# Copyright 1999-2000, The OpenLDAP Foundation, All Rights Reserved. # COPYING RESTRICTIONS APPLY, see COPYRIGHT. H1: Using SASL OpenLDAP clients and servers are capable of authenticating via the {{TERM[expand]SASL}} ({{TERM:SASL}}) framework, which is detailed in {{REF:RFC2222}}. This chapter describes how to make use of SASL in OpenLDAP. There are several industry standard authentication mechanisms that can be used with SASL, including Kerberos V4, GSSAPI, and some of the Digest mechanisms. The standard client tools provided with OpenLDAP, such as {{ldapsearch}}(1) and {{ldapmodify}}(1), will by default attempt to authenticate the user to the {{slapd}}(8) server using SASL. Basic authentication service can be set up by the LDAP administrator with a few steps, allowing users to be authenticated to the slapd server as their LDAP entry. With a few extra steps, some users and services can be allowed to exploit SASL's authorization feature, allowing them to authenticate themselves and then switch their identity to that of another user or service. This chapter assumes you have read {{Cyrus SASL for System Administrators}}, provided with the {{PRD:Cyrus}} {{PRD:SASL}} package (in {{FILE:doc/sysadmin.html}}). Note that in the following text the term {{user}} is used to describe a person or application entity who is connecting to the LDAP server via an LDAP client, such as {{ldapsearch}}(1). That is, the term {{user}} not ony applies to both an individual using an LDAP client, but to an application entity which issues LDAP client operations without direct user control. For example, an e-mail server which uses LDAP operations to access information held in an LDAP server is an application entity. H2: SASL Security Considerations SASL offers many different authentication mechanisms. This section briefly outlines security considerations. Some mechanisms, such as PLAIN and LOGIN, offer no greater security over LDAP "simple" authentication. Like "simple" authentication, such mechanisms should not be used unless you have adequate security protections in place. It is recommended that these mechanism be used only in conjunction with {{TERM[expand]TLS}} (TLS). Use of PLAIN and LOGIN are not discussed further in this document. The DIGEST-MD5 mechanism is the mandatory-to-implement authentication mechanism for LDAPv3. Though DIGEST-MD5 is not a strong authentication mechanism in comparison with trusted third party authentication systems (such as Kerberos or public key systems), yet it does offer significant protections against a number of attacks. Unlike the CRAM-MD5 mechanism, it prevents chosen plaintext attacks. DIGEST-MD5 is favored over the weaker and even more dangerous use of plaintext password mechanisms. The CRAM-MD5 mechanism is deprecated in favor of DIGEST-MD5. Use of {{SECT:DIGEST-MD5}} is discussed below. The KERBEROS_V4 mechanism utilizes Kerberos IV to provide secure authentication services. There are also GSSAPI based mechanisms which is generally used in conjunction with Kerberos V. Kerberos is viewed as a secure, distributed authentication system suitable for both small and large enterprises. Use of {{SECT:KERBEROS_V4}} and {{SECT:GSSAPI}} are discussed below. The EXTERNAL mechanism utilizes authentication services provided by lower level network services such as {{TERM:TLS}} (TLS). When used in conjunction with TLS X.509-based public key technology, EXTERNAL offers strong authentication. Use of EXTERNAL is discussed in the {{SECT:Using TLS}} chapter. There are other strong authentication mechanisms to choose from, including OTP (one time passwords) and SRP (secure remote passwords). These mechanisms are not discussed in this document. H2: 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 next section after that describes the second step of mapping authentication identities to DN's. H3: 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 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 {{FILE:doc/gssapi}} file) and successfully experimented with the Cyrus provided sample_server and sample_client applications. General information about Kerberos is available at {{URL: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 your run {{slapd}} on {{EX:directory.example.com}} and your realm is {{EX: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, such as {{FILE:/etc/krb5.keytab}}. 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 {{EX:-Y GSSAPI}} as a command option. For the purposes of authentication and authorization, {{slapd}}(8) associates a non-mapped authentication DN of the form: > uid=principal,cn=GSSAPI,cn=auth If the user principal is within the same realm, the realm is trimmed from the principal. Continuting our example, a user with the Kerberos principal {{EX:kurt@EXAMPLE.COM}} would have the associated DN: > uid=kurt,cn=GSSAPI,cn=auth and the principal {{EX:ursula@@FORIEGN.REALM}} would have the associated DN: > uid=ursula@FOREIGN-REALM,cn=GSSAPI,cn=auth H3: 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 Kerberos IV security system, and that your site has Kerberos IV deployed. Your users should be familiar with authentication policy, are aware of how to receive credentials in a Kerberos ticket cache, and how to refresh expired credentials. 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 {{EX:EXAMPLE.COM}} and the slapd server is running on the host named {{EX:directory.example.com}}, the {{FILE:/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, SASL will 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=,cn=,cn=,cn=auth So in our above example, if the user's name were "adamson", the authentication request DN would be: > uid=ADAMSON,cn=EXAMPLE.COM,cn=KERBEROS_V4,cn=AUTH This authentication request DN by itself could be placed into ACL's and {{EX:groupOfNames}} "member" attributes, since it is of legitimate LDAP DN format. The next section, however, tells how to map that DN into the DN of a person's own LDAP entry. Also note that this example, being for Kerberos, shows the portion of the DN being filled in with the Kerberos realm of the company. Several other authentication mechanisms do not emply the concept of a realm, so the ",cn=" portion of the authentication request DN would not appear. H3: Mapping Authentication identities to LDAP entries 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 LDAP namespace. As stated in the section above, that username is reformatted into an authentication request DN of the form > uid=,cn=,cn=,cn=auth or > uid=,cn=,cn=auth depending on whether or not employs the concept of "realms". 