Smallstep SSH Host Quickstart

Before you begin

  • You'll need a account on the smallstep platform. Need one? Register here
  • You'll need the enrollment token you received upon signup.
  • We support the following host platforms:
    • CentOS 8
    • CentOS 7
    • Amazon Linux 2
    • Ubuntu 20.04 LTS (Focal)
    • Ubuntu 18.04 LTS (Bionic)
    • Ubuntu 16.04 LTS (Xenial)
    • Debian 10 (Buster)
    • Debian 9 (Stretch)
    • Debian 8 (Jessie)
  • Running this quickstart will modify config files related to systemd, PAM, NSS, and SSHD:
    • We add step-ssh-renew and step-ssh-metadata services to systemd:
      • step-ssh-renew rotates the SSH host certificate every eight hours.
      • step-ssh-metadata syncs user ACLs with your CA every 3 seconds.
    • And we modify the following files:
      • /etc/pam.d/sshd
      • /etc/pam.d/sudo
      • /etc/pam.d/su
      • /etc/ssh/sshd_config
      • /etc/nsswitch.conf
    • We suggest backing these up before you begin setup on a particular base machine type, so that you can quickly revert changes if needed.

Understanding Host Tags

This section only applies to multi-user environments.

Host Tags (key-value pairs) are the pillar our access control model. Rather than mapping people or groups directly to hosts, you'll map tag combinations to your hosts and to your user groups. First you'll put your hosts into logical groups using tags, eg. role:web or env:staging. Then, you'll grant user groups access to all hosts with a specific tag combination. Finally, you'll choose which user group tag combinations will allow sudo privileges on any matching hosts.

Let's look at an example:

  • The developers group will have access to myserver #1 only.
  • The data group will have access to myserver #2 and myserver #3.
  • The ops group will have sudo access to myserver #2 and myserver #3.

Of course, hosts and groups can have as many tag combinations as you like. Take a minute to think about how you'd like to use Host Tags in your environment.

Quick Installation Instructions

Our installation script will guide you through host setup. It will:

  • Install the step CLI and step-ssh utilities
  • Comment out the broken pam_systemd.so in common-session, which causes a delay at login (this is a known Ubuntu/Debian bug)
  • Bootstrap your host to your CA
  • Enroll your host with an SSH host certificate
  • Configure SSHD to use certificate authentication
  • Activate step-ssh PAM/NSS modules
  • Add the host to the CA's inventory

Step 1. Install Smallstep SSH

If your host is a bastion host

As root, run:

# curl -sSf http://files.smallstep.com/ssh-host.sh
# bash ssh-host.sh --is-bastion
If your host is behind a bastion host

As root, run:

# curl -sSf http://files.smallstep.com/ssh-host.sh
# bash ssh-host.sh --bastion=<bastion hostname>
For any other host

As root, run:

# curl -sSf http://files.smallstep.com/ssh-host.sh | bash

You'll be prompted for your team ID, enrollment token, hostname, and a list of space-separated host tags (eg. "db=dev region=us-west-1").

Passing flags to ssh-host.sh

The ssh-host.sh utility can also be run non-interactively, as long as you provide the following required flags:

  • --team="[your team ID]"
  • --token="[your enrollment token]"
  • --hostname="[the hostname]"
  • One or more --tag="key=value" to assign host tags

The following additional flags are available:

  • Any number of --principal="[additional hostname]", to add additional hostnames to the host certificate
  • --is-bastion to indicate a bastion host
  • --bastion="[bastion hostname]" to specify the bastion that sits in front of this host

Step 2. Test your installation

Before you sign out of your sudo session, test your installation by logging in and running sudo in a separate session.

This step is especially important if you have made any non-standard changes to your PAM or NSS stacks.

Now sign in at https://smallstep.com/app/[Team ID]

You should see your host listed under the "Hosts" tab.

Manual Instructions

If you wish to enroll your host without using our script, you can follow our step-by-step installation instructions instead.

Need to remove a host?

As root on the host, run:

# curl -sSf http://files.smallstep.com/ssh-host-uninstall.sh | bash

This script will:

  • Unregister the host from the host inventory on the CA
  • Deactivate Smallstep SSH
  • Revert SSHD configuration changes
  • Remove your $HOME/.step configuration directory
  • Remove the step-ssh package

Optional Hardening

  • Now that you're using certificates, you may wish to explicitly disallow the use of authorized_keys files on the host.

    You can do this by setting AuthorizedKeysFile none in sshd_config. You may want to allow authorized_keys for an emergency access account, however. A configuration for this might look like:

Match User *,!ubuntu
    AuthorizedKeysFile none

Troubleshooting Tips

  • Having trouble? You should be able to revert any changes by running step-ssh deactivate.

