Slurm Workload Manager -

# Authentication Plugins

## Overview

It is important for Slurm to know that the Remote Procedure Calls (RPCs) that it receives are coming from trusted sources. There are a few different authentication mechanisms available within Slurm to verify the legitimacy and integrity of the requests.

If you plan to change the authentication type, you will need to drain the cluster of running jobs and stop all Slurm daemons before making the change. Otherwise, requests can be made with the new type while the running daemons still operate with the old type that was present when they started.

## MUNGE

MUNGE can be used to create and validate credentials. It allows Slurm to authenticate the UID and GID of a request from another host that has matching users and groups. MUNGE libraries must exist when building Slurm in order for it to be able to use munge for authentication. All hosts in the cluster must have a shared cryptographic key.

#### Setup

1. MUNGE requires that there be a shared key on the machines running slurmctld, slurmdbd and the nodes. You can create a key file by entering your own text or by generating random data:

       dd if=/dev/random of=/etc/munge/munge.key bs=1024 count=1

2. This key should be owned by the "munge" user and should not be readable or writable by other users:

       chown munge:munge /etc/munge/munge.key
       chmod 400 /etc/munge/munge.key

3. Distribute the key file to the machines on the cluster. It needs to be on the machines running slurmctld, slurmdbd, slurmd and any submit hosts you have configured. Use the distribution method of your choice.
4. The "munge" service should be running on any machines that need to use it for authentication. It should be enabled and started on all the machines you distributed a key to:

       systemctl enable munge
       systemctl start munge

5. Changing the authentication mechanism requires a restart of Slurm daemons. The daemons need to be stopped before updating the slurm.conf so that client commands do not use a mechanism other than what the running daemons are expecting.
6. Update your slurm.conf and slurmdbd.conf to use MUNGE authentication.
   - slurm.conf:

         AuthType = auth/munge
         CredType = cred/munge

   - slurmdbd.conf:

         AuthType = auth/munge

7. Start the Slurm daemons back up with the appropriate method for your cluster.

## Slurm

Beginning with version 23.11, Slurm has its own plugin that can create and validate credentials. It validates that the requests come from legitimate UIDs and GIDs on other hosts with matching users and groups. All hosts in the cluster must have a shared cryptographic key.

#### Single Key Setup

1. For the authentication to happen correctly you must have a shared key on the machine running slurmctld, slurmdbd and the nodes. You can create a key file by entering your own text or by generating random data:

       dd if=/dev/random of=/etc/slurm/slurm.key bs=1024 count=1

2. The slurm.key or slurm.jwks should be owned by SlurmUser and should not be readable or writable by other users. This example assumes the SlurmUser is 'slurm':

       chown slurm:slurm /etc/slurm/slurm.key
       chmod 600 /etc/slurm/slurm.key

3. Distribute the key file to the machines on the cluster. It needs to be on the machines running slurmctld, slurmdbd, slurmd and sackd.
4. Update your slurm.conf and slurmdbd.conf to use the Slurm authentication type.
   - slurm.conf:

         AuthType = auth/slurm
         CredType = cred/slurm

   - slurmdbd.conf:

         AuthType = auth/slurm

5. If not using systemd, create the runtime directories. (Systemd takes care of this automatically if you're starting the daemons using the provided service files).

       mkdir /run/slurmctld /run/slurmdbd
       chown slurm:slurm /run/slurmctld /run/slurmdbd

6. Start the daemons back up with the appropriate method for your cluster.

Beginning with version 24.05, you may alternatively create a slurm.jwks file with multiple keys defined. The slurm.jwks file aids with key rotation, as the cluster does not need to be restarted at once when a key is rotated. Instead, an scontrol reconfigure is sufficient. There are no slurm.conf parameters required to use the slurm.jwks file, instead, the presence of the slurm.jwks file enables this functionality. If the slurm.jwks is not present or cannot be read, the cluster defaults to the slurm.key.

#### Multiple Key Setup

Setup for multiple keys is very similar to the single key setup with the exception of a richer key file. The key file is composed of one jwks-esque "keys" list, with a number of "key" entries into this list. The key entries have many different fields. An example file is below.

    {
      "keys": [
        {
          "alg": "HS256",
          "kty": "oct",
          "kid": "key-identifier",
          "k": "VGhlIGtleSBiZWxvdyBtZSBuZXZlciBsaWVz",
          "exp": 1718200800
        },
        {
          "alg": "HS256",
          "kty": "oct",
          "kid": "key-identifier-2",
          "k": "VGhlIGtleSBhYm92ZSBtZSBhbHdheXMgbGllcw==",
          "use": "default"
        }
      ]
    }

The following fields are used by auth/slurm. (Additional fields can be present, but will be ignored.)

- alg — The cryptographic algorithm used with the key. This field is required, and the value MUST be HS256.
- kty — The family of cryptographic algorithms used to sign the key. This field is required, and the value MUST be oct.
- kid — The case-sensitive text identifier used for key matching. This field is required, and the text must be unique.
- k — The actual key, represented in a Base64 or Base64url encoded binary blob. This field is required, and must be longer than 16 bytes.
- use — Determines whether this key is the default key. Acceptable values are only "default", which denotes this key as the default key. There can only be one default key. This field is optional.
- exp — The expiration date of the key as a Unix timestamp. This field is optional.

Once the slurm.jwks file has been created, use the same process for setting up auth/slurm as in the single key setup, except use the slurm.jwks file instead of the slurm.key file.

If the cluster does not have access to consistent user ids from LDAP or the operating system, you can use the use_client_ids option to allow it to use the Slurm authentication mechanism.

#### SACK

Slurm's internal authentication makes use of a subsystem — the Slurm Auth and Cred Kiosk (SACK) — that is responsible for handling requests from the auth/slurm and cred/slurm plugins. This subsystem is automatically started and managed internally by the slurmctld, slurmdbd, and slurmd daemons on each system with no need to run a separate daemon.

For login nodes not running one of these Slurm daemons, the sackd daemon should be run to allow the Slurm client commands to authenticate to the rest of the cluster. This daemon can also manage cached configuration files for configless environments.

Beginning with version 25.05, it's possible for multiple sackd daemons to co-exist on the same login node by changing the RuntimeDirectory option in separate systemd service files. Clients can authenticate against different sackd daemons on the same login node by managing the SLURM_CONF environment variable to point at the different cluster configuration files. Refer to the sackd(8) manual for more information.

## JWT

Slurm can be configured to use JSON Web Tokens (JWT) for authentication purposes. This is configured with the AuthAltType parameter and is used only for client to server communication. You can read more about this authentication mechanism and how to install it here.