A "Connection_Dropped" event message is logged in the Httperr.log file on a Windows Server 2003-based computer that is running IIS 6.0

The "Connection_Dropped" event message occurs if the following conditions are true:

• An HTTP client has sent an HTTP request to IIS.
• IIS has received the request and has begun to process the response.
• The connection between the client and the server is closed before a complete response is sent.
  
  Note A complete response includes logging data.

In these conditions, the Http.sys driver cannot resolve the connection. However, the Http.sys driver may not immediately destroy the original connection object. Connections in this state are also known as "zombie connections."

The Http.sys driver adds the "zombie connection" to a list. Because the original connection object is still available, the original connection object information can be included in the logging information when the request is completed. If the response is completed before the time-out value that is used by the Http.sys driver is reached, no information is logged in the Httperr.log file. Instead, the status code is logged in the IIS log. For example, an "HTTP 200-OK" status code is logged in the IIS log when the request succeeds.

However, if response processing takes longer than the time-out value that is used by the Http.sys driver, the Http.sys driver destroys the original connection object. Then, the Http.sys driver writes a "Connection_Dropped" event message to the Httperr.log file.

Typically, a "Connection_Dropped" event message is logged in the Httperr.log file when a client closes its connection to the Web server before the complete response message is sent. For example, a "Connection_Dropped" event message is logged in the Httperr.log file if a user views a Web page, and then the user closes the browser window before the complete response message is sent by IIS.

It is very unlikely that a default installation of IIS would close a connection to a client before the complete response message is sent. However, a custom ISAPI application that does not handle response messages correctly may cause a "Connection_Dropped" event message to be logged in the Httperr.log file.