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docs	ACL System	docs-guides-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.

ACL System Overview

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

ACL Tokens

The ACL system is based on tokens, which are managed by Consul operators via Consul's ACL API, or systems like HashiCorp's Vault.

Every token has an ID, name, type, and rule set. The ID is a randomly generated UUID, making it infeasible 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. Consul's HTTP endpoints can accept tokens via the token query string parameter, or the X-Consul-Token request header. Consul's CLI commands can accept tokens via the token argument, or the CONSUL_HTTP_TOKEN environment variable.

If no token is provided, the rules associated with a special, configurable anonymous token are automatically applied. The anonymous token is managed using the ACL API like any other ACL token, but using anonymous for the ID.

ACL Rules and Scope

Tokens are bound to a set of rules that control which Consul resources the token has access to. Policies can be defined 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.

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

Policy	Scope
agent	Utility operations in the Agent API, other than service and check registration
event	Listing and firing events in the Event API
key	Key/value store operations in the KV Store API
keyring	Keyring operations in the Keyring API
node	Node-level catalog operations in the Catalog API, Health API, Prepared Query API, Network Coordinate API, and Agent API
operator	Cluster-level operations in the Operator API, other than the Keyring API
query	Prepared query operations in the Prepared Query API
service	Service-level catalog operations in the Catalog API, Health API, Prepared Query API, and Agent API
session	Session operations in the Session API

Since Consul snapshots actually contain ACL tokens, the Snapshot API requires a management token for snapshot operations and does not use a special policy.

The following resources are not covered by ACL policies:

1. The Status API is used by servers when bootstrapping and exposes basic IP and port information about the servers, and does not allow modification of any state.

2. The datacenter listing operation of the Catalog API similarly exposes the names of known Consul datacenters, and does not allow modification of any state.

Constructing rules from these policies is covered in detail in the Rule Specification section below.

ACL Datacenter

All nodes (clients and servers) must be configured with an acl_datacenter which enables ACL enforcement but also specifies the authoritative datacenter. Consul 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 consistency issues, a single datacenter is considered authoritative and stores the canonical set of tokens.

When a request is made to an agent in a non-authoritative datacenter, 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 an RPC call.

Enabling ACLs

Enabling ACLs is done by setting up the following configuration options. These are marked as to whether they are set on servers, clients, or both.

Configuration Option	Servers	Clients	Purpose
acl_datacenter	REQUIRED	REQUIRED	Master control that enables ACLs by defining the authoritative Consul datacenter for ACLs
acl_default_policy	OPTIONAL	N/A	Determines whitelist or blacklist mode
acl_down_policy	OPTIONAL	OPTIONAL	Determines what to do when the ACL datacenter is offline
acl_ttl	OPTIONAL	OPTIONAL	Determines time-to-live for cached ACLs

There are some additional configuration items related to ACL replication and Version 8 ACL support. These are discussed in those respective sections below.

A number of special tokens can also be configured which allow for bootstrapping the ACL system, or accessing Consul in special situations:

Special Token	Servers	Clients	Purpose
acl_agent_master_token	OPTIONAL	OPTIONAL	Special token that can be used to access Agent API when the ACL datacenter isn't available, or servers are offline (for clients); used for setting up the cluster such as doing initial join operations
acl_agent_token	OPTIONAL	OPTIONAL	Special token that is used for an agent's internal operations with the Catalog API; this needs to have at least node policy access so the agent can self update its registration information
acl_master_token	REQUIRED	N/A	Special token used to bootstrap the ACL system, see details below.
acl_token	OPTIONAL	OPTIONAL	Default token to use for client requests where no token is supplied; this is often configured with read-only access to services to enable DNS service discovery on agents

Bootstrapping the ACL system is done by providing an initial acl_master_token which will be created as a "management" type token if it does not exist. 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.

Once the ACL system is bootstrapped, ACL tokens can be managed through the ACL API.

Rule Specification

A core part of the ACL system is the rule language which is used to describe the policy that must be enforced. Most of the ACL rules are prefix-based, allowing operators to define different namespaces within Consul's resource areas like the catalog and key/value store, in order to delegate responsibility for these namespaces. Policies can have several dispositions:

• read: allow the resource to be read but not modified
• write: allow the resource to be read and modified
• deny: do not allow the resource to be read or modified

With prefix-based rules, the most specific prefix match determines the action. This allows for flexible rules like an empty prefix to allow read-only access to all resources, along with some specific prefixes that allow write access or that are denied all access.

We make use of the HashiCorp Configuration Language (HCL) to specify rules. This language is human readable and interoperable with JSON making it easy to machine-generate. Rules can make use of one or more policies.

