Ifconfig.co – Hacker-friendly “What is my IP address”

For the benefit of anyone interested: for a "self-hosted" solution, you can do this entirely within Nginx. Here's an example config:

    server {
      listen 80 default_server;
      listen [::]:80 default_server;

      listen 443 default_server;
      listen [::]:443 default_server;

      # Use Letsencrypt for SSL. This part will depend on your own setup.
      ssl_certificate /etc/letsencrypt/live/<my domain>/fullchain.pem;
      ssl_certificate_key /etc/letsencrypt/live/<my domain>/privkey.pem;

      server_name <my domain>;

      # Deny all access at all paths; useful if you're hosting other stuff behind
      # the same Nginx server (e.g. reverse proxy)
      location / {
        deny all;
      }

      # At /ip, return 200 with the client IP address in the body
      location = /ip {
        default_type text/plain;
        return 200 '$remote_addr';
      }
    }

Caddyfile:

  example.com {
      respond "{remote_host}"
  }

[1] https://caddyserver.com/docs/caddyfile/concepts#placeholders