Hi guys,
Can a TCP port be open without any "service" running behind, like a web server?
I am trying to get a certificate from Let's encrypt, using acme.sh client (standalone option). on a given host behind my net.
Set up a NAT rule, yet when trying to verify if port 80 is open, say by port checker as this.
https://www.yougetsignal.com/tools/open-ports
but it displays info - port 80 is closed. Verifcation run on a public IP.
Yet, when I start Mikrotik www service than suddenly the checker says "open", but for the verification it must be that acme.sh web server is internally started, started by acme.sh run with proper option. The standalone option means that the acme.sh will start the web server,
The doc https://github.com/acmesh-official/acme.sh says
Port 80 (TCP) MUST be free to listen on, otherwise you will be prompted to free it and try again.
But this is somehow I cannot see with a given port checker, even after I set up a NAT rule.
Hope somebody will explain to clear up the confusion.
Thanks,
Adam