Bastions are a cluster of machines used as the unique entry point by operational teams to securely connect to devices using ssh.
The Bastion provides mechanisms for authentication, authorization, traceability and auditability for the whole infrastructure.

Remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo and line editing of user keystrokes.

Mosh is a replacement for interactive SSH terminals. It's more robust and responsive, especially over Wi-Fi, cellular, and long-distance links.

Mosh is free software, available for GNU/Linux, BSD, macOS, Solaris, Android, Chrome, and iOS.

Snowflake is a graphical SSH client. It has a enhanced SFTP file browser, SSH terminal emulator, remote resource/process manager, server disk space analyzer, remote text editor, huge remote log viewer and lots of other helpful tools, which makes it easy to work with remote servers. Snowflake provides functionality similar to web based control panels but, it works over SSH from local computer, hence no installation required on server. It runs on Linux and Windows. Snowflake has been tested with serveral Linux and UNIX servers, like Ubuntu server, CentOS, RHEL, OpenSUSE, FreeBSD, OpenBSD, NetBSD and HP-UX.

Browsh is a fully-modern text-based browser. It renders anything that a modern browser can; HTML5, CSS3, JS, video and even WebGL. Its main purpose is to be run on a remote server and accessed via SSH/Mosh or the in-browser HTML service in order to significantly reduce bandwidth and thus both increase browsing speeds and decrease bandwidth costs.

The wargames offered by the OverTheWire community can help you to learn and practice security concepts in the form of fun-filled games.

Gravitational Teleport is a modern SSH server for remotely accessing clusters of Linux servers via SSH or HTTPS. It is intended to be used instead of sshd for organizations who need:

SSH audit with session recording/replay.
Easily manage SSH trust between teams, organizations and data centers.
SSH into behind-firewall clusters without any open ports.
Role-based access control (RBAC) for SSH protocol.

Gate One is an HTML5-powered terminal emulator and SSH client