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Block Web Crawlers With Rails

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Block Web Crawlers With Rails

Sometimes you just need to block web crawlers from accessing your web site or web app. In this post we take a look at how to do just that using Rails.

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Search engines “crawl” and “index” web content through programs called robots (a.k.a. crawlers or spiders). This may be problematic for our projects in situations such as:

  • a staging environment
  • migrating data from a legacy system to new locations
  • rolling out alpha or beta features

Approaches to blocking crawlers in these scenarios include:

  • authentication (best)
  • robots.txt (crawling)
  • X-Robots-Tag (indexing)

Problem: Duplicate Content

With multiple environments or during a data migration period, duplicate content may be accessible to crawlers. Search engines will have to guess which version to index, assign authority, and rank in query results.

For example, we periodically backup our production data to the staging environment using Parity:

production backup
staging restore production

Things Search Engines Do

In order to provide results, a search engine may prepare by doing these things:

  1. check a domain’s robots settings (e.g. http://example.com/robots.txt)
  2. request a web page on the domain (e.g. http://example.com/)
  3. check the webpage’s X-Robots-Tag HTTP response header
  4. cache the web page (saving its response body)
  5. index the web page (extract keywords from the response body for fast lookup)
  6. follow links on the web page to other web pages and repeat

Steps 1, 2, 3, and 6 are generally “crawling” steps and steps 4 and 5 are generally “indexing” steps.

Solution: Authentication (Best)

The most reliable way to hide content from a crawler is with authentication such as HTTP Basic authentication:

class ApplicationController < ActionController::Base
      name: ENV.fetch("BASIC_AUTH_USERNAME"),
      password: ENV.fetch("BASIC_AUTH_PASSWORD"),

This often is all we need for situations such as a staging environment. The following approaches are more limited but may be more suitable for other situations.

Notice we can control whether crawlers are allowed to access content via config in the environment. We can use Parity again to add configuration to Heroku staging:

staging config:set DISALLOW_ALL_WEB_CRAWLERS=true

Solution: robots.txt (Crawling)

The robots exclusion standard helps robots decide what action to take. A robot first looks at the /robots.txt file on the domain before crawling it.

It is a de-facto standard (not owned by a standards body) and is opt-in by robots. Mainstream robots such as Googlebot respect the standard, but bad actors may not.

An example /robots.txt file looks like this:

User-agent: *
Disallow: /

This blocks (i.e. disallows) all content (/) to all crawlers (User-agents). See this list of Google crawlers for examples of user agent tokens.

Globbing and regular expressions are not supported in this file. See what can go in it.

Add Climate Control to the


to control environment variables in tests:

gem "climate_control"

In spec/requests/robots_txt_spec.rb:

require "rails_helper"

describe "robots.txt" do
  context "when not blocking all web crawlers" do
    it "allows all crawlers" do
      get "/robots.txt"

      expect(response.code).to eq "404"
      expect(response.headers["X-Robots-Tag"]).to be_nil

  context "when blocking all web crawlers" do
    it "blocks all crawlers" do
      ClimateControl.modify "DISALLOW_ALL_WEB_CRAWLERS" => "true" do
        get "/robots.txt"

      expect(response).to render_template "disallow_all"
      expect(response.headers["X-Robots-Tag"]).to eq "none"

Google recommends no robots.txt if we want all our content to be crawled.

In config/routes.rb:

get "/robots.txt" => "robots_txts#show"

In app/controllers/robots_txts_controller.rb:

class RobotsTxtsController < ApplicationController
  def show
    if disallow_all_crawlers?
      render "disallow_all", layout: false, content_type: "text/plain"
      render nothing: true, status: 404


  def disallow_all_crawlers?

If we’re using an authentication library such as Clearance site-wide, we’ll want to skip its filter in our controller:

class ApplicationController < ActionController::Base
  before_action :require_login

class RobotsTxtsController < ApplicationController
  skip_before_action :require_login

Remove the default Rails robots.txt and prepare the custom directory:

rm public/robots.txt
mkdir app/views/robots_txts

In app/views/robots_txts/disallow_all.erb:

User-agent: *
Disallow: /

Solution: X-Robots-Tag (Indexing)

It is possible for search engines to index content without crawling it, because websites might link to it. So, our robots.txt technique blocked crawling, but not indexing.

Adding a X-Robots-Tag header to our responses short-circuits the entire process; well-behaved crawlers won’t make HTTP requests at all to content on the domain.

You may have seen meta tags like this in projects you’ve worked on:

<meta name="robots" content="noindex,nofollow">

The X-Robots-Tag header has the same effect as the robots meta tag, but applies it to all content types in our app (e.g. images, scripts, styles), not only HTML files.

To block robots in our environment, we want a header like this:

X-Robots-Tag: none

The none directive is equivalent to noindex, nofollow. It tells robots not to index, follow links, or cache.

In lib/rack_x_robots_tag.rb:

module Rack
  class XRobotsTag
    def initialize(app)
      @app = app

    def call(env)
      status, headers, response = @app.call(env)

      if ENV["DISALLOW_ALL_WEB_CRAWLERS"].present?
        headers["X-Robots-Tag"] = "none"

      [status, headers, response]

In config/application.rb:

require_relative "../lib/rack_x_robots_tag"

module YourAppName
  class Application < Rails::Application

    config.middleware.use Rack::XRobotsTag

Our specs will now pass.


Our environment’s content can be blocked in three different ways from crawling and indexing by web robots that respect the robots exclusion standard (most importantly Google).

Use authentication to entirely hide it, or robots.txt plus the X-Robots-Tag for more granular control.

With SDKs for all major client and server side platforms, you can experiment on any platform with Optimizely Full Stack.

google ,web crawling ,block ,ruby ,rails

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