How to Configure Global Catalog and DNS for Single Forest with Single Domain

• The article explains what global catalog and DNS servers are and how they work in Active Directory.
• The article presents three options for configuring global catalog and DNS servers for a single forest single domain, along with their advantages and disadvantages.
• The article provides some best practices and frequently asked questions for configuring global catalog and DNS servers for a single forest single domain.

If you have a single forest with a single domain and multiple sites, you might wonder how to configure your domain controllers (DCs) as global catalog servers and DNS servers. In this article, we will explain the benefits and drawbacks of different options and provide some best practices for optimal performance and security.

What is a Global Catalog Server?

A global catalog server is a DC that stores a partial, read-only copy of every object in every domain in the forest. It also stores the full, writable copy of the objects in its own domain, just like any other DC.

The main purpose of a global catalog server is to speed up searches across the forest and facilitate logon authentication for users who are members of universal groups. A global catalog server can answer queries about any object in the forest without having to contact other DCs, which reduces network traffic and improves efficiency.

A global catalog server also holds a special role in the forest: it is the only DC that can process complex queries involving multiple domains, such as finding all users who have a certain attribute value or belong to a certain group. This is useful for applications that need to access information from different domains, such as Exchange or Active Directory Users and Computers.

What is a DNS Server?

A DNS server is a server that resolves host names to IP addresses and vice versa. It also provides other information about hosts, such as their canonical names, aliases, mail servers, and service records.

DNS is essential for Active Directory because it enables clients and servers to locate each other on the network. Every DC registers its host name and IP address in DNS, along with other records that identify its roles and capabilities. For example, a DC that is also a global catalog server registers a record with the name _gc._tcp.<domain> that points to its IP address. This allows clients to find a global catalog server by querying DNS for this name.

DNS also supports Active Directory replication by allowing DCs to find their replication partners by their host names. For example, if DC1 wants to replicate with DC2, it queries DNS for the IP address of DC2 and then establishes a connection with it.

How to Configure Global Catalog and DNS for a Single Forest Single Domain

There are several factors to consider when configuring global catalog and DNS for a single forest single domain, such as:

• The number of sites and DCs in the forest
• The network bandwidth and latency between sites
• The size and frequency of universal group membership changes
• The application requirements and user expectations

Based on these factors, you can choose one of the following options:

Option 1: Configure all DCs as global catalog servers and DNS servers

This option is recommended for small to medium-sized forests with few sites and DCs, high network bandwidth and low latency between sites, low universal group membership changes, and simple application requirements.

The advantages of this option are:

• It simplifies administration by eliminating the need to designate specific DCs as global catalog servers or DNS servers.
• It improves performance by ensuring that every site has at least one global catalog server and one DNS server available locally.
• It enhances availability by providing redundancy for both global catalog and DNS services in case of a DC failure or network outage.

The disadvantages of this option are:

• It increases disk space usage by storing a partial copy of every object in every domain on every DC.
• It increases CPU usage by processing global catalog queries on every DC.
• It increases replication traffic by replicating universal group membership changes to every DC.

Option 2: Configure one or two DCs per site as global catalog servers and DNS servers

This option is recommended for large forests with many sites and DCs, low network bandwidth or high latency between sites, high universal group membership changes, and complex application requirements.

The advantages of this option are:

• It reduces disk space usage by storing a partial copy of every object in every domain only on selected DCs.
• It reduces CPU usage by processing global catalog queries only on selected DCs.
• It reduces replication traffic by replicating universal group membership changes only to selected DCs.

The disadvantages of this option are:

• It complicates administration by requiring you to designate specific DCs as global catalog servers or DNS servers.
• It reduces performance by forcing some clients to contact remote global catalog servers or DNS servers if none are available locally.
• It reduces availability by creating single points of failure for both global catalog and DNS services if no redundancy is provided.

Option 3: Configure one or two DCs per site as global catalog servers only and configure all DCs as DNS servers

This option is recommended for forests with mixed network conditions, moderate universal group membership changes, and moderate application requirements.

The advantages of this option are:

• It balances disk space usage, CPU usage, replication traffic, performance, availability, and administration complexity by combining aspects of option 1 and option 2.
• It ensures that every site has at least one DNS server available locally, which is important for Active Directory functionality and security.

The disadvantages of this option are:

• It still requires you to designate specific DCs as global catalog servers.
• It still forces some clients to contact remote global catalog servers if none are available locally.

