Configuring DNS for Exchange 2000

by Amit Zinman [Published on 6 Jan. 2004 / Last Updated on 6 Jan. 2004]

Exchange 2000, internally, unlike traditional SMTP mail servers, does not rely on DNS MX records for mail delivery within the organization. Instead, it maintains a dynamic routing table used to transport mail around the Exchange organization. However, Exchange 2000 does depend on DNS for resolving host names, finding global catalog servers and finding other mail servers on the Internet. Also, without MX records on Internet DNSs, you wouldn't get mail from the Internet into your corporate environment.

Exchange 2000 relies heavily on global catalog servers for basic routines such directory lookups. Special entries in the Windows 2000 DNS called SRV tell Exchange 2000 where to locate global catalog servers. You don't have to worry about creating these records seeing that they are created automatically using dynamic registration. Like all Windows 2000 machines, Exchange 2000 server should have their DNS client point to a Windows 2000 DNS server. Following is an example of an IP configuration of an Exchange server:

Connection-specific DNS Suffix........... test.lab
Description........................................ 3Com 3C905TX-based
Physical Address.............................. 00-60-08-53-AA-EC
Dhcp Enabled.................................... No
IP Address........................................ 192.168.0.5
Subnet Mask..................................... 255.255.255.0
Default Gateway................................ 192.168.0.254
DNS Servers..................................... 192.168.0.1
192.168.0.2
Primary WINS Server......................... 192.168.0.1

Note that the DNS client points to an internet DNS. However, Exchange 2000 needs to send mail to the Internet. For that, internal records might not be enough. There three ways to achieve this.

#1 Configure local DNS to forward to the Internet

Windows 2000 DNSs come configured with root hints and no forwarders. Root hints are used to find the servers at the root of the Internet hierarchal system. Using root hints is not really effective for most corporations. It is just too slow to start looking through the entire hierarchy unless you're an ISP. Instead, forwarders can be configured forward request from clients to DNS servers on the Internet, typically your ISP DNSs. ISP DNSs are usually fast and employ large cached information on Internet domains.

The benefits of using this configuration are that the Exchange 2000 machine can also connect to the Internet also for updates and online error reporting. The local DNS also does some caching by itself so some queries don't even go the ISP DNSs and resolve locally.

To configure forwarders and delete root hints, using the Administrative Tools DNS MMC, go to the property sheet of your DNS server.

Configuring Forwarders

Deleting Root Hints by using the "Remove" button.

#2 Configure a smart host

A smart host is a mail server is used to deliver your mail and is also called a mail relay. This eliminated the need for DNS MX record resolution so you save the time it takes to do external resolutions. That said, DNS traffic is usually very fast and hardly consumes any bandwidth. It is more likely that your mail will actually be delivered slower if you use an ISP smart host which usually is very busy and has a large queue. The upside of using an ISP smart host is you send a single e-mail with a large attachment to say twenty recipients that mail will only be sent once through your wires. The smart host will create twenty separate e-mails to be sent to their destinations. This is especially useful for companies using ADSL which has a limited upload ratio. However, not all ISPs are generous enough to let you route all your mail through their servers.

To set up a smart host go to Exchange System Manager and open the property page of your SMTP Connector.

#3 Using Different DNSs for External Mail Delivery

Exchange 2000 lets you use different DNS servers for resolving MX records. I haven't found this feature to be of much use but I guess that in organizations that do not want workstations to resolve Internet hostnames (See #1) this could come in handy. One might ask, why not just add external DNSs to the IP configuration?

In the following configuration I've added an ISP DNS to the list of DNS servers.

Connection-specific DNS Suffix...........

test.lab

Description.......................................

3Com 3C905TX-based

Physical Address...............................

00-60-08-53-AA-EC

Dhcp Enabled...................................

No

IP Address.......................................

192.168.0.5

Subnet Mask.....................................

255.255.255.0

Default Gateway...............................

192.168.0.254

DNS Servers....................................

192.168.0.1

192.168.0.2

192.116.202.99

Primary WINS Server........................

192.168.0.1

This actually does not work. Windows 2000 (and other DNS clients) resolve hostnames using the first available DNS. This means that after querying 192.16.0.1, the internal DNS server, the DNS client will not go on to ask the other DNS server for more results. It will just give up. This differs from WINS behavior. WINS queries are done using all available WINS servers. This sort of makes sense since DNS is a hierarchal system while WINS is a flat one.

Instead, you can configure external DNS server for Exchange 2000, using ESM, in the SMTP virtual sever property pages.

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