Connecting to the remote HPC system
Last updated on 2024-12-10 | Edit this page
Overview
Questions
- How do I open a terminal?
- How do I connect to a remote computer?
- What is an SSH key?
Objectives
- Connect to a remote HPC system.
Opening a Terminal
Connecting to an HPC system is most often done through a tool known as “SSH” (Secure SHell) and usually SSH is run through a terminal. So, to begin using an HPC system we need to begin by opening a terminal. Different operating systems have different terminals, none of which are exactly the same in terms of their features and abilities while working on the operating system. When connected to the remote system the experience between terminals will be identical as each will faithfully present the same experience of using that system.
Here is the process for opening a terminal in each operating system.
Creating a SSH key
SSH keys are an alternative method for authentication to obtain access to remote computing systems. They can also be used for authentication when transferring files or for accessing version control systems. In this section you will create a pair of SSH keys, a private key which you keep on your own computer and a public key which is placed on the remote HPC system that you will log in to.
Linux, Mac and Windows Subsystem for Linux
Once you have opened a terminal check for existing SSH keys and filenames since existing SSH keys are overwritten,
then generate a new public-private key pair,
-
-o(no default): use the OpenSSH key format, rather than PEM. -
-a(default is 16): number of rounds of passphrase derivation; increase to slow down brute force attacks. -
-t(default is rsa): specify the “type” or cryptographic algorithm. ed25519 is faster and shorter than RSA for comparable strength. -
-f(default is /home/user/.ssh/id_algorithm): filename to store your keys. If you already have SSH keys, make sure you specify a different name:ssh-keygenwill overwrite the default key if you don’t specify!
The flag -b sets the number of bits in the key. The
default is 2048. EdDSA uses a fixed key length, so this flag would have
no effect.
When prompted, enter a strong password that you will remember. Cryptography is only as good as the weakest link, and this will be used to connect to a powerful, precious, computational resource.
Take a look in ~/.ssh (use ls ~/.ssh). You
should see the two new files: your private key
(~/.ssh/key_ARCHER2_rsa) and the public key
(~/.ssh/key_ARCHER2_rsa.pub). If a key is requested by the
system administrators, the public key is the one to
provide.
PRIVATE KEYS ARE PRIVATE
A private key that is visible to anyone but you should be considered compromised, and must be destroyed. This includes having improper permissions on the directory it (or a copy) is stored in, traversing any network in the clear, attachment on unencrypted email, and even displaying the key (which is ASCII text) in your terminal window.
Protect this key as if it unlocks your front door. In many ways, it does.
Further information
For more information on SSH security and some of the flags set here, an excellent resource is Secure Secure Shell.
Windows
On Windows you can use
- puttygen, see the Putty documentation
- MobaKeyGen, see the MobaXterm documentation
Logging onto the system
With all of this in mind, let’s connect to a remote HPC system. In this workshop, we will connect to ARCHER2 — an HPC system located at the University of Edinburgh. Although it’s unlikely that every system will be exactly like ARCHER2, it’s a very good example of what you can expect from an HPC installation. To connect to our example computer, we will use SSH (if you are using PuTTY, see above).
SSH allows us to connect to UNIX computers remotely, and use them as
if they were our own. The general syntax of the connection command
follows the format
ssh -i ~/.ssh/key_for_remote_computer yourUsername@remote.computer.address
when using SSH keys and
ssh yourUsername@some.computer.address if only password
access is available. Let’s attempt to connect to the HPC system now:
or
or if SSH keys have not been enabled
OUTPUT
This node is running Cray's Linux Environment version 1.3.2
#######################################################################################
@@@@@@@@@
@@@ @@@ _ ____ ____ _ _ _____ ____ ____
@@@ @@@@@ @@@ / \ | _ \ / ___| | | | | | ____| | _ \ |___ \
@@@ @@ @@ @@@ / _ \ | |_) | | | | |_| | | _| | |_) | __) |
@@ @@ @@@ @@ @@ / ___ \ | _ < | |___ | _ | | |___ | _ < / __/
@@ @@ @@@ @@ @@ /_/ \_\ |_| \_\ \____| |_| |_| |_____| |_| \_\ |_____|
@@@ @@ @@ @@@
@@@ @@@@@ @@@ https://www.archer2.ac.uk/support-access/
@@@ @@@
@@@@@@@@@
- U K R I - E P C C - H P E C r a y -
Hostname: uan01
Distribution: SLES 15.1 1
CPUS: 256
Memory: 257.4GB
Configured: 2021-04-27
######################################################################################If you’ve connected successfully, you should see a prompt like the one below. This prompt is informative, and lets you grasp certain information at a glance. (If you don’t understand what these things are, don’t worry! We will cover things in depth as we explore the system further.)
Telling the Difference between the Local Terminal and the Remote Terminal
You may have noticed that the prompt changed when you logged into the
remote system using the terminal (if you logged in using PuTTY this will
not apply because it does not offer a local terminal). This change is
important because it makes it clear on which system the commands you
type will be run when you pass them into the terminal. This change is
also a small complication that we will need to navigate throughout the
workshop. Exactly what is reported before the $ in the
terminal when it is connected to the local system and the remote system
will typically be different for every user. We still need to indicate
which system we are entering commands on though so we will adopt the
following convention:
-
[local]$when the command is to be entered on a terminal connected to your local computer -
userid@ln03:~>when the command is to be entered on a terminal connected to the remote system -
$when it really doesn’t matter which system the terminal is connected to.
Being certain which system your terminal is connected to
If you ever need to be certain which system a terminal you are using
is connected to then use the hostname command.
Keep two terminal windows open
It is strongly recommended that you have two terminals open, one
connected to the local system and one connected to the remote system,
that you can switch back and forth between. If you only use one terminal
window then you will need to reconnect to the remote system using one of
the methods above when you see a change from [local]$ to userid@ln03:~> and
disconnect when you see the reverse.