Hello there,
Just to add: This challenge has recently been altered. Currently, the use of dns.lookup should not be used to achieve the functionality of the 4th user story.
dns.lookup is a hint to add as an optional extra, and works by (essentially) pinging a domain, and either returning a response or a bad gateway.
The 4th user story only expects the http|https format to be checked.
I hope this clarifies.