Specification in the HCL format looks like:

# These control access to the key/value store.
key "" {
  policy = "read"
}
key "foo/" {
  policy = "write"
}
key "foo/private/" {
  policy = "deny"
}

# This controls access to cluster-wide Consul operator information.
operator = "read"

This is equivalent to the following JSON input:

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

The ACL API allows either HCL or JSON to be used to define the content of the rules section.

Here's a sample request using the HCL form:

$ curl \
    --request PUT \
    --data \
'{
  "Name": "my-app-token",
  "Type": "client",
  "Rules": "key \"\" { policy = \"read\" } key \"foo/\" { policy = \"write\" } key \"foo/private/\" { policy = \"deny\" } operator = \"read\""
}' https://consul.rocks/v1/acl/create?token=<management token>

Here's an equivalent request using the JSON form:

$ curl \
    --request PUT \
    --data \
'{
  "Name": "my-app-token",
  "Type": "client",
  "Rules": "{\"key\":{\"\":{\"policy\":\"read\"},\"foo/\":{\"policy\":\"write\"},\"foo/private\":{\"policy\":\"deny\"}},\"operator\":\"read\"}"
}' https://consul.rocks/v1/acl/create?token=<management token>

On success, the token ID is returned:

{
  "ID": "adf4238a-882b-9ddc-4a9d-5b6758e4159e"
}

This token ID can then be passed into Consul's HTTP APIs via the token query string parameter, or the X-Consul-Token request header, or Consul's CLI commands via the token argument, or the CONSUL_HTTP_TOKEN environment variable.

Agent Rules

The agent policy controls access to the utility operations in the Agent API, such as join and leave. All of the catalog-related operations are covered by the node and service policies instead.

Agent rules look like this:

agent "" {
  policy = "read"
}
agent "foo" {
  policy = "write"
}
agent "bar" {
  policy = "deny"
}

Agent rules are keyed by the node name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to any node name with the empty prefix, allow read-write access to any node name that starts with "foo", and deny all access to any node name that starts with "bar".

Since Agent API utility operations may be required before an agent is joined to a cluster, or during an outage of the Consul servers or ACL datacenter, a special token may be configured with acl_agent_master_token to allow write access to these operations even if no ACL resolution capability is available.

Event Rules

The event policy controls access to event operations in the Event API, such as firing events and listing events.

Event rules look like this:

event "" {
  policy = "read"
}
event "deploy" {
  policy = "write"
}

Event rules are keyed by the event name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to any event, and firing of any event that starts with "deploy".

The consul exec command uses events with the "_rexec" prefix during operation, so to enable this feature in a Consul environment with ACLs enabled, you will need to give agents a token with access to this event prefix, in addition to configuring disable_remote_exec to false.

Key/Value Rules

The key policy controls access to key/value store operations in the KV API. Key rules look like this:

key "" {
  policy = "read"
}
key "foo" {
  policy = "write"
}
key "bar" {
  policy = "deny"
}

Key rules are keyed by the key name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to any key name with the empty prefix, allow read-write access to any key name that starts with "foo", and deny all access to any key name that starts with "bar".

Keyring Rules

The keyring policy controls access to keyring operations in the Keyring API.

Keyring rules look like this:

keyring = "write"

There's only one keyring policy allowed per rule set, and its value is set to one of the policy dispositions. In the example above, the keyring may be read and updated.

Node Rules

The node policy controls node-level registration and read access to the Catalog API, service discovery with the Health API, and filters results in Agent API operations like fetching the list of cluster members.

Node rules look like this:

node "" {
  policy = "read"
}
node "app" {
  policy = "write"
}
node "admin" {
  policy = "deny"
}

Node rules are keyed by the node name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to any node name with the empty prefix, allow read-write access to any node name that starts with "app", and deny all access to any node name that starts with "admin".

Agents need to be configured with an acl_agent_token with at least "write" privileges to their own node name in order to register their information with the catalog, such as node metadata and tagged addresses. If this is configured incorrectly, the agent will print an error to the console when it tries to sync its state with the catalog.

Consul's DNS interface is also affected by restrictions on node rules. If the acl_token used by the agent does not have "read" access to a given node, then the DNS interface will return no records when queried for it.

When reading from the catalog or retrieving information from the health endpoints, node rules are used to filter the results of the query. This allows for configurations where a token has access to a given service name, but only on an allowed subset of node names.

Node rules come into play when using the Agent API to register node-level checks. The agent will check tokens locally as a check is registered, and 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:

1. Using the acl_token configuration directive. This allows a single token to be configured globally and used during all check registration operations.
2. 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.

Operator Rules

The operator policy controls access to cluster-level operations in the Operator API, other than the Keyring API.

Operator rules look like this:

operator = "read"

There's only one operator policy allowed per rule set, and its value is set to one of the policy dispositions. In the example above, the token could be used to query the operator endpoints for diagnostic purposes but not make any changes.

