# USE1.4 Remote Access

An HPC system is accessed remotely and has its particular file systems that contain data and programs to execute.
Learning the ways of general interaction with the remote system and the tools involved is essential for newcomers.

Users must connect to an HPC system typically using the Secure Shell (SSH), which then starts a shell and allows the interactive access.
When the user disconnects, such a session is terminated.
A server-sided session that persists after disconnection enables the user to execute long-running programs remotely and allows the continuation of a previous session seamlessly.
Data transfer between a local user system (e.g., desktop or laptop) and a remote system requires special tools. 
Interactive access is also allowed.

The tools discussed here are generally valid for systems using Linux, Mac, Windows, and also mobile devices.

## Requirements

## Learning Objectives

  * Describe how SSH-authentication works:
    * Password authentication.
    * Public-key authentication.
    * The role of an authentication agent and the security implications.
  * Generate an SSH public/private key under Linux.
  * Register a key for public-key authentication using ssh-copy-id.
  * Perform a remote login from Linux using SSH.
  * Use SSH-agent or Windows equivalents.
  * Use Agent forwarding to connect to a third HPC system from an HPC system that you logged into with ssh from your computer.
  * Know when to use and how to create a config file.
  * Utilize tools to transfer data between the local and remote system:
    * scp
    * sftp
    * rsync (-avz)
  * Describe how SSHFS allows mounting a remote directory to a local directory for interactive usage (Mac/Linux only), e.g. for copying files or to transparently use your favourite graphical text editor on the local computer for editing files on the remote cluster.
  * Utilize screen and tmux to preserve a session between logins:
    * Creation of a session.
    * Detaching from the current session.
    * Resuming a previous session.

## Subskills