<refsect1 id='idmap'>
    <title>ID MAPPING</title>
    <para>
        The ID-mapping feature allows SSSD to act as a client of Active
        Directory without requiring administrators to extend user attributes
        to support POSIX attributes for user and group identifiers.
    </para>
    <para>
        NOTE: When ID-mapping is enabled, the uidNumber and gidNumber
        attributes are ignored. This is to avoid the possibility of conflicts
        between automatically-assigned and manually-assigned values. If you
        need to use manually-assigned values, ALL values must be
        manually-assigned.
    </para>
    <para>
        Please note that changing the ID mapping related configuration
        options will cause user and group IDs to change. At the moment,
        SSSD does not support changing IDs, so the SSSD database must
        be removed. Because cached passwords are also stored in the
        database, removing the database should only be performed while
        the authentication servers are reachable, otherwise users might
        get locked out. In order to cache the password, an authentication
        must be performed. It is not sufficient to use
        <citerefentry>
            <refentrytitle>sss_cache</refentrytitle>
            <manvolnum>8</manvolnum>
        </citerefentry>
        to remove the database, rather the process
        consists of:
            <itemizedlist>
                <listitem>
                    <para>
                        Making sure the remote servers are reachable
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Stopping the SSSD service
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Removing the database
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Starting the SSSD service
                    </para>
                </listitem>
            </itemizedlist>
        Moreover, as the change of IDs might necessitate the adjustment
        of other system properties such as file and directory ownership,
        it's advisable to plan ahead and test the ID mapping configuration
        thoroughly.
    </para>

    <refsect2 id='idmap_algorithm'>
        <title>Mapping Algorithm</title>
        <para>
            Active Directory provides an objectSID for every user and group
            object in the directory. This objectSID can be broken up into
            components that represent the Active Directory domain identity and
            the relative identifier (RID) of the user or group object.
        </para>
        <para>
            The SSSD ID-mapping algorithm takes a range of available UIDs and
            divides it into equally-sized component sections - called
            "slices"-. Each slice represents the space available to an Active
            Directory domain.
        </para>
        <para>
            When a user or group entry for a particular domain is encountered
            for the first time, the SSSD allocates one of the available slices
            for that domain. In order to make this slice-assignment repeatable
            on different client machines, we select the slice based on the
            following algorithm:
        </para>
        <para>
            The SID string is passed through the murmurhash3 algorithm to
            convert it to a 32-bit hashed value. We then take the modulus of
            this value with the total number of available slices to pick the
            slice.
        </para>
        <para>
            NOTE: It is possible to encounter collisions in the hash and
            subsequent modulus. In these situations, we will select the next
            available slice, but it may not be possible to reproduce the same
            exact set of slices on other machines (since the order that they
            are encountered will determine their slice). In this situation, it
            is recommended to either switch to using explicit POSIX attributes
            in Active Directory (disabling ID-mapping) or configure a default
            domain to guarantee that at least one is always consistent. See
            <quote>Configuration</quote> for details.
        </para>
    </refsect2>

