Windows Server 2008 R2 : Add and Remove DNS Servers (part 2) - Add Query Forwarding & Configure a Caching-Only DNS Server

3. Add Query Forwarding

Once the DNS Server is installed and configured, you will want to consider how to get name resolution for names that your DNS servers are not authoritative for. If you think about the way DNS servers resolve names, you'll realize they use a process of recursive queries to find an authoritative server that will resolve a name to an IP address. Your DNS servers simply do not know all the possible host names and IP addresses in the world's networks. To resolve host names for domains outside of the ones your servers are authoritative for, you will need to configure query forwarding.

Your network will likely have several DNS servers. If you configure one of those servers to pass queries from inside your network to the Internet, you have really just designated that server as a forwarder. You would change your network firewall settings to allow that DNS traffic from the forwarder through the firewall and out to the Internet. Queries will be returned from the Internet to the forwarder, and then the forwarder will pass those responses to the appropriate internal server. Do not host a local DNS zone on your forwarder! It is exposed to the Internet, and any zone stored on the forwarder will also be exposed to the Internet. You really don't want your internal DNS zone data becoming available online.

Maybe you don't want to simply forward all your unresolved queries through a single forwarder. Maybe you want to forward requests for certain domains through a specific forwarder. This concept of setting conditions under which queries are forwarded and through which server they are forwarded is called conditional forwarding. It offers a little more flexibility than traditional forwarding, and it can be far more effective than traditional forwarding if you are in a private network that hosts multiple domains, each with their own DNS zones. To add forwarders to your DNS architecture, you will use DNS Manager:

Figure 3. Adding DNS forwarders

4. Configure a Caching-Only DNS Server

All DNS servers resolve queries and then cache the results of queries for a limited time. They also perform other functions such as updating records and doing zone database maintenance. You might want a server that simply resolves queries and caches the results. A caching-only server is especially useful when DNS resolution is needed but when you don't want to create a separate zone for that location.

With a caching-only server, query information is gathered over time from other DNS servers as the caching-only server resolves client queries. That information is then stored by the caching-only server for future use. This process usually results in a decreasing amount of network traffic over time between the location containing the caching-only server and those other locations that contain full-version DNS zones. The benefit comes in the reduced use of the WAN link for DNS resolution while increasing name resolution performance for the local clients. The caching-only server does not perform zone transfers like other DNS servers, and so the WAN is not impacted by this traffic.

This process really could not be much easier. You now have a caching-only DNS server that will take client requests and perform recursive DNS name queries. When the server resolves a query, it will store the answer locally. There are no zones to maintain or update. No zone transfers are necessary, and the clients get the benefit of a local DNS server.

4.1. Manage Root Hints

We know some of you out there are scratching your heads and thinking, "What in the world is a root hint?" By definition, a root hint is a piece of DNS data stored in the DNS database that identifies the authoritative servers for the root of a given DNS namespace. If you want to resolve a query for a namespace, you have to find the server responsible for resolving requests for that space. DNS names are hierarchical in structure, and each level of the hierarchy is separated by a period (or a dot). So, for example, if you had a client who was trying to resolve the hierarchical name www.microsoft.com, you would begin the process at the root, which in this case is com. Where is the com server? Wouldn't it be nice if you already had a list of commonly used roots (like com, mil, gov, edu, net, org, and so on) and their corresponding IP addresses? These are the root hints.

By default DNS contains a standard list of commonly used root hints. The root hints contain the name server (NS) records and the host (A) resource records for the internet root servers. All of this works very well if you are on the Internet. What if you are on a private network and want to configure your own root servers? You can configure your own root zone and add the associated NS and A records to root hints as follows:

Figure 4. Managing root hints

At this point you can add, edit, or remove root hints, or you can choose to copy the root hints from another server.

5. Remove a DNS Server

There may come a time when you want to remove a DNS server from your network. When you remove a DNS server, it is important to remember that your DNS server is likely part of a larger DNS infrastructure, and it likely performs key functions and contains records referencing those functions to the other DNS servers and clients in your organization. You would not want to simply delete the server from DNS and remove the DNS Server role from Server Manager without first making sure that there will not be an interruption of the name resolution service. Make sure that as you remove a DNS server that its functions are being taken over by another server and that the records and references to those services have been updated in your DNS database.

This process consists of four steps: