Chapter 6. Backup Domain Control

-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 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 (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. Do not use an LDAP slave server for a PDC, this may cause serious -stability and operational problems. -

- -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 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. -

-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. - lists possible design configurations for a PDC/BDC infrastructure. - - - - -

Table 6.1. Domain Backend Account Distribution Options

PDC Backend	BDC Backend	Notes/Discussion
Master LDAP Server	Slave LDAP Server	The optimal solution that provides high integrity. The SAM will be - replicated to a common master LDAP server.
Single Central LDAP Server	Single Central LDAP Server	- A workable solution without fail-over ability. This is a useable solution, but not optimal. -
tdbsam	tdbsam + net rpc vampire	- Does not work with Samba-3.0.0; may be implemented in a later release. The downside of this solution - is that an external process will control account database integrity. This solution may appeal to sites - that wish to avoid the complexity of LDAP. The net rpc vampire is used to - synchronize domain accounts from the PDC to the BDC. -
tdbsam	tdbsam + rsync	- Do not use this configuration. - Does not work because the TDB files are live and data may not have been flushed to disk. - Use rsync to synchronize the TDB database files from the PDC to the BDC. -
smbpasswd file	smbpasswd file	- Do not use this configuration. - Not an elegant solution due to the delays in synchronization. - Use rsync to synchronize the TDB database files from the PDC to the BDC. - Can be made to work using a cron job to synchronize data from the PDC to the BDC. -

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 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. -

MS Windows NT4-style Domain Control

-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. -

-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:\Windows NT\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 localizes as much - of network to client interchange as possible will help to minimize wide area network - bandwidth needs (and thus costs). -

-The inter-operation of a PDC and its BDCs in a true Windows NT4 environemt 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. -

-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. -

-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. -

-The BDC is said to hold a read-only 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. -

-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. -

Example PDC Configuration

-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 [global]-section of the smb.conf have to be set. -Refer to for an example of the minimum required settings. -

Example 6.1. Minimal smb.conf for a PDC in Use With a BDC LDAP Server on PDC.

workgroup = MIDEARTH

passdb backend = ldapsam://localhost:389

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 user's home drive, and so on. This is not covered in this -chapter; for more information please refer to . -

LDAP Configuration Notes

-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. -

-When configuring a master LDAP server that will have slave LDAP servers, do not forget to configure -this in the /etc/openldap/slapd.conf 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. -

-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 /etc/ssl/certs/slapd.pem must be the -same as in /etc/openldap/sldap.conf. The Red Hat Linux startup script -creates the slapd.pem file with hostname “localhost.localdomain.” -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. -

-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 therfore 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. -

-Possible PDC/BDC plus LDAP configurations include: -

- PDC+BDC -> One Central LDAP Server. -
- PDC -> LDAP master server, BDC -> LDAP slave server. -
- PDC -> LDAP master, with secondary slave LDAP server. -
- BDC -> LDAP master, with secondary slave LDAP server. -
- PDC -> LDAP master, with secondary slave LDAP server. -
- BDC -> LDAP slave server, with secondary master LDAP server. -

-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 . -

Example 6.2. Multiple LDAP Servers in smb.conf

...

passdb backend = ldapsam:ldap://master.quenya.org

ldapsam:ldap://slave.quenya.org

...

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 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. -

What Qualifies a Domain Controller on the Network?

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

-Where a WINS server is not used, broadcast name registrations alone must suffice. Refer to - for more information regarding TCP/IP network protocols and how - SMB/CIFS names are handled. -

How does a Workstation find its Domain Controller?

-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. -

-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 -. -

NetBIOS Over TCP/IP Enabled

-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<#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 validation. -

NetBIOS Over TCP/IP Disabled

-An MS Windows NT4/200x/XP Professional workstation in the realm quenya.org -that has a need to affect user logon authentication will locate the Domain Controller by -requerying DNS servers for the _ldap._tcp.pdc.ms-dcs.quenya.org record. -More information regarding this subject may be found in . -

Backup Domain Controller Configuration

-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: - -

- 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 private/MACHINE.SID. - The domain SID is now stored in the file private/secrets.tdb. 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. -
- To retrieve the domain SID from the PDC or an existing BDC and store it in the - secrets.tdb, execute: -
```
-root# net rpc getsid
-
```
- Specification of the ldap admin dn is obligatory. - This also requires the LDAP administration password to be set in the secrets.tdb - using the smbpasswd -w mysecret. -
- Either ldap suffix or - ldap idmap suffix must be specified in - the smb.conf file. -
- - 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. - 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. -
- The Samba password database must be replicated from the PDC to the BDC. - Although it is possible to synchronize the smbpasswd - 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. -
- 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 cron job - that will replicate the directory structure in this share using a tool - like rsync. -

Example Configuration

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

Example 6.3. Minimal setup for being a BDC

workgroup = MIDEARTH

passdb backend = ldapsam:ldap://slave-ldap.quenya.org

domain master = no

domain logons = yes

idmap backend = ldapsam:ldap://slave-ldap.quenya.org

-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. -

- - -The idmap backend will redirect the winbindd utility to -use the LDAP database to resolve all UIDs and GIDs for UNIX accounts. -

Note

- -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 idmap backend. Please refer to the man page for smb.conf for more information -regarding its behavior. -

-The use of the idmap backend = ldap://master.quenya/org -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. -

Common Errors

-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 site. -

Machine Accounts Keep Expiring

- -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. -

-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. -

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

- -No. The native NT4 SAM replication protocols have not yet been fully implemented. -

-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. -

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. -

-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 -not recommended. Try using LDAP instead. -

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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