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+<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 6. Backup Domain Control</title><link rel="stylesheet" href="samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.60.1"><link rel="home" href="samba-doc.html" title="SAMBA Project Documentation"><link rel="up" href="type.html" title="Part II. Server Configuration Basics"><link rel="previous" href="samba-pdc.html" title="Chapter 5. Domain Control"><link rel="next" href="domain-member.html" title="Chapter 7. Domain Membership"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 6. Backup Domain Control</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="samba-pdc.html">Prev</a> </td><th width="60%" align="center">Part II. Server Configuration Basics</th><td width="20%" align="right"> <a accesskey="n" href="domain-member.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="samba-bdc"></a>Chapter 6. Backup Domain Control</h2></div><div><div class="author"><h3 class="author"><span class="firstname">John</span> <span class="othername">H.</span> <span class="surname">Terpstra</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><tt class="email">&lt;<a href="mailto:jht@samba.org">jht@samba.org</a>&gt;</tt></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Volker</span> <span class="surname">Lendecke</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><tt class="email">&lt;<a href="mailto:Volker.Lendecke@SerNet.DE">Volker.Lendecke@SerNet.DE</a>&gt;</tt></p></div></div></div></div></div><div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><a href="samba-bdc.html#id2889347">Features And Benefits</a></dt><dt><a href="samba-bdc.html#id2889536">Essential Background Information</a></dt><dd><dl><dt><a href="samba-bdc.html#id2889565">MS Windows NT4 Style Domain Control</a></dt><dt><a href="samba-bdc.html#id2889816">Active Directory Domain Control</a></dt><dt><a href="samba-bdc.html#id2889836">What qualifies a Domain Controller on the network?</a></dt><dt><a href="samba-bdc.html#id2889863">How does a Workstation find its domain controller?</a></dt></dl></dd><dt><a href="samba-bdc.html#id2889908">Backup Domain Controller Configuration</a></dt><dd><dl><dt><a href="samba-bdc.html#id2890011">Example Configuration</a></dt></dl></dd><dt><a href="samba-bdc.html#id2890167">Common Errors</a></dt><dd><dl><dt><a href="samba-bdc.html#id2890181">Machine Accounts keep expiring, what can I do?</a></dt><dt><a href="samba-bdc.html#id2890212">Can Samba be a Backup Domain Controller to an NT4 PDC?</a></dt><dt><a href="samba-bdc.html#id2890238">How do I replicate the smbpasswd file?</a></dt><dt><a href="samba-bdc.html#id2890283">Can I do this all with LDAP?</a></dt></dl></dd></dl></div><p>
+Before you continue reading in this section, please make sure that you are comfortable
+with configuring a Samba Domain Controller as described in <a href="samba-pdc.html" title="Chapter 5. Domain Control">chapter on setting up Samba as a PDC</a>.
+</p><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2889347"></a>Features And Benefits</h2></div></div><div></div></div><p>
+This is one of the most difficult chapters to summarise. 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 then please email
+<a href="mailto:jht@samba.org" target="_top">John H Terpstra</a> clearly setting out your requirements
+and / or question and we will do our best to provide a solution.
+</p><p>
+Samba-3 is capable of acting as a Backup Domain Controller to another Samba Primary Domain
+Controller. 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 iand is a very sweet (nice) solution
+for large organisations.
+</p><p>
+While it is possible to run a Samba-3 BDC with non-LDAP backend, the administrator will
+need to figure out precisely what is the best way to replicate (copy / distribute) the
+user and machine Accounts backend.
+</p><p>
+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 of the SAM that contains the updated (changed) trust account password with resulting
+breakage of the domain trust.
+</p><p>
+Considering the number of comments and questions raised concerning how to configure a BDC
+lets consider each possible option and look at the pro's and con's for each theoretical solution:
+</p><div class="itemizedlist"><p class="title"><b>Backup Domain Backend Account Distribution Options</b></p><ul type="disc"><li><p>
+ Solution: Passwd Backend is LDAP based, BDCs use a slave LDAP server
+ </p><p>
+ Arguments For: This is a neat and manageable solution. The LDAP based SAM (ldapsam)
+ is constantly kept up to date.
+ </p><p>
+ Arguments Against: Complexity
+ </p></li><li><p>
+ Passdb Backend is tdbsam based, BDCs use cron based <span class="emphasis"><em>net rpc vampire</em></span> to
+ obtain the Accounts database from the PDC and place them into the Samba SAM.
+ <span class="emphasis"><em>net rpc vampire</em></span> is a Samba function of the &quot;net&quot; command.
+ </p><p>
+ Arguments For: It would be a nice solution
+ </p><p>
+ Arguments Against: It does not work because Samba-3 does not support the required
+ protocols. This may become a later feature but is not available today.
