a Rails gem that allows you to validate a URL entered in a form. It validates if the URL exists by hitting it with a HEAD request. The improved version includes retries for common patterns when the head request is refused before giving a failure notice. It also looks up a SITE_URL constant to the user agent in the headers. Also has the option to also check that the URL returns content of a specified type.
Gemfile gem 'http_url_validation_improved', '~> 1.3.1'
install gem install http_url_validation_improved