Best Practices for Configuring Global Catalog and DNS for a Single Forest Single Domain

Regardless of which option you choose, here are some best practices to follow when configuring global catalog and DNS for a single forest single domain:

• Use Active Directory Sites and Services to define your sites and subnets and assign your DCs to the appropriate sites. This helps Active Directory to optimize replication and authentication traffic based on the physical network topology.
• Use the Universal Group Membership Caching feature to enable DCs that are not global catalog servers to cache the universal group memberships of users who log on to them. This reduces the need for DCs to contact global catalog servers for authentication requests, which improves performance and availability.
• Use the DNS Manager console or the dnscmd command-line tool to configure your DNS servers and zones. Make sure your DNS servers are authoritative for the zones that correspond to your Active Directory domains and forests. Also, make sure your DNS zones are integrated with Active Directory, which allows you to store and replicate DNS data in the Active Directory database.
• Use the DNS Client Settings on your DCs and clients to configure the preferred and alternate DNS servers. Make sure your DCs and clients point to local DNS servers as their preferred DNS servers and remote DNS servers as their alternate DNS servers. This ensures that they can resolve DNS queries even if their local DNS servers are unavailable.
• Use the dcdiag, repadmin, and nltest command-line tools to test and troubleshoot your global catalog and DNS configuration. These tools can help you verify the functionality and connectivity of your DCs, global catalog servers, and DNS servers.

Frequently Asked Questions

Here are some common questions and answers related to global catalog and DNS configuration for a single forest single domain:

Question: How do I enable or disable the global catalog role on a DC?

Answer: You can use the Active Directory Sites and Services console to enable or disable the global catalog role on a DC. To do so, follow these steps:

• Open the Active Directory Sites and Services console.
• Expand the Sites container, and then select the site that contains the DC you want to modify.
• Expand the Servers container, and then expand the server object for the DC you want to modify.
• Right-click NTDS Settings, and then click Properties.
• Select or clear the Global Catalog check box, depending on whether you want to enable or disable the global catalog role.
• Click OK.

Question: How do I find out which DCs are global catalog servers in my forest?

Answer: You can use the Active Directory Users and Computers console or the nltest command-line tool to find out which DCs are global catalog servers in your forest. To do so, follow these steps:

• Open the Active Directory Users and Computers console.
• Click View, and then click Advanced Features.
• Expand the Domain Controllers container, and then look for the DCs that have a GC icon next to them. These are the global catalog servers in your domain.

Alternatively, you can use the nltest command-line tool with the /dsgetdc parameter. To do so, follow these steps:

• Open a command prompt window.
• Type nltest /dsgetdc:<domain> /gc /force, where <domain> is the name of your domain or forest root domain.
• Press Enter.
• Look for the output that shows the name and IP address of a global catalog server in your domain or forest.

Question: How do I configure my DCs to register their global catalog records in DNS?

Answer: You don’t need to configure anything manually. By default, every DC that is also a global catalog server automatically registers its global catalog records in DNS when it starts up or when its IP address changes. These records include:

• A record with the name gc._msdcs.<forest>, where <forest> is the name of your forest root domain. This record points to the IP address of a random global catalog server in your forest.
• A record with the name _gc._tcp.<site>._sites.<forest>, where <site> is the name of your site and <forest> is the name of your forest root domain. This record points to the IP address of a random global catalog server in your site.
• A record with the name _ldap._tcp.gc._msdcs.<forest>, where <forest> is the name of your forest root domain. This record points to the IP address of every global catalog server in your forest.
• A record with the name _ldap._tcp.<site>._sites.gc._msdcs.<forest>, where <site> is the name of your site and <forest> is the name of your forest root domain. This record points to the IP address of every global catalog server in your site.

You can use the DNS Manager console or the nslookup command-line tool to verify that your global catalog records are registered correctly in DNS.

Question: How do I configure my clients to locate a global catalog server in their site or in the forest?

Answer: You don’t need to configure anything manually. By default, every client that is joined to the domain automatically locates a global catalog server in its site or in the forest by querying DNS for the appropriate records. For example, if a client wants to find a global catalog server in its site, it queries DNS for the name _gc._tcp.<site>._sites.<forest>, where <site> is the name of its site and <forest> is the name of its forest root domain. DNS returns the IP address of a random global catalog server in that site, and the client contacts that server.

You can use the nltest command-line tool with the /dsgetsite and /dsgetdc parameters to verify that your clients are locating the correct global catalog servers in their site or in the forest.

Disclaimer

The author of this article is not affiliated with or endorsed by any organization mentioned in this article. The information provided in this article is for educational purposes only and does not constitute professional advice. The author of this article is not responsible for any damages or losses that may result from following or applying the information in this article. Use this information at your own risk.