PSL Domain Extractor

This gem provides methods for extracting domains, subdomains and public suffixes from hostnames using the Public Suffix List.

Note this repository contains a submodule in psl. To clone the repository, run git clone --recurse-submodules https://github.com/rubenarakelyan/psl-domain-extractor.

A hostname is what you provide to this gem. For example, www.example.com is a hostname. You can then extract the domain, subdomain or public suffix.

A label is one part of a hostname separated by a dot. For example, the labels for www.example.com are www, example and com.

The domain is the part of the hostname that is registrable. For example, the domain for www.example.com is example.com because you could register example.com with a domain registrar.

The subdomain is the part of the hostname that is under the control of the domain’s registrant. For example, the subdomain for www.example.com is www because as the registrant for example.com, you control www. Note that a subdomain may contain multiple labels. For example, the subdomain for foo.bar.example.com is foo.bar, which is made up of two labels, foo and bar.

The public suffix is the part of the hostname that is allocated to the domain registry and hence not registrable. For example, the public suffix for www.example.com is com. com is allocated to the domain registry (in this case, Verisign) and you cannot register it. The public suffix is always one level below the domain. For example, co.uk and uk are both a public suffix because you can register example.co.uk and example.uk, but not co.uk or uk.

The actual rules for the public suffix list are more complex than this since they also include wildcards, exceptions, and private multi-tenant domains. This gem takes all those into consideration. For example, Netlify has registered netlify.app as a domain name to host apps from multiple customers. netlify.app is in the public suffix list. Therefore, even though netlify.app is a registrable domain, in this case it is considered the public suffix. A customer’s app at bar.netlify.app would be considered the domain (in a sense, this has been registered with Netlify), and for foo.bar.netlify.app, the subdomain would be foo (not foo.bar).

Add to your Gemfile then run bundle install:

gem "psl-domain-extractor"

or install using gem:

gem install psl-domain-extractor

Extract the domain:

PslDomainExtractor.extract_domain(domain)

Extract the subdomain:

PslDomainExtractor.extract_subdomain(domain)

Extract the public suffix:

PslDomainExtractor.extract_public_suffix(domain)

Add the following to your Rails configuration:

config.action_dispatch.domain_extractor = PslDomainExtractor

request.domain and request.subdomain will then be set using this gem’s logic.

Of course if you don’t want to change the default logic for domain extraction, you can always call the gem’s standalone methods as required in your app.

This gem is cryptographically signed. To be sure the gem you install hasn’t been tampered with, run:

gem cert --add <(curl -Ls https://raw.github.com/rubenarakelyan/psl-domain-extractor/main/certs/rubena.pem)
gem install psl-domain-extractor -P MediumSecurity

The MediumSecurity trust profile will verify signed gems, but allow the installation of unsigned dependencies.

This is necessary because not all of this gem’s dependencies are signed, so we cannot use HighSecurity.

This gem is licensed under the MIT licence.