guide/admin/sasl.sdf

# OpenLDAP: pkg/openldap-guide/admin/sasl.sdf,v 1.34.2.10 2009/04/29 01:27:12 quanah Exp
# Copyright 1999-2009 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:RFC4422}}.   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 {{TERM:GSSAPI}} for {{TERM:Kerberos}}
V, {{TERM:DIGEST-MD5}}, and {{TERM:PLAIN}} and {{TERM:EXTERNAL}}
for use with {{TERM[expand]TLS}} (TLS).

The standard client tools provided with OpenLDAP Software, such as
{{ldapsearch}}(1) and {{ldapmodify}}(1), will by default attempt
to authenticate the user to the {{TERM:LDAP}} directory 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 proxy
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 SASL}}
package (in {{FILE:doc/sysadmin.html}}) and have a working Cyrus
SASL installation.  You should use the Cyrus SASL {{EX:sample_client}}
and {{EX:sample_server}} to test your SASL installation before
attempting to make use of it with OpenLDAP Software.

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 only 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 LDAP {{simple}}
authentication, such mechanisms should not be used unless you have
adequate security protections in place.  It is recommended that
these mechanisms 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 {{TERM:Kerberos}} or public key systems), it does
offer significant protections against a number of attacks.  Unlike
the {{TERM:CRAM-MD5}} mechanism, it prevents chosen plaintext
attacks.  DIGEST-MD5 is favored over the 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 GSSAPI mechanism utilizes {{TERM:GSS-API}} {{TERM:Kerberos}} V
to provide secure authentication services.  The KERBEROS_V4 mechanism
is available for those using Kerberos IV.  Kerberos is viewed as a
secure, distributed authentication system suitable for both small
and large enterprises.  Use of {{SECT:GSSAPI}} and {{SECT:KERBEROS_V4}}
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 {{TERM:TLS}} {{TERM: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 {{TERM:OTP}} (one time passwords) and {{TERM: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 {{TERM: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 {{SECT:Mapping
Authentication Identities}}.


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 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 {{FILE:doc/gssapi}} file) and successfully
experimented with the Cyrus provided {{EX:sample_server}} and
{{EX: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 you 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 file,
{{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 {{EX:-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
{{EX:kurt@EXAMPLE.COM}} would have the associated DN:

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

and the principal {{EX: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
{{EX:groupOfNames}} "member" attributes, since it is of legitimate
LDAP DN format.  Or alternatively, the authentication DN could be
mapped before use.  See the section {{SECT:Mapping Authentication
Identities}} for details.


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 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 {{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, 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 {{SECT:Mapping
Authentication Identities}} for details.


H3: 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 {{EX:userPassword}} attribute.  It will be necessary to add
an option to {{EX: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 {{EX: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 {{SECT: 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 the password stored in {{sasldb}} or in
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.
{{EX:-X}}) was provided.   Unless you are attempting {{SECT:SASL
Proxy Authorization}}, no authorization identity should be specified.
The server will infer an authorization identity from authentication
identity (as described below).


H3: 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 {{EX: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 {{EX: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.

H3: 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 {{EX: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 {{EX: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 {{EX:authz-regexp}} directive for each case, with
the explicit-realm entry being listed first.

H3: 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 {{EX:ou=accounting}} tree and an {{EX: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 {{EX: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 {{EX: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 {{EX:authz-regexp}} internal search are subject
to access controls.  Specifically, the authentication identity
must have {{EX:auth}} access.

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


H2: SASL Proxy Authorization

The SASL offers a feature known as {{proxy 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. By default, the authorization
features are disabled, and must be explicitly configured by the
LDAP administrator before use.


H3: Uses of Proxy 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.

Proxy 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 privileged 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 proxy authorization, the original
authentication DN of 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 (or use a different
proxy authorization mechanism).  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: SASL Authorization Identities

The SASL authorization identity is sent to the LDAP server via the
{{EX:-X}} switch for {{ldapsearch}}(1) and other tools, or in the
{{EX:*authzid}} parameter to the {{lutil_sasl_defaults}}() call.
The identity can be in one of two forms, either

>	u:<username>

or

>	dn:<dn>

In the first form, the <username> is from the same namespace as
the authentication identities above. It is the user's username as
it is referred 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=<username>,cn=<realm>,cn=<mechanism>,cn=auth

That authorization request DN is then run through the same
{{EX:authz-regexp}} 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: Proxy 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:

>	authzTo
>	authzFrom

Both can be multivalued.  The {{EX:authzTo}} attribute is a
source rule, and it is placed into the entry associated with the
authentication DN to tell what authorization DNs the authenticated
DN is allowed to assume.  The second attribute is a destination
rule, and it is placed into the entry associated with the requested
authorization DN to tell which authenticated DNs may assume it.

The choice of which authorization policy attribute 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:authzTo}} rules
specify the authorization is permitted, the {{EX:authzFrom}}
rules in the authorization DN entry are then checked. If neither
case specifies that the request be honored, the request is denied.
Since the default behavior 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:authz-regexp}} directive:
either a DN or an LDAP URL. For example, if a {{EX:authzTo}}
value is a DN, that DN is one the authenticated user can authorize
to. On the other hand, if the {{EX:authzTo}} 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
>	authzTo: ldap:///dc=example,dc=com??sub?(objectclass=person)

then any user who authenticated as {{EX:cn=WebUpdate,dc=example,dc=com}}
could authorize to any other LDAP entry under the search base
{{EX:dc=example,dc=com}} which has an objectClass of {{EX:Person}}.


H4: Notes on Proxy Authorization Rules

An LDAP URL in a {{EX:authzTo}} or {{EX:authzFrom}} 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:authzFrom}} and
{{EX:authzTo}}, the values of these attributes are allowed to
be DNs with regular expression characters in them. This means a
source rule like

>	authzTo: dn.regex:^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
{{EX:(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 "{{EX:ldap://}}" is
taken as a DN. It is not permissible to enter another authorization
identity of the form "{{EX:u:<username>}}" as an authorization rule.


H4: Policy Configuration

The decision of which type of rules to use, {{EX:authzFrom}}
or {{EX:authzTo}}, 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 proxy authorization.

By default, processing of proxy authorization rules is disabled.
The {{EX:authz-policy}} directive must be set in the
{{slapd.conf}}(5) file to enable authorization. This directive can
be set to {{EX:none}} for no rules (the default), {{EX:to}} for
source rules, {{EX:from}} for destination rules, or {{EX:both}} for
both source and destination rules.

Source rules are extremely powerful. If ordinary users have
access to write the {{EX:authzTo}} attribute in their own
entries, then they can write rules that would allow them to authorize
as anyone else.  As such, when using source rules, the
{{EX:authzTo}} attribute should be protected with an ACL that
only allows privileged users to set its values.