rfc2829.txt

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            eavesdropping attacks, but does not provide protection
            against active intermediary attacks.

      (3)   For a directory needing session protection and
            authentication, the Start TLS extended operation [5], and
            either the simple authentication choice or the SASL EXTERNAL
            mechanism, are to be used together.  Implementations SHOULD
            support authentication with a password as described in
            section 6.2, and SHOULD support authentication with a
            certificate as described in section 7.1.  Together, these
            can provide integrity and disclosure protection of
            transmitted data, and authentication of client and server,
            including protection against active intermediary attacks.

   If TLS is negotiated, the client MUST discard all information about
   the server fetched prior to the TLS negotiation.  In particular, the
   value of supportedSASLMechanisms MAY be different after TLS has been
   negotiated (specifically, the EXTERNAL mechanism or the proposed
   PLAIN mechanism are likely to only be listed after a TLS negotiation
   has been performed).

   If a SASL security layer is negotiated, the client MUST discard all
   information about the server fetched prior to SASL.  In particular,
   if the client is configured to support multiple SASL mechanisms, it
   SHOULD fetch supportedSASLMechanisms both before and after the SASL
   security layer is negotiated and verify that the value has not
   changed after the SASL security layer was negotiated.  This detects
   active attacks which remove supported SASL mechanisms from the
   supportedSASLMechanisms list, and allows the client to ensure that it
   is using the best mechanism supported by both client and server
   (additionally, this is a SHOULD to allow for environments where the
   supported SASL mechanisms list is provided to the client through a
   different trusted source, e.g. as part of a digitally signed object).




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RFC 2829            Authentication Methods for LDAP             May 2000


5. Anonymous authentication

   Directory operations which modify entries or access protected
   attributes or entries generally require client authentication.
   Clients which do not intend to perform any of these operations
   typically use anonymous authentication.

   LDAP implementations MUST support anonymous authentication, as
   defined in section 5.1.

   LDAP implementations MAY support anonymous authentication with TLS,
   as defined in section 5.2.

   While there MAY be access control restrictions to prevent access to
   directory entries, an LDAP server SHOULD allow an anonymously-bound
   client to retrieve the supportedSASLMechanisms attribute of the root
   DSE.

   An LDAP server MAY use other information about the client provided by
   the lower layers or external means to grant or deny access even to
   anonymously authenticated clients.

5.1. Anonymous authentication procedure

   An LDAP client which has not successfully completed a bind operation
   on a connection is anonymously authenticated.

   An LDAP client MAY also specify anonymous authentication in a bind
   request by using a zero-length OCTET STRING with the simple
   authentication choice.

5.2. Anonymous authentication and TLS

   An LDAP client MAY use the Start TLS operation [5] to negotiate the
   use of TLS security [6].  If the client has not bound beforehand,
   then until the client uses the EXTERNAL SASL mechanism to negotiate
   the recognition of the client's certificate, the client is
   anonymously authenticated.

   Recommendations on TLS ciphersuites are given in section 10.

   An LDAP server which requests that clients provide their certificate
   during TLS negotiation MAY use a local security policy to determine
   whether to successfully complete TLS negotiation if the client did
   not present a certificate which could be validated.






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RFC 2829            Authentication Methods for LDAP             May 2000


6. Password-based authentication

   LDAP implementations MUST support authentication with a password
   using the DIGEST-MD5 SASL mechanism for password protection, as
   defined in section 6.1.

   LDAP implementations SHOULD support authentication with the "simple"
   password choice when the connection is protected against
   eavesdropping using TLS, as defined in section 6.2.

6.1. Digest authentication

   An LDAP client MAY determine whether the server supports this
   mechanism by performing a search request on the root DSE, requesting
   the supportedSASLMechanisms attribute, and checking whether the
   string "DIGEST-MD5" is present as a value of this attribute.

   In the first stage of authentication, when the client is performing
   an "initial authentication" as defined in section 2.1 of [4], the
   client sends a bind request in which the version number is 3, the
   authentication choice is sasl, the sasl mechanism name is "DIGEST-
   MD5", and the credentials are absent.  The client then waits for a
   response from the server to this request.

   The server will respond with a bind response in which the resultCode
   is saslBindInProgress, and the serverSaslCreds field is present.  The
   contents of this field is a string defined by "digest-challenge" in
   section 2.1.1 of [4].  The server SHOULD include a realm indication
   and MUST indicate support for UTF-8.

   The client will send a bind request with a distinct message id, in
   which the version number is 3, the authentication choice is sasl, the
   sasl mechanism name is "DIGEST-MD5", and the credentials contain the
   string defined by "digest-response" in section 2.1.2 of [4].  The
   serv-type is "ldap".

   The server will respond with a bind response in which the resultCode
   is either success, or an error indication.  If the authentication is
   successful and the server does not support subsequent authentication,
   then the credentials field is absent.  If the authentication is
   successful and the server supports subsequent authentication, then
   the credentials field contains the string defined by "response-auth"
   in section 2.1.3 of [4].   Support for subsequent authentication is
   OPTIONAL in clients and servers.







