Redirect non-www requests to www URLs in Ruby on Rails

For rails 4, use it -

  before_filter :add_www_subdomain

  private
  def add_www_subdomain
    unless /^www/.match(request.host)
      redirect_to("#{request.protocol}www.#{request.host_with_port}",status: 301)
    end
  end

While John's answer is perfectly fine, if you are using Rails >= 2.3 I would suggest to create a new Metal. Rails Metals are more efficient and they offers better performance.

$ ruby script/generate metal NotWwwToWww

Then open the file and paste the following code.

# Allow the metal piece to run in isolation
require(File.dirname(__FILE__) + "/../../config/environment") unless defined?(Rails)

class NotWwwToWww
  def self.call(env)
    if env["HTTP_HOST"] != 'www.example.org'
      [301, {"Content-Type" => "text/html", "Location" => "www.#{env["HTTP_HOST"]}"}, ["Redirecting..."]]
    else
      [404, {"Content-Type" => "text/html"}, ["Not Found"]]
    end
  end
end

Of course, you can customize further the Metal.

If you want to use Apache, here's a few configurations.

There is a better Rails 3 way - put this in your routes.rb file:

  constraints(:host => "example.com") do
    # Won't match root path without brackets around "*x". (using Rails 3.0.3)
    match "(*x)" => redirect { |params, request|
      URI.parse(request.url).tap { |x| x.host = "www.example.com" }.to_s
    }
  end

Update

Here is how to make it domain agnostic:

  constraints(subdomain: '') do
    match "(*x)" => redirect do |params, request|
      URI.parse(request.url).tap { |x| x.host = "www.#{x.host}" }.to_s
    end
  end

Ideally you'd do this in your web server (Apache, nginx etc.) configuation so that the request doesn't even touch Rails at all.

Add the following before_filter to your ApplicationController:

class ApplicationController < ActionController::Base
  before_filter :add_www_subdomain

  private
  def add_www_subdomain
    unless /^www/.match(request.host)
      redirect_to("#{request.protocol}x.com#{request.request_uri}",
                  :status => 301)
    end
  end
end

If you did want to do the redirect using Apache, you could use this:

RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.x\.com [NC]
RewriteRule ^(.*)$ http://www.x.com/$1 [R=301,L]