<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="ldap">
    <info>
        <title>LDAP Authentication</title>
    </info>
    <section xml:id="ldap-overview">
        <info>
            <title>Overview</title>
        </info>
        <para>LDAP is often used by organizations as a central repository for user information and
            as an authentication service. It can also be used to store the role information for
            application users.</para>
        <para>There are many different scenarios for how an LDAP server may be configured so Spring
            Security's LDAP provider is fully configurable. It uses separate strategy interfaces for
            authentication and role retrieval and provides default implementations which can be
            configured to handle a wide range of situations.</para>
        <para>You should be familiar with LDAP before trying to use it with Spring Security. The
            following link provides a good introduction to the concepts involved and a guide to
            setting up a directory using the free LDAP server OpenLDAP:
            <uri xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.zytrax.com/books/ldap/">http://www.zytrax.com/books/ldap/</uri>.
            Some familiarity with the JNDI APIs used to access LDAP from Java may also be useful. We don't use any third-party LDAP libraries
            (Mozilla, JLDAP etc.) in the LDAP provider, but extensive use is made of Spring LDAP, so
            some familiarity with that project may be useful if you plan on adding your own
            customizations.</para>
    </section>
    <section>
        <info>
            <title>Using LDAP with Spring Security</title>
        </info>
        <para>
            LDAP authentication in Spring Security can be roughly divided into the following stages.
            <orderedlist inheritnum="ignore" continuation="restarts">
                <listitem>
                    <para>Obtaining the unique LDAP
                        <quote>Distinguished Name</quote>, or DN, from the login name. This will
                        often mean performing a search in the directory, unless the exact mapping of
                        usernames to DNs is known in advance.</para>
                </listitem>
                <listitem>
                    <para>Authenticating the user, either by binding as that user or by performing a
                        remote
                        <quote>compare</quote>
                        operation of the user's password against the password attribute in the
                        directory entry for the DN.</para>
                </listitem>
                <listitem>
                    <para>Loading the list of authorities for the user.</para>
                </listitem>
            </orderedlist>
            The exception is when the LDAP directory is just being used to retrieve user information
            and authenticate against it locally. This may not be possible as directories are often
            set up with limited read access for attributes such as user passwords.
        </para>
        <para>
            We will look at some configuration scenarios below. For full information on available
            configuration options, please consult the security namespace schema (information from
            which should be available in your XML editor).
        </para>
    </section>
    <section xml:id="ldap-server">
        <info>
            <title>Configuring an LDAP Server</title>
        </info>
        <para>
            The first thing you need to do is configure the server against which authentication
            should take place. This is done using the
            <literal>&lt;ldap-server&gt;</literal>
            element from the security namespace. This can be configured to point at an external LDAP
            server, using the
            <literal>url</literal>
            attribute:
            <programlisting><![CDATA[
  <ldap-server url="ldap://springframework.org:389/dc=springframework,dc=org" />
]]> 
            </programlisting>
        </para>
        <section>
            <info>
                <title>Using an Embedded Test Server</title>
            </info>
            <para>
                The
                <literal>&lt;ldap-server&gt;</literal>
                element can also be used to create an embedded server, which can be very useful for
                testing and demonstrations. In this case you use it without the
                <literal>url</literal>
                attribute:
                <programlisting><![CDATA[
  <ldap-server root="dc=springframework,dc=org"/>
 ]]> 
    </programlisting>
                Here we've specified that the root DIT of the directory should be
                <quote>dc=springframework,dc=org</quote>, which is the default. Used this way, the
                namespace parser will create an embedded Apache Directory server and scan the
                classpath for any LDIF files, which it will attempt to load into the server. You can
                customize this behaviour using the
                <literal>ldif</literal>
                attribute, which defines an LDIF resource to be loaded:
                <programlisting><![CDATA[
  <ldap-server ldif="classpath:users.ldif" />
        ]]></programlisting>
                This makes it a lot easier to get up and running with LDAP, since it can be
                inconvenient to work all the time with an external server. It also insulates the
                user from the complex bean configuration needed to wire up an Apache Directory
                server. Using plain Spring Beans the configuration would be much more cluttered. You
                must have the necessary Apache Directory dependency jars available for your
                application to use. These can be obtained from the LDAP sample application.
            </para>
        </section>
        <section>
            <info>
                <title>Using Bind Authentication</title>
            </info>
            <para>
                This is the most common LDAP authentication scenario.
