If Log Reverse DNS Lookup is set to “True” any entry in a log file related to a client connection will include the hostname of the client as well as its IP address. True is the default.
If Log Reverse DNS Lookup is set to “False” only the IP address will be logged to the log file.
To change the Log Reverse DNS Lookup behavior, change the value of the “LogReverseDNSLookup” key in the appsettings section of the web.config file For example:
<add key="LogReverseDNSLookup" value="True"/>