Rack middleware replacement for mod_rewrite
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spec Be smart about when to include port in redirected host. Jan 31, 2010



Refraction is a Rack middleware replacement for mod_rewrite. It can rewrite URLs before they are processed by your web application, and can redirect using 301 and 302 status codes. Refraction is thread-safe, so it doesn't need to be guarded by Rack::Lock.

The best thing about Refraction is that rewrite rules are written in plain old Ruby code, not some funky web server config syntax. That means you can use Ruby regular expressions, case statements, conditionals, and whatever else you feel like.

For example:

Refraction.configure do |req|
  feedburner  = "http://feeds.pivotallabs.com/pivotallabs"

  if req.env['HTTP_USER_AGENT'] !~ /FeedBurner|FeedValidator/ && req.host =~ /pivotallabs\.com/
    case req.path
    when %r{^/(talks|blabs|blog)\.(atom|rss)$}        ; req.found! "#{feedburner}/#{$1}.#{$2}"
    when %r{^/users/(chris|edward)/blog\.(atom|rss)$} ; req.found! "#{feedburner}/#{$1}.#{$2}"
    case req.host
    when 'tweed.pivotallabs.com'
      req.rewrite! "http://pivotallabs.com/tweed#{req.path}"
    when /([-\w]+\.)?pivotallabs\.com/
      # passthrough with no change
    else  # wildcard domains (e.g. pivotalabs.com)
      req.permanent! :host => "pivotallabs.com"

Notice the use of regular expressions, the $1, $2, etc pseudo-variables, and string interpolation. This is an easy way to match URL patterns and assemble the new URL based on what was matched.

Installation (Rails)

Refraction can be installed in a Rails application as a plugin.

$ script/plugin install git://github.com/pivotal/refraction.git

It can also be used as a gem:

$ gem install refraction

In environments/production.rb, add Refraction at or near the top of your middleware stack.

config.middleware.insert_before(::Rack::Lock, ::Refraction, {})

You may want to occasionally turn on Refraction in the development environment for testing purposes, but if your rules redirect to other servers that can be a problem.

Put your rules in config/initializers/refraction_rules.rb (see example above). The file name doesn't actually matter, but convention is useful.

Server Configuration

If your application is serving multiple virtual hosts, it's probably easiest to configure your web server to handle a wildcard server name and let Refraction handle managing the virtual hosts. For example, in nginx, that is done with a server_name _; directive.

Writing Rules

Set up your rewrite/redirection rules during your app initialization using Refraction.configure. The configure method takes a block which is run for every request to process the rules. The block is passed a RequestContext object that contains information about the request URL and environment. The request object also has a small API for effecting rewrites and redirects.

Important note: don't do a return from within the configuration block. That would be bad (meaning your entire application would break). That's just how blocks work in Ruby.


The set method takes an options hash that sets pieces of the rewritten URL or redirect location header.

  • :scheme - Usually http or https.
  • :host - The server name.
  • :port - The server port. Usually not needed, as the scheme implies a default value.
  • :path - The path of the URL.
  • :query - Added at the end of the URL after a question mark (?)

Any URL components not explicitly set remain unchanged from the original request URL. You can use set before calls to rewrite!, permanent!, or found! to set common values. Subsequent methods will merge their component values into values from set.


The rewrite! method modifies the request URL and relevant pieces of the environment. When Refraction rule processing results in a rewrite!, the request is passed on down the Rack stack to the app or the next middleware component. rewrite! can take a single argument, either an options hash that uses the same options as the set method, or a string that sets all components of the URL.


The permanent! method tells Refraction to return a response with a 301 Moved Permanently status, and sets the URL for the Location header. Like rewrite! it can take either a string or hash argument to set the URL or some of its components.


The found! method tells Refraction to return a response with a 302 Found status, and sets the URL for the Location header. Like #rewrite! it can take either a string or hash argument to set the URL or some of its components.

RequestContext#respond!(status, headers, content)

Use respond! to return an arbitrary response to the request. This is useful for responding with the contents of a static file. For example:

req.respond!(503, {'Content-Type' => 'text/html'}, File.read(MAINTENANCE_PATH))

The args are largely the same as the contents of a standard Rack response array with the exceptions that you don't need to wrap the content in an array, and the Content-Length header is generated so it should not be supplied.

URL components

The request object provides the following components of the URL for matching requests: scheme, host, port, path, and query. It also provides a full environment hash as the env attribute. For example, req.env['HTTP_USER_AGENT'] can be used to access the request's user agent property.


  • Josh Susser (maintainer)
  • Sam Pierson
  • Wai Lun Mang
  • Brian Morearty

Copyright (c) 2009-2010 Pivotal Labs. This software is licensed under the MIT License.