openldap/doc/guide/admin/sasl.sdf

# Copyright 1999-2000, The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.

H1: SASL

  OpenLDAP clients and servers are capable of providing authentication
via the Simple Authentication Security Layer (SASL) system, which is
explained in RFC 2222. 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.

  Note that in the following text the term "{{user}}" is used to
describe a person who is connecting to the LDAP server via a client
program, such as {{ldapsearch}}(1). The term can also be used to
describe a computer program that runs itself and accesses the LDAP
database, such as a sendmail program or a nightly update program run
out of cron. Thus {{"user"}} refers to any computer process connecting
to the LDAP server, whether or not it has a human monitoring it.


H2: SASL Authentication

  Getting basic SASL authentication running involves a few simple
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: MIT Kerberos V4

  It will be assumed that you are familiar with the workings of MIT's
Kerberos security system, and that your site has this mechanism in
place. 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 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 a SASL client is authenticating a user to LDAP, 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 take the
user's username 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=authzid

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=AUTHZID

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 <realm>
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=<realm>" 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=<username>,cn=<realm>,cn=<mechanism>,cn=authzid

or

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

depending on whether or not <mechanism> 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=authzid. 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   <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: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 URI, 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=AUTHZID

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=authzid
>	  uid=$1,ou=person,dc=example,dc=com

An even more lenient rule could be written as

>	saslRegexp
>	  uid=(.*),.*cn=authzid 
>	  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 URI,
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 URI.  This URI will then be used to perform an internal search of
the LDAP database to find the person's authentication DN.

  An LDAP URI, similar to other URI'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 on the
local machine, the <host> portion is ignored. By the same token the
<attrs> field is also ignored since only the DN is of concern.  These
two elements are left in the format of the URI 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=authzid  
>	  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 <scope> in the URI is "base", then the
only LDAP entry that will be returned is the searchbase DN <base>, 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 <filter> in the
URI 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 library 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:<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
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=<username>,cn=<realm>,cn=authzid

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 URI, 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 "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 saslAuthzTo
rules specify the authorization is permitted, the 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 URI. 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 saslAuthzTo value is an LDAP URI, the URI 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 URI in a saslAuthzTo or saslAuthzFrom attribute will return
a list of DN's, and that list must be linearly scanned. Searches
which return a long list 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 saslAuthzFrom and
saslAuthzTo, the values of these attributes are allowed to be DN's
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 URI 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:<username>" 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.