Nginx X-Accel-Redirect with proxy_pass and Host change..

I already wrote about using Nginx and X-Accel-Redirect. But today I would like to get back to it, since recently I’ve spent a lot of time trying to solve similar problem..

The problemo

We have a cdn in front of our sites and multiple pointers to static assets like a.static-example.com b.static-example.com etc. Pretty common thing. We also have our normal sites like www.fastcompany.com, www.fastcodesign.com and so forth.

One of our developers came to me saying that he needs to load a csv file from one of our static domains on the client to read it with jQuery and display a nice chart. Of course you can’t just load a file from another hostname (or even a subdomain), because of AJAX Same-origin policy.

The solution

You probably can solve this situation in multiple ways, for example one could put a file to the website assets dir and serve it from the same domain, or try to utilize JSONP. We didn’t want to do this because we store all our assets on S3, so we decided to create a route with Nginx which internally would fetch file from our static hostname using X-Accel-Redirect.

In simple terms it looks like this:
GET to www.fastcompany.com/assets/my.scv ---> (internal Nginx magic) ---> static.fc.com/assets/my.scv

Now to the magic:

Now it may seem simple on the surface and in some case it will be simple. But in our scenario when static.fc.com would receive a request, it would pass it to the application and base on its logic it will return X-Accel-Redirect header with the final location of the asset on s3. Essentially it’s a chain of internal redirects.

It’s very important that you use destination hostaname with resolver in proxy_pass ( not the pointer to the upstream! ). Otherwise Nginx will ignore proxy_set_header and use original Host header for X-Accel-Redirect request. I confirmed that with tcpdump. I know it sounds odd and unnecessary complicated, but just try to stick with example above.

For some more info you can read my Nginx ticket (which was sort of rejected), probably because of my poor problem explanation..