+ </p></li><li><p>
+ Make use of rsync to replicate (pull down) copies of the essential account files
+ </p><p>
+ Arguments For: It is a simple solution, easy to set up as a scheduled job
+ </p><p>
+ Arguments Against: This will over-write the locally changed machine trust account
+ passwords. This is a broken and flawed solution. Do NOT do this.
+ </p></li><li><p>
+ Operate with an entirely local accounts database (not recommended)
+ </p><p>
+ Arguments For: Simple, easy to maintain
+ </p><p>
+ Arguments Against: All machine trust accounts and user accounts will be locally
+ maintained. Domain users will NOT be able to roam from office to office. This is
+ a broken and flawed solution. Do NOT do this.
+ </p></li></ul></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2889536"></a>Essential Background Information</h2></div></div><div></div></div><p>
+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.
+</p><p>
+When MS Windows NT3.10 was first released, it supported an 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 very complex array of
+services that are implemented over a complex spectrum of technologies.
+</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2889565"></a>MS Windows NT4 Style Domain Control</h3></div></div><div></div></div><p>
+Whenever a user logs into a Windows NT4 / 200x / XP Professional Workstation,
+the workstation connects to a Domain Controller (authentication server) to validate
+the username and password that the user entered are valid. If the information entered
+does not validate against the account information that has been stored in the Domain
+Control database (the SAM, or Security Account Manager database) then a set of error
+codes is returned to the workstation that has made the authentication request.
+</p><p>
+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).
+</p><p>
+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 <tt class="filename">C:\WinNT\System32\config</tt> directory. These
+are the files that are involved in replication of the SAM database where Backup Domain
+Controllers are present on the network.
+</p><p>
+There are two situations in which it is desirable to install Backup Domain Controllers:
+</p><div class="itemizedlist"><ul type="disc"><li><p>
+ 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.
+ </p></li><li><p>
+ 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 localises as much
+ of network to client interchange as possible will help to minimise wide area network
+ bandwidth needs (and thus costs).
+ </p></li></ul></div><p>
+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 synchronisation. 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.
+</p><p>
+Thus the BDC is said to hold a <span class="emphasis"><em>read-only</em></span> of the SAM from which
+it is able to process network logon requests and to authenticate users. The BDC can
+continue to provide this service, particularly while, for example, the wide area
+network link to the PDC is down. Thus a BDC plays a very important role in both
+maintenance of Domain security as well as in network integrity.
+</p><p>
+In the event that the PDC should need to be taken out of service, or if it dies, then
+one of the BDCs can be promoted to a PDC. If this happens while the original PDC is on
+line then it is automatically demoted to a BDC. This is an important aspect of Domain
+Controller management. The tool that is used to affect a promotion or a demotion is the
+Server Manager for Domains.
+</p><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2889716"></a>Example PDC Configuration</h4></div></div><div></div></div><p>
+Since 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 <i class="parameter"><tt>[global]</tt></i>-section of the <tt class="filename">smb.conf</tt> have to be set:
+</p><div class="example"><a name="id2889747"></a><p class="title"><b>Example 6.1. Minimal smb.conf for being a PDC</b></p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>workgroup = MIDEARTH</tt></i></td></tr><tr><td><i class="parameter"><tt>domain master = yes</tt></i></td></tr><tr><td><i class="parameter"><tt>domain logons = yes</tt></i></td></tr></table></div><p>
+Several other things like a <i class="parameter"><tt>[homes]</tt></i> and a
+<i class="parameter"><tt>[netlogon]</tt></i> share also need to be set along with
+settings for the profile path, the users home drive, etc.. This will not be covered in this
+chapter, for more information please refer to <a href="samba-pdc.html" title="Chapter 5. Domain Control">the chapter about samba as a PDC</a>.
+</p></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2889816"></a>Active Directory Domain Control</h3></div></div><div></div></div><p>
+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 can not be an Active Directory server. This means that Samba-3 also can NOT
+act as a Backup Domain Controller to an Active Directory Domain Controller.
+</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2889836"></a>What qualifies a Domain Controller on the network?</h3></div></div><div></div></div><p>
+Every machine that is a Domain Controller for the domain SAMBA has to register the NetBIOS
+group name SAMBA&lt;#1c&gt; with the WINS server and/or by broadcast on the local network.
+The PDC also registers the unique NetBIOS name SAMBA&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.
+</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2889863"></a>How does a Workstation find its domain controller?</h3></div></div><div></div></div><p>
+An MS Windows NT4 / 200x / XP Professional workstation in the domain SAMBA that wants a
+local user to be authenticated has to find the domain controller for SAMBA. It does this
+by doing a NetBIOS name query for the group name SAMBA&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.
