Accessing Nimbus

Overview:

• Teaching: 10 min
• Exercises: 0 min

Questions

• Where can I run the shell?
• What is Open on Demand?
• How do I connect to Nimbus

Objectives

• Get access to the Nimbus in a terminal, either locally or through the Open On Demand service

This episode details how to access The University of Bath's Cloud HPC Facility: Nimbus

We can access Nimbus in several ways:

• From the terminal on your local machine
• From the Open On Demand Service
• From an ssh client on your local machine

Accessing the new cloud HPC service is through ssh (secure shell).

For the purposes of this workshop you can access the terminal on your own machine, use an ssh client or we can use the the new Open on Demand service to access the Nimbus login nodes in the terminal.

Getting Started

Bath's portfolio of Research Computing environments are open to all academics and doctoral students.

Undergraduate students can also use the environments as part of their final year project.

To use Nimbus, you must have funding and be assigned to a resource allocation within the Research Computing Account Management (RCAM) portal.

If you do not have grant funding available to use Nimbus, you must reach out to your departmental champion who will discuss your needs and assign you some funding from the departmental research computing budget, and arrange access through a resource allocation linked to the departmental RCAM account.

If you have grant funding to use on Nimbus, you must first allocate this funding to HPC through Agresso, then you must create an RCAM account on the RCAM portal using that Agresso code, finally you will be able to create resource allocations linked to that RCAM account and assign funding to yourself or other researchers. For more information on RCAM accounts and resource allocations visit the RCAM training material.

Once you are assigned to a resource allocation within the RCAM portal, your Bath credentials will be authorised to allow you to log in and use Nimbus.

SSH

As HPC services are remote machines, interaction is done over an encrypted communication channel called Secure Shell version 2 (SSH-2). This allows command-line access to one of the login nodes of a HPC service, from which you can run commands or use a command-line text editor to edit files.

Security

Users and system operators have a shared responsibility to ensure the security of resources. System operators monitor the system, ensure security patches are up-to-date and that the systems is configured to restrict access correctly e.g. so that users cannot see each others data/files by default.

Users should follow best practice to mitigate the risk of accounts and access details being compromised, e.g. we all know that we must keep our passwords secret. You shouldn't make your files accessible to others unless necessary.

Strong passwords

A strong password describes a password that is difficult to detect by both humans and computer programs, effectively protecting data from unauthorized access. A strong password consists of at least twelve characters (and the more characters, the stronger the password) that are a combination of letters, numbers and symbols (@, !, &, ^ etc) if allowed. Passwords are typically case-sensitive, so a strong password contains letters in both uppercase and lowercase. Strong passwords also do not contain words that can be found in a dictionary or parts of the user's own name.

SSH key pairs

An alternative to using a password is to use an ssh key-pair. A public key pair consists of two parts, a public part and a private part which are related. One is used to lock the message the other is used to unlock the message:

• The public part is used to lock (encrypt) the message so that it can be sent over the internet and can be shared.
• The private part is used to unlock (decrpyt) the message and should not be shared.

Set up a key pair

The SSH key pair consists of a private part and a public part. The public key can be put on remote machines to allow you to log-in without the use of a password. You keep the private part of the key secure on your local machine protected with a passphrase.

Public key encryption uses fancy maths to enable secure communication over an open channel. There are a number of methods the most common being RSA) which uses prime numbers. If you are going to use this you should use a key size of at least 2048 and preferably 4096. Public keys can be broken with brute force computation and the longer the key the more secure it is.

An alternative encryption method and the one we recommend you to use are based on EdDSA (Ed25519). For our purposes the key pair works in the same way. Your public key can go on the remote resource or service and the private key is kept protected on your local machine. You can generate a key pair with:

ssh-keygen -o -a 100 -t ed25519

• ssh-keygen is the command to generate the key pair
• -o specifies to use a strong format to save the key
• -a 100 increases the strength of encryption with your passphrase
• -t ed25519 specifies the encryption method used

When you create a SSH key pair you will be prompted to provide a passphrase. This is effectively password for your private key and like a password should be kept secret. Now when you try to use the key, you should be asked for your key pair passphase (which you entered when you created the key pair) rather than your remote machine password.

Access to the HPC cloud service can be through key pairs, but ensure your key pair is protected by a passphrase.

More details on ssh and security

For further details on ssh, and security you can visit the Research Computing lesson:

https://arc-lessons.github.io/security/00_schedule.html

ssh from Windows & Linux/Mac

In order to access Nimbus via ssh first connect to the University of Bath VPN.

Access from Windows

Recommended access via Microsoft Powershell, PuTTY or MobaXterm.

Alternatively you can install Windows Subsystem for Linux on your Windows machine and run a Linux distribution e.g: https://ubuntu.com/wsl.

To use the OpenSSH client on Windows to connect to Nimbus, simply open a PowerShell window or a command prompt window and run the ssh command:

ssh username@nimbus.hpc.bath.ac.uk

Access from Linux/Mac:

Simply open a terminal and use the following command:

ssh username@nimbus.hpc.bath.ac.uk

Open on Demand

Research Computing have set up a new way to access the University's HPC services using Open on Demand. Open on Demand provides a web based portal to the University's HPC clusters, allowing users to create and monitor workloads, run GUI applications and connect via SSH.

To log into the service open https://ood.hpc.bath.ac.uk in your web browser and log in using your University credentials:

The first time you log-on you will need to follow the following steps in order to create your home directory:

Click the `Open Shell to create home directory:

Type yes in the shell window:

Insert your university password:

Close the window and click Restart web Server in the original window:

To Open up an ssh session click Clusters at the top of the screen and in the drop down menu select >_Nimbus HPC Shell Access:

You will now have a secure shell connected to Nimbus open in a web based terminal hosted in the Open on Demand service:

Congratulations: You are now in the Cloud!

Key Points:

• We can access the Nimbus cluster via ssh to nimbus.hpc.bath.ac.uk
• Open on Demand can be used to get a secure shell session directly in your web browser

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