# How to Act as a Backup Domain Controller in a Purely Samba Controlled Domain

# 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.

# 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:

    [global]
         workgroup = SAMBA
         domain master = yes
         domain logons = yes
         encrypt passwords = yes
         security = user
         ....

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.

# 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.

## 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.

## 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.

# 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.

# How do I set up a Samba BDC?

Several things have to be done:

- The file `private/MACHINE.SID` identifies the domain. When a samba server is first started, it is created on the fly and must never be changed again. This file has to be the same on the PDC and the BDC, so the MACHINE.SID has to be copied from the PDC to the BDC. Note that in the latest Samba 2.2.x releases, the machine SID (and therefore domain SID) is stored in the `private/secrets.tdb` database. This file cannot just be copied because Samba looks under the key `SECRETS/SID/`DOMAIN``. where `DOMAIN` is the machine's netbios name. Since this name has to be unique for each SAMBA server, this lookup will fail.

  A new option has been added to the smbpasswd(8) command to help ease this problem. When running smbpasswd -S as the root user, the domain SID will be retrieved from a domain controller matching the value of the `workgroup` parameter in `smb.conf` and stored as the new Samba server's machine SID. See the smbpasswd(8) man page for more details on this functionality.

- 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. LDAP is also a potential vehicle for sharing this information.

- 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

    [global]
         workgroup = SAMBA
         domain master = no
         domain logons = yes
         encrypt passwords = yes
         security = user
         ....

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.

## 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 (including machine trust account password changes) 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(1). rsync can use ssh(1) as a transport. ssh itself can be set up to accept only rsync transfer without requiring the user to type a password. Refer to the man pages for these two tools for more details.

Another solution with high potential is to use Samba's `--with-ldapsam` for sharing and/or replicating the list of `sambaAccount` entries. This can all be done over SSL to ensure security. See the Samba-LDAP-HOWTO for more details.