Trusted Clusters

If you haven't already looked at the introduction to Trusted Clusters in the Admin Guide we recommend you review that for an overview before continuing with this guide.

The Trusted Clusters chapter in the Admin Guide offers an example of a simple configuration which:

  • Uses a static cluster join token defined in a configuration file.
  • Does not cover inter-cluster role based access control (RBAC).

This guide's focus is on more in-depth coverage of trusted clusters features and will cover the following topics:

  • How to add and remove trusted clusters using CLI commands.
  • Enable/disable trust between clusters.
  • Establish permissions mapping between clusters using Teleport roles.

Introduction

As explained in the architecture document, Teleport can partition compute infrastructure into multiple clusters. A cluster is a group of SSH nodes connected to the cluster's auth server acting as a certificate authority (CA) for all users and nodes.

To retrieve an SSH certificate, users must authenticate with a cluster through a proxy server. So, if users want to connect to nodes belonging to different clusters, they would normally have to use different --proxy flags for each cluster. This is not always convenient.

The concept of trusted clusters allows Teleport administrators to connect multiple clusters together and establish trust between them. Trusted clusters allow users of one cluster to seamlessly SSH into the nodes of another cluster without having to "hop" between proxy servers. Moreover, users don't even need to have a direct connection to other clusters' proxy servers. The user experience looks like this:

# login using the "main" cluster credentials:
$ tsh login --proxy=main.example.com

# SSH into some host inside the "main" cluster:
$ tsh ssh host

# SSH into the host located in another cluster called "east"
# The connection is established through main.example.com:
$ tsh --cluster=east ssh host

# See what other clusters are available
$ tsh clusters

Trusted clusters also have their own restrictions on user access, i.e. permissions mapping takes place.

Join Tokens

Lets start with the diagram of how connection between two clusters is established:

Tunnels

The first step in establishing a secure tunnel between two clusters is for the trusting cluster "east" to connect to the trusted cluster "main". When this happens for the first time, clusters know nothing about each other, thus a shared secret needs to exist in order for "main" to accept the connection from "east".

This shared secret is called a "join token". There are two ways to create join tokens: to statically define them in a configuration file, or to create them on the fly using tctl tool.

Important

It is important to realize that join tokens are only used to establish the connection for the first time. The clusters will exchange certificates and won't be using the token to re-establish the connection in the future.

Static Tokens

To create a static join token, update the configuration file on "main" cluster to look like this:

# fragment of /etc/teleport.yaml:
auth_service:
  enabled: true
  tokens:
  - trusted_cluster:join-token

This token can be used unlimited number of times.

Dynamic Tokens

Creating a token dynamically with a CLI tool offers the advantage of applying a time to live (TTL) interval on it, i.e. it will be impossible to re-use such token after a specified period of time.

To create a token using the CLI tool, execute this command on the auth server of cluster "main":

$ tctl nodes add --ttl=5m --roles=trustedcluster --token=join-token

Users of Teleport will recognize that this is the same way you would add any node to a cluster.

  • The token created above can be used multiple times and has an expiration time of 5 minutes.
  • If you omit the --token flag tctl will generate one for you.

Security Implications

Consider the security implications when deciding which token method to use. Short lived tokens decrease the window for attack but make automation a bit more complicated.

RBAC

Version Warning

The RBAC section is applicable only to Teleport Enterprise. The open source version does not suppport SSH roles.

When a trusting cluster "east" from the diagram above establishes trust with the trusted cluster "main", it needs a way to configure which users from "main" should be allowed in and what permissions should they have. Teleport Enterprise uses role mapping to achieve this.

Consider the following:

  • Both clusters "main" and "east" have their own locally defined roles.
  • Every user in Teleport Enterprise is assigned a role.
  • When creating a trusted cluster resource, the administrator of "east" must define how roles from "main" map to roles on "east".

Example

Lets make a few assumptions for this example:

  • The cluster "main" has two roles: user for regular users and admin for local administrators.

  • We want administrators from "main" (but not regular users!) to have restricted access to "east". We want to deny them access to machines with "environment=production" label.

