SSH mandatory at SURF

The only way to access SURF's HPC systems is by using ssh, scp or sftp, which use an encrypted and secured connection.

The reason for this is that an unencrypted telnet or ftp connection to a remote system is very unsafe. In many cases it is possible to use a computer to intercept all tcp packets that are sent on the local network. All data that is sent via the local network to a remote (or local) computer system can be read by anyone who has access to a computer that is linked to the local network. This is especially very undesirable in the case of passwords that are sent to other systems.

The SSH standard circumvents this problem, by encrypting all data that is sent over the network, including the passwords needed for authentication.

Installation on your work station

Installation of the software involved is, as far as we experienced, rather simple. The central site for the open SSH is There one finds implementations for Unix-es, Windows, Macintosh.In the following we will elaborate somewhat on the different platforms:

  • Unix
    On SSH versions for many Unix-es are available. You will also need SSL, to be found on If unsure, check for the file libssl.a in /usr/local/ssl/lib/ or other "standard" locations. Most, if not all, linux distributions support SSH and include a package for an SSH client. Otherwise , installation from source is easy, one doesn't even need root permission to install the ssh  and scp  commands (replacements for telnet and ftp) in one's home directory.
  • Windows
    There are some implementations of SSH for Windows, see We tried PuTTY as a terminal emulator and it works satisfactorily. For secure file transfer there is WinSCP, with a graphical user interface. 
    For a full terminal solution with intergrated openSSH and  graphical environment (X11) support and many advanced options, we recommend MobaXterm.
  • Macintosh
    Look at for references. We tried MACSSH, which works well.


Logging in on one of SURF's HPC systems

ssh -X -i <my_private_key> <myname>@<hpc-system>
ssh -X -i <my_private_key> <myname>@<hpc-system>


ssh -X -i .ssh/psmith_lisa

Where <myname>  is the login name on the HPC system, and is the internet address of the HPC system. The -X flag is for the automatic arranging of your X environment. It depends on the configuration of ssh on your local workstation if this flag is necessary. The -i <my_private_key>  flag indicates to the SSH client what private key you are going to use (with the full path, but usually .ssh). I may ask you for the passphrase the first time you use it (warning)

Copying files from your workstation to an HPC system

On your workstation type:
scp infile <myname>@<hpc-system>

Where infile is the file you want to copy (you can specify more than one file and use wild charts if you like) and directory is the destination directory on the HPC system. When you omit directory, the file(s) will be copied to your home directory. tofile is the name the file gets on the HPC system. When omitted, the name will be the same as infile.

Copying files from an HPC system to your workstation:

On your workstation, type:

scp <myname>@<hpc-system> outfile

infile is the name of the file on the HPC system, outfile is the name of the file on your workstation.

Copying directories

You can copy a directory by specifying the -r  flag after scp, which will recursively copy anything in and below the given directory:

scp -r <myname>@<hpc-system> outdir

Bonus 1: automatic defining X11 environment in a safe way

There is a famous security problem connected with using X11 applications if your workstation is running some kind of Unix. Many people used the xhost command to produce X11 output on their screens. Not everybody is aware of the fact, however, that using the xhost command one creates very likely a situation where somebody, also logged in on the remote system can get access to the X-server running on the workstation and is able, for example, to read out every keystroke.

So we cannot stress enough: DON'T USE XHOST.

An alternative and more approved approach is the use of 'cookies' with the 'xauth' command.

Using ssh, neither of xhost or xauth is necessary, as ssh takes care of the X-window traffic, and setting the DISPLAY environment variable. Once you have a ssh connection from your Unix workstation with a SURF system, X-applications will automatically put their windows on your workstation, and all traffic is encrypted. Depending on the configuration of your system, you have to add the -X flag to the ssh command. When using PuTTY under Windows, choose 'Tunnels' from the 'Category' list and check the option 'Enable X11 forwarding'. Please note that you need a separate X-server, e.g.

Bonus 2: public-key authentication

Another bonus when using SSH is the possibility to login without using a password, using public-key authentication.

With this method a public-private key pair is generated once on your local workstation and the public key part (only) of that pair is then registered with SURF. Whenever you log into Snellius or Lisa the public key at SURF is then used together with the private key on your local workstation to authenticate, instead of having to enter your password.

The public-private key pair is automatically generated and much longer than a user-chosen password, and so provides much more safety against brute-force password guessing.


Note that public-key authentication is currently mandatory with two-factor authentication (2FA). This means that when 2FA is active on your login you will have to use public key authentication.

When 2FA is active on your login your public keys as registered in the SURF User Portal are used (those stored in ~/.ssh/authorized_keys are ignored in this case).

Creating an SSH keypair

You can use the following HOWTO in order to set up and use public key authentication on the SURF HPC facilities.

Host key fingerprints

 When you make an SSH connection to a remote system for the first time, SSH recognizes this and asks if you want to proceed.If you enter yes, the hostkey of the remote system gets stored in $HOME/.ssh/known_hosts on your local system. When you later make an SSH connection again to the same system SSH compares the remote hostkey with the locally stored hostkey. If there is a difference, you will get a warning and some functionality is not available. This is to protect you from possible attacks: you think you are connected to a particular host, but in reality you could be connected to another host (the so called "Man in the middle attack").

Most of the time, however, there is no problem because the hostkey of the host system has been changed for some reason, perhaps by an upgrade of the system. If you are sure that you are really connecting to the system you want, simply remove the line with the name of the remote host in the file $HOME/.ssh/known_hosts and connect again. You could also try a more drastic approach: remove the file $HOME/.ssh/known_hosts altogether. The above is for Unix systems. In general, the SSH implementations for Windows and Macintosh issue a warning and let you choose to change the stored hostkey.

Hostkey fingerprints for our systems are listed here, for verification:

systemhost(s)hostkey fingerprint reference Snellius Hostkey Fingerprints
Cartesiuscartesius.surfsara.nlSee Cartesius Hostkey Fingerprints
Lisalisa.surfsara.nlSee Lisa Hostkey Fingerprints e9:88:fa:9a:a7:99:85:5a:b8:62:5e:04:da:7c:96:13
LSGall LSG sites(RSA) 9c:ed:a5:89:f8:91:fc:08:51:cb:6f:a7:c9:89:ee:47


SURF considers easy and secure access to the HPC systems important, so in case of difficulties, please contact our Service Desk.