generally, slapd(8) listens on port 389/tcp for LDAP over TCP (e.g. ldap://) and port 636/tcp for LDAP over SSL (e.g. ldaps://). Note that LDAP over TCP sessions can be protected by TLS through the use of StartTLS. StartTLS is the Standard Track mechanism for protecting LDAP sessions with TLS.

As specifics of how to configure IP firewall are dependent on the particular kind of IP firewall used, no examples are provided here. See the document associated with your IP firewall.

9.1.3. TCP Wrappers

OpenLDAP supports TCP Wrappers. TCP Wrappers provide a rule-based access control system for controlling TCP/IP access to the server. For example, the host_options(5) rule:

        slapd: : ALLOW
        slapd: ALL : DENY

allows only incoming connections from the private network and localhost ( to access the directory service. Note that IP addresses are used as slapd(8) is not normally configured to perform reverse lookups.

It is noted that TCP wrappers require the connection to be accepted. As significant processing is required just to deny a connection, it is generally advised that IP firewall protection be used instead of TCP wrappers.

See hosts_access(5) for more information on TCP wrapper rules.

9.2. Integrity and Confidentiality Protection

Transport Layer Security (TLS) can be used to provide integrity and confidentiality protection. OpenLDAP supports both StartTLS and ldaps://. See the Using TLS chapter for more information.

A number of Simple Authentication and Security Layer (SASL) mechanisms, such as DIGEST-MD5 and GSSAPI, also provide integrity and confidentiality protection. See the Using SASL chapter for more information.

9.2.1. Security Strength Factors

The server uses Security Strength Factors (SSF) to indicate the relative strength of protection. A SSF of zero (0) indicates no protections are in place. A SSF of one (1) indicates integrity protection are in place. A SSF greater than one (>1) roughly correlates to the effective encryption key length. For example, DES is 56, 3DES is 112, and AES 128, 192, or 256.

A number of administrative controls rely on SSFs associated with TLS and SASL protection in place on an LDAP session.

security controls disallow operations when appropriate protections are not in place. For example:

        security ssf=1 update_ssf=112

requires integrity protection for all operations and encryption protection, 3DES equivalent, for update operations (e.g. add, delete, modify, etc.). See slapd.conf(5) for details.

For fine-grained control, SSFs may be used in access controls. See Access Control section of the The slapd Configuration File for more information.

9.3. Authentication Methods

9.3.1. "simple" method

The LDAP "simple" method has three modes of operation:

Anonymous access is obtained by providing no name and no password to the "simple" bind operation. Unauthenticated access is obtained by providing a name but no password. Authenticated access is obtain by providing a valid name and password.

An anonymous bind results in an anonymous authorization. Anonymous bind mechanism is enabled by default, but can be disabled by specifying "disallow bind_anon" in slapd.conf(5).

An unauthenticated bind results in an anonymous authorization. Unauthenticated bind mechanism is disabled by default, but can be enabled by specifying "allow bind_anon_cred" in slapd.conf(5). As a number of LDAP applications mistakenly generate unauthenticated bind request when authenticated access was intended (that is, they do not ensure a password was provided), this mechanism should generally not be enabled.

A successful user/password authenticated bind results in a user authorization identity, the provided name, being associated with the session. User/password authenticated bind is enabled by default. However, as this mechanism offers no evesdropping protection (e.g., the password is set in the clear), it is recommended that it be used only in tightly controlled systems or when the LDAP session is protected by other means (e.g., TLS, IPSEC). Where the administrator relies on TLS to protect the password, it is recommended that unprotected authentication be disabled. This is done by setting "disallow bind_simple_unprotected" in slapd.conf(5). The security directive's simple_bind option provides fine grain control over the level of confidential protection to require for simple user/password authentication.

The user/password authenticated bind mechanism can be completely disabled by setting "disallow bind_simple".

Note: An unsuccessful bind always results in the session having an anonymous authorization state.

9.3.2. SASL method

The LDAP SASL method allows use of any SASL authentication mechanism. The Using SASL discusses use of SASL.