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. teleport SSH servers have support for multiple features that are incompatible with OpenSSH:

• RBAC and resource filtering based on dynamically updated labels
• Session recording without SSH connection termination
• Advanced session recording
• Restricting outbound network connections in SSH sessions

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 version 7.4 or above installed, but not Teleport. The SSH port on this host must be open to traffic from the Teleport Proxy Service host.
• Make sure you can connect to Teleport. Log in to your cluster using tsh, then use tctl remotely:

  tsh login --proxy=teleport.example.com [email protected]
  tctl status
  Cluster  teleport.example.com
  Version  13.0.3
  CA pin   sha256:abdc1245efgh5678abdc1245efgh5678abdc1245efgh5678abdc1245efgh5678

  You can run subsequent tctl commands in this guide on your local machine.

  For full privileges, you can also run tctl commands on your Auth Service host.

Step 1/5. Add a node resource to your Teleport cluster

When you request an SSH connection to a OpenSSH node, Teleport needs to be able to find the node's IP address so it can establish a connection to it.

Declare a node resource so Teleport knows how to reach your OpenSSH server. On your local machine, create a file called openssh-node-resource.yaml with the following content:

kind: node
version: v2
sub_kind: openssh
metadata:
  name: a100fdd0-52db-4eca-a7ab-c3afa7a1564a
  labels:
    env: prod
spec:
  addr: 1.2.3.4:22
  hostname: openssh-node

spec.addr and spec.hostname are mandatory. Assign spec.addr to the address and port of your node and spec.hostname to the name of the node as you would like users to see it in Teleport.

The metadata.labels field labels the SSH Service instance so you can apply RBAC rules to it.

The metadata.name field isn't mandatory, but setting it here will save you some work later.

To generate a new universal unique identifier (UUID) suitable for a node name, use the uuidgen on Linux or MacOS, or use the New-Guid cmdlet in Powershell on Windows.

Create the node resource:

tctl create openssh-node-resource.yaml

Step 2/5. 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 the host where you are running sshd, run the following commands, assigning proxy to the address of your Teleport Proxy Service:

export KEY=$(curl 'https://proxy/webapi/auth/export?type=openssh' | sed "s/cert-authority\ //")

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 3/5. 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

Assign the host-certifier role to your Teleport user by running the following commands, depending on whether you authenticate as a local Teleport user or via the github, saml, or oidc authentication connectors:

tctl get users/$(tsh status -f json | jq -r '.active.username') > out.yaml

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

tctl create -f out.yaml

tctl get github/github --with-secrets > github.yaml

  teams_to_roles:
    - organization: octocats
      team: admins
      roles:
        - access
+       - host-certifier

tctl create -f github.yaml

tctl get --with-secrets saml/mysaml > saml.yaml

  attributes_to_roles:
    - name: "groups"
      value: "my-group"
      roles:
        - access
+       - host-certifier

tctl create -f saml.yaml

tctl get oidc/myoidc --with-secrets > oidc.yaml

  claims_to_roles:
    - name: "groups"
      value: "my-group"
      roles:
        - access
+       - host-certifier

tctl create -f oidc.yaml

Log out of your Teleport cluster and log in again to assume the new role.

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

Issue a host certificate

tctl get node/openssh-node --format text

tctl get node/openssh-node --format=json | jq -r ".[0].metadata.name"

tctl get node/openssh-node

Create the host certificate

When creating host certificates, it is important to specify all the domain names and addresses that refer to your node. If you try to connect to a node with a name or address that was not specified when creating it's host certificate, Teleport will reject the SSH connection.

On your local machine, assign the IP address, fully qualified domain name of your node, and the node's UUID to an environment variable. If you won't be connecting to your node with its hostname, you can safely omit it.

ADDR=1.2.3.4,openssh-node,a100fdd0-52db-4eca-a7ab-c3afa7a1564a

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: 
                1.2.3.4
                openssh-node
                a100fdd0-52db-4eca-a7ab-c3afa7a1564a
        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 4/5. 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 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 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

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 5/5. 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 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 OpenSSH 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.