Configuring DNS for Instant Messaging | Microsoft Exchange 2000 Server Adminstrator's Companion

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Configuring DNS for Instant Messaging is a straightforward, three-step process. First, you must enter an address (A) record for each IM server into the database. You've probably created this type of record for other servers, so it should not be a new activity.

Second, if you're going to have routing servers, consider giving them the host name of "im." Although this is not a standard, nor is it required, Microsoft recommends that the routing servers be given this name so that their fully qualified domain name (FQDN) is im.domainame.com (for example, im.microsoft.com). If everyone follows this standard, users can guess at the SMTP address to which they should send instant messages for recipients in other domains. For instance, you could send an instant message to a friend at Microsoft, user1, making an educated guess that the internal routing server's FQDN is im.microsoft.com. Thus, you would send the instant message to user1@im.microsoft.com, and if Microsoft has given its routing servers the name of "im," the message will be routed to that routing server over port 80 and will land in the instant messaging client of user1.

TIP
In a single-server environment, you may want to alias your server's host name with "im," thereby retaining your naming convention internally while still being compatible with other instant messaging installations on the Internet. Be sure that your ISP's DNS records have an FQDN mapping for the IM alias to your incoming IP address for port 80. You will set up the SRV record on your own DNS and will host two virtual servers on the same physical server: a virtual routing server and a virtual home server. If you do not want to configure a routing server, use the "im" FQDN as the host header for your default Web site to have messages routed into your server.

Third you need to create an _rvp SRV resource record for the RVP protocol to enable it to work over port 80. This record should map directly to the routing server. When the instant messaging client attempts to log on, the client first queries DNS for an _rvp SRV resource record. If such a record exists, the client contacts that server, and is then referred to its home server. This SRV record makes it possible for a user's e-mail address and IM address to be the same.

Contrary to what you might have read elsewhere, CNAME records are not necessary when configuring DNS for Instant Messaging. Moreover, aliases are not needed, although in a single-server environment, an alias might be helpful for interoperability on the Internet, as was mentioned in the previous tip.

MORE INFO
See the Windows 2000 Advanced Server DNS documentation for specific information about creating DNS resource records.

NOTE
If you have multiple IM routers identified with the same IM domain, you will need to create multiple A records, one for each router. You can use a DNS round-robin scheme to advertise a single FQDN, with each server binding to a unique IP address.

REAL WORLD Advanced Configuration of DNS to Work with Instant Messaging
If you meet any of the following criteria, you'll want to be sure to read this section:

You're an Internet service provider (ISP)

You're hosting several different companies or domains on the same physical server, all of which want IM services

You want to implement Instant Messaging without using a routing server

We will look at three different scenarios, describing how best to use three of the basic IM components—the routing server, the home server, and the SRV record—to achieve a specific goal.

Scenario 1: You want to host multiple domains for separate companies on a single physical server, and you're either unwilling or unable to use SRV records and routing servers.

You can implement Instant Messaging in this scenario as follows: Give the physical server the host name "im," and then place an A record in each DNS lookup zone that maps the server's IP address to each FQDN (im.domainname.com). Be sure that the name of the server after the @ sign is identical to the FQDN of the home server. This configuration causes the instant messaging client to contact the home server and establish a connection for IM.

This technique is not the best way or even a recommended way to implement IM services. Consider this configuration only if it is the only choice available to you.

Scenario #2: You want to host multiple domains for separate companies on a single physical server, and you're using SRV records, one home server, and no routing servers.

To implement IM given this scenario, first give all of the IM servers an A record in DNS. Second, create one SRV resource record in each DNS lookup zone that references one home server. Then have the users log on with their domain name minus the "im" designation. Creating a SRV resource record for each domain name causes the users in each DNS domain to be routed to the same home server.

Scenario #3: You want to host multiple domains for separate companies on a single physical server and you're using routing servers with SRV records and one home server.

This scenario is the preferred method of implementing IM. Give the routing server a host name—again, "im" is recommended—and map the SRV resource record in DNS to this server. Give the home server an A record in DNS. As user accounts are enabled for Instant Messaging, their URLs will be created, indicating to the routing server where to refer the client when they log on.