                <programlisting><![CDATA[
  <ldap-authentication-provider user-dn-pattern="uid={0},ou=people"/>
                     ]]></programlisting>
                This simple example would obtain the DN for the user by substituting the user login
                name in the supplied pattern and attempting to bind as that user with the login
                password. This is OK if all your users are stored under a single node in the
                directory. If instead you wished to configure an LDAP search filter to locate the
                user, you could use the following:
                <programlisting><![CDATA[
  <ldap-authentication-provider user-search-filter="(uid={0})" 
          user-search-base="ou=people"/>
                    ]]></programlisting>
                If used with the server definition above, this would perform a search under the DN
                <literal>ou=people,dc=springframework,dc=org</literal>
                using the value of the
                <literal>user-search-filter</literal>
                attribute as a filter. Again the user login name is substituted for the parameter in
                the filter name. If
                <literal>user-search-base</literal>
                isn't supplied, the search will be performed from the root.
            </para>
        </section>
        <section>
            <info>
                <title>Loading Authorities</title>
            </info>
            <para>
                How authorities are loaded from groups in the LDAP directory is controlled by the
                following attributes.
                <itemizedlist>
                    <listitem>
                        <para>
                            <literal>group-search-base</literal>. Defines the part of the directory
                            tree under which group searches should be performed.</para>
                    </listitem>
                    <listitem>
                        <para>
                            <literal>group-role-attribute</literal>. The attribute which contains
                            the name of the authority defined by the group entry. Defaults to
                            <literal>cn</literal>
                        </para>
                    </listitem>
                    <listitem>
                        <para>
                            <literal>group-search-filter</literal>. The filter which is used to
                            search for group membership. The default is
                            <literal>uniqueMember={0}</literal>, corresponding to the
                            <literal>groupOfUniqueMembers</literal>
                            LDAP class. In this case, the substituted parameter is the full
                            distinguished name of the user. The parameter
                            <literal>{1}</literal>
                            can be used if you want to filter on the login name.</para>
                    </listitem>
                </itemizedlist>
                So if we used the following configuration
                <programlisting><![CDATA[
  <ldap-authentication-provider user-dn-pattern="uid={0},ou=people" 
          group-search-base="ou=groups" />
    ]]></programlisting>
                and authenticated successfully as user
                <quote>ben</quote>, the subsequent loading of authorities would perform a search
                under the directory entry
                <literal>ou=groups,dc=springframework,dc=org</literal>, looking for entries which
                contain the attribute
                <literal>uniqueMember</literal> with value <literal>uid=ben,ou=people,dc=springframework,dc=org</literal>.
                By default the authority names will have the prefix <literal>ROLE_</literal> prepended. You can
                change this using the <literal>role-prefix</literal> attribute. If you don't want any prefix, use
                <literal>role-prefix="none"</literal>. For more information
                on loading authorities, see the Javadoc for the
                <classname>DefaultLdapAuthoritiesPopulator</classname>
                class.
            </para>
        </section>
    </section>
    <section>
        <info>
            <title>Implementation Classes</title>
        </info>
        <para>The namespace configuration options we've used above are simple to use and much more
            concise than using Spring beans explicitly. There are situations when you may need to
            know how to configure Spring Security LDAP directly in your application context. You may
            wish to customize the behaviour of some of the classes, for example. If you're happy
            using namespace configuration then you can skip this section and the next one.
        </para>
        <para>
            The main LDAP provider class, <classname>LdapAuthenticationProvider</classname>,
            doesn't actually do much itself but delegates the work to two other beans, an
            <interfacename>LdapAuthenticator</interfacename>
            and an
            <interfacename>LdapAuthoritiesPopulator</interfacename>
            which are responsible for authenticating the user and retrieving the user's set of
            <interfacename>GrantedAuthority</interfacename>s respectively.</para>
        <section xml:id="ldap-ldap-authenticators">
            <info>
                <title>LdapAuthenticator Implementations</title>
            </info>
            <para>The authenticator is also responsible for retrieving any required user attributes.
