Rails unified host override for use with apps behind a CDN (Rack middleware, supports OmniAuth)

set_default_host.rb

# Rack middleware that reads environment variable HOST (value such as: app.example.com)
# to override request Host headers, forcing the app to use this value for URLs & redirects.
#
# If env var HOST is unset, then this middleware does nothing.
#
# Useful for when a Rails app is the origin behind a CDN/proxy, so that all generated
# URLs point to the canonical hostname of the CDN, and not the origin itself.
#
# For a Rails app,
# - save this file in config/initializers/set_default_host.rb
# - set YourRailsAppModuleName to the app's module name (see config/application.rb)
#
module YourRailsAppModuleName
  class SetHostMiddleware
    def initialize(app)
      @app = app
      @new_host = ENV['HOST']
    end

    def call(env)
      if @new_host
        env[:Host] = @new_host
        env[:default_host] = @new_host
        env["HTTP_HOST"] = @new_host
      end
      @app.call(env)
    end
  end

  class Application < Rails::Application
    config.middleware.insert_before(Rack::Runtime, SetHostMiddleware)
  end
end