Configuring DNS to Support Exchange Servers

Because DNS is already required and integrated with Active Directory before Exchange Server is installed, most companies already have a robust DNS environment in place. Exchange by itself accesses DNS servers to find resources on the local network, such as Global Catalog servers and domain controllers. It also uses DNS to search for MX records of other domains.

External DNS Servers for the Internet

The external DNS server for Exchange (or any other mail system) is responsible for giving out the correct MX and A records for the domain for which it is authoritative . Administrators should take security precautions regarding who can change these records ”and how. Intentionally or accidentally changing these records can result in undelivered mail.

Most companies let their ISP host the external DNS entries for their domain. ISPs provide internal administrators with methods of managing DNS entries for their domain. In some cases it has to be done over the phone, but normally a secure Web interface is provided for management. Although this setup is convenient and ISPs usually take care of load balancing and redundancy, some companies opt to host their own zone records for the Internet. In this case companies have to host their own DNS server in-house with the ISP responsible only for forwarding all requests to their DNS server. When hosting an external DNS server, in-house administrators have to think about security issues and DNS configuration issues.

Internal DNS Servers for Outbound Mail Routing

Exchange SMTP gateways are responsible for delivering mail to external hosts . As with any name process involving resolving names to IP addresses, DNS plays a major part in successful mail delivery.

Exchange can route mail to outbound destinations two ways. One is by using smarthosts to offload all processing of messages destined to other domains. As seen in the previous section, an NLB cluster can be used to route Internet mail to its final destination.

The second way is the default, with Exchange Server 2003 taking care of delivering messages to other domains. In this scenario, Exchange queries DNS servers for other domains' MX records and A records for address resolution.

Internal DNS Servers for Internal Routing of Email Between Exchange Servers

Exchange 2003 depends entirely on SMTP for mail routing, both internally and externally. Internally, Exchange servers have to be able to resolve either the short name (for example, server1 ) or the FQDN (for example, server1.companyabc.com ).