SolutionBase: An introduction to DNS conditional forwarding

DNS is one of the most vital services on the network today. Getting proper performance and results from DNS can be problematic. Here's how you can use conditional forwarding in Windows Server 2003 to improve performance.

On the average Active Directory based network, DNS is one of the most heavily used services. Any time that a host on your network wants to contact another host on your network, query the Active Directory, access a Web page, or send an E-mail message, the request must first pass through a DNS server. Being that the DNS services are so heavily used and are such a critical networking component, you might be surprised to learn that they are extremely inefficient.

Sure, DNS offers a huge improvement over the WINS service that was previously used by Windows NT to perform some of the name resolution tasks that I mentioned above, but DNS still offers relatively poor performance because of the number of steps that it uses to resolve queries. However, there is a new feature in Windows Server 2003 called DNS conditional forwarding that you can use to enhance your DNS server's performance. In this article, I will show you how conditional forwarding works and how you can use it in your own network.

Why is DNS so slow?

Before you can really understand or appreciate conditional forwarding, you need to understand what it is that makes DNS so slow and inefficient. Throughout this article, I will be discussing various types of DNS implementations. To demonstrate how these various implementations work, let's assume that you have a small, Windows Server 2003 network. This network has a single domain controller named DC1 and that the domain controller doubles as a DNS server. The network also has one Windows XP workstation (I told you it was a small network), which we will call XP1. I realize that this example is absolutely ridiculous when compared against an enterprise class network, but the simplicity of my fictitious network will help to make the various explanations easier to follow.

OK, so let's go back to my original question of what makes DNS so slow? Let's assume for this example, that DC1 is running an out of the box configuration, butï¿? the server was connected to the Internet prior to being promoted to a domain controller / DNS server. Now, let's say that a user working off of XP1 opens Internet Explorer and enters Here's what happens.

  1. XP1 sends an iterative query to DC1 asking it to resolve into its corresponding IP address.
  2. DC1 checks its cache of recently queried domain names and determines that does not exist within the cache.
  3. DC1 opens its DNS database and finds zone information only for the local, Active Directory domain. Since is not a part of the local domain, the DNS server has no way of resolving the query.
  4. Since DC1 was connected to the Internet prior to being promoted to a domain controller / DNS server, DC1 contacts an Internet based root name server and downloads a list of all existing Internet based root name servers. This becomes DC1's list of root hints.
  5. DC1 sends a recursive query to the first root name server on the root hint list, asking for the IP address of the DNS server that is authoritative over the .COM domain.
  6. Once DC1 receives the response to this query, it sends a recursive query to the DNS server that is authoritative for the .COM domain, asking the server for the IP address corresponding to the DNS server that is authoritative for the domain.
  7. DC1 now sends yet another recursive query. This time, it sends the query to the authoritative name server for The query returns the IP address associated with the domain.
  8. DC1 receives the IP address and passes it to XP1.
  9. XP1 then accesses the address and the user sees the TechRepublic Web site displayed in Internet Explorer.
As you see, there are four separate DNS queries that have to happen before our user can access a Web site. That's a lot of overhead, especially when you consider that everything that I just described has to happen each time anybody in the organization tries to access a Web site.

DNS Forwarding

As you saw in the previous section, since the DNS server didn't know anything about the domain, it had to initiate a series of queries, all the way up to the root DNS level. One way that you can reduce overhead though and make DNS a little more efficient though is to use a technique called DNS forwarding (This technique also works with Windows 2000 Server).

The idea behind DNS forwarding is that when Windows installs the DNS services, it makes the DNS services authoritative over your local domains, but that's it. If your server was connected to the Internet at the time that the server was promoted to a domain controller / DNS server, then the DNS server has knowledge that the Internet exists, but the DNS server was never really intended to handle a large stream of Internet related queries.

