Upon success, the server must return a response code of HTTP 200 and set the response's content-type to a suitable MIME type, as described here.
Google Earth reads KML and KMZ files. The MIME type for KML files is
application/vnd.google-earth.kml+xml
The MIME type for KMZ files is
application/vnd.google-earth.kmz
For Apache, add these lines to the httpd.conf file:
AddType application/vnd.google-earth.kml+xml .kml
AddType application/vnd.google-earth.kmz .kmz
See the Microsoft documentation for details on setting up MIME types on Microsoft's IIS.
The body of the response must contain valid KML data, including the XML declaration (<?xml version="1.0" encoding="UTF-8"?>). If the server returns invalid KML, the Network Link will stop, deactivate, and output an error message.