openldap/doc/guide/admin/sasl.sdf

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

H1: Using SASL

OpenLDAP clients and servers are capable of providing authentication
via the {{TERM[expand]SASL}} ({{TERM:SASL}}) system, which is
explained in {{REF:RFC2222}}. 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: Security Considerations

SASL offers many different authentication mechanisms.  This section
breifly outlines security considerations.

Some mechanisms, such as PLAIN and LOGIN, offer no 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, 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 weaker and even more dangerous
use of plaintext password mechanisms.  The CRAM-MD5 mechanism is
deprecated in favor of DIGEST-MD5.

# Use of DIGEST-MD5 is discussed below.

The KERBEROS_V4 mechanism utilizes Kerberos IV services to provide
secure authentication services.  There are also GSSAPI based
mechanisms which utilize Kerberos V.  Kerberos is viewed as a
secure, distributed authentication system.
Use of KERBEROS_V4 is discussed below.
#Use of KERBEROS_V4 and 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 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: 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 V4 security system, and that your site has
either Kerberos V4 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 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 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 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 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=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
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://<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
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=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 URL 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
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:<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 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 "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 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 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 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 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:<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.