Altdns is a Subdomain Recon Tool in Python that allows for the discovery of subdomains that conform to patterns. The tool takes in words that could be present in subdomains under a domain (such as test, dev, staging) as well as takes in a list of subdomains that you know of.
From these two lists that are provided as input to altdns, the tool then generates a massive output of “altered” or “mutated” potential subdomains that could be present.