The way to re-use your field extractions is by applying them at the sourcetype level. This also means that you should set the same sourcetype for all logs of the same type.
While your setup may be better in some cases, this is probably not one of them, since you will have to maintain a bunch of identical field extractions. It may make more sense to change the host value, either through host_segment or host_regex , so that each website gets a unique host name as seen from splunk. You can look this up in the docs for inputs.conf
Or you could limit searches on the source when you want to differentiate between them, e.g.;
sourcetype=iis source=*W3SVC4* | the_rest_of_your_search
Hope this helps,
Kristian
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