Editing SSH (section)

= SSH Login =

To log in to a cluster, use the following command, replacing <code><username></code> with your username and <code><mycluster></code> with the fully qualified domain name (FQDN) of your cluster:
  ssh <username>@<mycluster>

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== Passwordless Login ==

If you want to bypass password authentication, you can use SSH key authentication. SSH keys allow you to connect to a server or multiple servers without entering your password each time.

To generate an SSH key pair, follow these steps:

<ol>
<li>On your local machine, generate an SSH key:</li>
</ol>

  ssh-keygen -t ed25519 -C "[$(whoami)@$(uname -n)-$(date -I)]"

<ol start="2">
<li>Automatically add your public key to the server by running the following command (use your ANDREW password):</li>
</ol>

  ssh-copy-id -i ${HOME}/.ssh/id_ed25519.pub <username>@<mycluster>.

  # for windows, first navigate to your .ssh folder, then run:
  type .\id_ed25519.pub | ssh <username>@<mycluster> "cat >> .ssh/authorized_keys"

<ol start="3">
<li>You can also configure your <code>~/.ssh/config</code> file to avoid typing the full cluster address every time. Add the following lines to the file:</li>
</ol>

  Host <mycluster>
  HostName <mycluster's hostname>
  User <username>
  IdentityFile ${HOME}/.ssh/id_ed25519

<ol start="4">
<li>Then, you can log in with the simple command <code>ssh ''mycluster''</code>.</li>
</ol>

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== Connecting to a Compute Node ==

You can also ssh directly to a compute node where you have a job already running by doing:

 ssh -J babel babel-X-XX

More on [[Port Forwarding]]

== Copying Data to Compute Nodes ==

To copy data to compute nodes when the data directories are not directly accessible from the login nodes, you can utilize the login node as a JumpHost, allowing direct connections to the compute nodes. Follow these steps to configure SSH and establish the connection:

<ol>
<li>Launch an interactive job on a compute node.</li>
<li>Determine the IP address of the compute node by executing the <code>ip a</code> command on the compute node itself.</li>
<li>Update your SSH configuration file located at <code>~/.ssh/config</code> with the following entries:</li>
</ol>

  Host <mycluster-short-name>
    HostName <mycluster-fqdn>
    User <username>
    IdentityFile <identityfile>

  Host <mycluster>-compute-node
    HostName <node ip addr>
    User <username>
    ProxyJump <mycluster>

Replace the relevant items in <code> < ></code> with your actual username, cluster name and the path to your SSH private key file.
For example, to connect to a node in the Babel cluster:

 Host babel
   HostName babel.lti.cs.cmu.edu
   User <username>
   IdentityFile /path/to/identityfile

 Host babel-compute-node
   HostName <node ip addr>
   User <username>
   ProxyJump babel

The updated configuration allows you to establish a connection to the compute node through the login node, acting as a JumpHost.

Refer to your cluster's dedicated page in the [[:Category:Clusters| Cluster's]] section for more detailed information.

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== Why is there an SSH key pair in the home directory upon first login? ==

When a user runs a job on two nodes, their script might need to SSH from node-1 to node-2. If the SSH connection fails or hangs, it is often because the fingerprint of the target node is not in the <code>~/.ssh/known_hosts</code> file. To prevent this issue, we automatically create SSH keys and add the necessary fingerprints to <code>~/.ssh/authorized_keys</code> when the user's home directory is created.

This process also eliminates password prompts when connecting to a node where you have a job running.

For more information on generating these keys, see: [[SSH#Passwordless_Login|Connecting to the Cluster - Passwordless Login]]