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0057-automatic-user-provisioning.md

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Alex McGrath ([email protected])
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RFD 57 - Automatic user and sudoers provisioning

What

Automatically create non-existing users and optionally add them to sudoers on Teleport nodes. Users will be removed after all sessions have logged out.

Why

Currently, when logging into an SSH node, the user must be pre-created. Adding automatic user and sudoer provisioning would make it so that any Teleport user would be able to login and have the account created automatically without manual intervention.

Details

The following are required for this feature:

  • Ability to automatically provision a Linux user if it's not present on the node.
  • Ability to automatically provision a Linux group if it's not present on the node.
  • Ability to add the provisioned user to existing Linux groups defined in the user traits/role.
  • Ability to add the provisioned user to sudoers.
  • Clean up the provisioned user / sudoers changes upon logout (being careful not to remove pre-existing users).

Config/Role Changes

Several new fields will need to be added to to the role options and allow sections:

kind: role
version: v5
metadata:
  name: example
spec:
  options:
    # Controls whether this role supports auto provisioning of users.
    create_host_user: true
  allow:
    # New field listing Linux groups to assign a provisioned user to.
    # Should support user and identity provider traits like other fields (e.g. "logins")
    host_groups: [ubuntu, "{{internal.groups}}", "{{external.xxx}}"]
    # host_sudoers is a list of entries to be included in a users sudoers file
    host_sudoers: ["{{internal.logins}} ALL=(ALL) ALL", ...]

An individual ssh_service can be configured disable auto user creation with the below config:

ssh_service:
    # when disabled, takes precedence over the role setting
    disable_create_host_user: true

User creation

In order to create users useradd will be executed from teleport after a user has tried to access a Teleport SSH node.

User Groups

When a user is created they will be added to the specified groups from the host_groups field in the role. In addition the user will be added to a special teleport-system group which can be used to indicate that the user was created by teleport and that its safe for it to be deleted. The groups will be created via groupadd at startup if they do not already exist and users will be added to groups via usermod -aG <list of groups> <username>

Valid user/group names

The set of valid names that are valid on Linux varies between distros and are generally more restrictive than the allowed usernames in Teleport. This will require that names containing invalid characters have those characters removed/replaced. Information on the valid characters between Linux distros is available here. The common core of valid characters is ^[a-z][a-z0-9-]{0,30}$.

Adding and removing users from sudoers

Each user with entries in host_sudoers will have a file created in /etc/sudoers.d, with one entry per line.

If a user is in multiple rules that specify host_sudoers they will be all be concatenated together.

sudoers file syntax validation

If a system has visudo present, validation could be performed by executing visudo -c -f path/to/sudoersfile, where if it fails to validate, the user fails to have the shell start and the error is reported.

sudoers security considerations

In order to stop users from being able to edit the sudoers file a command allow list must be used, as or equivalent to below:

${USERNAME} ALL = (${USER TO RUN AS}) NOPASSWD: /bin/cmd1 args, /bin/cmd2 args

Should a user be given root access to all commands, they will be able to modify any file, including sudoers files.

User and group deletion

After all of a users sessions are logged out the created user and any sudoers files that were created for that user will be deleted if that user is also a member of the teleport-system group.

Users can not be deleted while they have running processes so each time a session ends, an attempt to delete the user can happen, if it succeeds the sudoers file can also be removed.

If it does not succeed a cleanup process will run every 5 minutes, that will attempt to delete users if they no longer have running processes. This clean up process will also ensure that users with running sessions during a restart will be cleaned up appropriately.

Groups will not be cleaned up and will be created once and be reused this is to avoid files created with specified groups will remain accessible between sessions to users in those groups.

Multiple matching roles

Automatic user provisioning will require that all roles matching a node via labels have create_host_user=true

UX Examples

Teleport admin wants each user to have a dedicated host user defined by their Okta attributes

kind: role
version: v5
metadata:
  name: auto-user-groups
spec:
  options:
    # allow auto provisioning of users.
    create_host_user: true
  allow:
    # username from external okta attribute
    logins: [ "{{external.username}}" ]

Teleport admin wants to define which Linux groups each auto-created user will be added to

kind: role
version: v5
metadata:
  name: auto-user-groups
spec:
  options:
    # allow auto provisioning of users.
    create_host_user: true
  allow:
    # List of each group the user will be added to
    host_groups: [ubuntu, docker, ...]
    # username from external okta attribute
    logins: [ "{{external.username}}" ]

Teleport admin wants to make each auto-created user a sudoer

kind: role
version: v5
metadata:
  name: users-as-sudoers
spec:
  options:
    # allow auto provisioning of users.
    create_host_user: true
  allow:
    # add users to the wheel group
    host_groups: [wheel]
    # make it so users in the wheel group will be able to execute sudoers commands without a password
    host_sudoers: ["%wheel ALL=(ALL) NOPASSWD: ALL"]

Teleport admin wants to define particular commands user will be able to run as root

kind: role
version: v5
metadata:
  name: specify-commands-as-sudoers
spec:
  options:
    # allow auto provisioning of users.
    create_host_user: true
  allow:
    # make it so this specific user can execute `systemctl restart nginx.service `
    host_sudoers: ["{{internal.logins}} ALL = (root) NOPASSWD: /usr/bin/systemctl restart nginx.service"]

Teleport admin wants to prohibit some nodes from auto-creating users

Include the below config for the Teleport node that should not allow automatic user creation:

ssh_service:
  enabled: "yes"
  # stops a specific node from auto-creating users
  disable_create_host_user: true

Nodes where diable_create_host_user is false will still be able to have users be automatically created.

Teleport user has multiple roles but not all of them enable create_host_user

In the situtation where a user has roles as below, the user would not be able to make use of automatically provisioning users as both roles do not enable create_host_user.

kind: role
version: v5
metadata:
  name: allow-access-and-auto-create
spec:
  options:
    # allow auto provisioning of users.
    create_host_user: true
    node_labels:
      - 'env': 'example'	
kind: role
version: v5
metadata:
  name: specify-commands-as-sudoers
spec:
  options:
    node_labels:
      - 'env': 'example'