---
title: Configuring SSH Keys in Merlin

#tags:
keywords: Linux, connecting, client, configuration, SSH, Keys, SSH-Keys, RSA
last_updated: 15 Jul 2020
summary: "This document describes how to deploy SSH Keys in Merlin."
sidebar: merlin6_sidebar
permalink: /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).

```bash
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:
   ```bash
   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:
   ```bash
   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:
   ```bash
   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:
   ```bash
   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:
   ```bash
   ssh merlin-l-001
   ```
2. In the login node session, destroy any existing Kerberos ticket or active SSH Key:
   ```bash
   kdestroy
   ssh-add -D
   ```
3. Add the new private key identity to the authentication agent. You will be requested by the passphrase.
   ```bash
   ssh-add
   ```
4. Check that your key is active by the SSH agent:
   ```bash
   ssh-add -l
   ```
4. SSH to the second login node. No password should be requested:
   ```bash
   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:

```bash
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:

```bash
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
```