Another update.

[Samba/gebeck_regimport.git] / docs / Samba-HOWTO-Collection / TOSHARG-BDC.xml

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<chapter id="samba-bdc">

<chapterinfo>
        &author.jht;
        &author.vl;
        <author>&person.gd;<contrib>LDAP updates</contrib></author>
</chapterinfo>

<title>Backup Domain Control</title>

<para>
Before you continue reading this section, please make sure that you are comfortable
with configuring a Samba Domain Controller as described in <link linkend="samba-pdc">Domain Control</link>.
</para>

<sect1>
<title>Features and Benefits</title>

<para>
This is one of the most difficult chapters to summarize. It does not matter what we say here
for someone will still draw conclusions and/or approach the Samba Team with expectations
that are either not yet capable of being delivered, or that can be achieved far more
effectively using a totally different approach. In the event that you should have a persistent
concern that is not addressed in this book, please email <ulink url="mailto:jht@samba.org">John H. Terpstra</ulink>
clearly setting out your requirements and/or question and we will do our best to provide a solution.
</para>

<para>
<indexterm><primary>SAM backend</primary><secondary>LDAP</secondary></indexterm>
Samba-3 is capable of acting as a Backup Domain Controller (BDC) to another Samba Primary Domain
Controller (PDC). A Samba-3 PDC can operate with an LDAP Account backend. The LDAP backend can be
either a common master LDAP server, or a slave server. The use of a slave LDAP server has the
benefit that when the master is down, clients may still be able to log onto the network.
This effectively gives Samba a high degree of scalability and is an effective solution
for large organizations. If you use an LDAP slave server for a PDC,
you will need to ensure the master's continued availability - if the
slave finds it's master down at the wrong time, you will have 
stability and operational problems.
</para>

<para>
<indexterm><primary>replication</primary><secondary>SAM</secondary></indexterm>
While it is possible to run a Samba-3 BDC with non-LDAP backend, that
backend must allow some form of 'two way' propagation, of changes
from the BDC to the master.  Only LDAP is capable of this at this stage.
</para>

<para>
<indexterm><primary>SAM backend</primary><secondary>non-LDAP</secondary></indexterm>
The use of a non-LDAP backend SAM database is particularly problematic because Domain Member
servers and workstations periodically change the Machine Trust Account password. The new
password is then stored only locally. This means that in the absence of a centrally stored
accounts database (such as that provided with an LDAP-based solution) if Samba-3 is running
as a BDC, the BDC instance of the Domain Member trust account password will not reach the
PDC (master) copy of the SAM. If the PDC SAM is then replicated to BDCs, this results in 
overwriting the SAM that contains the updated (changed) trust account password with resulting
breakage of the domain trust.
</para>

<para>
Considering the number of comments and questions raised concerning how to configure a BDC,
let's consider each possible option and look at the pros and cons for each possible solution.
<link linkend="pdc-bdc-table">Following table</link> lists possible design configurations for a PDC/BDC infrastructure.
<indexterm><primary>net</primary><secondary>rpc</secondary></indexterm>
<indexterm><primary>SAM backend</primary><secondary>ldapsam</secondary></indexterm>
<indexterm><primary>SAM backend</primary><secondary>tdbsam</secondary></indexterm>
<indexterm><primary>replication</primary><secondary>SAM</secondary></indexterm>
</para>

<table frame="all" id="pdc-bdc-table"><title>Domain Backend Account Distribution Options</title>
<tgroup cols="3">
        <colspec align="center" colwidth="1*"/>
        <colspec align="center" colwidth="1*"/>
        <colspec align="left" colwidth="3*"/>

