HAproxy
HAproxy is fantastic reverse proxy with a massive amount of features. Knocknoc has supported HAproxy for years, and integrates with it natively. The vast amount of options for HAproxy can make it confusing at first, however we can walk through the bare essentials to be able to use Knocknoc, and you can extend it from there.
If you wanted to protect an arbitrary web application, for example a legacy PHP app, or Atlassian Confluence, we would recommend deploying HAproxy to sit in front of the application, and use Knocknoc to control access. This will often be an easier deployment path, and is a cost effective security measure for many reasons.