Add an SSH Server With Certificate Auth

Overview #

An SSH server is a combination of a specific SSH destination and the credentials to access it. This guide describes how to set up an SSH server with a certificate in the Admin UI.

Using certificate authentication eliminates the need to manage unique key pairs for each of your servers. By using a certificate authority managed by StrongDM, every connection is secured with a short-lived private/public key pair, thus eliminating the risk of lost keys being compromised.

Prerequisites #

Before you begin, ensure that the server you are attempting to add is 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 <YOUR_HOSTNAME>. If your gateway or relay can connect to this hostname, you can continue.

For more information see nodes.

Review StrongDM CA Public Key #

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.

As an alternative to the Strong CA, organizations that have the Enterprise bundle enabled may use an SSH CA issued by a third-party issuer (for example, HashiCorp). Third-party CAs are managed in the same way as the Strong CA.

All SSH CAs available to your organization are listed on the Admin UI’s Network > Certificate Authorities page. Selecting a CA opens that CA’s settings, which shows all certificates issued by the CA. Each certificate is identified by a unique fingerprint and the time and date when it was created.

Multiple certificates can exist, but only one can be active at a time. An active certificate is the one configured to authenticate to the resource. The active certificate is highlighted and shown in blue, whereas inactive certificates are shown in gray.

How to view and copy the SSH CA public key #

1. Log in to the StrongDM Admin UI.
2. Go to Network > Certificate Authorities and select either StrongDM SSH Certificate Authority or another SSH CA.
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 StrongDM 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
   

Restrict access by username #

Because the certificates generated by the CA use the resource settings, anyone with access to modify these settings can change the username and thus elevate their privileges, allowing them to assume root or any existing account on the target system. Of course, any actions are logged by StrongDM. However, we still strongly recommend disabling SSH access for the root account entirely.

Additionally, you can restrict what users or principals are allowed to authenticate with the CA. Every certificate created by the StrongDM CA contains two principals: the username specified in the datasource settings and the literal string strongdm. You can use the following steps to restrict access to only this user, replacing user-one with your desired username.

1. Create a folder to contain the user files.

   mkdir /etc/ssh/sdm_users
   

2. With your editor of choice, create a file that matches the desired username, as in the following example. Then type the string strongdm, save, and close.

   sudo vim /etc/ssh/sdm_users/user-one
   

3. Modify /etc/ssh/sshd_config and append the following line:

   AuthorizedPrincipalsFile /etc/ssh/sdm_users/%u
   

4. After making these changes, restart the sshd service. It then uses the updated version of the AuthorizedPrincipalsFile.

   sudo systemctl restart sshd
   

You can find additional ways to restrict access by username in Red Hat’s Creating SSH Certificates documentation.

Add an SSH Certificate-Based Server in the Admin UI #

To add your new SSH certificate-based server as a StrongDM resource, use the following steps.

1. Log in to the Admin UI and go to Infrastructure > Servers.
2. Click Add server.
3. Select SSH (Certificate Based) as the Server Type and set other resource properties to configure how the StrongDM relay connects to the server via SSH.
4. Click create to save the resource.
5. Click the resource name to view status, diagnostic information, and setting details. After the server is created, the Admin UI displays that resource as unhealthy until the health checks 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) 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.

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.