                This is because the permissions on the attributes may depend on the type of
                authentication being used. For example, if binding as the user, it may be necessary
                to read them with the user's own permissions.</para>
            <para>There are currently two authentication strategies supplied with Spring Security:
                <itemizedlist>
                    <listitem>
                        <para>Authentication directly to the LDAP server ("bind" authentication).</para>
                    </listitem>
                    <listitem>
                        <para>Password comparison, where the password supplied by the user is
                            compared with the one stored in the repository. This can either be done
                            by retrieving the value of the password attribute and checking it
                            locally or by performing an LDAP "compare" operation, where the supplied
                            password is passed to the server for comparison and the real password
                            value is never retrieved.</para>
                    </listitem>
                </itemizedlist>
            </para>
            <section xml:id="ldap-ldap-authenticators-common">
                <info>
                    <title>Common Functionality</title>
                </info>
                <para>Before it is possible to authenticate a user (by either strategy), the
                    distinguished name (DN) has to be obtained from the login name supplied to the
                    application. This can be done either by simple pattern-matching (by setting the
                    <property>setUserDnPatterns</property>
                    array property) or by setting the
                    <property>userSearch</property>
                    property. For the DN pattern-matching approach, a standard Java pattern format
                    is used, and the login name will be substituted for the parameter
                    <parameter>{0}</parameter>. The pattern should be relative to the DN that the
                    configured
                    <interfacename>SpringSecurityContextSource</interfacename>
                    will bind to (see the section on
                    <link linkend="ldap-context-source">connecting to the LDAP server</link>
                    for more information on this). For example, if you are using an LDAP server with
                    the URL
                    <literal>ldap://monkeymachine.co.uk/dc=springframework,dc=org</literal>, and
                    have a pattern
                    <literal>uid={0},ou=greatapes</literal>, then a login name of "gorilla" will map
                    to a DN
                    <literal>uid=gorilla,ou=greatapes,dc=springframework,dc=org</literal>. Each
                    configured DN pattern will be tried in turn until a match is found. For
                    information on using a search, see the section on
                    <link linkend="ldap-searchobjects">search objects</link>
                    below. A combination of the two approaches can also be used - the patterns will
                    be checked first and if no matching DN is found, the search will be used.</para>
            </section>
            <section xml:id="ldap-ldap-authenticators-bind">
                <info>
                    <title>BindAuthenticator</title>
                </info>
                <para>The class <classname>BindAuthenticator</classname> in the package
                    <filename>org.springframework.security.ldap.authentication</filename>
                    implements the bind authentication strategy. It simply attempts to bind as the
                    user.</para>
            </section>
            <section xml:id="ldap-ldap-authenticators-password">
                <info>
                    <title>PasswordComparisonAuthenticator</title>
                </info>
                <para>The class <classname>PasswordComparisonAuthenticator</classname>
                    implements the password comparison authentication strategy.</para>
            </section>
            <section xml:id="ldap-ldap-authenticators-active-directory">
                <info>
                    <title>Active Directory Authentication</title>
                </info>
                <para>In addition to standard LDAP authentication (binding with a DN), Active
                    Directory has its own non-standard syntax for user authentication.</para>
            </section>
        </section>
        <section xml:id="ldap-context-source">
            <info>
                <title>Connecting to the LDAP Server</title>
            </info>
            <para>The beans discussed above have to be able to connect to the server. They both have
                to be supplied with a
                <interfacename>SpringSecurityContextSource</interfacename>
                which is an extension of Spring LDAP's
                <interfacename>ContextSource</interfacename>. Unless you have special requirements,
                you will usually configure a
                <classname>DefaultSpringSecurityContextSource</classname>
                bean, which can be configured with the URL of your LDAP server and optionally with
                the username and password of a "manager" user which will be used by default when
                binding to the server (instead of binding anonymously). For more information read
                the Javadoc for this class and for Spring LDAP's
                <classname>AbstractContextSource</classname>.
            </para>
        </section>
        <section xml:id="ldap-searchobjects">
            <info>
                <title>LDAP Search Objects</title>
            </info>
            <para>Often more a more complicated strategy than simple DN-matching is required to
                locate a user entry in the directory. This can be encapsulated in an
                <interfacename>LdapUserSearch</interfacename>
                instance which can be supplied to the authenticator implementations, for example, to
                allow them to locate a user. The supplied implementation is
                <classname>FilterBasedLdapUserSearch</classname>.</para>
            <section xml:id="ldap-searchobjects-filter">
                <info>
                    <title xml:id="ldap-searchobjects-filter-based">
                        <classname>FilterBasedLdapUserSearch</classname>
                    </title>
                </info>
                <para>This bean uses an LDAP filter to match the user object in the directory. The
                    process is explained in the Javadoc for the corresponding search method on the
                    <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://java.sun.com/j2se/1.4.2/docs/api/javax/naming/directory/DirContext.html#search(javax.naming.Name,%20java.lang.String,%20java.lang.Object[],%20javax.naming.directory.SearchControls)">JDK
                        DirContext class</link>.
                    As explained there, the search filter can be supplied with parameters. For this class, the only valid parameter is
                    <parameter>{0}</parameter>
                    which will be replaced with the user's login name.</para>
            </section>
        </section>
        <section xml:id="ldap-authorities">
            <title>LdapAuthoritiesPopulator</title>
            <para>
                After authenticating the user successfully, the <classname>LdapAuthenticationProvider</classname>
                will attempt to load a set of authorities for the user by calling the configured 
                <interfacename>LdapAuthoritiesPopulator</interfacename> bean. The <classname>DefaultLdapAuthoritiesPopulator</classname>
                is an implementation which will load the authorities by searching the directory for groups of which the user is a member
                (typically these will be <literal>groupOfNames</literal> or <literal>groupOfUniqueNames</literal> entries in the directory).
                Consult the Javadoc for this class for more details on how it works. 