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RFC 2829            Authentication Methods for LDAP             May 2000


6.2. "simple" authentication choice under TLS encryption

   A user who has a directory entry containing a userPassword attribute
   MAY authenticate to the directory by performing a simple password
   bind sequence following the negotiation of a TLS ciphersuite
   providing connection confidentiality [6].

   The client will use the Start TLS operation [5] to negotiate the use
   of TLS security [6] on the connection to the LDAP server.  The client
   need not have bound to the directory beforehand.

   For this authentication procedure to be successful, the client and
   server MUST negotiate a ciphersuite which contains a bulk encryption
   algorithm of appropriate strength.  Recommendations on cipher suites
   are given in section 10.

   Following the successful completion of TLS negotiation, the client
   MUST send an LDAP bind request with the version number of 3, the name
   field containing the name of the user's entry, and the "simple"
   authentication choice, containing a password.

   The server will, for each value of the userPassword attribute in the
   named user's entry, compare these for case-sensitive equality with
   the client's presented password.  If there is a match, then the
   server will respond with resultCode success, otherwise the server
   will respond with resultCode invalidCredentials.

6.3. Other authentication choices with TLS

   It is also possible, following the negotiation of TLS, to perform a
   SASL authentication which does not involve the exchange of plaintext
   reusable passwords.  In this case the client and server need not
   negotiate a ciphersuite which provides confidentiality if the only
   service required is data integrity.

7. Certificate-based authentication

   LDAP implementations SHOULD support authentication via a client
   certificate in TLS, as defined in section 7.1.

7.1. Certificate-based authentication with TLS

   A user who has a public/private key pair in which the public key has
   been signed by a Certification Authority may use this key pair to
   authenticate to the directory server if the user's certificate is
   requested by the server.  The user's certificate subject field SHOULD
   be the name of the user's directory entry, and the Certification
   Authority must be sufficiently trusted by the directory server to



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RFC 2829            Authentication Methods for LDAP             May 2000


   have issued the certificate in order that the server can process the
   certificate.  The means by which servers validate certificate paths
   is outside the scope of this document.

   A server MAY support mappings for certificates in which the subject
   field name is different from the name of the user's directory entry.
   A server which supports mappings of names MUST be capable of being
   configured to support certificates for which no mapping is required.

   The client will use the Start TLS operation [5] to negotiate the use
   of TLS security [6] on the connection to the LDAP server.  The client
   need not have bound to the directory beforehand.

   In the TLS negotiation, the server MUST request a certificate.  The
   client will provide its certificate to the server, and MUST perform a
   private key-based encryption, proving it has the private key
   associated with the certificate.

   As deployments will require protection of sensitive data in transit,
   the client and server MUST negotiate a ciphersuite which contains a
   bulk encryption algorithm of appropriate strength.  Recommendations
   of cipher suites are given in section 10.

   The server MUST verify that the client's certificate is valid. The
   server will normally check that the certificate is issued by a known
   CA, and that none of the certificates on the client's certificate
   chain are invalid or revoked.  There are several procedures by which
   the server can perform these checks.

   Following the successful completion of TLS negotiation, the client
   will send an LDAP bind request with the SASL "EXTERNAL" mechanism.

8. Other mechanisms

   The LDAP "simple" authentication choice is not suitable for
   authentication on the Internet where there is no network or transport
   layer confidentiality.

   As LDAP includes native anonymous and plaintext authentication
   methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
   with LDAP.  If an authorization identity of a form different from a
   DN is requested by the client, a mechanism that protects the password
   in transit SHOULD be used.

   The following SASL-based mechanisms are not considered in this
   document: KERBEROS_V4, GSSAPI and SKEY.





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RFC 2829            Authentication Methods for LDAP             May 2000


   The "EXTERNAL" SASL mechanism can be used to request the LDAP server
   make use of security credentials exchanged by a lower layer. If a TLS
   session has not been established between the client and server prior
   to making the SASL EXTERNAL Bind request and there is no other
   external source of authentication credentials (e.g.  IP-level
   security [8]), or if, during the process of establishing the TLS
   session, the server did not request the client's authentication
   credentials, the SASL EXTERNAL bind MUST fail with a result code of
   inappropriateAuthentication.  Any client authentication and
   authorization state of the LDAP association is lost, so the LDAP
   association is in an anonymous state after the failure.

9. Authorization Identity

   The authorization identity is carried as part of the SASL credentials
   field in the LDAP Bind request and response.

   When the "EXTERNAL" mechanism is being negotiated, if the credentials
   field is present, it contains an authorization identity of the
   authzId form described below.

   Other mechanisms define the location of the authorization identity in
   the credentials field.

   The authorization identity is a string in the UTF-8 character set,
   corresponding to the following ABNF [7]:

   ; Specific predefined authorization (authz) id schemes are
   ; defined below -- new schemes may be defined in the future.

   authzId    = dnAuthzId / uAuthzId

   ; distinguished-name-based authz id.
   dnAuthzId  = "dn:" dn