        <thead>
        <row><entry>PDC Backend</entry><entry>BDC Backend</entry><entry>Notes/Discussion</entry></row>
        </thead>
        <tbody>
        <row>
        <entry><para>Master LDAP Server</para></entry>
        <entry><para>Slave LDAP Server</para></entry>
        <entry><para>The optimal solution that provides high integrity. The SAM will be
                replicated to a common master LDAP server.</para></entry>
        </row>
        <row>
        <entry><para>Single Central LDAP Server</para></entry>
        <entry><para>Single Central LDAP Server</para></entry>
        <entry><para>
        A workable solution without fail-over ability. This is a usable solution, but not optimal. 
        </para></entry>
        </row>
        <row>
        <entry><para>tdbsam</para></entry>
        <entry><para>tdbsam + <command>net rpc vampire</command></para></entry>
        <entry><para>
        Does not work with Samba-3.0; as Samba does not implement the
        server-side protocols required.
        </para></entry>
        </row>
        <row>
        <entry><para>tdbsam</para></entry>
        <entry><para>tdbsam + <command>rsync</command></para></entry>
        <entry><para>
        Do not use this configuration.
        Does not work because the TDB files are live and data may not
        have been flushed to disk.  Furthermore, this will cause
        domain trust breakdown.
        </para></entry>
        </row>
        <row>
        <entry><para>smbpasswd file</para></entry>
        <entry><para>smbpasswd file</para></entry>
        <entry><para>
        Do not use this configuration.
        Not an elegant solution due to the delays in synchronization
        and also suffers
        from the issue of domain trust breakdown.
        </para></entry>
        </row>
        </tbody>
</tgroup>
</table>

</sect1>

<sect1>
<title>Essential Background Information</title>

<para>
A Domain Controller is a machine that is able to answer logon requests from network
workstations. Microsoft LanManager and IBM LanServer were two early products that
provided this capability. The technology has become known as the LanMan Netlogon service.
</para>

<para>
When MS Windows NT3.10 was first released, it supported a new style of Domain Control
and with it a new form of the network logon service that has extended functionality.
This service became known as the NT NetLogon Service. The nature of this service has
changed with the evolution of MS Windows NT and today provides a complex array of
services that are implemented over an intricate spectrum of technologies.
</para>

<sect2>
<title>MS Windows NT4-style Domain Control</title>

<para>
Whenever a user logs into a Windows NT4/200x/XP Professional Workstation,
the workstation connects to a Domain Controller (authentication server) to validate that
the username and password the user entered are valid. If the information entered
does not match account information that has been stored in the Domain
Control database (the SAM, or Security Account Manager database), a set of error
codes is returned to the workstation that has made the authentication request.
</para>

<para>
When the username/password pair has been validated, the Domain Controller
(authentication server) will respond with full enumeration of the account information
that has been stored regarding that user in the User and Machine Accounts database
for that Domain. This information contains a complete network access profile for
the user but excludes any information that is particular to the user's desktop profile,
or for that matter it excludes all desktop profiles for groups that the user may
belong to. It does include password time limits, password uniqueness controls,
network access time limits, account validity information, machine names from which the
user may access the network, and much more. All this information was stored in the SAM
in all versions of MS Windows NT (3.10, 3.50, 3.51, 4.0).
</para>

<para>
<indexterm><primary>replication</primary><secondary>SAM</secondary></indexterm>
The account information (user and machine) on Domain Controllers is stored in two files,
one containing the Security information and the other the SAM. These are stored in files
by the same name in the <filename>%SystemRoot%\System32\config</filename> directory. 
This normally translates to the path <filename>C:\WinNT\System32\config</filename>. These
are the files that are involved in replication of the SAM database where Backup Domain
Controllers are present on the network.
</para>

<para>
There are two situations in which it is desirable to install Backup Domain Controllers:
</para>

<itemizedlist>
        <listitem><para>
        On the local network that the Primary Domain Controller is on, if there are many
        workstations and/or where the PDC is generally very busy. In this case the BDCs
        will pick up network logon requests and help to add robustness to network services.
        </para></listitem>

        <listitem><para>
        At each remote site, to reduce wide area network traffic and to add stability to
        remote network operations. The design of the network, the strategic placement of
        Backup Domain Controllers, together with an implementation that localizes as much
        of network to client interchange as possible will help to minimize wide area network
        bandwidth needs (and thus costs).
        </para></listitem>
</itemizedlist>

<para>
The inter-operation of a PDC and its BDCs in a true Windows NT4 environment is worth
mentioning here. The PDC contains the master copy of the SAM. In the event that an
administrator makes a change to the user account database while physically present
on the local network that has the PDC, the change will likely be made directly to
the PDC instance of the master copy of the SAM. In the event that this update may
be performed in a branch office, the change will likely be stored in a delta file
on the local BDC. The BDC will then send a trigger to the PDC to commence the process
of SAM synchronization. The PDC will then request the delta from the BDC and apply
it to the master SAM. The PDC will then contact all the BDCs in the Domain and
trigger them to obtain the update and then apply that to their own copy of the SAM.
</para>

