rack-url-canonicalizer is a lightweight, zero-dependency Rack middleware for Ruby applications (Rails, Sinatra, Hanami, Roda, or plain Rack).

It automatically canonicalizes incoming request URLs to prevent search engine Duplicate Content penalties by enforcing:

1. Removal of the www. sub-domain prefix.
2. Collapsing duplicate slashes (//path//to ➔ /path/to).
3. Stripping trailing slashes from paths (/path/ ➔ /path).
4. Validating locale GET query parameters against an allowed list and stripping invalid locales.

Search engines (Google, Yandex, Bing) treat every unique URL string as a distinct page. Without strict URL normalization, subtle variations in request paths create Duplicate Content, which splits link authority (PageRank), degrades search rankings, and wastes crawl budget.

This gem is essential when:

• Protecting SEO & Domain Authority: You need to prevent index cannibalization caused by variations like example.com/item vs. www.example.com/item, example.com/item/, or example.com//item.
• Enforcing Single Canonical URLs: You want incoming traffic from user typos or legacy backlinks to be redirected with an HTTP 301 (Moved Permanently) status to a single canonical address before reaching your application logic.
• Handling Multi-Locale Query Parameters: Your app supports localization (e.g. ?locale=en) and you want to sanitize or discard invalid/malformed locale parameters (e.g. ?locale=xyz) that would otherwise generate infinite duplicate URL variants.
• High-Performance Early Redirects: You want URL normalization to happen at the lightweight Rack middleware layer—avoiding unnecessary Rails controller instantiations, routing checks, or database queries.
• Simplified Deployment with Kamal & Modern Stacks: Ideal for containerized setups deployed with Kamal, Thruster, or Docker, where configuring and maintaining separate Nginx/Caddy redirect rules or complex DNS edge layers adds unnecessary overhead. This gem handles canonicalization directly inside your application stack for fast, zero-config deployments.

Add this line to your application's Gemfile:

gem "rack-url-canonicalizer"

And then execute:

bundle install

In config/application.rb:

config.middleware.use Rack::UrlCanonicalizer,
  strip_www: true,
  collapse_slashes: true,
  strip_trailing_slash: true,
  exclude_paths: %w[/api /assets /up],
  allowed_locales: -> { I18n.available_locales }

In config.ru:

require "rack/url_canonicalizer"

use Rack::UrlCanonicalizer,
  strip_www: true,
  exclude_paths: ["/api"]

run MyApp

Alternatively, set global defaults:

Rack::UrlCanonicalizer.configure do |config|
  config.strip_www = true
  config.collapse_slashes = true
  config.allowed_locales = %w[en ru es]
end

# In Rack stack:
use Rack::UrlCanonicalizer

Run tests with RSpec:

bundle exec rspec

The gem is available as open source under the terms of the MIT License.