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Chapter 7. How to Act as a Backup Domain Controller in a Purely Samba Controlled Domain

7.1. Prerequisite Reading

Before you continue reading in this chapter, please make sure -that you are comfortable with configuring a Samba PDC -as described in the Samba-PDC-HOWTO.

7.2. Background

What is a Domain Controller? It is a machine that is able to answer -logon requests from workstations in a Windows NT Domain. Whenever a -user logs into a Windows NT Workstation, the workstation connects to a -Domain Controller and asks him whether the username and password the -user typed in is correct. The Domain Controller replies with a lot of -information about the user, for example the place where the users -profile is stored, the users full name of the user. All this -information is stored in the NT user database, the so-called SAM.

There are two kinds of Domain Controller in a NT 4 compatible Domain: -A Primary Domain Controller (PDC) and one or more Backup Domain -Controllers (BDC). The PDC contains the master copy of the -SAM. Whenever the SAM has to change, for example when a user changes -his password, this change has to be done on the PDC. A Backup Domain -Controller is a machine that maintains a read-only copy of the -SAM. This way it is able to reply to logon requests and authenticate -users in case the PDC is not available. During this time no changes to -the SAM are possible. Whenever changes to the SAM are done on the PDC, -all BDC receive the changes from the PDC.

Since version 2.2 Samba officially supports domain logons for all -current Windows Clients, including Windows 2000 and XP. This text -assumes the domain to be named SAMBA. To be able to act as a PDC, some -parameters in the [global]-section of the smb.conf have to be set:

workgroup = SAMBA
-domain master = yes
-domain logons = yes

Several other things like a [homes] and a [netlogon] share also may be -set along with settings for the profile path, the users home drive and -others. This will not be covered in this document.

7.3. What qualifies a Domain Controller on the network?

Every machine that is a Domain Controller for the domain SAMBA has to -register the NetBIOS group name SAMBA#1c with the WINS server and/or -by broadcast on the local network. The PDC also registers the unique -NetBIOS name SAMBA#1b with the WINS server. The name type #1b 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.

7.3.1. How does a Workstation find its domain controller?

A NT 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#1c. 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 (TODO: How is the DC -chosen) domain controller authenticate each other. After that the -workstation sends the user's credentials (his name and password) to -the domain controller, asking for approval.

7.3.2. When is the PDC needed?

Whenever a user wants to change his password, this has to be done on -the PDC. To find the PDC, the workstation does a NetBIOS name query -for SAMBA#1b, assuming this machine maintains the master copy of the -SAM. The workstation contacts the PDC, both mutually authenticate and -the password change is done.

7.4. Can Samba be a Backup Domain Controller?

With version 2.2, no. The native NT 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 version 2.2.

Can I get the benefits of a BDC with Samba? Yes. 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.

7.5. How do I set up a Samba BDC?

Several things have to be done:

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 -anymore since Samba 2.2.5 or even earlier. 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.
To retrieve the domain SID from the PDC or an existing BDC and store it in the -secrets.tdb, execute 'net rpc getsid' on the BDC.
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.
The Samba password database in the file private/smbpasswd has to be -replicated from the PDC to the BDC. This is a bit tricky, see the -next section.
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.

Finally, the BDC has to be found by the workstations. This can be done -by setting

workgroup = samba
-domain master = no
-domain logons = yes

in the [global]-section of the smb.conf of the BDC. This makes the BDC -only register the name SAMBA#1c with the WINS server. This is no -problem as the name SAMBA#1c is a NetBIOS group name that is meant to +Chapter 6. Backup Domain Control

Chapter 6. Backup Domain Control
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Chapter 6. Backup Domain Control

John H. Terpstra

Samba Team

<jht@samba.org>

Volker Lendecke

Samba Team

<Volker.Lendecke@SerNet.DE>

Table of Contents

Features And Benefits

Essential Background Information

MS Windows NT4 Style Domain Control
Active Directory Domain Control
What qualifies a Domain Controller on the network?
How does a Workstation find its domain controller?

Backup Domain Controller Configuration

Example Configuration

Common Errors

Machine Accounts keep expiring, what can I do?
Can Samba be a Backup Domain Controller to an NT4 PDC?
How do I replicate the smbpasswd file?
Can I do this all with LDAP?