Prepared Query Rules

The query policy controls access to create, update, and delete prepared queries in the Prepared Query API. Executing queries is subject to node and service policies, as will be explained below.

Query rules look like this:

query "" {
  policy = "read"
}
query "foo" {
  policy = "write"
}

Query rules are keyed by the query name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to any query name with the empty prefix, and allow read-write access to any query name that starts with "foo". This allows control of the query namespace to be delegated based on ACLs.

There are a few variations when using ACLs with prepared queries, each of which uses ACLs in one of two ways: open, protected by unguessable IDs or closed, managed by ACL policies. These variations are covered here, with examples:

• Static queries with no Name defined are not controlled by any ACL policies. These types of queries are meant to be ephemeral and not shared to untrusted clients, and they are only reachable if the prepared query ID is known. Since these IDs are generated using the same random ID scheme as ACL Tokens, it is infeasible to guess them. When listing all prepared queries, only a management token will be able to see these types, though clients can read instances for which they have an ID. An example use for this type is a query built by a startup script, tied to a session, and written to a configuration file for a process to use via DNS.

• Static queries with a Name defined are controlled by the query ACL policy. Clients are required to have an ACL token with a prefix sufficient to cover the name they are trying to manage, with a longest prefix match providing a way to define more specific policies. Clients can list or read queries for which they have "read" access based on their prefix, and similar they can update any queries for which they have "write" access. An example use for this type is a query with a well-known name (eg. prod-master-customer-db) that is used and known by many clients to provide geo-failover behavior for a database.

• Template queries queries work like static queries with a Name defined, except that a catch-all template with an empty Name requires an ACL token that can write to any query prefix.

When prepared queries are executed via DNS lookups or HTTP requests, the ACL checks are run against the service being queried, similar to how ACLs work with other service lookups. There are several ways the ACL token is selected for this check:

• If an ACL Token was captured when the prepared query was defined, it will be used to perform the service lookup. This allows queries to be executed by clients with lesser or even no ACL Token, so this should be used with care.

• If no ACL Token was captured, then the client's ACL Token will be used to perform the service lookup.

• If no ACL Token was captured and the client has no ACL Token, then the anonymous token will be used to perform the service lookup.

In the common case, the ACL Token of the invoker is used to test the ability to look up a service. If a Token was specified when the prepared query was created, the behavior changes and now the captured ACL Token set by the definer of the query is used when looking up a service.

Capturing ACL Tokens is analogous to PostgreSQL’s SECURITY DEFINER attribute which can be set on functions, and using the client's ACL Token is similar to the complementary SECURITY INVOKER attribute.

Prepared queries were originally introduced in Consul 0.6.0, and ACL behavior remained unchanged through version 0.6.3, but was then changed to allow better management of the prepared query namespace.

These differences are outlined in the table below:

Operation	Version <= 0.6.3	Version > 0.6.3
Create static query without `Name`	The ACL Token used to create the prepared query is checked to make sure it can access the service being queried. This token is captured as the `Token` to use when executing the prepared query.	No ACL policies are used as long as no `Name` is defined. No `Token` is captured by default unless specifically supplied by the client when creating the query.
Create static query with `Name`	The ACL Token used to create the prepared query is checked to make sure it can access the service being queried. This token is captured as the `Token` to use when executing the prepared query.	The client token's `query` ACL policy is used to determine if the client is allowed to register a query for the given `Name`. No `Token` is captured by default unless specifically supplied by the client when creating the query.
Manage static query without `Name`	The ACL Token used to create the query, or a management token must be supplied in order to perform these operations.	Any client with the ID of the query can perform these operations.
Manage static query with a `Name`	The ACL token used to create the query, or a management token must be supplied in order to perform these operations.	Similar to create, the client token's `query` ACL policy is used to determine if these operations are allowed.
List queries	A management token is required to list any queries.	The client token's `query` ACL policy is used to determine which queries they can see. Only management tokens can see prepared queries without `Name`.
Execute query	Since a `Token` is always captured when a query is created, that is used to check access to the service being queried. Any token supplied by the client is ignored.	The captured token, client's token, or anonymous token is used to filter the results, as described above.

Service Rules

The service policy controls service-level registration and read access to the Catalog API and service discovery with the Health API.

Service rules look like this:

service "" {
  policy = "read"
}
service "app" {
  policy = "write"
}
service "admin" {
  policy = "deny"
}

Service rules are keyed by the service name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to any service name with the empty prefix, allow read-write access to any service name that starts with "app", and deny all access to any service name that starts with "admin".

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

When reading from the catalog or retrieving information from the health endpoints, service rules are used to filter the results of the query.

