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+<?xml version="1.0" encoding="iso-8859-1"?>
+<!DOCTYPE chapter PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">
+<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>