IIS includes a pointer to this structure when it is preprocessing a request's headers. If your filter should be notified for this event, it should register for the SF_NOTIFY_PREPROC_HEADERS event.
typedef struct _HTTP_FILTER_PREPROC_HEADERS
{
BOOL (WINAPI * GetHeader) ;
BOOL (WINAPI * SetHeader) ;
BOOL (WINAPI * AddHeader) ;
DWORD HttpStatus;
DWORD dwReserved;
} HTTP_FILTER_PREPROC_HEADERS, *PHTTP_FILTER_PREPROC_HEADERS;
When the server is about to process the client headers, this structure is pointed to by the pvNotification parameter in the HttpFilterProc when the notificationType parameter is set to SF_NOTIFY_PREPROC_HEADERS.
HttpFilterProc, GetHeader, SetHeader, AddHeader