This has just been merged into master. It’s an incredibly useful challenge since it takes place on port 443, so you don’t need to open or forward port 80 or use the DNS challenge. Like the HTTP-01 challenge, the TLS-ALPN-01 challenge can be solved by a cluster and the certificates shared among a fleet of Caddy instances.
It replaces the TLS-SNI challenge.
It will finally go out with the next release. Would appreciate it if people could build from source and give it a try before then! Especially if you run a cluster of Caddy instances, try out the distributed solving support. It’s pretty neat and zero-config (except you have to share the $CADDYPATH/.acme folder, but that’s an environment thing, external to Caddy.)