9.2. Setting a Minimum Strength Factor

For additional security, the Directory Server can be configured to require a certain level of encryption before it allows a connection. The Directory Server can define and require a specific Security Strength Factor (SFF) for any connection. The SSF sets a minimum encryption level, defined by its key strength, for any connection or operation.
To require a minimum SSF for any and all directory operations, set the nsslapd-minssf configuration attribute. When enforcing a minimum SSF, Directory Server looks at each available encryption type for an operation — TLS or SASL — and determines which has the higher SSF value and then compares the higher value to the minimum SSF. It is possible for both SASL authentication and TLS to be configured for some server-to-server connections, such as replication.


Alternatively, use the nsslapd-minssf-exclude-rootdse configuration attribute. This sets a minimum SSF setting for all connections to the Directory Server except for queries against the root DSE. A client may need to obtain information about the server configuration, like its default naming context, before initiating an operation. The nsslapd-minssf-exclude-rootdse attribute allows the client to get that information without having to establish a secure connection first.
The SSF for a connection is evaluated when the first operation is initiated on a connection. This allows STARTTLS and SASL binds to succeed, even though those two connections initially open a regular connection. After the TLS or SASL session is opened, then the SSF is evaluated. Any connection which does not meet the SSF requirements is closed with an LDAP unwilling to perform error.
Set a minimum SSF to disable insecure connections to a directory.


If you connect to the directory using the unencrypted LDAP protocol without SASL, the first LDAP message can contain the bind request. In this case, the credentials are sent unencrypted over the network before the server cancels the connection, because the SSF did not met the minimum value set.
Use the LDAPS protocol or SASL binds to ensure that the credentials are never sent unencrypted.
The default nsslapd-minssf attribute value is 0, which means there is no minimum SSF for server connections. The value can be set to any reasonable positive integer. The value represents the required key strength for any secure connection.
The following example sets the nsslapd-minssf parameter to 128:
# dsconf -D "cn=Directory Manager" ldap://server.example.com config replace nsslapd-minssf=128
Successfully replaced "nsslapd-minssf"


An ACI can be set to require an SSF for a specific type of operation, as in Section, “Requiring a Certain Level of Security in Connections”.
Secure connections can be required for bind operations by turning on the nsslapd-require-secure-binds attribute, as in Section 20.12.1, “Requiring Secure Binds”.