  • Suspect host or user certificates are not working? In other words, your ssh client fails to log in or shows "trust on first use" warning? Try this:

    • Be sure you can connect to our test server, with ssh-test.app.smallstep.com.
    • On your client run step ssh list and find your user certificate marked (ECDSA-CERT), e.g. 256 SHA256:Bb2TcimUYj8Nc5w4FhpZ/gmeNIIvLIzphTx35NzaRoA alice@smallstep.com (ECDSA-CERT), which you can inspect with step ssh list --raw alice@smallstep.com | step ssh inspect. Be sure the current time is neither before or after the period specified in Valid: .
    • On the destination host run step ssh inspect /etc/ssh/ssh_host_ecdsa_key-cert.pub. Make sure the current time is within the period of Valid:.
    • The destination host's sshd_config should show included the following lines for sshd (make sure the service has reloaded its config) to leverage certificate-based authentication:
    $ tail -n 7 /etc/ssh/sshd_config
    #       ForceCommand cvs server
    
    # autogenerated by step @ 2020-04-02T21:16:05Z
    TrustedUserCAKeys /etc/ssh/ca.pub
    HostCertificate /etc/ssh/ssh_host_ecdsa_key-cert.pub
    HostKey /etc/ssh/ssh_host_ecdsa_key
    # end
    
    • You can run sshd -t (as root) to test you SSHD configuration. No output means the file is OK; you'll get an error if any of the referenced files (eg. host keys) are not accessible to sshd.
  • Suspect your ssh client might have a problem? Use verbose logging ssh -v <hostname>. Look out for following key lines/events below in your log. If you don't see errors the absence of these events might indicate your config is invalid / not being applied.

    • Your team's config is applied for this specific host:

      debug1: Executing command: 'step ssh check-host ec2-52-200-74-193.compute-1.amazonaws.com'
      debug1: Reading configuration data /etc/ssh/ssh_config
      debug1: /etc/ssh/ssh_config line 48: Applying options for *
      debug1: /etc/ssh/ssh_config line 52: Applying options for *
      debug1: Executing proxy command: exec step ssh proxycommand --provisioner "okta" alice ec2-52-200-74-193.compute-1.amazonaws.com 22
      
    • The host certificate passed authentication on the client:

      debug1: Server host certificate: ecdsa-sha2-nistp256-cert-v01@openssh.com SHA256:ON/csaCQ4yDKfxblQnOLTiZIOoOtsHPAjQn6DrW+k9I, serial 2468062085372141930 ID "ec2-52-200-74-193.compute-1.ama
      debug1: Server host certificate: ecdsa-sha2-nistp256-cert-v01@openssh.com SHA256:46gC0CEzXWN4acTHGQldL6H+QlbhB4+KPZjkoRToI/w, serial 8551898981883739717 ID "ec2-52-200-74-193.compute-1.amazonaws.com" CA ecdsa-sha2-nistp256 SHA256:sqfZG6AOPUvcheFUIZDX+DEesnyfNZQ5JwqpcxUzY+0 valid from 2020-04-14T04:45:10 to 2020-05-14T04:46:10
      debug1: Host 'ec2-52-200-74-193.compute-1.amazonaws.com' is known and matches the ECDSA-CERT host certificate.
      debug1: Found CA key in /Users/alice/.step/ssh/known_hosts:1
      
    • The username being used for authentication:

      debug1: Authenticating to ec2-52-200-74-193.compute-1.amazonaws.com:22 as 'alice'
      
    • The ssh client offers the user's certificate to the destination host:

      debug1: Offering public key: sebastian@smallstep.com ECDSA-CERT SHA256:VIa1uWhBTjjtpW3IBkUG/aFGfqlUhjkXNQVk6Hc1lXc agent
      debug1: Server accepts key: alice@smallstep.com ECDSA-CERT SHA256:VIa1uWhBTjjtpW3IBkUG/aFGfqlUhjkXNQVk6Hc1lXc agent
      debug1: sign_and_send_pubkey: no separate private key for certificate "alice@smallstep.com"
      
  • The following endpoints are being used to deliver the SSH service

    • ssh-test.app.smallstep.com — For SSH test sessions
    • https://ssh.<team-name>.ca.smallstep.com — The CA internal PKI APIs (protected by mTLS)
    • https://smallstep.com/app/teams/sso/success — Single sign-on success page
    • https://smallstep.com/app/<team-name> — Admin Dashboard
    • https://api.smallstep.com — APIs to fetch team information
    • https://auth.smallstep.com — OpenID Connect flow, if you have no identity provider configured
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