No More Backdoors: Know Who Has Access to What, Right Now
Jun 13
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Machine ID Architecture

This section provides an overview of Teleport Machine ID's inner workings.

The initial specification and design for Machine ID can be found in the Request For Discussion.

What is a bot?

Within Teleport, "bot" refers to a special user that is intended to be used by a machine. Bots are similar to normal users, but do not authenticate using static username/password credentials.

A bot does not exist as a single distinct resource within Teleport. Instead, they comprise three linked resources. These are:

  • Bot user: this will be the user that the Machine ID agent authenticates as.
  • Bot role: the bot user is assigned the bot role, and the bot role contains various permissions that the bot will need to function. For example, the ability to watch the certificate authorities and the ability to impersonate roles.
  • Token: for onboarding, a token must exist that allows the Machine ID agent to initially authenticate as the bot user. If an existing token is not specified, then a single-use token will be created by the Auth Server.

The creation of these resources is managed by tctl bots add.

It is important to recognize the distinction between the bot and an instance of tbot. This is because this is not always a one-to-one relationship, in many cases there may be multiple tbot's using the same bot identity.

Role Impersonation

Role Impersonation is an RBAC feature of Teleport that is used heavily by Machine ID.

Role Impersonation allows a user to generate credentials with a set of requested roles. The user does not have to hold these roles, but must have been granted permission to impersonate them. The impersonated credentials still include the username of the user that generated them, so actions can be attributed to the user.

These credentials can then be used to complete any action that is allowed by the role's configured permissions.

In the case of Machine ID, the bot user is assigned a bot role, which includes permissions to impersonate the roles that the user has configured.

tbot

tbot is the binary that acts as the agent for Machine ID on your machines that need access to resources protected by Teleport. It is typically ran in one of two modes. By default, it is a daemon-like long-running process. This is suitable for situations where your machine is long running and will need continuous access to resources. tbot can also run in "oneshot" mode where it will fetch credentials for your machine once before exiting. This is ideal for short-lived environments such as CI/CD workflows.

Before tbot can be started, at least two parts of configuration will need to be provided via the configuration file or as arguments provided to tbot when it is executed. This consists of:

  • A join method that the bot can use to prove that it should be allowed to join the Teleport cluster.
  • A series of outputs. An output consists of the configuration settings that specify where a set of credentials should be output, and any options that should be applied to those credentials (for example, what roles should be impersonated).

For more detail about the configuration options, see the reference.

On initial load, tbot uses the configured join method to obtain a set of credentials for the bot user from the Teleport Auth Service. It can then use these credentials to communicate with the Teleport Auth Service as the bot.

Then on a configured regular period tbot begins its renewal process. It begins by refreshing the bot's own credentials, by renewing them or fetching a fresh set of credentials depending on the configured onboarding method.

For each output provided in the tbot configuration, the tbot program uses impersonation to obtain credentials from the Auth Service for the roles specified in the configuration for that output. After tbot fetches the credentials for its roles, they are then persisted to the output's destination in various formats along with other useful artifacts such as the current certificate authority certificates.

Concurrently to this, tbot monitors the Teleport certificate authorities to detect certificate rotations. When this occurs, it triggers additional renewals to ensure that output destinations continue to have certificates that are signed by the latest certificate authority.

Joining and Authentication

Joining is the process by which tbot initially authenticates as a bot with the Teleport Auth Service.

Machine ID leverages the existing token resource within Teleport, with the token containing an additional botName field that identifies the bot user associated with the token.

Machine ID currently supports two methods of joining that have some key differences.

Ephemeral token

  • The name of the token is used as an opaque secret needed to join the Teleport cluster. This means it must be stored and communicated securely.
  • Once used, the token resource self-destructs. This means it can only be used to join a single bot to a Teleport cluster.

As these tokens can only be used once, the certificates that are issued when using an ephemeral token are renewable. This allows the short-lived certificate to be used to request new short-lived certificate.

In order to mitigate the risk of bot user credentials being stolen, and then continually renewed by a malicious actor, renewable bot user certificates include a generation counter.

The generation counter is stored against the user in the database and within the certificate. This counter is incremented each time the user renews their certificate. When a bot attempts to renew, the Auth Server ensures that the value within the certificate and in the database match. If they do not match, then the bot user is automatically locked. This means that if certificates are stolen, and attempted to be renewed whilst the bot is still running, the next renewal will render them useless.

Dynamic join tokens (e.g AWS IAM)

  • These tokens rely on an external authority that allows the bot to prove it is allowed to join the cluster. The name of the token identifies the Token resource in Teleport that contains the configuration.
  • The token can be used to join as many bots as you want, and do not self destruct in the same way that ephemeral tokens do.
  • The certificates exchanged for the token are not renewable. When the bot wants to renew its certificates, it simply repeats the original join steps.

Where possible, you should prefer to use a dynamic join token over an ephemeral token as this eliminates the need to handle a secret.

File permissions

There are two types of folder in use by tbot:

  • The bot's own files: these store credentials belonging to the tbot process itself. As these credentials are potentially renewable, and will allow the impersonation of any roles you have assigned to the bot user, they should be treated as exceptionally sensitive. The bot's own files are stored by default at /var/lib/teleport/bot/.
  • Output destinations: when a directory destination is configured, the bot outputs the role impersonated credentials as files in the specified directory.

It is important that we ensure that these files can only be accessed by the fewest number of Linux processes and users on the system as is necessary.

In the case of the bot's own files, it is best practice to create a Linux user specifically for running tbot and to ensure that only this user has access to this directory.

In the case of directory destinations, the process the bot runs as requires read and write permissions, and processes that will need the credentials output by the bot require read permissions. We recommend that you create a Linux user specific to the process that needs to access these files. When using tbot init, specify this Linux user as the "reader" to grant it access to the destination.

In addition to basic POSIX filesystem permissions, tbot init also sets up Linux ACLs if the system supports it. This allows more granular control by granting individual users access.

Finally, on systems that support it, tbot will by default attempt to prevent the resolution of symbolic links when reading and writing files. This prevents a class of attacks sometimes known as symlink attacks. This behaviour can be disabled using the insecure symlink option when configuring your destination.