Serving Different Robots.txt Using Rack

While doing an SEO audit for the daily deal API I’m working on, the subject of the robots.txt came up. In addition to our production environment (what you and everyone else see), we also use an “edge” environment. It’s a place we can push the latest and greatest changes and test them before they go live. Edge is an exact copy of production just running on a different domain. Since we didn’t want to get dinged with content duplication we had to disallow spiders from crawling our edge environment. Here’s how we serve different robots.txt files based on environment using Rack within Rails.

  1. Move public/robots.txt to config/robots.txt. This is now your production robots.txt. Any other environment will disallow everything for all user-agents.
  2. Create a RobotsGenerator in lib
  3. Point /robots.txt to the generator in your routes


class RobotsGenerator
  # Use the config/robots.txt in production.
  # Disallow everything for all other environments.
    body = if Rails.env.production? Rails.root.join('config', 'robots.txt')
      "User-agent: *\nDisallow: /"

    # Heroku can cache content for free using Varnish
    headers = {
      'Content-Type'  => 'text/plain',
      'Cache-Control' => "public, max-age=#{1.year.seconds.to_i}"

    [200, headers, [body]]
  rescue Errno::ENOENT
    headers = { 'Content-Type' => 'text/plain' }
    body    = '# A robots.txt is not configured'

    [404, headers, [body]]


require 'robots_generator'

YourApplication::Application.routes.draw do
  # ...

  match '/robots.txt' => RobotsGenerator

  # ...

Update (Sept. 23, 2013): Thanks to @michaelbaudino for pointing out that routes.rb needs the require 'robots_generator' since Rails 3 does not autoload files in lib. Additionally, the request headers should always include Content-Type to avoid a Rack::Lint::LintError error.

Thanks for reading! I'm Avand.

I’ve been working on the web for over a decade and am passionate about building great products.

My last job was with Airbnb, where I focused on internal products that helped teams measure the quality of the software they were building. I also built internal tools for employees to stay more connected, especially after the COVID-19 pandemic. Before that, I was lead engineer at Mystery Science, the #1 way in which science is taught in U.S. elementary school classroms. For a while, I also taught with General Assembly, teaching aspiring developers the basics of front-end web development.

I was born in Boston, grew up in Salt Lake City, and spent many years living in Chicago. Now, I call San Francisco my home and Mariposa my home away from home.

I enjoy the great outdoors and absolutely love music and dance. Cars have been an lifelong obsession of mine, especially vintage BMWs and Volkswagens. I’m the proud owner of a 2002 E-250 Sportsmobile van, and he and I have enjoyed many trips to beautiful and remote parts of the West Coast to create good vibes.

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