<para>
Samba-3 can not participate in true SAM replication and is therefore not able to
employ precisely the same protocols used by MS Windows NT4. A Samba-3 BDC will
not create SAM update delta files. It will not inter-operate with a PDC (NT4 or Samba)
to synchronize the SAM from delta files that are held by BDCs.
</para>

<para>
Samba-3 cannot function as a BDC to an MS Windows NT4 PDC, and Samba-3 can not
function correctly as a PDC to an MS Windows NT4 BDC. Both Samba-3 and MS Windows
NT4 can function as a BDC to its own type of PDC.
</para>

<para>
The BDC is said to hold a <emphasis>read-only</emphasis> of the SAM from which
it is able to process network logon requests and authenticate users. The BDC can
continue to provide this service, particularly while, for example, the wide area
network link to the PDC is down. A BDC plays a very important role in both the
maintenance of Domain Security as well as in network integrity.
</para>

<para>
In the event that the NT4 PDC should need to be taken out of service, or if it dies, 
one of the NT4 BDCs can be promoted to a PDC. If this happens while the original NT4 PDC is on
line, it is automatically demoted to an NT4 BDC. This is an important aspect of Domain
Controller management. The tool that is used to effect a promotion or a demotion is the
Server Manager for Domains. It should be noted that Samba-3 BDCs can not be promoted
in this manner because reconfiguration of Samba requires changes to the &smb.conf; file.
</para>

<sect3>
<title>Example PDC Configuration</title>

<para>
Beginning with Version 2.2, Samba officially supports domain logons for all current Windows clients,
including Windows NT4, 2003 and XP Professional. For Samba to be enabled as a PDC, some
parameters in the <smbconfsection name="[global]"/>-section of the &smb.conf; have to be set.
Refer to <link linkend="minimalPDC">following configuration</link> for an example of the minimum required settings.
</para>

<para><smbconfexample id="minimalPDC">
<title>Minimal smb.conf for a PDC in Use With a BDC &smbmdash; LDAP Server on PDC.</title>
<smbconfoption name="workgroup">&example.workgroup;</smbconfoption>
<smbconfoption name="passdb backend">ldapsam://localhost:389</smbconfoption>
<smbconfoption name="domain master">yes</smbconfoption>
<smbconfoption name="domain logons">yes</smbconfoption>
</smbconfexample></para>

<para>
Several other things like a <smbconfsection name="[homes]"/> and a
<smbconfsection name="[netlogon]"/> share also need to be set along with
settings for the profile path, the user's home drive, and so on. This is not covered in this
chapter; for more information please refer to <link linkend="samba-pdc">Domain Control</link>.
</para>

</sect3>
</sect2>

<sect2>
<title>LDAP Configuration Notes</title>

<para>
When configuring a master and a slave LDAP server, it is advisable to use the master LDAP server
for the PDC and slave LDAP servers for the BDCs. It is not essential to use slave LDAP servers, however,
many administrators will want to do so in order to provide redundant services. Of course, one or more BDCs
may use any slave LDAP server. Then again, it is entirely possible to use a single LDAP server for the
entire network.
</para>

<para>
When configuring a master LDAP server that will have slave LDAP servers, do not forget to configure
this in the <filename>/etc/openldap/slapd.conf</filename> file. It must be noted that the DN of a
server certificate must use the CN attribute to name the server, and the CN must carry the servers'
fully qualified domain name. Additional alias names and wildcards may be present in the
subjectAltName certificate extension. More details on server certificate names are in RFC2830.
</para>

<para>
It does not really fit within the scope of this document, but a working LDAP installation is
basic to LDAP enabled Samba operation. When using an OpenLDAP server with Transport Layer Security
(TLS), the machine name in <filename>/etc/ssl/certs/slapd.pem</filename> must be the
same as in <filename>/etc/openldap/sldap.conf</filename>. The Red Hat Linux startup script
creates the <filename>slapd.pem</filename> file with hostname <quote>localhost.localdomain.</quote>
It is impossible to access this LDAP server from a slave LDAP server (i.e., a Samba BDC) unless the
certificate is recreated with a correct hostname.
</para>

<para>
For preference, do not install a Samba PDC on a OpenLDAP slave server. Joining client machines to the domain
will fail in this configuration because the change to the machine account in the LDAP tree
must take place on the master LDAP server. This is not replicated rapidly enough to the slave
server that the PDC queries. It therefore gives an error message on the client machine about
not being able to set up account credentials. The machine account is created on the LDAP server
but the password fields will be empty.  Unfortunately, some sites are
unable to avoid such configurations, and these sites should review the
<smbconfoption name="ldap replication sleep"/> parameter, intended to slow down Samba sufficiently
for the replication to catch up.  This is a kludge, and one that the
administrator must manually duplicate in any scripts (such as the
<smbconfoption name="add machine script"/>) that
they use.
</para>

