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LDAP Administration Guide
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13.2. SASL Authentication

Getting basic SASL authentication running involves a few steps. The first step configures your slapd server environment so that it can communicate with client programs using the security system in place at your site. This usually involves setting up a service key, a public key, or other form of secret. The second step concerns mapping authentication identities to LDAP DN's, which depends on how entries are laid out in your directory. An explanation of the first step will be given in the next section using Kerberos V4 as an example mechanism. The steps necessary for your site's authentication mechanism will be similar, but a guide to every mechanism available under SASL is beyond the scope of this chapter. The second step is described in the section Mapping Authentication Identities.

13.2.1. GSSAPI

This section describes the use of the SASL GSSAPI mechanism and Kerberos V with OpenLDAP. It will be assumed that you have Kerberos V deployed, you are familiar with the operation of the system, and that your users are trained in its use. This section also assumes you have familiarized yourself with the use of the GSSAPI mechanism by reading Configuring GSSAPI and Cyrus SASL (provided with Cyrus SASL in the doc/gssapi file) and successfully experimented with the Cyrus provided sample_server and sample_client applications. General information about Kerberos is available at http://web.mit.edu/kerberos/www/.

To use the GSSAPI mechanism with slapd(8) one must create a service key with a principal for ldap service within the realm for the host on which the service runs. For example, if you run slapd on directory.example.com and your realm is EXAMPLE.COM, you need to create a service key with the principal:

        ldap/directory.example.com@EXAMPLE.COM

When slapd(8) runs, it must have access to this key. This is generally done by placing the key into a keytab file, /etc/krb5.keytab. See your Kerberos and Cyrus SASL documentation for information regarding keytab location settings.

To use the GSSAPI mechanism to authenticate to the directory, the user obtains a Ticket Granting Ticket (TGT) prior to running the LDAP client. When using OpenLDAP client tools, the user may mandate use of the GSSAPI mechanism by specifying -Y GSSAPI as a command option.

For the purposes of authentication and authorization, slapd(8) associates an authentication request DN of the form:

        uid=<primary[/instance]>,cn=<realm>,cn=gssapi,cn=auth

Continuing our example, a user with the Kerberos principal kurt@EXAMPLE.COM would have the associated DN:

        uid=kurt,cn=example.com,cn=gssapi,cn=auth

and the principal ursula/admin@FOREIGN.REALM would have the associated DN:

        uid=ursula/admin,cn=foreign.realm,cn=gssapi,cn=auth

The authentication request DN can be used directly ACLs and groupOfNames "member" attributes, since it is of legitimate LDAP DN format. Or alternatively, the authentication DN could be mapped before use. See the section Mapping Authentication Identities for details.

13.2.2. KERBEROS_V4

This section describes the use of the SASL KERBEROS_V4 mechanism with OpenLDAP. It will be assumed that you are familiar with the workings of the Kerberos IV security system, and that your site has Kerberos IV deployed. Your users should be familiar with authentication policy, how to receive credentials in a Kerberos ticket cache, and how to refresh expired credentials.


Note: KERBEROS_V4 and Kerberos IV are deprecated in favor of GSSAPI and Kerberos V.

Client programs will need to be able to obtain a session key for use when connecting to your LDAP server. This allows the LDAP server to know the identity of the user, and allows the client to know it is connecting to a legitimate server. If encryption layers are to be used, the session key can also be used to help negotiate that option.

The slapd server runs the service called "ldap", and the server will require a srvtab file with a service key. SASL aware client programs will be obtaining an "ldap" service ticket with the user's ticket granting ticket (TGT), with the instance of the ticket matching the hostname of the OpenLDAP server. For example, if your realm is named EXAMPLE.COM and the slapd server is running on the host named directory.example.com, the /etc/srvtab file on the server will have a service key

        ldap.directory@EXAMPLE.COM

When an LDAP client is authenticating a user to the directory using the KERBEROS_IV mechanism, it will request a session key for that same principal, either from the ticket cache or by obtaining a new one from the Kerberos server. This will require the TGT to be available and valid in the cache as well. If it is not present or has expired, the client may print out the message:

        ldap_sasl_interactive_bind_s: Local error

When the service ticket is obtained, it will be passed to the LDAP server as proof of the user's identity. The server will extract the identity and realm out of the service ticket using SASL library calls, and convert them into an authentication request DN of the form

        uid=<username>,cn=<realm>,cn=<mechanism>,cn=auth

So in our above example, if the user's name were "adamson", the authentication request DN would be:

        uid=adamsom,cn=example.com,cn=kerberos_v4,cn=auth

This authentication request DN can be used directly ACLs or, alternatively, mapped prior to use. See the section Mapping Authentication Identities for details.

