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Configuring SSH Keys in Merlin	Linux, connecting, client, configuration, SSH, Keys, SSH-Keys, RSA	15 Jul 2020	This document describes how to deploy SSH Keys in Merlin.	merlin6_sidebar	/merlin6/ssh-keys.html

Merlin users sometimes will need to access the different Merlin services without being constantly requested by a password. One can achieve that with Kerberos authentication, however in some cases some software would require the setup of SSH Keys. One example is ANSYS Fluent, which, when used interactively, the way of communication between the GUI and the different nodes is through the SSH protocol, and the use of SSH Keys is enforced.

Setting up SSH Keys on Merlin

For security reason, users must always protect SSH Keys with a passphrase.

User can check whether a SSH key already exists. These would be placed in the ~/.ssh/ directory. RSA encryption is usually the default one, and files in there would be id_rsa (private key) and id_rsa.pub (public key).

ls ~/.ssh/id*   

For creating SSH RSA Keys, one should:

1. Run ssh-keygen, a password will be requested twice. You must remember this password for the future.
   • Due to security reasons, always add a password. Never leave an empty password.
   • This will generate a private key id_rsa, and a public key id_rsa.pub in your ~/.ssh directory.
2. Add your public key to the authorized_keys file, and ensure proper permissions for that file, as follows:

   cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys
   chmod 0600 ~/.ssh/authorized_keys
   

3. Configure the SSH client in order to force the usage of the psi.ch domain for trusting keys:

   echo "CanonicalizeHostname yes" >> ~/.ssh/config
   

Using the SSH Keys

By default, when login in the login node through SSH, it will automatically add your SSH Keys to the authentication agent. Hence, no actions are needed by the user.

However, there are some cases where it might not automatically work. For example, for NoMachine one always need to add the private key identity to the authentication agent. This can be done as follows:

1. Check whether the authentication agent has already the key added:

   ssh-add -l | grep "/psi/home/$(whoami)/.ssh"
   

2. If no key is returned in the previous step, you have to add the private key identity to the authentication agent. You will be requested for the passphrase of your key, and it can be done by running:

   ssh-add
   

When running ssh-add is needed (i.e. NoMachine session, or miss-behaving SSH access), you need to run it only once per new session. It is, for NoMachine, you just need to run it once, and it would apply to all terminal windows within that NoMachine session.

Testing SSH Keys

Once SSH Key is created, for testing that the SSH Key is valid, one can do the following:

1. Create a new SSH session in one of the login nodes:

   ssh merlin-l-001
   

2. In the login node session, destroy any existing Kerberos ticket or active SSH Key:

   kdestroy
   ssh-add -D
   

3. Add the new private key identity to the authentication agent. You will be requested by the passphrase.

   ssh-add
   

4. Check that your key is active by the SSH agent:

   ssh-add -l
   

5. SSH to the second login node. No password should be requested:

   ssh -vvv merlin-l-002
   

If the last step succeeds, then means that your SSH Key is properly setup.

Troubleshooting

Add/Update SSH RSA Key password

If an existing SSH Key does not have password, or you want to update an existing password with a new one, you can do it as follows:

ssh-keygen -p -f ~/.ssh/id_rsa

SSH Keys deployed but not working

Please ensure proper permissions of the involved files, as well as any typos in the file names involved:

chmod u+rwx,go-rwx,g+s ~/.ssh
chmod u+rw-x,go-rwx    ~/.ssh/authorized_keys
chmod u+rw-x,go-rwx    ~/.ssh/id_rsa
chmod u+rw-x,go+r-wx   ~/.ssh/id_rsa.pub