source: branches/samba-3.5.x/docs-xml/smbdotconf/winbind/idmapallocbackend.xml

<samba:parameter name="idmap alloc backend"
                 context="G"
                 type="string"
                 advanced="1" developer="1" hide="1"
                 xmlns:samba="http://www.samba.org/samba/DTD/samba-doc">
<description>
        <para>
        The idmap alloc backend provides a plugin interface for Winbind to use
        when allocating Unix uids/gids for Windows SIDs. This option refers
        to the name of the idmap module which will provide the id allocation
        functionality. Please refer to the man page for each idmap plugin to
        determine whether or not the module implements the allocation feature.
        The most common plugins are the tdb (<citerefentry>
        <refentrytitle>idmap_tdb</refentrytitle><manvolnum>8</manvolnum></citerefentry>)
        and ldap (<citerefentry><refentrytitle>idmap_ldap</refentrytitle>
        <manvolnum>8</manvolnum></citerefentry>) libraries.
        </para>

        <para>
        This parameter defaults to the value <smbconfoption name="idmap
        backend"/> was set to, so by default winbind will allocate Unix IDs
        from the default backend. You will only need to set this parameter
        explicitly if you have an external source for Unix IDs, like a central
        database service somewhere in your company.
        </para>

        <para>
        Also refer to the <smbconfoption name="idmap alloc config"/> option.
        </para>
</description>

<value type="example">tdb</value>
</samba:parameter>