<para>
Possible PDC/BDC plus LDAP configurations include:
</para>

<itemizedlist>
        <listitem><para>
        PDC+BDC -> One Central LDAP Server.
        </para></listitem>
        <listitem><para>
        PDC -> LDAP master server, BDC -> LDAP slave server.
        </para></listitem>
        <listitem><para>
        PDC -> LDAP master, with secondary slave LDAP server.
        </para><para>
        BDC -> LDAP master, with secondary slave LDAP server.
        </para></listitem>
        <listitem><para>
        PDC -> LDAP master, with secondary slave LDAP server.
        </para><para>
        BDC -> LDAP slave server, with secondary master LDAP server.
        </para></listitem>
</itemizedlist>

<para>
In order to have a fall-back configuration (secondary) LDAP server one would specify
the secondary LDAP server in the &smb.conf; file as shown in <link linkend="mulitldapcfg">following example</link>.
</para>

<para>
<smbconfexample id="mulitldapcfg">
<title>Multiple LDAP Servers in &smb.conf;</title>
<member>...</member>
<smbconfoption name="passdb backend"> </smbconfoption>
<member><parameter>ldapsam:"ldap://master.quenya.org ldap://slave.quenya.org"</parameter></member>
<member>...</member>
</smbconfexample>
</para>

</sect2>

<sect2>
<title>Active Directory Domain Control</title>

<para>
As of the release of MS Windows 2000 and Active Directory, this information is now stored
in a directory that can be replicated and for which partial or full administrative control
can be delegated. Samba-3 is not able to be a Domain Controller within an Active Directory
tree, and it cannot be an Active Directory server. This means that Samba-3 also cannot
act as a Backup Domain Controller to an Active Directory Domain Controller.
</para>

</sect2>

<sect2>
<title>What Qualifies a Domain Controller on the Network?</title>

<para>
Every machine that is a Domain Controller for the domain MIDEARTH has to register the NetBIOS
group name MIDEARTH&lt;#1c&gt; with the WINS server and/or by broadcast on the local network.
The PDC also registers the unique NetBIOS name MIDEARTH&lt;#1b&gt; with the WINS server.
The name type &lt;#1b&gt; name is normally reserved for the Domain Master Browser, a role
that has nothing to do with anything related to authentication, but the Microsoft Domain
implementation requires the Domain Master Browser to be on the same machine as the PDC.
</para>

<para>
Where a WINS server is not used, broadcast name registrations alone must suffice. Refer to
<link linkend="netdiscuss">Network Browsing: Discussion</link> for more information regarding TCP/IP network protocols and how
 SMB/CIFS names are handled.
</para>

</sect2>

<sect2>
<title>How does a Workstation find its Domain Controller?</title>

<para>
There are two different mechanisms to locate a domain controller, one method is used when
NetBIOS over TCP/IP is enabled and the other when it has been disabled in the TCP/IP
network configuration.
</para>

<para>
Where NetBIOS over TCP/IP is disabled, all name resolution involves the use of DNS, broadcast
messaging over UDP, as well as Active Directory communication technologies. In this type of
environment all machines require appropriate DNS entries. More information may be found in
<link linkend="adsdnstech">DNS and Active Directory</link>.
</para>

<sect3>
<title>NetBIOS Over TCP/IP Enabled</title>
<para>
An MS Windows NT4/200x/XP Professional workstation in the domain MIDEARTH that wants a
local user to be authenticated has to find the Domain Controller for MIDEARTH. It does this
by doing a NetBIOS name query for the group name MIDEARTH&lt;#1c&gt;. It assumes that each
of the machines it gets back from the queries is a Domain Controller and can answer logon
requests. To not open security holes, both the workstation and the selected Domain Controller
authenticate each other. After that the workstation sends the user's credentials (name and
password) to the local Domain Controller for validation.
</para>

</sect3>

<sect3>
<title>NetBIOS Over TCP/IP Disabled</title>

<para>
An MS Windows NT4/200x/XP Professional workstation in the realm <constant>quenya.org</constant>
that has a need to affect user logon authentication will locate the Domain Controller by 
re-querying DNS servers for the <constant>_ldap._tcp.pdc._msdcs.quenya.org</constant> record.
More information regarding this subject may be found in <link linkend="adsdnstech">DNS and Active Directory</link>.
</para>

