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An IFilter is a plugin that allows the Windows Indexing Service and the newer Windows Desktop Search to index various file formats so that they become searchable. Without an appropriate IFilter, contents of a file cannot be parsed and indexed by the search engine. IFilters are also used by SQL Server as a component of the SQL Server Full Text Search service.
IFilter is designed to help users to index a large number of documents and then quickly find text within these documents. The documents can be files, email attachments or database records.
Windows Indexing Service, MSN Desktop Search, Internet Information Server, SharePoint Portal Server, Windows SharePoint Services (WSS), Site Server, Exchange Server, SQL Server and all other products based on Microsoft Search technology support indexing technology. Hereafter, we use term "search engine" to refer to these products.
An IFilter acts as a plug-in for full-text search engines. A search engine usually works in two steps:
- The search engine goes through a designated place, e.g. a file folder or a database, and indexes all documents or newly modified documents, including the various types documents, in the background and creates internal data to store indexing result.
- A user specifies some keywords he would like to search and the search engine answers the query immediately by looking up the indexing result and responds to the user with all the documents that contains the keywords.
During Step 1, the search engine itself doesn't understand format of a document. Therefore, it looks in windows registry for an appropriate IFilter and finds the IFilter. IFilter understands the document format. It filters out embedded formatting and extracts text from the document and return text back to the search engine.
- Filter Central — Microsoft Search Filters Discussion Board;
- IFilter at MSDN — Interface documentation on MSDN;
- IFilter.org — Downloads and documentation;
- MSG IFilter — IFilter for Outlook Message Files (.MSG) for Windows Desktop Search;
- IFilterShop — Some IFilters available as free for non-commercial users.
- IFilter Explorer Displays all the IFilters currently installed on your computer.
- PDF IFilter Foxit PDF IFilter. Works on Windows OS.
- PDFlib TET PDF IFilter PDF IFilter from PDFlib. Works on Windows OS.
- IFilter Downloads iFilter Downloads.
-  Windows Search connector for IBM Lotus Notes.
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