» 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 which are associated with policies to determine which fine grained rules can be applied. Consul's capability based ACL system is very similar to the design of AWS IAM.

» ACL System Overview

The ACL system is designed to be easy to use and fast to enforce while providing administrative insight. At the highest level, there are two major components to the ACL system:

  • ACL Policies - Policies allow grouping of a set of rules into a logical unit that can be reused and linked with many tokens.

  • ACL Tokens - Requests to Consul are authorized by using bearer token. Each ACL token has a public Accessor ID which is used to name a token, and a Secret ID which is used as the bearer token used to make requests to Consul.

ACL Tokens and Policies are managed by Consul operators via Consul's ACL API, ACL CLI or systems like HashiCorp's Vault.

» ACL Policies

An ACL policy is a named set of rules and is composed of the following elements:

  • ID - The policies auto-generated public identifier.
  • Name - A unique meaningful name for the policy.
  • Rules - Set of rules granting or denying permissions. See the Rule Specification section for more details.
  • Datacenters - A list of datacenters the policy is valid within.

» Builtin Policies

  • Global Management - Grants unrestricted privileges to any token that uses it. When created it will be named global-management and will be assigned the reserved ID of 00000000-0000-0000-0000-000000000001. This policy can be renamed but modification of anything else including the rule set and datacenter scoping will be prevented by Consul.

» ACL Tokens

ACL tokens are used to determine if the caller is authorized to perform an action. An ACL Token is composed of the following elements:

  • Accessor ID - The token's public identifier.
  • Secret ID -The bearer token used when making requests to Consul.
  • Description - A human readable description of the token. (Optional)
  • Policy Set - The list of policies that are applicable for the token.
  • Locality - Indicates whether the token should be local to the datacenter it was created within or created in the primary datacenter and globally replicated.

» Builtin Tokens

During cluster bootstrapping when ACLs are enabled both the special anonymous and the master token will be injected.

  • Anonymous Token - The anonymous token is used when a request is made to Consul without specifying a bearer token. The anonymous token's description and policies may be updated but Consul will prevent this tokens deletion. When created, it will be assigned 00000000-0000-0000-0000-000000000002 for its Accessor ID and anonymous for its Secret ID.

  • Master Token - When a master token is present within the Consul configuration, it is created and will be linked With the builtin Global Management policy giving it unrestricted privileges. The master token is created with the Secret ID set to the value of the configuration entry.

» Authorization

The token Secret ID is passed along with each RPC request to the servers. Consul's HTTP endpoints can accept tokens via the token query string parameter, the X-Consul-Token request header, or Authorization Bearer token RFC6750. Consul's CLI commands can accept tokens via the token argument, or the CONSUL_HTTP_TOKEN environment variable.

If no token is provided for an HTTP request then Consul will use the default ACL token if it has been configured. If no default ACL token was configured then the anonymous token will be used.

» ACL Rules and Scope

The rules from all policies linked with a token are combined to form that token's effective rule set. Policy rules 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" access to all resources, then policy rules can be set to whitelist access to specific resources. Conversely, if the default policy is “allow” then policy rules can be used to explicitly deny access to resources.

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

Resource Scope
acl Operations for managing the ACL system ACL API
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 token with "write" privileges for the ACL system.

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.

  3. The connect CA roots endpoint exposes just the public TLS certificate which other systems can used to verify the TLS connection with Consul.

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

» Configuring ACLs

ACLs are configured using several different configuration options. These are marked as to whether they are set on servers, clients, or both.

Configuration Option Servers Clients Purpose
acl.enabled REQUIRED REQUIRED Controls whether ACLs are enabled
acl.default_policy OPTIONAL N/A Determines whitelist or blacklist mode
acl.down_policy OPTIONAL OPTIONAL Determines what to do when the remote token or policy resolution fails
acl.policy_ttl OPTIONAL OPTIONAL Determines time-to-live for cached ACL Policies
acl.token_ttl OPTIONAL OPTIONAL Determines time-to-live for cached ACL Tokens

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.tokens.agent_master OPTIONAL OPTIONAL Special token that can be used to access Agent API when remote bearer token resolution fails; used for setting up the cluster such as doing initial join operations, see the ACL Agent Master Token section for more details
acl.tokens.agent OPTIONAL OPTIONAL Special token that is used for an agent's internal operations, see the ACL Agent Token section for more details
acl.tokens.master OPTIONAL N/A Special token used to bootstrap the ACL system, see the Bootstrapping ACLs section for more details
acl.tokens.default 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

All of these tokens except the master token can all be introduced or updated via the /v1/agent/token API.

