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Most of the desktop machine at the STRW can be accessed through the ssh protocol. So when you know your machine name, use that (including the domain) to access that machine directly.

If you do not have a personal machine you can use the virtual machine to log into our systems and continue from there with an ssh to any of the science servers or cluster machines.

Special access

Some places we visit (e.g. China or Iran) or some hotels abroad limit the internet access to web browsing only. Because you want more in such cases the ssh server of the Sterrewacht now also serves the ssh protocol on web ports 80 and 443. So you can now get access to the Sterrewacht computer systems from those limiting environments using

ssh -p 80 -l <your STRW accountname>

With this type of connectivity you can add the tunnelling options (as indicated below) to gain connectivity to a windows remote desktop or your Linux VNC environment.

Instituut Lorentz

For Instituut Lorentz, the server is Desktops and servers cannot be reached directly from outside, so you will always have to go through the ssh server first. But see also our list of details and tricks


For LION, there is

SSH tunnels

For the Mac and Linux commandline ssh client, setting up a tunnel is usually a matter of using the option
-L local_port:remote_machine:remote_port, e.g. -L 3389:windows machine:3389
for forwarding a Windows remote desktop. More detail can be found in the vnc ssh tuning pages.

See putty for instructions about setting up a tunnel with putty (Windows, linux ssh client).

SSH client software

Linux and macOS come with a commandline client for ssh. For Windows, the recommended client is putty

SSH keys

Create a key pair

To create an ssh key pair, with the proper encryption, open up a console, and enter the following command:

$ ssh-keygen -t ed25519

This results in the following output:

 Generating public/private ed25519 key pair.
 Enter file in which to save the key (/home/testuser1/.ssh/id_ed25519): 
 Enter passphrase (empty for no passphrase): 
 Enter same passphrase again: 
 Your identification has been saved in /home/testuser1/.ssh/id_ed25519.
 Your public key has been saved in /home/testuser1/.ssh/
 The key fingerprint is:
 SHA256:gPD6FBuSJTpfkWCrpBPo7XoRqIEV+43g2sX2b6It2YI testuser1@ssh
 The key's randomart image is:
 +--[ED25519 256]--+
 |   .o*#*=.       |
 |  o..*+^o        |
 | . .++E.*        |
 |.   .@.= .       |
 | .  ..X S        |
 |  . ...= o       |
 |   . .  o        |
 |                 |
 |                 |

When asked for a “passphrase”, you should enter (a complex) one or optionally leave it blank. Note that without a passphrase your key pair will be free to use by anyone that has illegally gained access to your keys. A passphrase is known to you only.

The ssh-keygen program will now generate both your public and your private key. Your keys are stored in the .ssh/ directory in your home directory.

The file id_ed25519 contains your private key. YOU SHOULD GUARD THIS KEY WITH YOUR LIFE! This key is used to gain access on systems which have your private key listed in their authorized keys file. We cannot stress this enough, do not have your keys drifting around. Also, make sure your private key always is chmod 600, so other users on the system won't have access to it.

The file contains your public key, which can be added to other system's authorized keys files.

Simplified version in case of a shared home disk

This is how you authorize the key for use within a local network with shared home disk. See below for the general case of accessing a remote system.

Simply add the public part of the key to your .ssh/authorized_keys file, and make sure that that file is not accessible for others:

cat ~/.ssh/ >> ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

Copy public key to server

1. modern and easy, if it works

Nowadays, ssh comes with a utility to send a public key to a remote machine (requiring you to log in using your password once, or requiring a previous key to be already in place). This will take care adding the key to the authorized_keys on the remote system. To do this, simply use:

ssh-copy-id -i user@remotehost

Actually, if you only have one key pair, you can leave out the -i and the name of the key to be copied, so this will do:

ssh-copy-id user@remotehost

2. the old way

To be able to log in to remote systems using your pair of keys, you will first have to add your public key on the remote server to the authorized_keys file in the .ssh/ directory in your home directory on the remote machine.

In our example we will assume you don't have any keys in the authorized_keys files on the remote server. (Hint: If you do not have a remote shell, you can always use your own useraccount on your local machine as a remote shell (ssh localhost))

First we will upload the public keys to the remote server:

$ cd .ssh/
$ scp user@remotehost:./    100% |*****************************************************|   526       00:00

This will place your keys in your home directory on the remote server. After that we will login on the remote server using ssh the conventional way… with a password.

When you are logged in you should create a .ssh directory, and inside the .ssh/ directory create an authorized_keys file and add the keys to the file. Make sure the files are not readable for other users/groups. chmod 600 authorized_keys does the trick.

Placing the key works as follows:

$ cd .ssh
$ touch authorized_keys
$ chmod 600 authorized_keys
$ cat ../ >> authorized_keys
$ rm ../

From now on you can login from client to server without having to specify a password (just a passphrase).

ssh.txt · Last modified: 2021/03/24 20:20 by deul