Nomad ACL system fundamentals

The ACL system is designed to be intuitive, high-performance, and to provide administrative insight. At the highest level, there are three major components to the ACL system: tokens, policies, and capabilities. The components are illustrated in the diagram below.

Core objects overview

• Tokens - Requests to Nomad are authenticated using a bearer token. Each ACL token has a public Accessor ID which is used to name a token and a Secret ID which is used to make requests to Nomad. The Secret ID is provided using a request header (X-Nomad-Token) and is used to authenticate the caller. Tokens are either management or client types. The management tokens are effectively "root" in the system and can perform any operation. The client tokens are associated with one or more ACL policies which grant specific capabilities.

• Policies - Policies consist of a set of rules defining the capabilities or actions to be granted. For example, a "readonly" policy might only grant the ability to list and inspect running jobs, but not to submit new ones. No permissions are granted by default, making Nomad a default-deny system.

  • Rules - Policies are comprised of one or more rules. The rules define the capabilities of a Nomad ACL token for accessing the objects in a Nomad cluster, objects like namespaces, node, agent, operator, quota, etc. The full set of rules are discussed in a later section.

• Capabilities - Capabilities are the set of actions that can be performed. This includes listing jobs, submitting jobs, querying nodes, etc. A management token is granted all capabilities, while client tokens are granted specific capabilities via ACL policies. The full set of capabilities is discussed in the rule specifications.

ACL policies

An ACL policy is a named set of rules. Each policy must have a unique name, an optional description, and a rule set. A client ACL token can be associated with multiple policies; a request is allowed if any of the associated policies grant the capability. Management tokens cannot be associated with policies because they are granted all capabilities.

The special anonymous policy can be defined to grant capabilities to anonymous requests. An anonymous request is a request made to Nomad without the X-Nomad-Token header specified. This can be used to allow anonymous users to list jobs and view their status, while requiring authenticated requests to submit new jobs or modify existing jobs. By default, there is no anonymous policy set meaning all anonymous requests are denied.

ACL tokens

ACL tokens are used to authenticate requests and determine if the caller is authorized to perform an action. Each ACL token has a public Accessor ID which is used to identify the token, a Secret ID which is used to make requests to Nomad, and an optional human readable name. All client type tokens are associated with one or more policies and can perform an action if any associated policy allows it. Tokens can be associated with policies which do not exist, which are the equivalent of granting no capabilities. The management type tokens cannot be associated with policies, but can perform any action.

When ACL tokens are created, they can be optionally marked as Global. This causes them to be created in the authoritative region and replicated to all other regions. Otherwise, tokens are created locally in the region the request was made and not replicated. Local tokens cannot be used for cross-region requests since they are not replicated between regions.

Rules and scope

The following table summarizes the rules that are available for constructing ACL policies:

Constructing policies from these rules is covered in detail in the "Nomad ACL Policy Concepts" tutorial later in this collection.

Multi-Region configuration

Nomad supports multi-datacenter and multi-region configurations. A single region is able to service multiple datacenters, and all servers in a region replicate their state between each other. In a multi-region configuration, there is a set of servers per region. Each region operates independently and is loosely coupled to allow jobs to be scheduled in any region and requests to flow transparently to the correct region.

When ACLs are enabled, Nomad depends on an "authoritative region" to act as a single source of truth for ACL policies and global ACL tokens. The authoritative region is configured in the server stanza of agents, and all regions must share a single authoritative source. Any ACL policies or global ACL tokens are created in the authoritative region first. All other regions replicate ACL policies and global ACL tokens to act as local mirrors. This allows policies to be administered centrally, and for enforcement to be local to each region for low latency.

Global ACL tokens are used to allow cross-region requests. Standard ACL tokens are created in a single target region and not replicated. This means if a request takes place between regions, global tokens must be used so that both regions will have the token registered.