Using Teleport with OpenSSH

Teleport is fully compatible with OpenSSH and can be quickly set up to record and audit all SSH activity. 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?

Teleport is a standards-compliant SSH proxy and it can work in environments with existing SSH implementations, such as OpenSSH. This section will cover:

• Configuring 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.
• Configuring OpenSSH client ssh to login into nodes inside a Teleport cluster.

ssh -V

Overview

The recording proxy mode, although less secure, was added to allow Teleport users to enable session recording for OpenSSH's servers running sshd, which is helpful when gradually transitioning large server fleets to Teleport.

We consider the "recording proxy mode" to be less secure for two reasons:

• It grants additional privileges to the Teleport proxy. In the default "node recording" mode, the proxy stores no secrets and cannot "see" the decrypted data. This makes a proxy less critical to the security of the overall cluster. But if an attacker gains physical access to a proxy node running in the "recording" mode, they will be able to see the decrypted traffic and client keys stored in the proxy's process memory.
• Recording proxy mode requires the use of SSH agent forwarding. Agent forwarding is required because without it, a proxy will not be able to establish the 2nd connection to the destination node.

Teleport proxy should be available to clients and be set up with TLS.

Teleport OpenSSH supports:

• FQDN [email protected]
• IPv4 [email protected]
• IPv6 [email protected]:db8::2

Set up OpenSSH recording proxy mode

The first step is install and setup Teleport, we recommend starting with our Getting Started Guide and Admin Manual.

To enable session recording for sshd nodes, the cluster must be switched to "recording proxy" mode.

In this mode, the recording will be done on the proxy level:

# snippet from /etc/teleport.yaml
auth_service:
   # Session Recording must be set to Proxy to work with OpenSSH
   session_recording: "proxy"  # can also be "off" and "node" (default)

Next, sshd must be told to allow users to log in with certificates generated by the Teleport User CA. Start by exporting the Teleport CA public key:

Export the Teleport Certificate Authority certificate into a file and update SSH configuration to trust Teleport's CA:

tctl needs to be run on the auth server.
sudo tctl auth export --type=user | sed s/cert-authority\ // > teleport_user_ca.pub
sudo mv ./teleport_user_ca.pub /etc/ssh/teleport_user_ca.pub
echo "TrustedUserCAKeys /etc/ssh/teleport_user_ca.pub" | sudo tee -a /etc/ssh/sshd_config

Restart SSH daemon.

Now, sshd will trust users who present a Teleport-issued certificate. The next step is to configure host authentication.

When in recording mode, Teleport will check that the host certificate of any node a user connects to is signed by a Teleport CA. By default, this is a strict check. If the node presents just a key or a certificate signed by a different CA, Teleport will reject this connection with the error message saying "ssh: handshake failed: remote host presented a public key, expected a host certificate"

You can disable strict host checks as shown below. However, this opens the possibility for Man-in-the-Middle (MITM) attacks and is not recommended.

# snippet from /etc/teleport.yaml
auth_service:
  proxy_checks_host_keys: no

The recommended solution is to ask Teleport to issue valid host certificates for all OpenSSH nodes. To generate a host certificate, run this on your Teleport auth server:

Creating host certs, with an array of every host to be accessed.
Wildcard certs aren't supported by OpenSSH, must be full FQDN.
Management of the host certificates can become complex, this is another
reason we recommend using Teleport SSH on nodes.
sudo tctl auth sign \      --host=api.example.com,ssh.example.com,64.225.88.175,64.225.88.178 \      --format=openssh \      --out=api.example.com
The credentials have been written to api.example.com, api.example.com-cert.pub
You can use ssh-keygen to verify the contents.
ssh-keygen -L -f api.example.com-cert.pub
api.example.com-cert.pub:
Type: [email protected] host certificate
Public key: RSA-CERT SHA256:ireEc5HWFjhYPUhmztaFud7EgsopO8l+GpxNMd3wMSk
Signing CA: RSA SHA256:/6HSHsoU5u+r85M26Ut+M9gl+HventwSwrbTvP/cmvo
Key ID: ""
Serial: 0
Valid: after 2020-07-29T20:26:24
Principals:
api.example.com
ssh.example.com
64.225.88.175
64.225.88.178
Critical Options: (none)
Extensions:
x-teleport-authority UNKNOWN OPTION (len 47)
x-teleport-role UNKNOWN OPTION (len 8)

Then add the following lines to /etc/ssh/sshd_config on all OpenSSH nodes, and restart sshd.