First, we need to create a special role for main users on "east":

# save this into main-user-role.yaml on the east cluster and execute:
# tctl create main-user-role.yaml
- kind: role
  version: v3
  metadata:
    name: mainuser
  spec:
    allow:
      node_labels:
        '*': '*'
    deny:
      node_labels:
        "environment": "production"

Now, we need to establish trust between roles "main:admin" and "east:mainuser". This is done by creating a trusted cluster resource on "east" which looks like this:

# save this as main-cluster.yaml on the auth server of "east" and then execute:
# tctl create main-cluster.yaml
kind: trusted_cluster
version: v1
metadata:
  name: "main"
spec:
  enabled: true
  role_map:
    - remote: "admin"
      local: [mainuser]
  token: "join-token"
  tunnel_addr: main.example.com:3024
  web_proxy_addr: main.example.com:3080

What if we wanted to let any user from "main" to be allowed to connect to nodes on "east"? In this case we can use a wildcard * in the role_map like this:

role_map:
  - remote: "*"
    local: [mainuser]

Using Trusted Clusters

Now an admin from the main cluster can now see and access the "east" cluster:

# login into the main cluster:
$ tsh --proxy=proxy.main login admin
# see the list of available clusters
$ tsh clusters

Cluster Name   Status
------------   ------
main           online
east           online
# see the list of machines (nodes) behind the eastern cluster:
$ tsh --cluster=east ls

Node Name Node ID            Address        Labels
--------- ------------------ -------------- -----------
db1.east  cf7cc5cd-935e-46f1 10.0.5.2:3022  role=db-master
db2.east  3879d133-fe81-3212 10.0.5.3:3022  role=db-slave
# SSH into any node in "east":
$ tsh --cluster=east ssh [email protected]

Note

Trusted clusters work only one way. So, in the example above users from "east" cannot see or connect to the nodes in "main".

Disabling Trust

To temporarily disable trust between clusters, i.e. to disconnect the "east" cluster from "main", edit the YAML definition of the trusted cluster resource and set enabled to "false", then update it:

$ tctl create --force cluster.yaml

How does it work?

At a first glance, Trusted Clusters in combination with RBAC may seem complicated. However, it is based on certificate-based SSH authentication which is fairly easy to reason about:

One can think of an SSH certificate as a "permit" issued and time-stamped by a certificate authority. A certificate contains four important pieces of data:

  • List of allowed UNIX logins a user can use. They are called "principals" in the certificate.
  • Signature of the certificate authority who issued it (the auth server)
  • Metadata (certificate extensions): additional data protected by the signature above. Teleport uses the metadata to store the list of user roles and SSH options like "permit-agent-forwarding".
  • The expiration date.

Try executing tsh status right after tsh login to see all these fields in the client certificate.

When a user from "main" tries to connect to a node inside "east" cluster, her certificate is presented to the auth server of "east" and it performs the following checks:

  • Checks that the certificate signature matches one of the trusted clusters.
  • Tries to find a local role which maps to the list of principals found in the certificate.
  • Checks if the local role allows the requested identity (UNIX login) to have access.
  • Checks that the certificate is not expired.

Troubleshooting

There are three common types of problems Teleport administrators can run into when configuring trust between two clusters:

  • HTTPS configuration: when the main cluster uses a self-sgined or invalid HTTPS certificate.

  • Connectivity problems: when a trusting cluster "east" does not show up in tsh clusters output on "main".

  • Access problems: when users from "main" get "access denied" error messages trying to connect to nodes on "east".

HTTPS configuration

If the web_proxy_addr endpoint of the main cluster uses a self-signed or invalid HTTPS certificate, you will get an error: "the trusted cluster uses misconfigured HTTP/TLS certificate". For ease of testing the teleport daemon of "east" can be started with --insecure CLI flag to accept self-signed certificates. Make sure to configure HTTPS properly and remove the insecure flag for production use.

Connectivity Problems

To troubleshoot connectivity problems, enable verbose output for the auth servers on both clusters. Usually this can be done by adding --debug flag to teleport start --debug. You can also do this by updating the configuration file for both auth servers:

# snippet from /etc/teleport.yaml
teleport:
  log:
    output: stderr
    severity: DEBUG

On systemd-based distributions you can watch the log output via:

$ sudo journalctl -fu teleport

Most of the time you will find out that either a join token is mismatched/expired, or the network addresses for tunnel_addr or web_proxy_addr cannot be reached due to pre-existing firewall rules or how your network security groups are configured on AWS.

Access Problems

Troubleshooting access denied messages can be challenging. A Teleport administrator should check to see the following:

  • Which roles a user is assigned on "main" when they retreive their SSH certificate via tsh login. You can inspect the retreived certificate with tsh status command on the client side.
  • Which roles a user is assigned on "east" when the role mapping takes place. The role mapping result is reflected in the Teleport audit log. By default, it is stored in /var/lib/teleport/log on a auth server of a cluster. Check the audit log messages on both clusters to get answers for the questions above.