The need for X-Accel-Redirect (and it’s sibling X-Sendfile) comes from two distinct requirements
The need to deliver large files.
The need for those files to not be available to the public.
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                http://thedataasylum.com/articles/how-rails-nginx-x-accel-redirect-work-together.html
  
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                http://thedataasylum.com/articles/how-rails-nginx-x-accel-redirect-work-together.html