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docs	ACL System	docs-internals-acl	Consul provides an optional Access Control List (ACL) system which can be used to control access to data and APIs. The ACL system is a Capability-based system that relies on tokens which can have fine grained rules applied to them. It is very similar to AWS IAM in many ways.

ACL System

Consul provides an optional Access Control List (ACL) system which can be used to control access to data and APIs. The ACL is Capability-based, relying on tokens to which fine grained rules can be applied. It is very similar to AWS IAM in many ways.

Scope

When the ACL system was launched in Consul 0.4, it was only possible to specify policies for the KV store. In Consul 0.5, ACL policies were extended to service registrations.

ACL Design

The ACL system is designed to be easy to use, fast to enforce, and flexible to new policies, all while providing administrative insight.

Every token has an ID, name, type, and rule set. The ID is a randomly generated UUID, making it unfeasible to guess. The name is opaque to Consul and human readable. The type is either "client" (meaning the token cannot modify ACL rules) or "management" (meaning the token is allowed to perform all actions).

The token ID is passed along with each RPC request to the servers. Agents can be configured with an acl_token property to provide a default token, but the token can also be specified by a client on a per-request basis. ACLs were added in Consul 0.4, meaning prior versions do not provide a token. This is handled by the special "anonymous" token. If no token is provided, the rules associated with the anonymous token are automatically applied: this allows policy to be enforced on legacy clients.

Enforcement is always done by the server nodes. All servers must be configured to provide an acl_datacenter which enables ACL enforcement but also specifies the authoritative datacenter. Consul does not replicate data cross-WAN and instead relies on RPC forwarding to support Multi-Datacenter configurations. However, because requests can be made across datacenter boundaries, ACL tokens must be valid globally. To avoid replication issues, a single datacenter is considered authoritative and stores all the tokens.

When a request is made to a server in a non-authoritative datacenter server, it must be resolved into the appropriate policy. This is done by reading the token from the authoritative server and caching the result for a configurable acl_ttl. The implication of caching is that the cache TTL is an upper bound on the staleness of policy that is enforced. It is possible to set a zero TTL, but this has adverse performance impacts, as every request requires refreshing the policy via a cross-datacenter WAN call.

The Consul ACL system is designed with flexible rules to accommodate for an outage of the acl_datacenter or networking issues preventing access to it. In this case, it may be impossible for servers in non-authoritative datacenters to resolve tokens. Consul provides a number of configurable acl_down_policy choices to tune behavior. It is possible to deny or permit all actions or to ignore cache TTLs and enter a fail-safe mode. The default is to ignore cache TTLs for any previously resolved tokens and to deny any uncached tokens.

ACLs can also act in either a whitelist or blacklist mode depending on the configuration of acl_default_policy. If the default policy is to deny all actions, then token rules can be set to whitelist specific actions. In the inverse, the allow all default behavior is a blacklist where rules are used to prohibit actions. By default, Consul will allow all actions.

Blacklist mode and consul exec

If you set acl_default_policy to deny, the anonymous token won't have permission to read the default _rexec prefix; therefore, Consul agents using the anonymous token won't be able to perform consul exec actions.

Here's why: the agents need read/write permission to the _rexec prefix for consul exec to work properly. They use that prefix as the transport for most data.

You can enable consul exec from agents that are not configured with a token by allowing the anonymous token to access that prefix. This can be done by giving this rule to the anonymous token:

key "_rexec/" {
    policy = "write"
}

Alternatively, you can, of course, add an explicit acl_token to each agent, giving it access to that prefix.

Bootstrapping ACLs

Bootstrapping the ACL system is done by providing an initial acl_master_token configuration which will be created as a "management" type token if it does not exist. Note that the acl_master_token is only installed when a server acquires cluster leadership. If you would like to install or change the acl_master_token, set the new value for acl_master_token in the configuration for all servers. Once this is done, restart the current leader to force a leader election.

Rule Specification

A core part of the ACL system is a rule language which is used to describe the policy that must be enforced. Consul supports ACLs for both K/Vs and services.

Key policies are defined by coupling a prefix with a policy. The rules are enforced using a longest-prefix match policy: Consul picks the most specific policy possible. The policy is either "read", "write", or "deny". A "write" policy implies "read", and there is no way to specify write-only. If there is no applicable rule, the acl_default_policy is applied.

Service policies are defined by coupling a service name and a policy. The rules are enforced using an exact match policy. The default rule, applied to any service that doesn't have a matching policy, is provided using the empty string. A service policy is either "read", "write", or "deny". A "write" policy implies "read", and there is no way to specify write-only. If there is no applicable rule, the acl_default_policy is applied. Currently, only the "write" level is enforced for registration of services; services can always be read.

The policy for the "consul" service is always "write" as it is managed internally by Consul.

We make use of the HashiCorp Configuration Language (HCL) to specify policy. This language is human readable and interoperable with JSON making it easy to machine-generate.

Specification in the HCL format looks like:

# Default all keys to read-only
key "" {
  policy = "read"
}
key "foo/" {
  policy = "write"
}
key "foo/private/" {
  # Deny access to the dir "foo/private"
  policy = "deny"
}

# Default all services to allow registration
service "" {
    policy = "write"
}

service "secure" {
    # Deny registration access to service named "secure"
    policy = "read"
}

This is equivalent to the following JSON input:

{
  "key": {
    "": {
      "policy": "read"
    },
    "foo/": {
      "policy": "write"
    },
    "foo/private": {
      "policy": "deny"
    }
  },
  "service": {
      "": {
          "policy": "write"
      },
      "secure": {
          "policy": "read"
      }
  }
}