7  Bonus: Making it easier to log in to the cluster

Learning Objectives:

Make logging in to Xanadu easier

So far logging in to Xanadu has required significant typing: ssh username@xanadu-submit-ext.cam.uchc.edu followed by your password. But we can make this easier in a few ways! First, we can create ssh keys that will authenticate you, so you won’t have to enter a password. Second, we can edit a configuration file to create a shortcut (or several shortcuts) to Xanadu login nodes.

7.1 SSH keys

We at the CBC are certainly not experts in cryptography, so we won’t try to explain the details. In simple form, an ssh key is a means of verifying your identity that does not require a password. It consists of a pair of files, a public key, and a private key. You place the public key on a remote server, and keep the private key on your local machine. When you try to log in to the remote server, it essentially asks your machine a question that can only be answered by the private key. It’s extremely important that you keep your private key private and secure, as in some ways it serves as a password. The public key is less sensitive.

So how do we create and use SSH keys? Linux-based systems come with software for creating and managing SSH keys.

For this, we’ll start in a local terminal window (i.e. one that is not logged in to Xanadu). Go to the hidden directory .ssh that should be found in your home directory.

cd ~/.ssh

If you don’t have this directory, create it with mkdir ~/.ssh, and then cd into it. Any files in the Linux file system that begin with . are hidden by default from directory listings. You can see them with ls -a.

Now, to create an ssh key pair type:

ssh-keygen -b 2048

First will ask you to name the file, but you don’t have to. It will use the default name in parentheses.

Then it will ask you to create a passphrase. You don’t have to do this, but it will make the key more secure. If you don’t, anyone who gets your key pair will be able to log in to Xanadu.

If you used the default names, and you type ls you should have the files:

id_rsa 
id_rsa.pub 

id_rsa.pub is the public key and id_rsa, the private one. Depending on your OS and shell, you may alternatively have produced files with different names, such as id_ed25519 and id_ed25519.pub, .pub still indicates the public key. Now, use scp to upload the public key to Xanadu (you’ll have to make sure you also have a .ssh directory in your home on Xanadu):

scp id_rsa.pub username@transfer.cam.uchc.edu:~/.ssh/ 

If you don’t have a .ssh directory on Xanadu, log in and create one, then try uploading again.

Now log in to Xanadu and, in the .ssh directory, create a file called authorized_keys and paste the contents of your public key there. You could do this in one go with

cat id_rsa.pub >authorized_keys

Finally, back on your local machine, to avoid having to enter the ssh key passphrase (if you created one), you should do:

ssh-add id_rsa

And enter your passphrase at the prompt if you created one.

Now when you log in, you should not have to enter your password.

7.1.1 The SSH config

The second thing we can do to ease connecting to Xanadu is to modify our ssh configuration file, simply called config.

This file, if it already exists will also be in the .ssh hidden directory. Go there now. If the file does not exist, create it with

touch config

I’m going to suggest using nano to edit this file for now, but you can use any editor you like.

nano config

Then enter:

Host xanadu
Hostname xanadu-submit-ext.cam.uchc.edu
User username
IdentityFile ~/.ssh/id_rsa

Remember to subsitute username for your own username, and if you renamed the ssh keys, change the name here as well.

xanadu will be the name of the shortcut, you can change this if you want. You could cut it down to just x if you want to absolutely minimize typing.

Now you should be able to log in with ssh xanadu.

You can set one of these up for the transfer node as well:

Host transfer
Hostname transfer.cam.uchc.edu
User username
IdentityFile ~/.ssh/id_rsa

Or if you want to have shortcuts for specific login nodes (numbers 1-5):

Host hpc1
Hostname hpc-ext-1.cam.uchc.edu
User username
IdentityFile ~/.ssh/id_rsa