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Use the side-by-side browser

The side-by-side browser is a tool to preview redirections for sites that are being transitioned to GOV.UK.

Proxy a web site to preview redirections ahead of it being transitioned to GOV.UK.

sketchy explanation

Run the tests

$ npm install
$ npm test


$ node server.js

…then you can use to make it work with localhost, eg:


Run the server on Heroku. It requires a side-by-side wildcard subdomain pointed to it:

$ heroku domains:add *

Follow the returned instructions to CNAME the wildcard to Heroku.

By default, the application will take its list of allowed domains from the production instance of the GOV.UK Transition tool. This can be overridden with the SIDE_BY_SIDE_HOSTS environment variable. Examples:

  • SIDE_BY_SIDE_HOSTS= (the default)
  • SIDE_BY_SIDE_HOSTS=file:etc/hosts.json
This page was last reviewed on 21 October 2019. It needs to be reviewed again on 21 April 2020 by the page owner #govuk-platform-health .
This page was set to be reviewed before 21 April 2020 by the page owner #govuk-platform-health. This might mean the content is out of date.