Network Administration News

A simple problem may have rather a complex solution, and vice versa. Most complex situations are when cross-tool compatibility is required in the environment when tools can't be either configured according to the task, or there are other restrictions preventing from applying the necessary configuration. An example of such a situation is described on SysAdmin Adventures, in Forcing browsers to “open with..” or “save” attachments.

This is what the article states: there are times when you want to force web-browsers to handle a file differently to its default action. Specifically for me, I wanted browsers to not open certain text files themselves, but instead pass them to a text editor. Doing this is quite easy, but how is not well-known.

Obviously the immediate choice is to change the mime-type handling of the browser which is possible in Firefox, although not easily – I’m not sure if it’s possible in IE – but this has to be done manually for every browser. For Firefox you need to either get an extension for modifying the handlers, or you need to find a text file that has been forced into using the Open/Save dialogue (as per solution below) and then use the “Always perform this action” checkbox.

I decided to look at how the file is sent from the server as, often, when a txt file is passed from a CGI script, you get the Open/Save dialogue option. Although there are many different ways you can do it, the canonical way is for the server to pass a “Content-Disposition: attachment” response header to the browser which will stop it disposing of the file by opening within the browser regardless of the mime-type it sniffs from the file. As ALWAYS, Internet Explorer does not follow this rule and will often ignore the content-disposition header and act based on the mime-type it sniffs from the file regardless and so, to deal with IE you also need to force the mime-type to application/octet-stream.

This needs to be done in the Apache config, however the best way to do it is with .htaccess as it gives you directory-level control on the files you are affecting. Because it is being done with a regular expression being passed to the FilesMatch directive, you can choose to specify a particular file extension to affect as I have below, or you can specify a particular file, or anything else you can normally do with a regexp.

The following is the correct directive for a .htaccess file if you want to match any file with a .txt file extension, matching upper-case, lower-case or a mixture of cases. When matching the regexp, the mime-type is forced to “application/octet-stream” and the “Content-Disposition: attachment” header is passed.

<FilesMatch "\.(?i:txt)$">
  ForceType application/octet-stream
  Header set Content-Disposition attachment
</FilesMatch>

Exactly the same approach should be used with every type of document when the default behavior of a browser should be overridden. The above example illustrates the basic idea: the wider is the number of tools that should be affected by a change, the lower should be the level at which changes are made.

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