Each Communicator Web Access server must have a DNS host record that associates the Web site URL with the computer's IP address. In addition, each Communicator Web Access server must have a pair of canonical name (CNAME) records named asand download. For example, the URL im.contoso.com must have the following two DNS records:
These CNAME records are required in order to support desktop sharing.
If you are employing a hardware load balancer, your CNAME records must refer to the IP address of the load balancer rather than to individual Communicator Web Access server. For example, if you have four servers located behind a hardware load balancer, your CNAME records should point to the load balancer, and you should have a single asrecord that points to the load balancer rather than four separate as records, one for each server.
A similar approach is required if you are using a reverse proxy server to handle external logons. In that case, your CNAME records must refer to the IP address of the reverse proxy server. In addition, you will need to create a host name record for this server.
For details, including step-by-step information about creating DNS records, see in Deploying Communicator Web Access in the Deploying Office Communications Server 2007 R2 documentation.