Overview of DFS in Windows 2000 (812487)

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Administrators can use DFS to make it easier for users to access and manage files that are physically distributed across a network. With DFS, you can make files that are distributed on multiple servers appear to users as if they are in one location on the network.

Overview of DFS

In most environments, shared resources reside on multiple servers in various shared folders. To access a resource, a user or program must either map a drive to the server or specify the Universal Naming Convention (UNC) path of the shared resource. For example: DFS makes it possible for a share point on one server to host the shares that reside on other servers. It transparently links file servers and shared folders, and then maps them to a single hierarchy so they can be accessed from one location. This occurs although the data is actually distributed in different locations. Users no longer have to go to multiple locations on the network to find the information that they need. Users only have to connect to: When a user accesses a folder in this share, the user is redirected to the network location that contains the shared resource. In this way, users only must be aware of the DFS root share to have access to shared resources throughout the enterprise.

DFS topology starts with the root of the DFS tree. The DFS root at the top of the logical hierarchy is mapped to a physical share. A DFS link maps a Domain Name System (DNS) name to the UNC name of the target shared folder or target DFS root. When a DFS client accesses a DFS shared folder, the DFS server uses the DNS name to UNC mapping to return a referral to the client so that it can locate the shared folder. Mapping the DNS name to the UNC name makes the physical location of data transparent to users, who no longer have to remember the server where a folder is stored. When a DFS client requests a referral to a DFS share, the DFS server uses the Partition Knowledge Table (PKT) to direct the DFS client to the physical share. The PKT is stored in Active Directory for domain-based DFS and is stored in the registry for stand-alone DFS. In a network environment, the PKT maintains all information about DFS topology, including its mappings to the underlying physical shares. After the DFS server refers the DFS client to a list of replica shares that correspond to the requested DFS link, the DFS client uses Active Directory site topology to connect to a replica in the same site or, if one is not available, a replica that is outside the site.

Key Terms and Concepts

The following is a list of some of the key terms that are associated with DFS:

• DFS topology: The logical structure of a Distributed File System, including elements such as a DFS root, one or more DFS links, DFS shared folders, and replica sets. The DFS topology is different from "DFS namespace." DFS namespace is the logical view of shared resources as seen by the DFS client.
• DFS root: The DFS root is the share at the top of the DFS topology that is the starting point for the DFS links and DFS shared folders that make up the DFS namespace. A DFS root can be defined at the domain level for domain-based operation, or at the server level for stand-alone operation. Domain-based DFS can have multiple roots in the domain, but only one root on each server.
• DFS link: A link from a DFS root to one or more shared files, another DFS root, or a domain-based volume. DFS links appear as folders under the DFS root, and redirect the DFS client to a shared folder or to the DFS root. It does this by mapping a DNS name to the UNC of the target shared folder.
• DFS shared folder: Files or folders in the DFS namespace that are shared by users with the appropriate permissions. Shared folders can exist at the root level (domain-based DFS only), or can be referred to by DFS links.
• Partition knowledge table (PKT): A table that maps root and replica nodes in the DFS namespace to Active Directory sites and physical servers. For a domain-based DFS root, the PKT is stored in Active Directory, and is made available to each domain controller in a domain. For a stand-alone DFS root, the PKT is stored in the local registry. When a DFS client gains access to a shared folder in the DFS namespace, it caches that portion of the PKT for the length of time that is specified in the Time-To-Live (TTL).
• Referral: When a DFS client accesses a shared folder in the DFS namespace, the DFS server returns a referral for the DFS client to use to locate that shared folder. The referral contains the information that maps a DNS name in the logical namespace to the UNC equivalent name of a physical share.
• Replica: A DFS shared folder in a replica set . This is the share to which the DFS link points.
• Replica set: Two or more DFS roots or DFS shared folders that participate in replication.
• DFS Replication: The process of synchronizing data from one referral in a link to the other referrals in the link.
• Root replica: The server that duplicates a DFS root to provide fault-tolerance. The server that hosts the DFS root provides referrals to DFS clients for shared folders. The DFS root can be hosted on a member server or on a domain controller.

