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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. In Consul 0.6, ACL's were further extended to restrict the service discovery mechanisms and user events..

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.

Blacklist mode and Service Discovery

If your acl_default_policy is set to deny, the anonymous token will be unable to read any service information. This will cause the service discovery mechanisms in the REST API and the DNS interface to return no results for any service queries. This is because internally the API's and DNS interface consume the RPC interface, which will filter results for services the token has no access to.

You can allow all services to be discovered, mimicing the behavior of pre-0.6.0 releases, by configuring this ACL rule for the anonymous token:

service "" {
    policy = "read"
}

Note that the above will allow access for reading service information only. This level of access allows discovering other services in the system, but is not enough to allow the agent to sync its services and checks into the global catalog during anti-entropy.

The most secure way of handling service registration and discovery is to run Consul 0.6+ and issue tokens with explicit access for the services or service prefixes which are expected to run on each agent.

Blacklist mode and Events

Similar to the above, if your acl_default_policy is set to deny, the anonymous token will have no access to allow firing user events. This deviates from pre-0.6.0 builds, where user events were completely unrestricted.

Events have their own first-class expression in the ACL syntax. To restore access to user events from arbitrary agents, configure an ACL rule like the following for the anonymous token:

event "" {
    policy = "write"
}

As always, the more secure way to handle user events is to explicitly grant access to each API token based on the events they should be able to fire.

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 longest-prefix match policy (this was an exact match in 0.5, but changed in 0.5.1). 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. The "read" policy in a service ACL rule allows restricting access to the discovery of that service prefix. More information about service discovery and ACLs can be found below.

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

User event policies are defined by coupling an event name prefix with a policy. The rules are enforced using a longest-prefix match policy. The default rule, applied to any user event without a matching policy, is provided by an empty string. An event policy is one of "read", "write", or "deny". Currently, only the "write" level is enforced during event firing. Events can always be read.

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. Also permits all
# services to be discovered.
service "" {
    policy = "write"
}

# Deny registration access to services prefixed "secure-".
# Discovery of the service is still allowed in read mode.
service "secure-" {
    policy = "read"
}

# Allow firing any user event by default.
event "" {
    policy = "write"
}

# Deny firing events prefixed with "destroy-".
event "destroy-" {
    policy = "deny"
}

This is equivalent to the following JSON input:

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

Services and Checks with ACLs

Consul allows configuring ACL policies which may control access to service and check registration. In order to successfully register a service or check with these types of policies in place, a token with sufficient privileges must be provided to perform the registration into the global catalog. Consul also performs periodic anti-entropy syncs, which may require an ACL token to complete. To accommodate this, Consul provides two methods of configuring ACL tokens to use for registration events:

Using the acl_token configuration directive. This allows a single token to be configured globally and used during all service and check registration operations.
Providing an ACL token with service and check definitions at registration time. This allows for greater flexibility and enables the use of multiple tokens on the same agent. Examples of what this looks like are available for both services and checks. Tokens may also be passed to the HTTP API for operations that require them.

Restricting service discovery with ACLs

In Consul 0.6, the ACL system was extended to support restricting read access to service registrations. This allows tighter access control and limits the ability of a compromised token to discover other services running in a cluster.

The ACL system permits a user to discover services using the REST API or UI if the token used during requests has "read"-level access or greater. Consul will filter out all services which the token has no access to in all API queries, making it appear as though the restricted services do not exist.

Consul's DNS interface is also affected by restrictions to service registrations. If the token used by the agent does not have access to a given service, then the DNS interface will return no records when queried for it.

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