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 people 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 user's authentication username to their {{authentication DN}}. 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 {{EX:saslRegexp}} directives to the {{slapd.conf}}(5) file. This directive takes two arguments: > saslRegexp 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 {{EX:saslRegexp}} 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, in a legitimate LDAP DN format. It can also be an LDAP URL, which is discussed below. 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 matches one or more characters, and terms in parenthesis are remembered for the replacement pattern. The replacement pattern will produce the final authentication DN of 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 parenthesis in the search pattern, the variables $2, $3, etc are used. For example, suppose the user's authentication identity is written as the DN string > uid=ADAMSON,cn=EXAMPLE.COM,cn=KERBEROS_V4,cn=AUTH and the user's actual LDAP entry is > uid=ADAMSON,ou=PERSON,dc=EXAMPLE,dc=COM The {{EX:saslRegexp}} directive in {{slapd.conf}}(5) could be written > saslRegexp > uid=(.*),cn=example.com,cn=kerberos_v4,cn=auth > uid=$1,ou=person,dc=example,dc=com An even more lenient rule could be written as > saslRegexp > uid=(.*),.*cn=auth > uid=$1,ou=person,dc=example,dc=com Be careful about setting the search pattern too leniently, however, since it may mistakenly allow people 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 {{EX:saslRegexp}} directive. Some sites may have people's DN's spread to multiple areas of the LDAP tree, such as if there were an ou=accounting tree and an ou=engineering tree, with people interspersed between them. Or there may not be enough information in the authentication identity to isolate the DN, such as if the above person's LDAP entry looked like > dn: cn=mark adamson,ou=person,dc=example,dc=com > objectclass: Person > cn: mark adamson > uid: adamson In this case, the information in the authentication identity can only be used to search for the user's DN, not derive it directly. For both of these situations, and others, the replacement pattern in the {{EX:saslRegexp}} directives will need to produce an LDAP URL, described in the next section. H3: Performing searches for a person's DN When there is not enough information in the authentication identity to derive a person's authentication DN directly, the {{EX:saslRegexp}} directives in the {{slapd.conf}}(5) file will need to produce an LDAP URL. 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:///??? This contains all of the elements necessary to perform an LDAP search: the name of the server , the LDAP DN search base , the LDAP attributes to retrieve , the search scope which is one of the three options "base", "one", or "sub", and lastly an LDAP search filter . Since the search is for an LDAP DN on the local machine, the portion is ignored. By the same token the 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 {{EX:saslRegexp}} directive might be written as > saslRegexp > uid=(.*),cn=example.com,cn=kerberos_v4,cn=auth > ldap://localhost/ou=person,dc=example,dc=com??sub?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. Note that if the search scope in the URL is "base", then the only LDAP entry that will be returned is the searchbase DN , so the actual search of the database is skipped. This is equivalent to setting the replacement pattern in the directive to a DN directly, as in the section above. The attributes that are used in the search 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. H2: SASL Authorization The SASL offers a feature known as {{authorization}}, which allows an authenticated user to request that they act on the behalf of another user. This step occurs after the user has obtained an authentication DN, and involves sending an authorization identity to the server. The server will then make a decision on whether or not to allow the authorization to occur. If it is allowed, the user's LDAP connection is switched to have a binding DN derived from the authorization identity, and the LDAP session proceeds with the access of the new authorization DN. The decision to allow an authorization to proceed depends on the rules and policies of the site where LDAP is running, and thus cannot be made by SASL alone. The SASL library leaves it up to the server to make the decision. The LDAP administrator sets the guidelines of who can authorize to what identity by adding information into the LDAP database entries. H3: Uses of Authorization This sort of service is useful when one entity needs to act on the behalf of many other users. For example, users may be directed to a web page to make changes to their personal information in their LDAP entry. The users authenticate to the web server to establish their identity, but the web server CGI cannot authenticate to the LDAP server as that user to make changes for them. Instead, the web server authenticates itself to the LDAP server as a service identity, say, > cn=WebUpdate,dc=example,dc=com and then it will SASL authorize to the DN of the user. Once so authorized, the CGI makes changes to the LDAP entry of the user, and as far as the slapd server can tell for its ACLs, it is the user themself on the other end of the connection. The user could have connected to the LDAP server directly and authenticated as themself, but that would require the user to have more knowledge of LDAP clients, knowledge which the web page provides in an easier format. Authorization can also be used to limit access to an account that has greater access to the database. Such an account, perhaps