OpenSSH (also known as OpenBSD Secure Shell) is a suite of security-related network-level utilities based on the Secure Shell (SSH) protocol, which help to secure network communications via the encryption of network traffic over multiple authentication methods and by providing secure tunneling capabilities.
OpenSSH started as a fork of the free SSH program, developed by Tatu Ylönen; later versions of Ylönen's SSH were proprietary software, offered by SSH Communications Security. OpenSSH was first released as part of the OpenBSD operating system in 1999.
OpenSSH is not a single computer program, but rather a suite of programs that serve as alternatives to unencrypted network communication protocols like Telnet and FTP. Active development primarily takes place within the OpenBSD source tree. OpenSSH is integrated into the base system of several other BSD projects, while the portable version is available as a package in other Unix-like systems.
To help connecting to other machines using tmux and ssh check 2.2. Configure Identities section, at the end shows how to attach to remote with only one command.
Test sshd server from internet https://www.infobyip.com/sshservertest.php
SSH proxy server can be used to protect some services.