Why am I getting infinite redirect loop with force_ssl in my Rails app?

I want to have my API controller use SSL, so I added another listen directive to my nginx.conf

upstream unicorn {
  server unix:/tmp/unicorn.foo.sock fail_timeout=0;
}

server {
  listen 80 default deferred;
  listen 443 ssl default;
  ssl_certificate /etc/ssl/certs/foo.crt;
  ssl_certificate_key /etc/ssl/private/foo.key;

  server_name foo;
  root /var/apps/foo/current/public;

  try_files $uri/system/maintenance.html $uri/index.html $uri @unicorn;

  location @unicorn {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;
    proxy_pass http://unicorn;
  }

  error_page 502 503 /maintenance.html;
  error_page 500 504 /500.html;
  keepalive_timeout 5;
}

which passes the nginx conftest without any problems. I also added a force_ssl directive to my ApiController

class ApiController < ApplicationController
  force_ssl if Rails.env.production?

  def auth
    user = User.authenticate(params[:username], params[:password])
    respond_to do |format|
      format.json do
        if user
          user.generate_api_key! unless user.api_key.present?
          render json: { key: user.api_key }
        else
          render json: { error: 401 }, status: 401
        end
      end
    end
  end

  def check
    user = User.find_by_api_key(params[:api_key])
    respond_to do |format|
      format.json do
        if user
          render json: { status: 'ok' }
        else
          render json: { status: 'failure' }, status: 401
        end
      end
    end
  end
end

which worked just fine when I wasn't using SSL, but now when I try to curl --LI http://foo/api/auth.json, I get properly redirected to https, but then I keep on getting redirected to http://foo/api/auth ending in an infinite redirect loop.

My routes simply have

get "api/auth"
get "api/check"

I'm using Rails 3.2.1 on Ruby 1.9.2 with nginx 0.7.65

You're not forwarding any information about whether this request was an HTTPS-terminated request or not. Normally, in a server, the "ssl on;" directive will set these headers, but you're using a combined block.

Rack (and force_ssl) determines SSL by:

• If the request came in on port 443 (this is likely not being passed back to Unicorn from nginx)
• If ENV['HTTPS'] == "on"
• If the X-Forwarded-Proto header == "HTTPS"

See the force_ssl source for the full story.

Since you're using a combined block, you want to use the third form. Try:

proxy_set_header X-Forwarded-Proto $scheme;

in your server or location block per the nginx documentation.

This will set the header to "http" when you come in on a port 80 request, and set it to "https" when you come in on a 443 request.

Try setting this directive in your nginx location @unicorn block:

proxy_set_header X-Forwarded-Proto https;

I had this same issue and investigating the Rack middleware handler (not force_ssl but similar) I could see that it was expecting that header to be set to determine if the request was already processed as being SSL by nginx.