Service rules come into play when using the Agent API to register services or checks. The agent will check tokens locally as a service or check is registered, and 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:

1. Using the acl_token configuration directive. This allows a single token to be configured globally and used during all service and check registration operations.
2. 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.

Session Rules

The session policy controls access to Session API operations.

Session rules look like this:

session "" {
  policy = "read"
}
session "app" {
  policy = "write"
}
session "admin" {
  policy = "deny"
}

Session rules are keyed by the node name prefix they apply to, using the longest prefix match rule. In the example above, the rules allow read-only access to sessions on node name with the empty prefix, allow creating sessions on any node name that starts with "app", and deny all access to any sessions on a node name that starts with "admin".

Advanced Topics

Outages and ACL Replication

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 agents 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.

Consul 0.7 added an ACL Replication capability that can allow non-authoritative datacenter agents to resolve even uncached tokens. This is enabled by setting an acl_replication_token in the configuration on the servers in the non-authoritative datacenters. With replication enabled, the servers will maintain a replica of the authoritative datacenter's full set of ACLs on the non-authoritative servers. The ACL replication token needs to be a valid ACL token with management privileges, it can also be the same as the master ACL token.

Replication occurs with a background process that looks for new ACLs approximately every 30 seconds. Replicated changes are written at a rate that's throttled to 100 updates/second, so it may take several minutes to perform the initial sync of a large set of ACLs.

If there's a partition or other outage affecting the authoritative datacenter, and the acl_down_policy is set to "extend-cache", tokens will be resolved during the outage using the replicated set of ACLs. An ACL replication status endpoint is available to monitor the health of the replication process.

Locally-resolved ACLs will be cached using the acl_ttl setting of the non-authoritative datacenter, so these entries may persist in the cache for up to the TTL, even after the authoritative datacenter comes back online.

ACL replication can also be used to migrate ACLs from one datacenter to another using a process like this:

1. Enable ACL replication in all datacenters to allow continuation of service during the migration, and to populate the target datacenter. Verify replication is healthy and caught up to the current ACL index in the target datacenter using the ACL replication status endpoint.
2. Turn down the old authoritative datacenter servers.
3. Rolling restart the agents in the target datacenter and change the acl_datacenter servers to itself. This will automatically turn off replication and will enable the datacenter to start acting as the authoritative datacenter, using its replicated ACLs from before.
4. Rolling restart the agents in other datacenters and change their acl_datacenter configuration to the target datacenter.

Complete ACL Coverage in Consul 0.8

Consul 0.8 added many more ACL policy types and brought ACL enforcement to Consul agents for the first time. To ease the transition to Consul 0.8 for existing ACL users, there's a configuration option to disable these new features. To disable support for these new ACLs, set the acl_enforce_version_8 configuration option to false on Consul clients and servers.

Here's a summary of the new features:

• Agents now check node and service ACL policies for catalog-related operations in /v1/agent endpoints, such as service and check registration and health check updates.
• Agents enforce a new agent ACL policy for utility operations in /v1/agent endpoints, such as joins and leaves.
• A new node ACL policy is enforced throughout Consul, providing a mechanism to restrict registration and discovery of nodes by name. This also applies to service discovery, so provides an additional dimension for controlling access to services.
• A new session ACL policy controls the ability to create session objects by node name.
• Anonymous prepared queries (non-templates without a Name) now require a valid session, which ties their creation to the new session ACL policy.
• The existing event ACL policy has been applied to the /v1/event/list endpoint.

Two new configuration options are used once version 8 ACLs are enabled:

• acl_agent_master_token is used as a special access token that has agent ACL policy write privileges on each agent where it is configured. This token should only be used by operators during outages when Consul servers aren't available to resolve ACL tokens. Applications should use regular ACL tokens during normal operation.
• acl_agent_token is used internally by Consul agents to perform operations to the service catalog when registering themselves or sending network coordinates to the servers. This token must at least have node ACL policy write access to the node name it will register as in order to register any node-level information like metadata or tagged addresses.

Since clients now resolve ACLs locally, the acl_down_policy now applies to Consul clients as well as Consul servers. This will determine what the client will do in the event that the servers are down.

Consul clients must have acl_datacenter configured in order to enable agent-level ACL features. If this is set, the agents will contact the Consul servers to determine if ACLs are enabled at the cluster level. If they detect that ACLs are not enabled, they will check at most every 2 minutes to see if they have become enabled, and will start enforcing ACLs automatically. If an agent has an acl_datacenter defined, operators will need to use the acl_agent_master_token to perform agent-level operations if the Consul servers aren't present (such as for a manual join to the cluster), unless the acl_down_policy on the agent is set to "allow".

Non-server agents do not need to have the acl_master_token configured; it is not used by agents in any way.