iframe_allowing_proxy.rb

# This proxy is to allow an Iframe to display content from gov.uk, which is a different domain, into this application.
module Proxies
  class IframeAllowingProxy < Rack::Proxy
    PROXY_BASE_PATH = "/iframe-proxy/".freeze

    def rewrite_env(env)
      env["HTTP_HOST"] = "www.gov.uk:443"
      env["SERVER_NAME"] = "www.gov.uk"
      env["SERVER_PORT"] = 443
      env["SCRIPT_NAME"] = ""
      env["REQUEST_PATH"] = env["REQUEST_URI"] = env["PATH_INFO"]
      env["rack.url_scheme"] = "https"

      # Ensure the target returns an uncompressed body so that the response can easily be rewritten
      env.delete("HTTP_ACCEPT_ENCODING")
      env
    end

    # Links from gov.uk are rewritten to point to the proxy
    def rewrite_response(triplet)
      status, headers, body = triplet

      result = []
      if headers.fetch("content-type", []).any? { |header| header.include?("text/html") }
        body.each do |body_part|
          result << body_part.gsub(%r{(href|src)=["'](https://(www\.)?gov\.uk)?/([^'"]*)["']}, %(\\1="#{Proxies::IframeAllowingProxy::PROXY_BASE_PATH}\\4"))
        end
      else
        result = body
      end

      # Without this, I get `Rack::Lint::LintError: header must not contain Status`
      # when testing locally
      headers.delete("status")

      [status, headers.tap { |h| h["x-frame-options"] = "ALLOWALL" }, result]
    end
  end
end