Connecting to Tesseract

On the Tesseract system, interactive access can be achieved via SSH, either directly from a command line terminal or using an SSH client. In addition data can be transferred to and from the Tesseract system using scp from the command line or by using a file transfer client.

This section covers the basic connection methods.

Access credentials

To access Tesseract, you need to use two credentials: your password and an SSH key pair protected by a passphrase. You can find more detailed instructions on how to set up your credentials to access Tesseract from Windows, macOS and Linux below.

SSH Key Pairs

You will need to generate an SSH key pair protected by a passphrase to access Tesseract.

Using a terminal (the command line), set up a key pair that contains your e-mail address and enter a passphrase you will use to unlock the key:

ssh-keygen -t rsa -C "your@email.com"
...
-bash-4.1$ ssh-keygen -t rsa -C "your@email.com"
Generating public/private rsa key pair.
Enter file in which to save the key (/Home/user/.ssh/id_rsa): [Enter]
Enter passphrase (empty for no passphrase): [Passphrase]
Enter same passphrase again: [Passphrase]
Your identification has been saved in /Home/user/.ssh/id_rsa.
Your public key has been saved in /Home/user/.ssh/id_rsa.pub.
The key fingerprint is:
03:d4:c4:6d:58:0a:e2:4a:f8:73:9a:e8:e3:07:16:c8 your@email.com
The key's randomart image is:
+--[ RSA 2048]----+
|    . ...+o++++. |
| . . . =o..      |
|+ . . .......o o |
|oE .   .         |
|o =     .   S    |
|.    +.+     .   |
|.  oo            |
|.  .             |
| ..              |
+-----------------+

(remember to replace “your@email.com” with your e-mail address).

Upload public part of key pair to DiRAC SAFE

You should now upload the public part of your SSH key pair to the DiRAC SAFE by following the instructions at:

Once you have done this, your SSH key will be added to your Tesseract account.

Remember, you will need to use both an SSH key and password to log into Tesseract so you will also need to collect your initial password before you can log into Tesseract. We cover this next.

Initial passwords

The SAFE web interface is used to provide your initial password for logging onto Tesseract (see the DiRAC SAFE Documentation for more details on requesting accounts and picking up passwords).

Note: you may now change your password on the Tesseract machine itself using the passwd command or when you are prompted the first time you login. This change will not be reflected in the SAFE. If you forget your password, you should use the SAFE to request a new one-shot password.

SSH Clients

Interaction with Tesseract is done remotely, over an encrypted communication channel, Secure Shell version 2 (SSH-2). This allows command-line access to one of the login nodes of a Tesseract, from which you can run commands or use a command-line text editor to edit files. SSH can also be used to run graphical programs such as GUI text editors and debuggers when used in conjunction with an X client.

Logging in from Linux and MacOS

Linux distributions and MacOS each come installed with a terminal application that can be use for SSH access to the login nodes. Linux users will have different terminals depending on their distribution and window manager (e.g. GNOME Terminal in GNOME, Konsole in KDE). Consult your Linux distribution’s documentation for details on how to load a terminal.

MacOS users can use the Terminal application, located in the Utilities folder within the Applications folder.

You can use the following command from the terminal window to login into Tesseract:

ssh username@tesseract.dirac.ed.ac.uk

You will first be prompted for the passphrase associated with your SSH key pair. Once you have entered your passphrase successfully, you will then be prompted for your password. You need to enter both correctly to be able to access Tesseract.

Note

If your SSH key pair is not stored in the default location (usually ~/.ssh/id_rsa) on your local system, you may need to specify the path to the private part of the key wih the -i option to ssh. For example, if your key is in a file called keys/id_rsa_tesseract you would use the command ssh -i keys/id_rsa_tesseract username@tesseract.dirac.ed.ac.uk to log in.

Note

When you first log into Tesseract, you will be prompted to change your initial password. This is a three step process:

  1. When promoted to enter your ldap password: Re-enter the password you retrieved from SAFE
  2. When prompted to enter your new password: type in a new password
  3. When prompted to re-enter the new password: re-enter the new password

Your password has now been changed

To allow remote programs, especially graphical applications to control your local display, such as being able to open up a new GUI window (such as for a debugger), use:

ssh -X username@tesseract.dirac.ed.ac.uk

Some sites recommend using the -Y flag. While this can fix some compatibility issues, the -X flag is more secure.

Current MacOS systems do not have an X window system. Users should install the XQuartz package to allow for SSH with X11 forwarding on MacOS systems:

Logging in from Windows using MobaXterm

A typical Windows installation will not include a terminal client, though there are various clients available. We recommend all our Windows users to download and install MobaXterm to access Tesseract. It is very easy to use and includes an integrated X server with SSH client to run any graphical applications on Tesseract.

You can download MobaXterm Home Edition (Installer Edition) from the following link:

Double-click the downloaded Microsoft Installer file (.msi), and the Windows wizard will automatically guides you through the installation process. Note, you might need to have administrator rights to install on some Windows OS. Also make sure to check whether Windows Firewall hasn’t blocked any features of this program after installation.

Start MobaXterm using, for example, the icon added to the Start menu during the installation process.

If you would like to run any small remote GUI applications, then make sure to use -X option along with the ssh command (see above) to enable X11 forwarding, which allows you to run graphical clients on your local X server.

Making access more convenient using the SSH configuration file

Typing in the full command to login or transfer data to Tesseract can become tedious as it often has to be repeated many times. You can use the SSH configuration file, usually located on your local machine at .ssh/config to make things a bit more convenient.

Each remote site (or group of sites) can have an entry in this file which may look something like:

Host tesseract
  HostName tesseract.dirac.ed.ac.uk
  User username

(remember to replace username with your actual username!).

The Host tesseract line defines a short name for the entry. In this case, instead of typing ssh username@tesseract.dirac.ed.ac.uk to access the Tesseract login nodes, you could use ssh tesseract instead. The remaining lines define the options for the tesseract host.

  • Hostname tesseract.dirac.ed.ac.uk - defines the full address of the host
  • User username - defines the username to use by default for this host (replace username with your own username on the remote host)

Now you can use SSH to access Tesseract without needing to enter your username or the full hostname every time:

-bash-4.1$ ssh tesseract

You can set up as many of these entries as you need in your local configuration file. Other options are available. See the ssh_config man page (or man ssh_config on any machine with SSH installed) for a description of the SSH configuration file. You may find the IdentityFile option useful if you have to manage multiple SSH key pairs for different systems as this allows you to specify which SSH key to use for each system.