            </para>
            <para>If you want to use LDAP only for authentication, but load the authorities from a difference source (such as a database) 
                then you can provide your own implementation of this interface and inject that instead.</para>
        </section>
        <section xml:id="ldap-bean-config">
            <info>
                <title>Spring Bean Configuration</title>
            </info>
            <para>A typical configuration, using some of the beans we've discussed here, might look
                like this:
                <programlisting><![CDATA[
<bean id="contextSource"
        class="org.springframework.security.ldap.DefaultSpringSecurityContextSource">
  <constructor-arg value="ldap://monkeymachine:389/dc=springframework,dc=org"/>
  <property name="userDn" value="cn=manager,dc=springframework,dc=org"/>
  <property name="password" value="password"/>
</bean>

<bean id="ldapAuthProvider"
    class="org.springframework.security.ldap.authentication.LdapAuthenticationProvider">
 <constructor-arg>
   <bean class="org.springframework.security.ldap.authentication.BindAuthenticator">
     <constructor-arg ref="contextSource"/>
     <property name="userDnPatterns">
       <list><value>uid={0},ou=people</value></list>
     </property>
   </bean>
 </constructor-arg>
 <constructor-arg>
   <bean 
     class="org.springframework.security.ldap.userdetails.DefaultLdapAuthoritiesPopulator">
     <constructor-arg ref="contextSource"/>
     <constructor-arg value="ou=groups"/>
     <property name="groupRoleAttribute" value="ou"/>
   </bean>
 </constructor-arg>
</bean>]]>
                </programlisting>
                This would set up the provider to access an LDAP server with URL
                <literal>ldap://monkeymachine:389/dc=springframework,dc=org</literal>.
                Authentication will be performed by attempting to bind with the DN
                <literal>uid=&lt;user-login-name&gt;,ou=people,dc=springframework,dc=org</literal>.
                After successful authentication, roles will be assigned to the user by searching
                under the DN
                <literal>ou=groups,dc=springframework,dc=org</literal>
                with the default filter
                <literal>(member=&lt;user's-DN&gt;)</literal>. The role name will be taken from the
                <quote>ou</quote>
                attribute of each match.</para>
            <para>To configure a user search object, which uses the filter
                <literal>(uid=&lt;user-login-name&gt;)</literal>
                for use instead of the DN-pattern (or in addition to it), you would configure the
                following bean
                <programlisting><![CDATA[
<bean id="userSearch"
    class="org.springframework.security.ldap.search.FilterBasedLdapUserSearch">
  <constructor-arg index="0" value=""/>
  <constructor-arg index="1" value="(uid={0})"/>
  <constructor-arg index="2" ref="contextSource" />
</bean> ]]>                    
                </programlisting>
                and use it by setting the <classname>BindAuthenticator</classname> bean's
                <property>userSearch</property>
                property. The authenticator would then call the search object to obtain the correct
                user's DN before attempting to bind as this user.</para>
        </section>
        <section xml:id="ldap-custom-user-details">
            <title>LDAP Attributes and Customized UserDetails</title>
            <para>
                The net result of an authentication using <classname>LdapAuthenticationProvider</classname> is the
                same as a normal Spring Security authentication using the standard <interfacename>UserDetailsService</interfacename>
                interface. A <interfacename>UserDetails</interfacename> object is created and stored in the 
                returned <interfacename>Authentication</interfacename> object. As with using a 
                <interfacename>UserDetailsService</interfacename>, a common requirement is to be able to customize this
                implementation and add extra properties. When using LDAP, these will normally be attributes from the user entry.
                The creation of the <interfacename>UserDetails</interfacename> object is controlled by the provider's 
                <interfacename>UserDetailsContextMapper</interfacename> strategy, which is responsible for mapping user objects
                to and from LDAP context data:
                <programlisting><![CDATA[
public interface UserDetailsContextMapper {
  UserDetails mapUserFromContext(DirContextOperations ctx, String username, 
          GrantedAuthority[] authority);

  void mapUserToContext(UserDetails user, DirContextAdapter ctx);
}]]>                    
                </programlisting>
                Only the first method is relevant for authentication. If you provide an implementation of this interface, you can
                control exactly how the UserDetails object is created. The first parameter is an instance of Spring LDAP's
                <interfacename>DirContextOperations</interfacename> which gives you access to the LDAP attributes which were loaded.
                The <literal>username</literal> parameter is the name used to authenticate and the final parameter is the list of authorities
                loaded for the user.
            </para>
            <para>
                The way the context data is loaded varies slightly depending on the type of authentication you are using. With the
                <classname>BindAuthenticatior</classname>, the context returned from the bind operation will be used to read the attributes, 
                otherwise the data will be read using the standard context obtained from the configured 
                <interfacename>ContextSource</interfacename> (when a search is configured to locate the user, 
                this will be the data returned by the search object).
            </para>
        </section>
    </section>
</chapter>