Application Handles Responses Received from DNS

• Microsoft Windows NT Workstation versions 4.0
• Microsoft Windows NT Server versions 4.0
• Microsoft Internet Explorer 2.0 and 3.0

SUMMARY

Using Domain Name System (DNS) to resolve multihomed clients can fail even if one of the IP addresses returned by DNS is correct.

MORE INFORMATION

When a Windows NT 4.0 client uses DNS to resolve a name of a machine that is defined with multiple IP addresses, the application may fail if one or more of the IP addresses mapped to the name do not exist.

Most HOST utilities (FTP, Telnet, and so forth) make a separate request to DNS each time an attempt is made to resolve the name. The round robin feature of DNS enables DNS to return the IP addresses of the name in a different order each time. The application will use the first IP address of the list each time and attempt to make a connection using that IP address.

NetBT connectivity handles the responses differently. When the first query is made through DNS, the name is then cached for the time defined as the CacheTimeout (10 minutes). The request is made for the first entry in the returned list of IP addresses. Any attempts made within 10 minutes of the first are then addressed through the cache first and DNS will not be used. Any additional requests made after the cache is empty will (more likely) then also receive the response in a new order.

Microsoft Internet Explorer works differently in that it will go through each IP address returned until either a response is received or all IP addresses are attempted. The responsibility of performing in this manner lies in the application. It is the responsibility of the resolver to return the addresses from DNS, not define the manner in which the application uses the addresses.