Using Teleport with OpenSSH

In this guide, we will show you how to configure the OpenSSH server sshd to join a Teleport cluster. Existing fleets of OpenSSH servers can be configured to accept SSH certificates dynamically issued by a Teleport CA.

Using Teleport and OpenSSH has the advantage of getting you up and running, but in the long run, we would recommend replacing sshd with teleport. We've outlined these reasons in OpenSSH vs Teleport SSH for Servers?

Prerequisites

• OpenSSH version 6.9 or above on your local machine. View your OpenSSH version with the command:

  ssh -V

• A Linux host with the OpenSSH server sshd installed, but not Teleport. The SSH port on this host must be open to traffic from the Teleport Proxy Service host.

tsh login --proxy=teleport.example.com [email protected]
tctl status
Cluster  teleport.example.com
Version  10.2.2
CA pin   sha256:abdc1245efgh5678abdc1245efgh5678abdc1245efgh5678abdc1245efgh5678

tsh login --proxy=myinstance.teleport.sh [email protected]
tctl status
Cluster  myinstance.teleport.sh
Version  10.1.9
CA pin   sha256:sha-hash-here

Step 1/4. Configure sshd to trust the Teleport CA

Later in this guide, we will generate an SSH client configuration that will use a certificate signed by the Teleport Auth Service to authenticate to your SSH server. For this to work, sshd must be told to allow users to log in with certificates generated by the Teleport Auth Service.

Start by exporting the Teleport CA public key.

On your local machine, print the Teleport certificate authority certificate to stdout:

tctl auth export --type=user | sed "s/cert-authority\ //"

Copy the output.

On the host where you are running sshd, run the following commands.

Assign the output of the tctl auth export command to an environment variable:

export KEY="<pasted output>"

Make the public key accessible to sshd:

sudo bash -c "echo \"$KEY\" > /etc/ssh/teleport_user_ca.pub"
sudo bash -c "echo 'TrustedUserCAKeys /etc/ssh/teleport_user_ca.pub' >> /etc/ssh/sshd_config"

Restart sshd. For systemd-enabled hosts, run the following command:

sudo systemctl restart sshd

Now, sshd will trust users who present a Teleport-issued certificate.

Step 2/4. Configure host authentication

Next, ask Teleport to issue a valid host certificate for your sshd host. Later in this guide, we will configure your SSH client to trust the certificate, authenticating your sshd host for your SSH client. Like the user certificate we created earlier, the host certificate will be signed by the Teleport Auth Service.

Ensure that your user has the correct privileges

Your user must be authorized to read and write host certificates.

On your local machine, create a file called host-certifier.yaml with the following content:

kind: role
version: v5
metadata:
  name: host-certifier
spec:
  allow:
    rules:
      - resources:
          - host_cert
        verbs:
          - list
          - create
          - read
          - update
          - delete

Create the role resource:

tctl create host-certifier.yaml
role 'host-certifier' has been created

Fetch the configuration for your user. Replace USERNAME with the name of the Teleport username that you used to log in to your cluster.

tctl get user/USERNAME > myuser.yaml

Make the following change to myuser.yaml:

  roles:
   - access
   - auditor
   - editor
+  - host-certifier

Apply your change:

tctl create -f myuser.yaml

Log out of your Teleport cluster and log in again.

You will now have the required permissions to export a host key for your sshd host.

Issue a host certificate

On your local machine, assign the IP address or fully qualified domain name of your Node to an environment variable.

ADDR=203.0.113.0

Run the following tctl command to generate a host certificate:

tctl auth sign \      --host=${ADDR?} \      --format=openssh \      --out=myhost
The credentials have been written to myhost, myhost-cert.pub

The above command will result in a private key and certificate.

Use ssh-keygen to verify the contents of the certificate:

ssh-keygen -L -f myhost-cert.pub

The Principals section should contain the address you assigned to ADDR earlier:

myhost-cert.pub:
        Type: [email protected] host certificate
        Public key: RSA-CERT SHA256:nHkp6SnrAW4AV00VUaqPgR6SgdyvV9MmjUrYnwZ779A
        Signing CA: RSA SHA256:euqx2Y8Pq+r0c94GKVNXAklBVTmAJtaQUn3/ehrfEJE (using rsa-sha2-512)
        Key ID: ""
        Serial: 0
        Valid: after 2022-04-22T11:14:16
        Principals: 
                203.0.113.0
        Critical Options: (none)
        Extensions: 
                x-teleport-authority UNKNOWN OPTION (len 33)
                x-teleport-role UNKNOWN OPTION (len 8)

Copy the host key and certificate to your sshd host, placing them in the directory /etc/ssh.

Make sure these files have the correct permissions:

sudo chmod 0600 /etc/ssh/myhost
sudo chmod 0600 /etc/ssh/myhost-cert.pub

Then add the following lines to /etc/ssh/sshd_config on your sshd host:

HostKey /etc/ssh/myhost
HostCertificate /etc/ssh/myhost-cert.pub

Restart sshd.