13.2.3. DIGEST-MD5

This section describes the use of the SASL DIGEST-MD5 mechanism using secrets stored either in the directory itself or in Cyrus SASL's own database. DIGEST-MD5 relies on the client and the server sharing a "secret", usually a password. The server generates a challenge and the client a response proving that it knows the shared secret. This is much more secure than simply sending the secret over the wire.

Cyrus SASL supports several shared-secret mechanisms. To do this, it needs access to the plaintext password (unlike mechanisms which pass plaintext passwords over the wire, where the server can store a hashed version of the password).

The server's copy of the shared-secret may be stored in Cyrus SASL's own sasldb database, in an external system accessed via saslauthd, or in LDAP database itself. In either case it is very important to apply file access controls and LDAP access controls to prevent exposure of the passwords. The configuration and commands discussed in this section assume the use of Cyrus SASL 2.1.

To use secrets stored in sasldb, simply add users with the saslpasswd2 command:

       saslpasswd2 -c <username>

The passwords for such users must be managed with the saslpasswd2 command.

To use secrets stored in the LDAP directory, place plaintext passwords in the userPassword attribute. It will be necessary to add an option to slapd.conf to make sure that passwords set using the LDAP Password Modify Operation are stored in plaintext:

       password-hash   {CLEARTEXT}

Passwords stored in this way can be managed either with ldappasswd(1) or by simply modifying the userPassword attribute. Regardless of where the passwords are stored, a mapping will be needed from authentication request DN to user's DN.

The DIGEST-MD5 mechanism produces authentication IDs of the form:

        uid=<username>,cn=<realm>,cn=digest-md5,cn=auth

If the default realm is used, the realm name is omitted from the ID, giving:

        uid=<username>,cn=digest-md5,cn=auth

See Mapping Authentication Identities below for information on optional mapping of identities.

With suitable mappings in place, users can specify SASL IDs when performing LDAP operations and sldb}} and the directory itself will be used to verify the authentication. For example, the user identified by the directory entry:

       dn: cn=Andrew Findlay+uid=u000997,dc=example,dc=com
       objectclass: inetOrgPerson
       objectclass: person
       sn: Findlay
       uid: u000997
       userPassword: secret

can issue commands of the form:

       ldapsearch -Y DIGEST-MD5 -U u000997 ...


Note: in each of the above cases, no authorization identity (e.g. -X) was provided. Unless you are attempting SASL Proxy Authorization, no authorization identity should be specified. The server will infer an authorization identity from authentication identity (as described below).

13.2.4. Mapping Authentication Identities

The authentication mechanism in the slapd server will use SASL library calls to obtain the authenticated user's "username", based on whatever underlying authentication mechanism was used. This username is in the namespace of the authentication mechanism, and not in the normal LDAP namespace. As stated in the sections above, that username is reformatted into an authentication request DN of the form

        uid=<username>,cn=<realm>,cn=<mechanism>,cn=auth

or

        uid=<username>,cn=<mechanism>,cn=auth

depending on whether or not <mechanism> employs the concept of "realms". Note also that the realm part will be omitted if the default realm was used in the authentication.

The ldapwhoami(1) command may be used to determine the identity associated with the user. It is very useful for determining proper function of mappings.

It is not intended that you should add LDAP entries of the above form to your LDAP database. Chances are you have an LDAP entry for each of the persons that will be authenticating to LDAP, laid out in your directory tree, and the tree does not start at cn=auth. But if your site has a clear mapping between the "username" and an LDAP entry for the person, you will be able to configure your LDAP server to automatically map a authentication request DN to the user's authentication DN.


Note: it is not required that the authentication request DN nor the user's authentication DN resulting from the mapping refer to an entry held in the directory. However, additional capabilities become available (see below).

The LDAP administrator will need to tell the slapd server how to map an authentication request DN to a user's authentication DN. This is done by adding one or more authz-regexp directives to the slapd.conf(5) file. This directive takes two arguments:

        authz-regexp   <search pattern>   <replacement pattern>

The authentication request DN is compared to the search pattern using the regular expression functions regcomp() and regexec(), and if it matches, it is rewritten as the replacement pattern. If there are multiple authz-regexp directives, only the first whose search pattern matches the authentication identity is used. The string that is output from the replacement pattern should be the authentication DN of the user or an LDAP URL. If replacement string produces a DN, the entry named by this DN need not be held by this server. If the replace string produces an LDAP URL, that LDAP URL must evaluate to one and only one entry held by this server.

The search pattern can contain any of the regular expression characters listed in regexec(3C). The main characters of note are dot ".", asterisk "*", and the open and close parenthesis "(" and ")". Essentially, the dot matches any character, the asterisk allows zero or more repeats of the immediately preceding character or pattern, and terms in parenthesis are remembered for the replacement pattern.

The replacement pattern will produce either a DN or URL referring to the user. Anything from the authentication request DN that matched a string in parenthesis in the search pattern is stored in the variable "$1". That variable "$1" can appear in the replacement pattern, and will be replaced by the string from the authentication request DN. If there were multiple sets of parentheses in the search pattern, the variables $2, $3, etc are used.

