This topic describes provides an overview of the optional access control list (ACL) system shipped with Consul. The ACL system authenticates requests and authorizes access to resources. It is used by the UI, API, and CLI for service-to-service communication and agent-to-agent communication.
This topic describes core concepts associated with the optioal access control list (ACL) system shipped with Consul. ACLs authenticate requests and authorize access to resources. They also control access to the Consul UI, API, and CLI, as well as secure service-to-service and agent-to-agent communication.
Implementations may vary depending on the needs of the organization, but the following procedure describes the basic workflow for for creating and implementing ACLs:
1. The person responsible for administrating ACLs in your organization specifies one or more authentication rules to define a [policy](#policies).
1. The ACL administrator uses the Consul API to generate and link a [token](#tokens) to one or more policies. The following diagram illustrates the relationship between rules, policies, and tokens:
The ACL administrator can create and link additional artifacts to tokens, such as [service identities](#service-identities), [node identities](#node-identities), and [roles](#roles) that enable policies to accommodate more complex requirements.
ACL tokens are the core method of authentication in Consul. Tokens contain several attributes, but the value of the `SecretID` field (sometimes referred to as the ACL token) is the attribute that you or your service must include to identify the person or system making the request. Your ACL administrator may also use the token's `AccessorID` for audit logging purposes.
A role is a collection of policies that your ACL administrator can link to a token.
They enable you to reuse policies by decoupling the policies from the token distributed to team members.
Instead, the token is linked to the role, which is able to hold several policies that can be updated asynchronously without distributing new tokens to users.
As a result, roles can provide a more convenient authentication infrastrcture than creating unique policies and tokens for each requester.
Service identities are configuration blocks that you can add to role configurations or specify when linking tokens to policies. The are used during the authorization process to automatically generate a policy for the service(s) specifed. The policy will be linked to the role or token so that the service(s) can _be discovered_ and _discover other healthy service instances_ in a service mesh.
Node identities are configuration blocks that you can add to role configurations or specify when linking tokens to policies. The are used during the authorization process to automatically generate a policy for the node(s) specifed. You can specify the token linked to the policy in the [`acl_tokens_agent`](/docs/agent/options#acl_tokens_agent) field when configuring the agent.
Node identities enable you to quickly construct policies for nodes, rather than creating identical polices for each service.
