5.2 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.

5.2.1 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.

5.2.1.1 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 Example 5.1 for an example of the minimum required settings.

Example 5.1 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 Chapter 4, Domain Control .

5.2.2 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 5.2.

Example 5.2 Multiple LDAP Servers in smb.conf

 ...  passdb backend = ldapsam:"ldap:ldap://master.quenya.org \   ldapsam:ldap://slave.quenya.org  ... 

5.2.3 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.

5.2.4 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 Section 9.3 for more information regarding TCP/IP network protocols and how SMB/CIFS names are handled.

5.2.5 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 Section 9.3.3.

5.2.5.1 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.

5.2.5.2 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 Section 9.3.3.