13.2.5. Direct Mapping

Where possible, direct mapping of the authentication request DN to the user's DN is generally recommended. Aside from avoiding the expense of searching for the user's DN, it allows mapping to DNs which refer to entries not held by this server.

Suppose the authentication request DN is written as:

        uid=adamson,cn=example.com,cn=gssapi,cn=auth

and the user's actual LDAP entry is:

        uid=adamson,ou=people,dc=example,dc=com

then the following authz-regexp directive in slapd.conf(5) would provide for direct mapping.

        authz-regexp
          uid=([^,]*),cn=example.com,cn=gssapi,cn=auth
          uid=$1,ou=people,dc=example,dc=com

An even more lenient rule could be written as

        authz-regexp
          uid=([^,]*),cn=[^,]*,cn=auth
          uid=$1,ou=people,dc=example,dc=com

Be careful about setting the search pattern too leniently, however, since it may mistakenly allow persons to become authenticated as a DN to which they should not have access. It is better to write several strict directives than one lenient directive which has security holes. If there is only one authentication mechanism in place at your site, and zero or one realms in use, you might be able to map between authentication identities and LDAP DN's with a single authz-regexp directive.

Don't forget to allow for the case where the realm is omitted as well as the case with an explicitly specified realm. This may well require a separate authz-regexp directive for each case, with the explicit-realm entry being listed first.

13.2.6. Search-based mappings

There are a number of cases where mapping to a LDAP URL may be appropriate. For instance, some sites may have person objects located in multiple areas of the LDAP tree, such as if there were an ou=accounting tree and an ou=engineering tree, with persons interspersed between them. Or, maybe the desired mapping must be based upon information in the user's information. Consider the need to map the above authentication request DN to user whose entry is as follows:

        dn: cn=Mark Adamson,ou=People,dc=Example,dc=COM
        objectclass: person
        cn: Mark Adamson
        uid: adamson

The information in the authentication request DN is insufficient to allow the user's DN to be directly derived, instead the user's DN must be searched for. For these situations, a replacement pattern which produces a LDAP URL can be used in the authz-regexp directives. This URL will then be used to perform an internal search of the LDAP database to find the person's authentication DN.

An LDAP URL, similar to other URL's, is of the form

        ldap://<host>/<base>?<attrs>?<scope>?<filter>

This contains all of the elements necessary to perform an LDAP search: the name of the server <host>, the LDAP DN search base <base>, the LDAP attributes to retrieve <attrs>, the search scope <scope> which is one of the three options "base", "one", or "sub", and lastly an LDAP search filter <filter>. Since the search is for an LDAP DN within the current server, the <host> portion should be empty. The <attrs> field is also ignored since only the DN is of concern. These two elements are left in the format of the URL to maintain the clarity of what information goes where in the string.

Suppose that the person in the example from above did in fact have an authentication username of "adamson" and that information was kept in the attribute "uid" in their LDAP entry. The authz-regexp directive might be written as

        authz-regexp
          uid=([^,]*),cn=example.com,cn=gssapi,cn=auth
          ldap:///ou=people,dc=example,dc=com??one?(uid=$1)

This will initiate an internal search of the LDAP database inside the slapd server. If the search returns exactly one entry, it is accepted as being the DN of the user. If there are more than one entries returned, or if there are zero entries returned, the authentication fails and the user's connection is left bound as the authentication request DN.

The attributes that are used in the search filter <filter> in the URL should be indexed to allow faster searching. If they are not, the authentication step alone can take uncomfortably long periods, and users may assume the server is down.

A more complex site might have several realms in use, each mapping to a different subtree in the directory. These can be handled with statements of the form:

        # Match Engineering realm
        authz-regexp
           uid=([^,]*),cn=engineering.example.com,cn=digest-md5,cn=auth
           ldap:///dc=eng,dc=example,dc=com??one?(&(uid=$1)(objectClass=person))

        # Match Accounting realm
        authz-regexp
           uid=([^,].*),cn=accounting.example.com,cn=digest-md5,cn=auth
           ldap:///dc=accounting,dc=example,dc=com??one?(&(uid=$1)(objectClass=person))

        # Default realm is customers.example.com
        authz-regexp
           uid=([^,]*),cn=digest-md5,cn=auth
           ldap:///dc=customers,dc=example,dc=com??one?(&(uid=$1)(objectClass=person))

Note that the explicitly-named realms are handled first, to avoid the realm name becoming part of the UID. Also note the use of scope and filters to limit matching to desirable entries.

Note as well that authz-regexp internal search are subject to access controls. Specifically, the authentication identity must have auth access.

See slapd.conf(5) for more detailed information.


LDAP Administration Guide
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