5.6 Exchange s URL namespace

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5.6 Exchange's URL namespace

The Store automatically generates a URL for every item that it holds. Initially, the URL for an item comprises the server name, the alias for the mailbox, the folder name, and the subject. The Store adds an .eml extension,^[9] to let the browser know that it is able to process the file by reference to the application that controls the file system where the item is located. In this case, the application is Exchange, which takes responsibility for displaying information for any of the standard item types, such as note, appointment, or message. For example, if I post an item with a subject of "OK" in a folder, the URL is:

http://server_name/exchange/alias/folder_name/OK.eml

If you know it, you can use an item's URL just like any other URL, and, as long as you have permission to access the item, OWA will display its contents. In Figure 5.20, you can see an item in the Drafts folder displayed by OWA after I typed in its URL. Note that OWA does not display some of the command buttons that you would expect to see if you opened the Drafts folder normally (such as Reply and Reply All).

Figure 5.20: Accessing an item with a URL.

The Store does not finalize an item's URL until a client first accesses the item through the ExIFS interface. At this stage, the Store writes the file name part of the URL into the owning EDB as a property of the item. Even if the subject of the item changes after this time, the file name part of the URL will remain constant. For example, if an item starts with a subject called "Treasure Island," its initial URL is http://server_name/exchange/ alias/folder_name/Treasure Island.eml. If you then rename the item to "Treasure Cove," the Store automatically adjusts the file name part to be "Treasure Cove.eml." If ExIFS is used to access the item (from a Web browser or IMAP client, for instance), the file name is committed into the Store and remains constant from that point on, even if the item was then renamed back to "Treasure Island." Deferring writing the file name prop- erty in this manner avoids the need to update the store with properties for all known items during either the Exchange installation procedure or when a user opens a mailbox the first time after the upgrade. In the case of duplicate subjects for items held within a folder, Exchange adds a number to create a unique URL for each item. For example, if I have two items called "Operating Budget" in the "Plans" folder in my mailbox, the Store generates the following URLs:

http://server_name/exchange/tonyr/Budgets/Operating  Budget.eml     http://server_name/exchange/tonyr/Budgets/Operating  Budget-2.eml 

If other items are added with the same subject, the trailing number is incremented by one each time, resulting in URLs ending in -3, -4, -5, and so on. The Store updates item URLs as they move between folders. Interestingly, if the Store gives an item a trailing number to create its URL, the item keeps the number as part of the URL unless some other reason occurs to force a readjustment, such as the discovery that an item with the same name already exists in the folder. Thus, you can take an item with a URL of "Budget-3.elm" and move it into another folder and the Store keeps the same file name. Figure 5.21 shows the scheme in action. The "%20" characters in the URL represent space characters.

Figure 5.21: Creating unique URLs for duplicate item names.

The Store generates slightly different URLs for items that originate from Office and other applications. Instead of the .eml extension, Exchange uses the extension that already exists when the user imports the file into the Store. For example, if you drag and drop a BMP file into a folder, the generated URL uses the .bmp extension. The same occurs for common file types, such as Word (.doc), Excel (.xls), PowerPoint (.ppt), and Abode Acrobat (.pdf). When a browser requests items that belong to an application outside Exchange, OWA either loads them into the browser window (if the browser supports this feature) or downloads the content to a local file and launches the appropriate application.

5.6.1 FreeDocs

FreeDocs is the Microsoft term for documents that users add to public folders by dragging and dropping from Windows Explorer or the desktop. Users can then double-click on the document to launch the associated application and load the content from the public folder. The same type of access is possible through OWA if you pass a URL pointing to the document in the public folder. For example:

http://server-name/public/Budgets/2004-Budget.doc

In this instance, the URL points to the file 2004-Budget.doc in the top- level Budgets public folder. Exchange 2000 supports this type of access (referred to as a translated HTTP request) by default, but Microsoft disabled it in Exchange 2003 to close off a potential route where a user could put some malicious content in a public folder and infect other users with a virus or take another action to damage files or steal data. Some applications depend on translated HTTP requests, so you can reenable this feature by adding a new DWORD registry value at the key:

HKLM\System\CurrentControlSet\Services\MSExchangeWEB\OWA

The supported values are:

• 0: Default. OWA does not support translated HTTP requests.

• 1: OWA supports translated HTTP requests, but only from a back- end server.

• 2: OWA supports translated HTTP requests, but only from a back- end server or via a front-end server when the host header matches one of the format types specified in the AcceptedAttachmentFrontEnds registry value.

• 3: OWA supports translated HTTP requests everywhere.

The best course of action is to leave OWA in its default state unless you discover that an application needs to use this feature, in which case you can test various scenarios to discover the setting you need to make on the server.

^[9] . The .eml extension stands for "Exchange Message Link."

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