</sect3>
</sect2>
</sect1>

<sect1>
<title>Backup Domain Controller Configuration</title>

<para>
The creation of a BDC requires some steps to prepare the Samba server before
&smbd; is executed for the first time. These steps are outlines as follows:
<indexterm><primary>SID</primary></indexterm>
</para>

<itemizedlist>
<listitem><para>
        The domain SID has to be the same on the PDC and the BDC. In Samba versions
        pre-2.2.5, the domain SID was stored in the file <filename>private/MACHINE.SID</filename>.
        The domain SID is now stored in the file <filename>private/secrets.tdb</filename>. This file
        is unique to each server and can not be copied from a PDC to a BDC, the BDC will generate
        a new SID at start-up. It will over-write the PDC domain SID with the newly created BDC SID.
        There is a procedure that will allow the BDC to aquire the Domain SID. This is described here.
        </para>

        <para>
        To retrieve the domain SID from the PDC or an existing BDC and store it in the
        <filename>secrets.tdb</filename>, execute:
        </para>
<screen>
&rootprompt;<userinput>net rpc getsid</userinput>
</screen>
        </listitem>

        <listitem><para>
        Specification of the <smbconfoption name="ldap admin dn"/> is obligatory.
        This also requires the LDAP administration password to be set in the <filename>secrets.tdb</filename>
        using the <command>smbpasswd -w <replaceable>mysecret</replaceable></command>.
        </para></listitem>

        <listitem><para>
        Either <smbconfoption name="ldap suffix"/> or
        <smbconfoption name="ldap idmap suffix"/> must be specified in
        the &smb.conf; file.
        </para></listitem>

        <listitem><para>
<indexterm><primary>replication</primary><secondary>SAM</secondary></indexterm>
        The UNIX user database has to be synchronized from the PDC to the
        BDC. This means that both the <filename>/etc/passwd</filename> and
        <filename>/etc/group</filename> have to be replicated from the PDC
        to the BDC. This can be done manually whenever changes are made. 
        Alternately, the PDC is set up as an NIS master server and the BDC as an NIS slave
        server. To set up the BDC as a mere NIS client would not be enough,
        as the BDC would not be able to access its user database in case of
        a PDC failure. NIS is by no means the only method to synchronize
        passwords. An LDAP solution would also work.
        </para>
        </listitem>

        <listitem><para>
        The Samba password database must be replicated from the PDC to the BDC.
        Although it is possible to synchronize the <filename>smbpasswd</filename>
        file with <command>rsync</command> and <command>ssh</command>, this method
        is broken and flawed, and is therefore not recommended. A better solution
        is to set up slave LDAP servers for each BDC and a master LDAP server for the PDC.
        </para></listitem>

        <listitem><para>
        The netlogon share has to be replicated from the PDC to the
        BDC. This can be done manually whenever login scripts are changed,
        or it can be done automatically using a <command>cron</command> job
        that will replicate the directory structure in this share using a tool
        like <command>rsync</command>.
        </para></listitem>

</itemizedlist>

<sect2>
<title>Example Configuration</title>

<para> Finally, the BDC has to be found by the workstations. This can be
done by setting Samba as shown in <link linkend="minim-bdc">the next example</link>.
</para>

<para><smbconfexample id="minim-bdc">
<title>Minimal setup for being a BDC</title>
<smbconfoption name="workgroup">&example.workgroup;</smbconfoption>
<smbconfoption name="passdb backend">ldapsam:ldap://slave-ldap.quenya.org</smbconfoption>
<smbconfoption name="domain master">no</smbconfoption>
<smbconfoption name="domain logons">yes</smbconfoption>
<smbconfoption name="idmap backend">ldap:ldap://slave-ldap.quenya.org</smbconfoption>
</smbconfexample></para>

<para>
In the <smbconfsection name="[global]"/>-section of the &smb.conf; of the BDC. This makes the BDC
only register the name MIDEARTH&lt;#1c&gt; with the WINS server. This is no
problem as the name MIDEARTH&lt;#1c&gt; is a NetBIOS group name that is meant to
be registered by more than one machine. The parameter
<smbconfoption name="domain master">no</smbconfoption>
forces the BDC not to register <?latex \linebreak ?>MIDEARTH&lt;#1b&gt; which as a unique NetBIOS
name is reserved for the Primary Domain Controller.
</para>

