StrongDM Linux Agent

Overview #

The StrongDM Linux Agent is a just-in-time (JIT) provisioning service to enforce least privilege within Linux compute resources. The StrongDM Linux Agent receives requests from StrongDM prior to facilitating a user connection, which sets up the environment. This allows customers to configure constrained environments with user fidelity on the target resources.

Capabilities #

• JIT provisioning: The StrongDM Linux Agent provisions users on demand and adds or updates their group membership to align with the Remote Identity being used for the connection.
• Privilege escalation: Access workflows can enable privilege escalation through the StrongDM Linux Agent.
• Secure sandbox: The connection is isolated to a specific session used for the duration of the connection. Controls can be put on the session to limit the activities that the user can perform.

Prerequisites #

• The server that is added must be accessible from your StrongDM gateways or relays.
  You must have a properly functioning gateway or relay up and running, and it must be able to reach the target server before you can proceed. To verify, go to the gateway or relay server and from a command prompt, type ping <SERVER_HOSTNAME>. If your gateway or relay can connect to this hostname, you can continue. For more information see nodes.
• Remote Identities must be configured for users intending to connect.

Configuration #

Follow these steps to configure the StrongDM Linux Agent.

Configure the Trusted CA #

Every organization in StrongDM is automatically assigned a unique Certificate Authority (CA) for SSH, known as a Strong CA. The Strong CA is a cryptographic key pair in charge of issuing and validating certificates for every SSH session. You need to add the SSH CA public key as a trusted source on any hosts you want to access with this option.

View and copy the StrongDM SSH CA public key #

1. Log in to the StrongDM Admin UI.
2. Go to Network > Certificate Authorities and select an SSH CA to use.
3. From the SSH CA’s Settings tab, select one of the certificates shown, or create a new certificate.
4. Click the i button beside the certificate’s fingerprint.

   

5. From the SSH Certificate dialog that displays, use the Copy button to copy the SSH CA public key to your clipboard.

   

Add the CA to your host #

The next step is to tell your host to trust certificates issued by your organization’s CA. If you have not already, add your organization’s SSH CA public key to the targeted host.

1. Create a /etc/ssh/sdm_ca.pub file and add the SSH CA public key.

2. SSH can sometimes be unpredictable if permissions are not set correctly. Therefore, update the file’s permissions:

   sudo chmod 600 /etc/ssh/sdm_ca.pub
   

3. With your editor of choice, modify /etc/ssh/sshd_config by appending the following lines:

   # StrongDM CA
   TrustedUserCAKeys /etc/ssh/sdm_ca.pub
   

4. Restart the SSH service on this host for the changes to take effect. Note that the command you execute may differ based on your system configuration, and you may need to restart sshd or ssh, as in the following example:

   sudo systemctl restart ssh
   

Configure Privileged User #

A privileged user must be created on the end resource to allow the StrongDM Linux Agent to provision users.

1. On the server, create a privileged user called strongdm.
2. Configure that user to use a restricted bash with access to the following:
   • useradd
   • usermod
   • userdel

Add a Certificate-Based SSH Server With User Provisioning in the Admin UI #

To add your new SSH (Certificate Based with User Provisioning) server as a StrongDM resource, use the following steps.

1. In the Admin UI, go to Infrastructure > Servers.
2. Click Add server.
3. For Server Type, select SSH (Certificate Based with User Provisioning).
4. For Authentication, select Remote Identities and enter the healthcheck username, which is the username that will be utilized to verify StrongDM’s connection to the server.
5. Set other required resource properties to configure how the StrongDM relay connects to the server via SSH.
6. Click Create to save the resource.

Configuration is now complete.

After the server is created, the Admin UI displays that resource as unhealthy until the healthchecks run successfully. When the resource is ready, the Health icon indicates a positive, green status.

Resource properties #

Configuration properties are visible when you add a Server Type or when you click to view the server’s settings. The following table describes the settings available for your SSH (Certificate Based with User Provisioning) server.

Certificate Authority options #

By default, server credentials are stored in Strong CA. If the Enterprise bundle is enabled for your organization, you may use a third-party CA, instead of the default Strong CA, to issue certificates for authentication to your certificate-based SSH resources.

For more information, please see Certificate Authorities.

Key type options #

The following table describes the different key types StrongDM can use to encrypt and secure SSH connections.

Connect to the Server #

You can now connect to the server via SSH (for example, in the CLI, use sdm ssh <SERVER_DISPLAY_NAME>) and verify that a user is provisioned upon connection.

Once connected, you are logged in with the username that was set as the Remote Identity Username in your StrongDM user profile. That username did not exist previously—it was provisioned when the SSH session started. Moreover, if you specified a Remote Identity Role on your StrongDM user profile, your user is placed in a group with that same role name.

Additional Considerations #

• You may have an existing CA key pair and certificate to perform direct SSH or other tasks. You can continue to use this key pair, but you cannot import this CA into StrongDM.
• You do not need to sign these keys, or any user keys. StrongDM handles that for you and for your users.
• Session-based certificates for users are automatically renewed every 3 minutes.