Domain Name System and Its Role in Exchange Server 2007

For computer systems to communicate with each other, whether you are talking about a local area network (LAN), a wide area network (WAN), or the Internet, they must have the ability to identify one another using some type of name resolution. Several strategies have been developed over the years, but the most reliable one to date (and the current industry standard) is the use of a DNS.

Accurate name resolution is critical in a mail environment as well. For a message to reach its destination, it might pass through several systems that need to know where it came from and where it is going.

In the past, Microsoft has continued to support the Windows Internet Naming Service, commonly known as WINS, as an alternative way of performing name resolution within an environment. WINS provided a distributed database for registering and querying dynamic mappings of NetBIOS names for computers and groups. WINS mapped these NetBIOS names to IP addresses, and was originally designed to resolve problems that surrounded NetBIOS name resolution in routed networks.

However, in Microsoft Exchange Server 2007, support for WINS/NetBIOS broadcasts has been done away with. This makes the importance of DNS in Exchange 2007 greater than ever because if DNS is not configured and working properly, Exchange 2007 will not work at all.

Even Lightweight Directory Access Protocol (LDAP) queries for local mailbox users require the DNS client to be properly configured and functioning on your Exchange 2007 servers.

Domain Name System Defined

The Internet, as well as most home and business networks, rely on Internet Protocol (IP) addresses to allow computers to connect to one another. If we had to remember the IP addresses of every website, server, workstation, and printer that we connect to on a daily basis, it would be very difficult to accomplish anything!

The domain name system, commonly abbreviated as DNS, is a hierarchical, distributed database used to resolve, or translate, domain and host names to IP addresses. Using DNS, users, computers, and applications that query DNS can specify remote systems by fully qualified domain names (FQDNs).

DNS is the primary method for name resolution for the Microsoft Windows Server platforms. DNS is also a requirement for deploying Active Directory (AD), though Active Directory is not a requirement for deploying DNS. That being said, in a Microsoft Windows environment, integrating DNS and Active Directory enables DNS servers to take advantage of the security, performance, and fault-tolerance capabilities designed into Active Directory.

Using DNS

DNS is composed of two components: clients and servers. Servers store information about specific components.

When a DNS client needs to contact a host system, it first attempts to do so by using local resources. The client first checks its local cache, which is created by saving the results of previous queries. Items in the local cache remain until one of three things occurs:

1. The Time-to-Live (TTL) period, which is set on each item, expires.

2. The client runs the ipconfig /flushdns command.

3. The DNS client is shut down.

Next, the client attempts to resolve the query using the local HOSTS file, which, on Windows systems, is located in the %systemroot%\system32\drivers\etc directory. This file is used to manually map host names to IP addresses, and remains in place even if the system is rebooted.

Finally, if the client is unable to resolve the query locally, it forwards the request to a DNS server for resolution. The DNS server attempts to resolve the client’s query as detailed next:

- If the query result is found in any of the zones for which the DNS server is authoritative, the server responds to the host with an authoritative answer.

- If the result is in the zone entries of the DNS server, the server checks its own local cache for the information.

If the DNS server is unable to resolve the query, it forwards the request to other DNS servers, sending what is known as a recursive query. The server forwards to other servers that are listed as “forwarders,” or to a set of servers configured in the DNS server’s “Root Hints” file.

The DNS query is forwarded through communications channels on the Internet until it reaches a DNS server that is listed as being authoritative for the zone listed in the query. That DNS server then sends back a reply—either an “affirmative,” with the IP address requested, or a “negative” stating that the host in question could not be resolved.

Understanding Who Needs DNS

Not all situations require the use of DNS. There are other name resolution mechanisms that exist besides DNS, some of which come standard with the operating system (OS) that companies deploy. While not all scenarios have the requirement of a complex name resolution structure, DNS makes life easier by managing name servers in a domain sometimes with little overhead.

In the past, an organization with a standalone, noninterconnected network could get away with using only host files or WINS to provide NetBIOS-to-IP address name translation. Some very small environments could also use broadcast protocols such as NetBEUI to provide name resolution. In modern networks, however, DNS becomes a necessity, especially in Active Directory environments.

As stated before, WINS is no longer used by Exchange with the release of Exchange 2007. The proper installation and configuration of DNS is critical to the successful deployment of Exchange 2007.