even the root DN specified in {{slapd.conf}}(5), can have a strict list of people who can authorize to that DN. Changes to the LDAP database could then be only allowed by that DN, and in order to become that DN, users must first authenticate as one of the persons on the list. This allows for better auditing of who made changes to the LDAP database. If people were allowed to authenticate directly to the priviliged account, possibly through the {{EX:rootpw}} {{slapd.conf}}(5) directive or through a {{EX:userPassword}} attribute, then auditing becomes more difficult. Note that after a successful authorization, the original authentication DN in the LDAP connection is overwritten by the new DN from the authorization request. If a service program is able to authenticate itself as its own authentication DN and then authorize to other DN's, and it is planning on switching to several different identities during one LDAP session, it will need to authenticate itself each time before authorizing to another DN. The slapd server does not keep record of the service program's ability to switch to other DN's. On authentication mechanisms like Kerberos this will not require multiple connections being made to the Kerberos server, since the user's TGT and "ldap" session key are valid for multiple uses for the several hours of the ticket lifetime. H3: Authorization Identities The authorization identity is sent to the slapd server via the -X switch for {{ldapsearch}}(1) and other tools, or in the *authzid parameter to the {{lutil_sasl_defaults}}() call. The identity can be in one of two forms, either > u: or > dn: In the first form, the is from the same namespace as the authentication identities above. It is the user's username as it is refered to by the underlying authentication mechanism. Authorization identities of this form are converted into a DN format by the same function that the authentication process used, producing an {{authorization request DN}} of the form > uid=,cn=,cn=auth That authorization request DN is then run through the same {{EX:saslRegexp}} process to convert it into a legitimate authorization DN from the database. If it cannot be converted due to a failed search from an LDAP URL, the authorization request fails with "inappropriate access". Otherwise, the DN string is now a legitimate authorization DN ready to undergo approval. If the authorization identity was provided in the second form, with a {EX:"dn:"}} prefix, the string after the prefix is already in authorization DN form, ready to undergo approval. H3: Authorization rules Once slapd has the authorization DN, the actual approval process begins. There are two attributes that the LDAP administrator can put into LDAP entries to allow authorization: > saslAuthzTo > saslAuthzFrom Both can be multivalued. The first is called a source rule, and it is placed into a person's authentication DN entry to tell what other authorization DN's the person is allowed to change to. The second form is called a destination rule, and it is placed into an authorization DN's entry to tell what authenticated DN a person must be coming from in order to switch to that authorization DN. The choice of which form to use is up to the administrator. Source rules are checked first in the person's authentication DN entry, and if none of the {{EX:saslAuthzTo}} rules specify the authorization is permitted, the {{EX:saslAuthzFrom}} rules in the authorization DN entry are then checked. If neither case specifies that the request be honored, the request is denied with an "inappropriate access" message. Since the default behaviour is to deny authorization requests, rules only specify that a request be allowed; there are no negative rules telling what authorizations to deny. The value(s) in the two attributes are of the same form as the output of the replacement pattern of a {{EX:saslRegexp}} directive: either a DN or an LDAP URL. For example, if a saslAuthzTo value is a DN, that DN is one the authenticated user can authorize to. On the other hand, if the {{EX:saslAuthzTo}} value is an LDAP URL, the URL is used as an internal search of the LDAP database, and the authenticated user can become ANY DN returned by the search. If an LDAP entry looked like: > dn: cn=WebUpdate,dc=example,dc=com > saslAuthzTo: ldap://host/dc=example,dc=com??sub?objectclass=Person then any user who authenticated as cn=WebUpdate,dc=example,dc=com could authorize to any other LDAP entry under the search base "dc=example,dc=com" which has an objectClass of "Person". H4: Notes on Authorization rules An LDAP URL in a {{EX:saslAuthzTo}} or {{EX:saslAuthzFrom}} attribute will return a set of DNs. Each DN returned will be checked. Searches which return a large set can cause the authorization process to take an uncomfortably long time. Also, searches should be performed on attributes that have been indexed by slapd. To help produce more sweeping rules for {{EX:saslAuthzFrom}} and {{EX:saslAuthzTo}}, the values of these attributes are allowed to be DNs with regular expression characters in them. This means a source rule like > saslAuthzTo: uid=.*,dc=example,dc=com would allow that authenticated user to authorize to any DN that matches the regular expression pattern given. This regular expression comparison can be evaluated much faster than an LDAP search for "uid=*". Also note that the values in an authorization rule must be one of the two forms: an LDAP URL or a DN (with or without regular expression characters). Anything that does not begin with "ldap://" is taken as a DN. It is not permissable to enter another authorization identity of the form "u:" as an authorization rule. The decision of which type of rules to use, saslAuthzFrom or saslAuthzTo, will depend on the site's situation. For example, if the set of people who may become a given identity can easily be written as a search filter, then a single destination rule could be written. If the set of people is not easily defined by a search filter, and the set of people is small, it may be better to write a source rule in the entries of each of those people who should be allowed to perform the authorization.