+</p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2889908"></a>Backup Domain Controller Configuration</h2></div></div><div></div></div><p>
+Several things have to be done:
+</p><div class="itemizedlist"><ul type="disc"><li><p>
+ The domain SID has to be the same on the PDC and the BDC. This used to
+ be stored in the file private/MACHINE.SID. This file is not created
+ since Samba 2.2.5. Nowadays the domain SID is stored in the file
+ private/secrets.tdb. Simply copying the secrets.tdb
+ from the PDC to the BDC does not work, as the BDC would
+ generate a new SID for itself and override the domain SID with this
+ new BDC SID.</p><p>
+ To retrieve the domain SID from the PDC or an existing BDC and store it in the
+ secrets.tdb, execute:
+ </p><pre class="screen">
+<tt class="prompt">root# </tt><b class="userinput"><tt>net rpc getsid</tt></b>
+</pre></li><li><p>
+ The UNIX user database has to be synchronized from the PDC to the
+ BDC. This means that both the /etc/passwd and /etc/group have to be
+ replicated from the PDC to the BDC. This can be done manually
+ whenever changes are made, or the PDC is set up as a NIS master
+ server and the BDC as a 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 work
+ as well.
+ </p></li><li><p>
+ The Samba password database has to be replicated from the PDC to the BDC.
+ As said above, though possible to synchronise the <tt class="filename">smbpasswd</tt>
+ file with rsync and ssh, 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.
+ </p></li><li><p>
+ Any 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 together with the smbpasswd
+ synchronization.
+ </p></li></ul></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2890011"></a>Example Configuration</h3></div></div><div></div></div><p>
+Finally, the BDC has to be found by the workstations. This can be done by setting:
+</p><div class="example"><a name="id2890026"></a><p class="title"><b>Example 6.2. Minimal setup for being a BDC</b></p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>workgroup = MIDEARTH</tt></i></td></tr><tr><td><i class="parameter"><tt>domain master = no</tt></i></td></tr><tr><td><i class="parameter"><tt>domain logons = yes</tt></i></td></tr><tr><td><i class="parameter"><tt>idmap backend = ldapsam://slave-ldap.quenya.org</tt></i></td></tr></table></div><p>
+In the <i class="parameter"><tt>[global]</tt></i>-section of the <tt class="filename">smb.conf</tt> of the BDC. This makes the BDC
+only register the name SAMBA&lt;#1c&gt; with the WINS server. This is no
+problem as the name SAMBA&lt;#1c&gt; is a NetBIOS group name that is meant to
+be registered by more than one machine. The parameter
+<a class="indexterm" name="id2890094"></a><i class="parameter"><tt>domain master</tt></i> = no
+forces the BDC not to register SAMBA&lt;#1b&gt; which as a unique NetBIOS
+name is reserved for the Primary Domain Controller.
+</p><p>
+The <i class="parameter"><tt>idmap backend</tt></i> will redirect the <b class="command">winbindd</b> utility to
+use the LDAP database to resolve all UIDs and GIDs for UNIX accounts.
+</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
+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 of 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 <i class="parameter"><tt>idmap backend</tt></i>. Please refer to the man page for <tt class="filename">smb.conf</tt> for more information
+regarding it's behaviour. Do NOT set this parameter except where an LDAP backend (ldapsam) is in use.
+</p></div></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2890167"></a>Common Errors</h2></div></div><div></div></div><p>
+As this is a rather new area for Samba there are not many examples that we may refer to. Keep
+watching for updates to this section.
+</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2890181"></a>Machine Accounts keep expiring, what can I do?</h3></div></div><div></div></div><p>
+This problem will occur when occur when the passdb (SAM) files are copied from a central
+server but the local Backup Domain Controllers. Local machine trust account password updates
+are not copied back to the central server. The newer machine account password is then over
+written when the SAM is copied from the PDC. The result is that the Domain member machine
+on start up will find that it's passwords does 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.
+</p><p>
+The solution: use a more robust passdb backend, such as the ldapsam backend, setting up
+an slave LDAP server for each BDC, and a master LDAP server for the PDC.
+</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2890212"></a>Can Samba be a Backup Domain Controller to an NT4 PDC?</h3></div></div><div></div></div><p>
+With version 2.2, no. The native NT4 SAM replication protocols have not yet been fully
+implemented. The Samba Team is working on understanding and implementing the protocols,
+but this work has not been finished for Samba-3.
+</p><p>
+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.
+</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2890238"></a>How do I replicate the smbpasswd file?</h3></div></div><div></div></div><p>
+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.
+</p><p>
+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.
+Ssh itself can be set up to accept <span class="emphasis"><em>only</em></span> rsync transfer without requiring the user
+to type a password.
+</p><p>
+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 very broken domain. This method is
+<span class="emphasis"><em>not</em></span> recommended. Try using LDAP instead.
+</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2890283"></a>Can I do this all with LDAP?</h3></div></div><div></div></div><p>
+The simple answer is YES. Samba's pdb_ldap code supports binding to a replica
+LDAP server, and will also follow referrals and rebind 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).
+</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="samba-pdc.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="type.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="domain-member.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 5. Domain Control </td><td width="20%" align="center"><a accesskey="h" href="samba-doc.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 7. Domain Membership</td></tr></table></div></body></html>