<para>
<indexterm><primary>idmap backend</primary></indexterm>
<indexterm><primary>winbindd</primary></indexterm>
The <parameter>idmap backend</parameter> will redirect the <command>winbindd</command> utility to
use the LDAP database to resolve all UIDs and GIDs for UNIX accounts.
</para>

<note><para>
<indexterm><primary>Server Type</primary><secondary>Domain Member</secondary></indexterm>
Samba-3 has introduced a new ID mapping facility. One of the features of this facility is that it
allows greater flexibility in how user and group IDs are handled in respect to NT Domain User and Group
SIDs. One of the new facilities provides for explicitly ensuring that UNIX/Linux UID and GID values
will be consistent on the PDC, all BDCs and all Domain Member servers. The parameter that controls this
is called <parameter>idmap backend</parameter>. Please refer to the man page for &smb.conf; for more information
regarding its behavior.
</para></note>

<para>
The use of the <smbconfoption name="idmap backend">ldap:ldap://master.quenya.org</smbconfoption>
option on a BDC only make sense where ldapsam is used on a PDC. The purpose for an LDAP based idmap backend is
also to allow a domain-member (without its own passdb backend) to use winbindd to resolve Windows network users
and groups to common UID/GIDs. In other words, this option is generally intended for use on BDCs and on Domain
Member servers.
</para>

</sect2>
</sect1>

<sect1>
<title>Common Errors</title>

<para>
As this is a rather new area for Samba, there are not many examples that we may refer to.
Updates will be published as they become available and may be found in later Samba releases or
from the Samba web <ulink url="http://samba.org">site.</ulink>
</para>

<sect2>
<title>Machine Accounts Keep Expiring</title>

<para>
<indexterm><primary>Machine Trust Accounts</primary></indexterm>
This problem will occur when the passdb (SAM) files are copied  from a central
server but the local Backup Domain Controller is acting as a PDC. This results in the application of
Local Machine Trust Account password updates to the local SAM. Such updates 
are not copied back to the central server. The newer machine account password is then over
written when the SAM is re-copied from the PDC. The result is that the Domain Member machine
on start up will find that its passwords do not match the one now in the database and
since the startup security check will now fail, this machine will not allow logon attempts
to proceed and the account expiry error will be reported.
</para>

<para>
The solution is to use a more robust passdb backend, such as the ldapsam backend, setting up
a slave LDAP server for each BDC, and a master LDAP server for the PDC.
</para>

</sect2>

<sect2>
<title>Can Samba Be a Backup Domain Controller to an NT4 PDC?</title>

<para>
<indexterm><primary>replication</primary><secondary>SAM</secondary></indexterm>
No. The native NT4 SAM replication protocols have not yet been fully implemented.
</para>

<para>
Can I get the benefits of a BDC with Samba?  Yes, but only to a Samba PDC.The
main reason for implementing a BDC is availability. If the PDC is a Samba
machine, a second Samba machine can be set up to service logon requests whenever
the PDC is down.
</para>

</sect2>

<sect2>
<title>How Do I Replicate the smbpasswd File?</title>

<para>
<indexterm><primary>replication</primary><secondary>SAM</secondary></indexterm>
Replication of the smbpasswd file is sensitive. It has to be done whenever changes
to the SAM are made. Every user's password change is done in the smbpasswd file and
has to be replicated to the BDC. So replicating the smbpasswd file very often is necessary.
</para>

<para>
As the smbpasswd file contains plain text password equivalents, it must not be
sent unencrypted over the wire. The best way to set up smbpasswd replication from
the PDC to the BDC is to use the utility rsync. rsync can use ssh as a transport.
<command>ssh</command> itself can be set up to accept <emphasis>only</emphasis>
<command>rsync</command> transfer without requiring the user to type a password.
</para>

<para>
As said a few times before, use of this method is broken and flawed. Machine trust 
accounts will go out of sync, resulting in a broken domain. This method is
<emphasis>not</emphasis> recommended. Try using LDAP instead.
</para>

</sect2>

<sect2>
<title>Can I Do This All with LDAP?</title>

<para>
The simple answer is yes. Samba's pdb_ldap code supports binding to a replica
LDAP server, and will also follow referrals and re-bind to the master if it ever
needs to make a modification to the database. (Normally BDCs are read only, so
this will not occur often).
</para>

</sect2>
</sect1>
</chapter>