Add general terminal page

doc.zih.tu-dresden.de/docs/access/ssh_terminal.md

+# Connecting via terminal
+
+Connecting via terminal works on every operating system. For Linux and Macintosh operating systems
+no additional software is required. For users of a Windows OS a recent version of Windows is
+required (Windows 10, Build 1809 and higher). It is possible to use
+[Command Prompt](https://en.wikipedia.org/wiki/Cmd.exe) or [PowerShell](https://en.wikipedia.org/wiki/PowerShell)).
+Ensure that [OpenSSH](https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/factoryos/connect-using-ssh?view=windows-10)
+is installed on the system.
+
+SSH establishes secure connections using authentication and encryption. The login nodes accept
+the following encryption algorithms: `aes128-ctr`, `aes192-ctr`, `aes256-ctr`,
+`aes128-gcm@openssh.com`, `aes256-gcm@openssh.com`, `chacha20-poly1305@openssh.com`,
+`chacha20-poly1305@openssh.com`.
+
+## Before Your First Connection
+
+We suggest to create an SSH key pair before you work with the ZIH systems. This ensures high
+connection security.
+
+```console
+marie@local$ mkdir -p ~/.ssh
+marie@local$ ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519
+Generating public/private ed25519 key pair.
+Enter passphrase (empty for no passphrase):
+Enter same passphrase again:
+[...]
+```
+
+Type in a passphrase for the protection of your key. The passphrase should be **non-empty**.
+Copy the public key to the ZIH system (Replace placeholder `marie` with your ZIH login):
+
+```console
+marie@local$ ssh-copy-id -i ~/.ssh/id_ed25519.pub marie@taurus.hrsk.tu-dresden.de
+The authenticity of host 'taurus.hrsk.tu-dresden.de (141.30.73.104)' can't be established.
+RSA key fingerprint is SHA256:HjpVeymTpk0rqoc8Yvyc8d9KXQ/p2K0R8TJ27aFnIL8.
+Are you sure you want to continue connecting (yes/no)?
+```
+
+Compare the shown fingerprint with the [documented fingerprints](key_fingerprints.md). Make sure
+they match. Then you can accept by typing `yes`.
+
+!!! info
+    If `ssh-copy-id` is not available, you need to do additional steps:
+
+    ```console
+    marie@local$ scp ~/.ssh/id_ed25519.pub marie@taurus.hrsk.tu-dresden.de:
+    The authenticity of host 'taurus.hrsk.tu-dresden.de (141.30.73.104)' can't be established.
+    RSA key fingerprint is SHA256:HjpVeymTpk0rqoc8Yvyc8d9KXQ/p2K0R8TJ27aFnIL8.
+    Are you sure you want to continue connecting (yes/no)?
+    ```
+
+    After that, you need to manually copy the key to the right place:
+
+    ```console
+    marie@local$ ssh marie@taurus.hrsk.tu-dresden.de
+    [...]
+    marie@login$ mkdir -p ~/.ssh
+    marie@login$ touch ~/.ssh/authorized_keys
+    marie@login$ cat id_ed25519.pub >> ~/.ssh/authorized_keys
+    ```
+
+### Configuring Default Parameters for SSH
+
+After you have copied your key to the ZIH system, you should be able to connect using:
+
+```console
+marie@local$ ssh marie@taurus.hrsk.tu-dresden.de
+[...]
+marie@login$ exit
+```
+
+However, you can make this more comfortable if you prepare an SSH configuration on your local
+workstation. Navigate to the subdirectory `.ssh` in your home directory and open the file `config`
+(`~/.ssh/config`) in your favorite editor. If it does not exist, create it. Put the following lines
+in it (you can omit lines starting with `#`):
+
+```bash
+Host taurus
+  #For login (shell access)
+  HostName taurus.hrsk.tu-dresden.de
+  #Put your ZIH-Login after keyword "User":
+  User marie
+  #Path to private key:
+  IdentityFile ~/.ssh/id_ed25519
+  #Don't try other keys if you have more:
+  IdentitiesOnly yes
+  #Enable X11 forwarding for graphical applications and compression. You don't need parameter -X and -C when invoking ssh then.
+  ForwardX11 yes
+  Compression yes
+Host taurusexport
+  #For copying data without shell access
+  HostName taurusexport.hrsk.tu-dresden.de
+  #Put your ZIH-Login after keyword "User":
+  User marie
+  #Path to private key:
+  IdentityFile ~/.ssh/id_ed25519
+  #Don't try other keys if you have more:
+  IdentitiesOnly yes
+```
+
+Afterwards, you can connect to the ZIH system using:
+
+```console
+marie@local$ ssh taurus
+```
+
+If you want to copy data from/to ZIH systems, please refer to [Export Nodes: Transfer Data to/from
+ZIH's Filesystems](../data_transfer/export_nodes.md) for more information on export nodes.
+
+## X11-Forwarding
+
+If you plan to use an application with graphical user interface (GUI), you need to enable
+X11-forwarding for the connection. If you use the SSH configuration described above, everything is
+already prepared and you can simply use:
+
+```console
+marie@local$ ssh taurus
+```
+
+If you have omitted the last two lines in the default configuration above, you need to add the
+option `-X` or `-XC` to your SSH command. The `-C` enables compression which usually improves
+usability in this case:
+
+```console
+marie@local$ ssh -XC taurus
+```
+
+!!! info
+
+    Also consider to use a [DCV session](desktop_cloud_visualization.md) for remote desktop
+    visualization at ZIH systems.