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Since we are proposing a new filter, we can and should take the opportunity to rationalise all aspects of filter configuration. The new mod_filter will implement

FilterDeclare filter-name level
The handler for this will call ap_register_output_filter to register filter_harness under the name supplied and at the level supplied.
FilterDispatcher filter-name header-name match-criteria
This specifies an HTTP header and a pattern-matching criterion to be used on its value to extract a key on which we dispatch.
FilterProvider filter-name match-value handler
This specifies a match on the criterion supplied by FilterDispatcher, and a filter handler (as provided by any Apache 2.0 filter module) to be used as the dispatcher when the match is successful.

The existing directives (SetOutputFilter and family) can then be used to insert or remove filters in the output chain as before. Likewise, ap_add_output_filter can be used programmatically to insert a smart filter, just as it can a traditional filter.

[ Question: do we want to abstract FilterDispatcher further to enable RewriteRule/RewriteCond-like definition of dispatch criteria ? ]