source: branches/samba-3.2.x/docs-xml/smbdotconf/security/directorysecuritymask.xml

<samba:parameter name="directory security mask"
                 context="S"
                                 type="string"
                 xmlns:samba="http://www.samba.org/samba/DTD/samba-doc">
<description>
    <para>This parameter controls what UNIX permission bits
    will be set when a Windows NT client is manipulating the UNIX
    permission on a directory using the native NT security dialog
    box.</para>

    <para>
        This parameter is applied as a mask (AND'ed with) to the incoming permission bits, thus resetting
        any bits not in this mask.  Make sure not to mix up this parameter with <smbconfoption name="force
        directory security mode"/>, which works similar like this one but uses logical OR instead of AND.
        Essentially, zero bits in this mask are a set of bits that will always be set to zero.
        </para>

    <para>
        Essentially, all bits set to zero in this mask will result in setting to zero the corresponding bits on the
        file permissions regardless of the previous status of this bits on the file.
    </para>

    <para>If not set explicitly this parameter is set to 0777
    meaning a user is allowed to set all the user/group/world
    permissions on a directory.</para>

    <para><emphasis>Note</emphasis> that users who can access the 
    Samba server through other means can easily bypass this restriction, 
    so it is primarily useful for standalone &quot;appliance&quot; systems.  
    Administrators of most normal systems will probably want to leave
        it as the default of <constant>0777</constant>.</para>
</description>

<related>force directory security mode</related>
<related>security mask</related>
<related>force security mode</related>
<value type="default">0777</value>
<value type="example">0700</value>
</samba:parameter>