Types of DFS Implementation

There are two ways that you can implement DFS in Windows 2000:

• Stand-alone DFS:
  
  Stand-alone DFS stores its configuration in the registry of the local computer. A stand-alone DFS server does not use Active Directory, cannot have replicas at the root level, and can only have a single level of DFS links. It is intended for backward compatibility with previous versions of DFS.
• Domain-based DFS:
  
  Domain-based DFS stores its configuration information in Active Directory. Because this information is made available on multiple domain controllers in the domain, domain-based DFS provides fault-tolerance for any DFS in the domain. A domain-based DFS root must be hosted on a domain member server or domain controller, can have shared folders at the root level, and supports root and file replication through Microsoft File Replication service (FRS).

How to Create a DFS Root

To create a DFS root:

1. Click Start, point to Programs, point to Administrative Tools, and then click Distributed File System.
2. On the Action menu, click New DFS Root to start the New DFS Root Wizard. Click Next.
3. On the Select the DFS Root Type page, do one of the following, and then click Next:
   • To create a domain DFS root, click Create a domain DFS root.
   • To create a stand-alone DFS root, click Create a standalone DFS root.
4. If you are creating a domain-based DFS root, click the name of the domain where you want to create the DFS root in the Trusting domains box, and then click Next.
5. On the Specify the Host Server for the DFS Root page, type the name of the host computer for the DFS root in the Server Name box, or click Browse to click the server that you want, and then click Next.
6. On the Specify the DFS Root Share page, click an existing shared folder or specify the path and name of a new shared folder, and then click Next.
7. Accept the default name for the DFS root or type a new name, and then click Next.
8. Click Finish. You may have to restart the computer.

How to Add DFS Links

To add a DFS link:

1. Click Start, point to Programs, point to Administrative Tools, and then click Distributed File System.
2. In the console tree, right-click the DFS root where you want to create a DFS link, and then click New DFS Link.
3. In the Create a New DFS Link dialog box, type the name and path for the DFS link in UNC format, and then click OK. The link is created and appears under the DFS root in the left pane. When you click the link, the replica that is associated with it is displayed in the right pane. The replica is represented by the UNC path to the share point to which the link is configured.

How to Configure a Replication Policy

To configure replication for DFS link replicas:

1. Click Start, point to Programs, point to Administrative Tools, point to Administrative Tools, and then click Distributed File System.
2. In the console tree, right-click a DFS link that has more than one replica, and then click Replication Policy.
3. In the Replication Policy dialog box, click the DFS shared folder in the list of shared folders that you want to use as the master folder for replication.
4. Click each shared folder in the list, clicking either Enable or Disable, and then click OK.

How to Access DFS roots and DFS Shared Folders

To access a DFS shared folder in stand-alone DFS, use the following UNC path, where Server is the name of the DFS server, and Dfsroot is the name of DFS root: For example, to access the Share1 share on a member server that is named Server1 and that is hosted on a stand-alone DFS root that is named Root1, use the following UNC path: In this example, a link that is named Office is created in the stand-alone DFS root, and the following two replicas are created for the Office link: When you connect to \\Server1\Root1, you see one folder that is named Office. When you access the Office folder, a referral that contains the list of replicas that are configured for the link is sent from the DFS server. The referral contains the \\Flat1\Office and \\Flat2\Office replica information. One of these replicas is selected, and you are connected to the share on the server.

To access a DFS shared folder in a domain-based DFS, use either of the following UNC paths, where DomainName is the domain name, Server is the name of the DFS server, and Dfsroot is the DFS root:Note The following connections are not supported on Windows 2000:
\\<DomainName> will fail with the following error: \\<DomainName.com> will fail with the following error: