How to link two domains using HTTP Via headers domain hashes

HTTP requests and responses can contain a number of extra headers, which you can use to send along extra metadata about the server serving the request.

Carbon.txt supports using the HTTP Via header, to declare that a HTTP response has been sent along by one or more intermediate entities. In our case usually a managed hosting provider.

  1. Follow the steps above to create your carbon.txt file

    Follow Steps 1 to 4 of the Getting Started guide above to create a carbon.txt file for your organisation.

  2. Create a domain hash for the domain you want to show as green

    Create a domain hash. This is a SHA256 hash of your shared secret and the domain you want to establish a link to.

    Use the form below to create one.

    Green Web Foundation credentials:
  3. Set the via header on HTTP responses to requests for the domain you want to show as green

    For example: also owns In order to link to the main carbon.txt file, when a request comes in for, you would configure the server serving the request to add the following Via header.

    Via: 1.1 <generated_domain_hash>

    Note: the domain hash would be a 64 character hash of and your shared secret.

    We maintain a set of server config setup examples folder on github, for popular open source servers like Nginx, Caddy, Apache, and so on (pull requests gratefully accepted).