HostKey /etc/ssh/api.example.com
HostCertificate /etc/ssh/api.example.com-cert.pub

Now you can use tsh ssh --port=22 [email protected] to login into any sshd node in the cluster and the session will be recorded.

tsh ssh to use default ssh port:22
tsh ssh --port=22 [email protected]
Example for a Amazon EC2 Host
tsh ssh --port=22 [email protected]

If you want to use OpenSSH ssh client for logging into sshd servers behind a proxy in "recording mode", you have to tell the ssh client to use the jump host and enable SSH agent forwarding, otherwise, a recording proxy will not be able to terminate the SSH connection to record it:

Note that agent forwarding is enabled twice: one from a client to a proxy
(mandatory if using a recording proxy), and then optionally from a proxy
to the end server if you want your agent running on the end server or not
ssh -o "ForwardAgent yes" \    -o "ProxyCommand ssh -o 'ForwardAgent yes' -p 3023 %[email protected] -s proxy:%h:%p" \    [email protected]

Setup SSH agent forwarding

It's important to remember that SSH agent forwarding must be enabled on the client. Verify that a Teleport certificate is loaded into the agent after logging in:

Login as Joe
tsh login --proxy=proxy.example.com --user=joe
see if the certificate is present (look for "teleport:joe") at the end of the cert
ssh-add -L

Use OpenSSH client

It is possible to use the OpenSSH client ssh to connect to nodes within a Teleport cluster. Teleport supports SSH subsystems and includes a proxy subsystem that can be used like netcat is with ProxyCommand to connect through a jump host.

OpenSSH client configuration may be generated automatically by tsh, or it can be configured manually. In either case, make sure you are running OpenSSH's ssh-agent, and have logged in to the Teleport proxy:

eval `ssh-agent`
tsh --proxy=root.example.com login

ssh-agent will print environment variables into the console. Either eval the output as in the example above, or copy and paste the output into the shell you will be using to connect to a Teleport node. The output exports the SSH_AUTH_SOCK and SSH_AGENT_PID environment variables that allow OpenSSH clients to find the SSH agent.

Automatic Setup

tsh can automatically generate the necessary OpenSSH client configuration to connect using the standard OpenSSH client:

on the machine where you want to run the ssh client
tsh --proxy=root.example.com config

This will generate an OpenSSH client configuration block for the root cluster and all currently-known leaf clusters. Append this to your local OpenSSH config file (usually ~/.ssh/config) using your text editor of choice.

Once configured, log into any node in the root.example.com cluster as any principal listed in your Teleport profile:

ssh [email protected]

If any trusted clusters exist, they are also configured:

ssh [email protected]

When connecting to nodes with Teleport daemons running on non-standard ports (other than 3022), a port may be specified:

ssh -p 4022 [email protected]

Manual Setup

On your client machine, you need to import the public key of Teleport's host certificate. This will allow your OpenSSH client to verify that host certificates are signed by Teleport's trusted host CA:

on the Teleport auth server
tctl auth export --type=host > teleport_host_ca.pub
on the machine where you want to run the ssh client
cat teleport_host_ca.pub >> ~/.ssh/known_hosts

If you have multiple Teleport clusters, you have to export and set up these certificate authorities for each cluster individually.

Lastly, configure the OpenSSH client to use the Teleport proxy when connecting to nodes with matching names. Edit ~/.ssh/config for your user or /etc/ssh/ssh_config for global changes:

# root.example.com is the jump host (proxy). credentials will be obtained from the
# openssh agent.
Host root.example.com
    HostName 192.168.1.2
    Port 3023

# connect to nodes in the root.example.com cluster through the jump
# host (proxy) using the same. credentials will be obtained from the
# openssh agent.
Host *.root.example.com
    HostName %h
    Port 3022
    ProxyCommand ssh -p 3023 %[email protected] -s proxy:%h:%p

# when connecting to a node within a trusted cluster with the name "leaf1.example.com",
# add the name of the cluster to the invocation of the proxy subsystem.
Host *.leaf1.example.com
   HostName %h
   Port 3022
   ProxyCommand ssh -p 3023 %[email protected] -s proxy:%h:%[email protected]

When everything is configured properly, you can use ssh to connect to any node behind root.example.com :

ssh [email protected]

To connect to the OpenSSH server via tsh, add --port=<ssh port> with the tsh ssh command:

Example ssh to database.work.example.com as root with an OpenSSH server on port 22 via tsh:

tsh ssh --port=22 [email protected]

OpenSSH rate limiting

When using a Teleport proxy in "recording mode", be aware of OpenSSH built-in rate-limiting. On large numbers of proxy connections, you may encounter errors like:

channel 0: open failed: connect failed: ssh: handshake failed: EOF

See MaxStartups setting in man sshd_config. This setting means that by default OpenSSH only allows 10 unauthenticated connections at a time and starts dropping connections 30% of the time when the number of connections goes over 10. When it hits 100 authentication connections, all new connections are dropped.

To increase the concurrency level, increase the value to something like MaxStartups 50:30:100. This allows 50 concurrent connections and a max of 100.

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.