Step 3/4. Generate an SSH client configuration

The next step is to configure your OpenSSH client to connect to your sshd host using credentials managed by Teleport. This configuration will use the SSH agent and your user's Teleport-issued certificate to authenticate to the sshd host. It will also authenticate the sshd host using the host certificate you generated earlier.

First, make sure you are running OpenSSH's ssh-agent and have logged in to your Teleport cluster:

tsh status
> Profile URL:        https://teleport.example.com:443
  Logged in as:       myuser
  Cluster:            teleport.example.com
  Roles:              access, auditor, editor, host-certifier
  Logins:             ubuntu, root
  Kubernetes:         enabled
  Valid until:        2022-05-06 22:54:01 -0400 EDT [valid for 11h53m0s]
  Extensions:         permit-agent-forwarding, permit-port-forwarding, permit-pty
eval `ssh-agent`
Agent pid 5931

The ssh-agent command prints additional commands to export the SSH_AUTH_SOCK and SSH_AGENT_PID environment variables. These variables allow OpenSSH clients to find the SSH agent. Running ssh-agent with eval executes these commands.

On your local machine, run the following tsh command. This will print a configuration block that tells your SSH client to use credentials managed by Teleport to connect to hosts in your cluster.

tsh config > ssh_config_teleport

This command creates an SSH configuration file at a nonstandard location in order to make it easier to clean up, but you can append the output of tsh config to the default SSH config file (~/.ssh/config) if you wish.

# Common flags for all {{ .ClusterName }} hosts
Host *.{{ .ClusterName }} {{ .ProxyHost }}
    UserKnownHostsFile "{{ .KnownHostsPath }}"
    IdentityFile "{{ .IdentityFilePath }}"
    CertificateFile "{{ .CertificateFilePath }}"

# Flags for all {{ .ClusterName }} hosts except the proxy
Host *.{{ .ClusterName }} !{{ .ProxyHost }}
    Port 3022
    ProxyCommand "{{ .TSHPath }}" proxy ssh --cluster={{ .ClusterName }} --proxy={{ .ProxyHost }} %[email protected]%h:%p

/usr/bin/ssh -l root -A -o UserKnownHostsFile=/root/.tsh/known_hosts -p 11105 teleport.example.com -s proxy:mynode:[email protected]

tsh.exe config | out-file .ssh\config -encoding utf8 -append

Host *.{{ .NodeName }}.leaf1.example.com
   Port 3022
   ProxyCommand tsh proxy ssh -J proxy.leaf1.example.com:443 %[email protected]%h:%p

Host *.example.com
   Port 3022
   ProxyCommand tsh proxy ssh -J {{proxy}} %[email protected]%h:%p

proxy_templates:
- template: '^(\w+)\.(leaf1\.example\.com):([0-9]+)$'
  proxy: "$2:443"

proxy_templates:
- template: '^(\w+)\.(leaf1\.example\.com):([0-9]+)$'
  proxy: "$2:443"
- template: '^(\w+)\.(leaf2\.example\.com):([0-9]+)$'
  proxy: "$2:3080"
  host: "$1:$3"
- template: '(\w+(\.\w+)*)\.(example\.com):([0-9]+)$'
  proxy: "leaf1.example.com:443"
  host: "$1:22"

Given the configuration above, the following command will connect to the Node
`node-1.leaf1.example.com:3022` through the Proxy Service `leaf1.example.com:443`:

```code
$ ssh [email protected]

ssh [email protected]

ssh [email protected]

Step 4/4. Connect to your sshd host

Once you have appended the new text to your OpenSSH client configuration file, you can log in to your sshd host using the configuration we generated earlier.

First, define environment variables for the address of your Teleport cluster, the username you will use to log in to your sshd host, and the port on your sshd host you are using for SSH traffic:

See the available logins you can use to access your sshd host
tsh status | grep Logins
Logins:             ubuntu, root
USER=ubuntu
CLUSTER=teleport.example.com
PORT=22

Next, SSH in to your remote host:

ssh -p ${PORT?} -F ssh_config_teleport "${USER?}@${ADDR?}.${CLUSTER?}"

This will connect to the node node1 on your Teleport cluster. This name does not need to be resolvable via DNS as the connection will be routed through your Teleport Proxy Service.

ssh -F ssh_config_teleport ${USER?}@node2.leafcluster.${CLUSTER}

Revoke an SSH certificate

To revoke the current Teleport CA and generate a new one, run tctl auth rotate. Unless you've highly automated your infrastructure, we would suggest you proceed with caution as this will invalidate the user and host CAs, meaning that the new CAs will need to be exported to every OpenSSH-based machine again using tctl auth export as above.