# $OpenLDAP$ # Copyright 1999-2020 The OpenLDAP Foundation, All Rights Reserved. # COPYING RESTRICTIONS APPLY, see COPYRIGHT. H1: Using TLS OpenLDAP clients and servers are capable of using the {{TERM[expand]TLS}} ({{TERM:TLS}}) framework to provide integrity and confidentiality protections and to support LDAP authentication using the {{TERM:SASL}} {{TERM:EXTERNAL}} mechanism. TLS is defined in {{REF:RFC4346}}. Note: For generating certificates, please reference {{URL:http://www.openldap.org/faq/data/cache/185.html}} H2: TLS Certificates TLS uses {{TERM: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 {{PRD:OpenSSL}} or {{PRD:GnuTLS}} documentation, depending on which TLS implementation libraries you are using. H3: Server Certificates The {{TERM:DN}} of a server certificate must use the {{EX:CN}} attribute to name the server, and the {{EX:CN}} must carry the server's fully qualified domain name. Additional alias names and wildcards may be present in the {{EX:subjectAltName}} certificate extension. More details on server certificate names are in {{REF:RFC4513}}. H3: Client Certificates The DN of a client certificate can be used directly as an authentication DN. Since X.509 is a part of the {{TERM: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 {{SECT:Mapping Authentication Identities}} can be applied to these DNs as well. H2: 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 {{TERM[expand]CA}} (CA) certificates it will trust. The server must be configured with the {{TERM: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. H3: Server Configuration The configuration directives for slapd belong in the global directives section of {{slapd.conf}}(5). H4: TLSCACertificateFile This directive specifies the {{TERM: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. H4: TLSCACertificatePath This directive specifies the path of a directory that contains individual {{TERM: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 {{EX:TLSCACertificateFile}} directive instead. H4: TLSCertificateFile This directive specifies the file that contains the slapd server certificate. Certificates are generally public information and require no special protection. H4: TLSCertificateKeyFile This directive specifies the file that contains the private key that matches the certificate stored in the {{EX: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. H4: TLSCipherSuite This directive configures what ciphers will be accepted and the preference order. {{EX:}} 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 {{EX:HIGH}}, {{EX:MEDIUM}}, {{EX:LOW}}, {{EX:EXPORT}}, and {{EX:EXPORT40}} may be helpful, along with {{EX:TLSv1}}, {{EX:SSLv3}}, and {{EX:SSLv2}}. To obtain the list of ciphers in GnuTLS use: > gnutls-cli -l H4: TLSRandFile This directive specifies the file to obtain random bits from when {{FILE:/dev/urandom}} is not available. If the system provides {{FILE:/dev/urandom}} then this option is not needed, otherwise a source of random data must be configured. Some systems (e.g. Linux) provide {{FILE:/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 {{EX:RANDFILE}} can also be used to specify the filename. Also, in the absence of these options, the {{EX:.rnd}} file in the slapd user's home directory may be used if it exists. To use the {{EX:.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. H4: TLSDHParamFile 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. {{EX: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 or > certtool --generate-dh-params --bits --outfile H4: TLSECName 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. H4: 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 {{EX:never}} by default, in which case the server never asks the client for a certificate. With a setting of {{EX: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 {{EX: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 {{EX: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 {{EX: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. H3: 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 {{EX:-Z}} and {{EX:-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 ({{EX:ldaps://}}) instead of the normal LDAP URI scheme ({{EX:ldap://}}). OpenLDAP command line tools allow either scheme to used with the {{EX:-H}} flag and with the {{EX:URI}} {{ldap.conf}}(5) option. H4: TLS_CACERT This is equivalent to the server's {{EX:TLSCACertificateFile}} option. As noted in the {{SECT:TLS Configuration}} section, a client typically may need to know about more CAs than a server, but otherwise the same considerations apply. H4: TLS_CACERTDIR This is equivalent to the server's {{EX:TLSCACertificatePath}} option. The specified directory must be managed with the OpenSSL {{c_rehash}} utility as well. H4: TLS_CERT This directive specifies the file that contains the client certificate. This is a user-only directive and can only be specified in a user's {{.ldaprc}} file. H4: TLS_KEY This directive specifies the file that contains the private key that matches the certificate stored in the {{EX:TLS_CERT}} file. The same constraints mentioned for {{EX:TLSCertificateKeyFile}} apply here. This is also a user-only directive. H4: TLS_RANDFILE This directive is the same as the server's {{EX:TLSRandFile}} option. H4: TLS_REQCERT { never | allow | try | demand } This directive is equivalent to the server's {{EX:TLSVerifyClient}} option. However, for clients the default value is {{EX:demand}} and there generally is no good reason to change this setting.