Message "X-Accel-Mapping header missing" in Nginx error log

Question

I am running a Rails 3 site on Ubuntu 8.04 with Nginx 1.0.0 and Passenger 3.0.7.

In my Nginx error.log I started seeing the message X-Accel-Mapping header missing quite a lot. Googling lead me to the docs of Rack::Sendfile and to the Nginx docs.

Now, my app can be accessed through several domains and I am using send_file in my app to deliver some files specific to the domain they are requested from, e.g., if you come to domain1.com/favicon.ico I look up the favicon in at public/websites/domain1/favicon.ico. This works fine and I don't think I need/want to get Nginx involved and create some private area where I store those files, as the samples in the Rack::Sendfile docs suggest.

How can I get rid of the error message?

Answer 1

this message means that Rack::Sendfile disabled X-Accel-Redirect for you, because you have missing configuration for it in nginx.conf...

I'm using Nginx + Passenger 3 + Rails 3.1.

Gathered information from this pages I've figured it out:

http://wiki.nginx.org/X-accel

http://greenlegos.wordpress.com/2011/09/12/sending-files-with-nginx-x-accel-redirect

http://code.google.com/p/substruct/source/browse/trunk/gems/rack-1.1.0/lib/rack/sendfile.rb?r=355

Serving Large Files Through Nginx via Rails 2.3 Using x-sendfile

I have controller which maps /download/1 requests to storage files which have their own directory structure, like this: storage/00/00/1, storage/01/0f/15 etc. So I need to pass this through Rails, but then I need to use send_file method which will use X-Accel-Redirect to send the final file to the browser through nginx directly.

Within the code I have this:

send_file(
  '/var/www/shared/storage/00/00/01', 
  :disposition => :inline, 
  :filename => @file.name # an absolute path to the file which you want to send
)

I replaced the filename for this example purposes

Now I had to add these lines to my nginx.conf:

server {
    # ... 

    passenger_set_cgi_param HTTP_X_ACCEL_MAPPING /var/www/shared/storage/=/storage/; 
    passenger_pass_header X-Accel-Redirect;

    location /storage {
      root /var/www/shared;
      internal;
    }

    # ...
}

The path /storage is not visible from outside world, it is internal only.

Rack::Sendfile gets the header X-Accel-Mapping, extracts the path from it and replaces /var/www/shared/storage with /storage.... Then it spits out the modified header:

X-Accel-Redirect: /storage/00/00/01

which is then processed by nginx.

I can see this works correctly as the file is downloaded 100x faster than before and no error is shown in the logs.

Hope this helps.