    <refsect2 id='idmap_config'>
        <title>Configuration</title>
        <para>
            Minimum configuration (in the <quote>[domain/DOMAINNAME]</quote>
            section):
        </para>
        <para>
<programlisting>
ldap_id_mapping = True
ldap_schema = ad
</programlisting>
        </para>
        <para>
            The default configuration results in configuring 10,000 slices,
            each capable of holding up to 200,000 IDs, starting from 10,001
            and going up to 2,000,100,000. This should be sufficient for
            most deployments.
        </para>
        <refsect3 id='idmap_advanced_config'>
            <title>Advanced Configuration</title>
            <variablelist>
                <varlistentry>
                    <term>ldap_idmap_range_min (integer)</term>
                    <listitem>
                        <para>
                            Specifies the lower bound of the range of POSIX IDs to
                            use for mapping Active Directory user and group SIDs.
                        </para>
                        <para>
                            NOTE: This option is different from
                            <quote>min_id</quote> in that <quote>min_id</quote>
                            acts to filter the output of requests to this domain,
                            whereas this option controls the range of ID
                            assignment. This is a subtle distinction, but the
                            good general advice would be to have
                            <quote>min_id</quote> be less-than or equal to
                            <quote>ldap_idmap_range_min</quote>
                        </para>
                        <para>
                            Default: 200000
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>ldap_idmap_range_max (integer)</term>
                    <listitem>
                        <para>
                            Specifies the upper bound of the range of POSIX IDs to
                            use for mapping Active Directory user and group SIDs.
                        </para>
                        <para>
                            NOTE: This option is different from
                            <quote>max_id</quote> in that <quote>max_id</quote>
                            acts to filter the output of requests to this domain,
                            whereas this option controls the range of ID
                            assignment. This is a subtle distinction, but the
                            good general advice would be to have
                            <quote>max_id</quote> be greater-than or equal to
                            <quote>ldap_idmap_range_max</quote>
                        </para>
                        <para>
                            Default: 2000200000
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>ldap_idmap_range_size (integer)</term>
                    <listitem>
                        <para>
                            Specifies the number of IDs available for each slice.
                            If the range size does not divide evenly into the min
                            and max values, it will create as many complete slices
                            as it can.
                        </para>
                        <para>
                            NOTE: The value of this option must be at least as large as the
                            highest user RID planned for use on the Active Directory server. User
                            lookups and login will fail for any user whose RID is greater than
                            this value.
                        </para>
                        <para>
                            For example, if your most recently-added Active Directory user has
                            objectSid=S-1-5-21-2153326666-2176343378-3404031434-1107,
                            <quote>ldap_idmap_range_size</quote> must be at least 1108 as
                            range size is equal to maximal SID minus minimal SID plus one
                            (e.g. 1108 = 1107 - 0 + 1).
                        </para>
                        <para>
                            It is important to plan ahead for future expansion, as changing this
                            value will result in changing all of the ID mappings on the system,
                            leading to users with different local IDs than they previously had.
                        </para>
                        <para>
                            Default: 200000
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>ldap_idmap_default_domain_sid (string)</term>
                    <listitem>
                        <para>
                            Specify the domain SID of the default domain. This
                            will guarantee that this domain will always be
                            assigned to slice zero in the ID map, bypassing
                            the murmurhash algorithm described above.
                        </para>
                        <para>
                            Default: not set
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>ldap_idmap_default_domain (string)</term>
                    <listitem>
                        <para>
                            Specify the name of the default domain.
                        </para>
                        <para>
                            Default: not set
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>ldap_idmap_autorid_compat (boolean)</term>
                    <listitem>
                        <para>
                            Changes the behavior of the ID-mapping algorithm
                            to behave more similarly to winbind's
                            <quote>idmap_autorid</quote> algorithm.
                        </para>
                        <para>
                            When this option is configured, domains will be
                            allocated starting with slice zero and increasing
                            monatomically with each additional domain.
                        </para>
                        <para>
                            NOTE: This algorithm is non-deterministic (it
                            depends on the order that users and groups are
                            requested). If this mode is required for
                            compatibility with machines running winbind, it
                            is recommended to also use the
                            <quote>ldap_idmap_default_domain_sid</quote>
                            option to guarantee that at least one domain is
                            consistently allocated to slice zero.
                        </para>
                        <para>
                            Default: False
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>ldap_idmap_helper_table_size (integer)</term>
                    <listitem>
                        <para>
                          Maximal number of secondary slices that is tried when
                          performing mapping from UNIX id to SID.
                        </para>
                        <para>
                          Note: Additional secondary slices might be generated
                          when SID is being mapped to UNIX id and RID part of
                          SID is out of range for secondary slices generated so
                          far. If value of ldap_idmap_helper_table_size is equal
                          to 0 then no additional secondary slices are
                          generated.
                        </para>
                        <para>
                            Default: 10
                        </para>
                    </listitem>
                </varlistentry>
            </variablelist>
        </refsect3>
    </refsect2>

    <refsect2 id='well_known_sids'>
        <title>Well-Known SIDs</title>
        <para>
            SSSD supports to look up the names of Well-Known SIDs, i.e. SIDs
            with a special hardcoded meaning. Since the generic users and groups
            related to those Well-Known SIDs have no equivalent in a Linux/UNIX
            environment no POSIX IDs are available for those objects.
        </para>
        <para>
            The SID name space is organized in authorities which can be seen as
            different domains. The authorities for the Well-Known SIDs are
            <itemizedlist>
                <listitem><para>Null Authority</para></listitem>
                <listitem><para>World Authority</para></listitem>
                <listitem><para>Local Authority</para></listitem>
                <listitem><para>Creator Authority</para></listitem>
                <listitem><para>NT Authority</para></listitem>
                <listitem><para>Built-in</para></listitem>
            </itemizedlist>
            The capitalized version of these names are used as domain names when
            returning the fully qualified name of a Well-Known SID.
        </para>
        <para>
            Since some utilities allow to modify SID based access control
            information with the help of a name instead of using the SID
            directly SSSD supports to look up the SID by the name as well. To
            avoid collisions only the fully qualified names can be used to look
            up Well-Known SIDs. As a result the domain names <quote>NULL
            AUTHORITY</quote>, <quote>WORLD AUTHORITY</quote>, <quote> LOCAL
            AUTHORITY</quote>, <quote>CREATOR AUTHORITY</quote>, <quote>NT
            AUTHORITY</quote> and <quote>BUILTIN</quote> should not be used as
            domain names in <filename>sssd.conf</filename>.
        </para>
    </refsect2>

</refsect1>