The other thing that you need to understand in order to understand DNS forwarding is that in most cases, your company's network is not directly connected to an Internet backbone. Your company gets Internet access from an Internet Service provider. At the time that your company signed up with your Internet service provider, they probably assigned you some IP addresses, but they also gave you a list of IP addresses that could be used for things like SMTP, POP3, and DNS. Your ISP has their own DNS server that is specifically configured to handle Internet related queries. So why not let your ISP deal with DNS queries instead of burdening your own network with them?

If your network is Active Directory based (and our example network is), then you don't have this luxury because Active Directory is totally dependant on the DNS services. Any machine that is a part of the Active Directory (including workstations) must be pointed to a DNS server that contains the zone information specific to your network. This is where forwarding comes into play. Forwarding is a technique by which you can tell your DNS server that if queries come in that do not pertain to local domains then those queries should be resolved by the ISP's DNS server instead. This greatly simplifies the name resolution process from your network's point of view. Here is what happens when the user at XP1 enters into Internet Explorer:

  1. XP1 sends an iterative query to DC1 asking it to resolve
  2. DC1 checks its DNS cache and determines that is not currently cached
  3. DC1 opens the DNS database and determines that it only contains information for the local domains, and that is not a local domain.
  4. Since DC1 has no way of resolving, it checks its list of forwarders for a forwarding address.
  5. Since the list of forwarders contains the IP address of your ISP's DNS server, DC1 forwards the query to the ISP's DNS server.
  6. The ISP's DNS server resolves into its corresponding IP address, and sends the result back to DC1.
  7. DC1 passes the resolved IP address back to XP1
  8. XP1 then goes to the specified address and Internet Explorer displays the TechRepublic Web site.

This process looks a lot simpler than the process that I showed you earlier, but don't let it fool you. In the first example that I showed you, your DNS server had to make a root level query, followed by a few additional queries.

In the second example, your DNS server passes the unresolved query to your ISP's DNS server and it resolves the query. The query isn't resolved by magic though. The ISP's DNS server works exactly the same way that your DNS server does. If your ISP's DNS server doesn't have knowledge of the site that's being queried, then it will have to perform multiple queries starting at the root, just like your server would have.

If your ISP's DNS server is forced to make a root level query, then there are actually more steps involved in DNS forwarding then if your DNS server had just resolved the query on its own. So why on earth would you want to use DNS forwarding if it is potentially less efficient than just allowing your DNS server to make the necessary queries on its own?

There are a couple of reasons why forwarding is preferred. First, forwarding frees up your system resources. Using four different queries to resolve a single address consumes a lot of processor time and bandwidth. If you forward these requests to your ISP, you can use their CPU and bandwidth resources rather than your own.

A second reason why forwarding is preferred is that even though forwarding involves more steps than resolving queries yourself, forwarding is often much faster. One reason why forwarding is often faster is because there is a good chance that your ISP will have the resolved address cached (remember, your company isn't the only one making queries through your ISP's DNS). More importantly though, your ISP has a whole lot more bandwidth than you do. Your ISP can probably make all of the necessary queries in less time than it would take your DNS server to make even the first query.

As you can see, your network will usually resolve DNS queries more quickly if you use DNS forwarding than if you do not. To implement DNS forwarding, open the DNS console on your DNS server. When the console opens, right click on your server and select the Properties command from the resulting shortcut menu. When the DNS server's properties sheet appears, select the Forwarders tab. Now, enter the IP address corresponding to your ISP's DNS server into the place provided and click the Add button, followed by OK.

Conditional forwarding

So now that you have some background in how DNS works and in DNS forwarding, let's talk about conditional forwarding. Conditional forwarding is designed to get queries to their destination in a fraction of the time. The process works by associating a domain name with the IP address of the name server that is authoritative for the domain.

This means that if your DNS server is unable to resolve the IP address for a particular domain, the query would be automatically sent directly to the DNS server that is authoritative for that domain, and the name resolution would be returned instantly.

