Question: About `ACMEIssuer.AltTLSALPNPort` parameter about certmagic HOT 3 CLOSED

It changes the port that would be used on the server for listening for TLS requests. It's only useful if you need to listen on a non-standard port (e.g. 8443 because you don't have sudo and can't bind to low ports) while mapping incoming connections on 443 to that port (using router port forwarding from 443 -> 8443, for example).

In other words, no, this isn't useful for accepting TLS-ALPN challenges on a non-standard port, it's just a way to control the listeners created automatically by certmagic.

from certmagic.

I understand, this parameter just tells CerMagic which port it should listen on, but port 443 will still be requested for the acme challenge;

The difference is that this is to put the CertMagic application behind the firewall, we can use port 443 externally Traffic is mapped to the AltTLSALPNPort.

from certmagic.

Yeah, the external port must still be 443. If you forward or map port 443 to something else like 8443 with your router or firewall then you'd put 8443 as the AltTLSALPNPort.

from certmagic.