+Before you continue reading in this section, please make sure that you are comfortable +with configuring a Samba Domain Controller as described in the +Domain Control Chapter. +

Features And Benefits

+This is one of the most difficult chapters to summarise. It matters not 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 for more +effectively using a totally different approach. Since this HOWTO is already so large and +extensive, we have taken the decision to provide sufficient (but not comprehensive) +information regarding Backup Domain Control. In the event that you should have a persistent +concern that is not addressed in this HOWTO document then please email +John H Terpstra clearly setting out your requirements +and / or question and we will do our best to provide a solution. +

+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 Samba-3 BDC can +operate with a slave LDAP server for the Account backend. This effectively gives samba a high +degree of scalability. This is a very sweet (nice) solution for large organisations. +

+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. +

+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 PDC 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. +

+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: +

Backup Domain Backend Account Distribution Options

+ Solution: Passwd Backend is LDAP based, BDCs use a slave LDAP server +
+ Arguments For: This is a neat and manageable solution. The LDAP based SAM (ldapsam) + is constantly kept up to date. +
+ Arguments Against: Complexity +
+ Passdb Backend is tdbsam based, BDCs use cron based "net rcp vampire" to + suck down the Accounts database from the PDC +
+ Arguments For: It would be a nice solution +
+ 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. +
+ Make use of rsync to replicate (pull down) copies of the essential account files +
+ Arguments For: It is a simple solution, easy to set up as a scheduled job +
+ Arguments Against: This will over-write the locally changed machine trust account + passwords. This is a broken and flawed solution. Do NOT do this. +
+ Operate with an entirely local accounts database (not recommended) +
+ Arguments For: Simple, easy to maintain +
+ 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. +

Essential Background Information

+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. +

+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. +

MS Windows NT4 Style Domain Control

+Whenever a user logs into a Windows NT4 / 200x / XP Profresional 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 Accounts Manager database) then a set of error +codes is returned to the workstation that has made the authentication request. +

+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). +

+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 C:\WinNT\System32\config directory. These +are the files that are involved in replication of the SAM database where Backup Domain +Controllers are present on the network. +

+There are two situations in which it is desirable to install Backup Domain Controllers: +

+ 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. +
+ 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). +

+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. +

+Thus the BDC is said to hold a read-only 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. +

+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. +

Example PDC Configuration

+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 [global]-section of the smb.conf have to be set: +

+	workgroup = SAMBA
+	domain master = yes
+	domain logons = yes
+

+Several other things like a [homes] and a [netlogon] 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 the chapter on Domain Control. +

Active Directory Domain Control

+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 Contoller to an Active Directory Domain Controller. +

What qualifies a Domain Controller on the network?

+Every machine that is a Domain Controller for the domain SAMBA has to register the NetBIOS +group name SAMBA<#1c> with the WINS server and/or by broadcast on the local network. +The PDC also registers the unique NetBIOS name SAMBA<#1b> with the WINS server. +The name type <#1b> 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. +

How does a Workstation find its domain controller?

+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<#1c>. 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 valdation. +

Backup Domain Controller Configuration

+Several things have to be done: +

+ 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 + anymore since Samba 2.2.5 or even earlier. 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.
+ To retrieve the domain SID from the PDC or an existing BDC and store it in the + secrets.tdb, execute 'net rpc getsid' on the BDC. +
+ 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. +
+ The Samba password database in the file private/smbpasswd has to be + replicated from the PDC to the BDC. This is a bit tricky, see the + next section. +
+ 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. +

Example Configuration

+Finally, the BDC has to be found by the workstations. This can be done by setting: +

+	workgroup = SAMBA
+	domain master = no
+	domain logons = yes
+

+in the [global]-section of the smb.conf of the BDC. This makes the BDC +only register the name SAMBA<#1c> with the WINS server. This is no +problem as the name SAMBA<#1c> is a NetBIOS group name that is meant to be registered by more than one machine. The parameter 'domain master = -no' forces the BDC not to register SAMBA#1b which as a unique NetBIOS -name is reserved for the Primary Domain Controller.

7.5.1. How do I replicate the smbpasswd file?

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.

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 *only* rsync transfer without requiring the user to type a -password.

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How to Configure Samba as a NT4 Primary Domain Controller	Up	Samba as a ADS domain member

\ No newline at end of file +no' forces the BDC not to register SAMBA<#1b> which as a unique NetBIOS +name is reserved for the Primary Domain Controller. +

Common Errors

+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. +

Machine Accounts keep expiring, what can I do?

+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 procede and the account expiry error will be reported. +

Can Samba be a Backup Domain Controller to an NT4 PDC?

+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 version 2.2. +

+With version 3.0, the work on both the replication protocols and a suitable storage +mechanism has progressed, and some form of NT4 BDC support is expected soon. +

+Can I get the benefits of a BDC with Samba? Yes. 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. +

How do I replicate the smbpasswd file?

+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. +

+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 *only* rsync transfer without requiring the user +to type a password. +

Can I do this all with LDAP?

+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). +

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