ngrok HLD: Exposing localhost to the Internet With Reverse Tunnels

You run a web app on localhost:3000 and want to show it to someone across the world — or let Stripe send a webhook to it. But your laptop has no public IP, sits behind a NAT, and you can't open a firewall port. ngrok solves this with one elegant move: instead of the world connecting to you, your machine dials out to a public edge server and holds that connection open; the edge then pipes inbound public requests back down it. The genius is in the direction (outbound, so no firewall hole) and the efficiency (one connection carries all your requests via multiplexing). This is the inside of ngrok …