Add an SSH Server With Certificate Auth

Last modified on February 7, 2024

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). This 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.

The SSH CA public key can be seen under Network > Certificate Authorities > StrongDM SSH Certificate Authority. The StrongDM SSH Certificate Authority page shows all certificates issued by the CA. Each certificate is identified by a unique fingerprint and the time and date when it was created.

Your organization may have multiple certificates for each CA, but only one SSH certificate and one RDP certificate may be active at any given 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 > StrongDM SSH Certificate Authority.
  3. From the StrongDM 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.
    StrongDM SSH Certificate Authority Page Showing Copy Button
    StrongDM SSH Certificate Authority Page Showing Copy Button
  5. From the SSH Certificate dialog that displays, use the Copy button to copy the SSH CA public key to your clipboard.
    Example SSH Certificate Dialog
    Example SSH Certificate Dialog

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.

PropertyRequirementDescription
Display NameRequiredMeaningful name to display the resource throughout StrongDM; exclude special characters like quotes (") or angle brackets (< or >)
Server TypeRequiredSelect SSH (Certificate Based)
HostnameRequiredHostname or IP address to which you are connecting, such as testserver-01.example.org; relay server should be able to connect to your target server or hostname
PortRequiredPort to connect to the resource; default port value 22
Bind InterfaceRead onlyAutomatically generated IP address value in the 127.0.0.1 to 127.255.255.254 IP address range; default is 127.0.0.1; preferred bind interface value can be modified later under Settings > Port Overrides
Port OverrideRead onlyAutomatically generated with a value between 1024 to 59999 as long as that port is not used by another resource; preferred port can be modified later under Settings > Port Overrides
Certificate AuthorityRequiredWhere the credentials for the server are stored; defaults to Strong CA; to learn more, see Certificate Authority options
Key TypeRequiredSigning algorithm with default value set to RSA-2048; other options include RSA-4096, ECDSA-256, ECDSA-384, ECDSA-521, and ED25519; to learn more, see Key Type options
AuthenticationRequiredSelect Leased Credentials (default) or Remote Identities
UsernameRequiredDisplays if Authentication is set to Leased Credentials; enter the username the relay should utilize to connect to the server via SSH (for example, bob.belcher)
Healthcheck UsernameRequiredDisplays if Authentication is set to Remote Identity; enter the username that will be utilized to verify StrongDM’s connection to the server; username must exist on the target server
Resource TagsOptionalResource tags consisting of key-value pairs <KEY>=<VALUE> (for example, env=dev)

Certificate Authority options

By default, server credentials are stored in Strong CA. The Strong CA is configured in the Admin UI’s Certificate Authorities page. 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.

Key typeDescriptionAdditional information
RSA-20482048-bit key generated with RSA algorithmLowest security guarantees, but has broad support across hosts
RSA-40964096-bit key generated with RSA algorithmSlightly better than RSA-2048; still uses RSA, but larger keys can prolong the time to crack if an attacker gains access
ECDSA-256Key generated with 256-bit elliptic curve and ECDSA algorithmProvides better security guarantees than RSA
ECDSA-384Key generated with 384-bit elliptic curve and ECDSA algorithmSlightly better than ECSDA-256
ECDSA-521Key generated with 521-bit elliptic curve and ECDSA algorithmServes as the best ECDSA choice from a security standpoint
ED25519Key generated with ED25519 algorithmProvides the best performance and comparable security to ECDSA; much smaller keys, but not as widely supported as other options

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.
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