Here is exactly what would happen if you were to create a conditional forwarding for

  1. The user at XP1 enters into Internet Explorer.
  2. XP1 sends a DNS query to DC1
  3. DC1 checks the DNS cache to see if there is a cached entry for
  4. if no cached entry is found, DC1 searches the DNS database for information related to the domain.
  5. Since DC1 has no knowledge of the domain, DC1 checks its list of forwarders to see if there are any conditional forwarders for
  6. The forwarders list supplies the IP address of the DNS server that is authoritative for
  7. The query is sent to the DNS server, where the query is resolved.
  8. The remote DNS server passes the resolved IP address to DC1, which passes the IP address to XP1.
  9. The user is able to access the Web site

OK, so let's assume that you wanted to create a conditional forwarder that pointed to To do so, you would have to go to and do a who is search on When you do, the following information is returned:

               CNET Networks, Inc (DOM-311974)
               235 Second Street San Francisco CA 94105 US
    Domain Name:
               Registrar Name:
               Registrar Whois:
               Registrar Homepage:
    Administrative Contact:
               Host Master (NIC-1500677)  CNET Networks, Inc
               235 Second Street San Francisco CA 94105 US
      +1.4153442000 Fax- +1.4153442000
    Technical Contact, Zone Contact:
               Host Master (NIC-1500677)  CNET Networks, Inc
               235 Second Street San Francisco CA 94105 US
      +1.4153442000 Fax- +1.4153442000
    Created on..............: 1998-Nov-20.
    Expires on..............: 2013-Nov-19.
    Record last updated on..: 2004-Jul-22 13:09:25.
    Domain servers in listed order:

What you are interested in is the authoritative DNS server for The authoritative DNS server is usually the first one in the Domain Servers In Listed Order Section. In this case, it's NS.CNET.COM, In this case we got lucky because we were provided with the DNS server's IP address and fully qualified domain name. Sometimes though, only the fully qualified domain name is provided and you will have to ping the DNS server in order to determine its IP address.

Now that you know the authoritative DNS server's IP address, go back to the Forwarders tab of the DNS server's properties sheet (This is the same tab that you used to set up a normal forwarder). This time instead of directly entering an IP address, click the New button. When prompted to enter a DNS domain, enter and click OK. will now appear just beneath the All Other DNS Domains entry in the list of forwarders. At this point, select the entry and enter the DNS server's IP address into the space provided and click Add, followed by OK. You have now created a conditional forwarder.

Words of caution

Now that I have shown you how to create conditional forwarders, and you know just how much time conditional forwarders can save, I feel obligated to give you a few words of caution. Right about now, you might be thinking that it would be a good idea to check the logs on your router to determine the 50 Web sites that are visited the most often and then create conditional forwarders for those sites. This is actually a really bad idea for a couple of reasons.

First, having more than a handful of conditional forwarders can make the DNS query process run more slowly than if conditional forwarders were not used at all, because every outbound query has to be compared against every item on the list of forwarders.

Another reason why big lists of conditional forwarders are a bad idea is because DNS servers sometimes change, as do IP addresses. For example, if I were to become dissatisfied with my ISP and moved my Web site to a different ISP for hosting, a completely different DNS server would become authoritative for my domain. If someone had a conditional forwarder set up to point to the previously authoritative DNS server, the entry would no longer point to the correct location. Worse yet, if the DNS server's IP address were to change, a network with a conditional forwarder pointing to that DNS server would no longer be able to access the domain in question.

Just because there are some serious issues to consider in regard to conditional forwarders doesn't mean that conditional forwarding doesn't have its place though. There's certainly no harm in creating conditional forwarders for a couple of Web sites (assuming that the DNS doesn't change). For example, you might create a conditional forwarder that will help the Windows update service to work more efficiently. You might create another one that points to your anti virus company since your anti virus software regularly updates itself. Finally, you might create a conditional entry for a search engine such as Google. That's about as far as I would recommend taking conditional forwarding for Web sites though.

Conditional forwarding is much more appropriate in situations in which you have a working relationship with another company and access their network from your network on a regular basis. For example, if a domain in your forest had an external trust relationship to a domain in a remote forest, then it might be appropriate to create a conditional forwarding entry for that domain (assuming that it is heavily used). Likewise, if you regularly access a database on a supplier's server, then this would be another good time to use a conditional forwarding entry.

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