» ACL Agent Master Token

Since the acl.tokens.agent_master is designed to be used when the Consul servers are not available, its policy is managed locally on the agent and does not need to have a token defined on the Consul servers via the ACL API. Once set, it implicitly has the following policy associated with it

agent "<node name of agent>" {
  policy = "write"
}
node_prefix "" {
  policy = "read"
}

» ACL Agent Token

The acl.tokens.agent is a special token that is used for an agent's internal operations. It isn't used directly for any user-initiated operations like the acl.tokens.default, though if the acl.tokens.agent_token isn't configured the acl.tokens.default will be used. The ACL agent token is used for the following operations by the agent:

  1. Updating the agent's node entry using the Catalog API, including updating its node metadata, tagged addresses, and network coordinates
  2. Performing anti-entropy syncing, in particular reading the node metadata and services registered with the catalog
  3. Reading and writing the special _rexec section of the KV store when executing consul exec commands

Here's an example policy sufficient to accomplish the above for a node called mynode:

node "mynode" {
  policy = "write"
}
service_prefix "" {
  policy = "read"
}
key_prefix "_rexec" {
  policy = "write"
}

The service_prefix policy needs read access for any services that can be registered on the agent. If remote exec is disabled, the default, then the key_prefix policy can be omitted.

» Bootstrapping ACLs

Bootstrapping ACLs on a new cluster requires a few steps, outlined in the examples in this section.

» Enable ACLs on the Consul Servers

The first step for bootstrapping ACLs is to enable ACLs on the Consul servers in the primary datacenter. In this example, we are configuring the following:

  1. A primary datacenter of "dc1", which is where these servers are.
  2. An ACL master token of "b1gs33cr3t"; see below for an alternative using the /v1/acl/bootstrap API
  3. A default policy of "deny" which means we are in whitelist mode
  4. A down policy of "extend-cache" which means that we will ignore token TTLs during an outage

Here's the corresponding JSON configuration file:

{
  "primary_datacenter": "dc1",
  "acl" : {
    "enabled": true,
    "default_policy": "deny",
    "down_policy": "extend-cache",
    "tokens" : {
      "master" : "b1gs33cr3t",
    }
  }
}

The servers will need to be restarted to load the new configuration. Please take care to start the servers one at a time, and ensure each server has joined and is operating correctly before starting another.

The acl.tokens.master will be created and assigned the global-management policy. acl.tokens.master is only installed when a server acquires cluster leadership. If you would like to install or change the acl.tokens.master, set the new value for acl.tokens.master in the configuration for all servers. Once this is done, restart the current leader to force a leader election.

In Consul 0.9.1 and later, you can use the /v1/acl/bootstrap API to make the initial master token, so a token never needs to be placed into a configuration file. To use this approach, omit acl.tokens.master from the above config and then call the API:

$ consul acl bootstrap
AccessorID:   1ee820ce-e149-829f-caba-77ec37be3c98
SecretID:     be13b885-ddd4-830a-857c-d5fec72bbe8b
Description:  Bootstrap Token (Global Management)
Local:        false
Create Time:  2018-10-19 11:48:25.614214 -0400 EDT
Policies:
   00000000-0000-0000-0000-000000000001 - global-management

It's only possible to bootstrap one time, and bootstrapping will be disabled if a master token was configured and created.

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

» Create an Agent Token

After the servers are restarted above, you will see new errors in the logs of the Consul servers related to permission denied errors:

2017/07/08 23:38:24 [WARN] agent: Node info update blocked by ACLs
2017/07/08 23:38:44 [WARN] agent: Coordinate update blocked by ACLs

These errors are because the agent doesn't yet have a properly configured acl.tokens.agent that it can use for its own internal operations like updating its node information in the catalog and performing anti-entropy syncing. We can create a token using the ACL API, and the ACL master token we set in the previous step:

The first step is to create a policy for your agent tokens.

# Assumes agent-policy.hcl contains the following:
# node_prefix "" {
#    policy = "write"
# }
# service_prefix "" {
#    policy = "read"
# }
#
$ consul acl policy create  -name "agent-token" -description "Agent Token Policy" -rules @agent-policy.hcl

ID:           5102b76c-6058-9fe7-82a4-315c353eb7f7
Name:         agent-policy
Description:  Agent Token Policy
Datacenters:
Rules:
node_prefix "" {
   policy = "write"
}

service_prefix "" {
   policy = "read"
}

The returned value is the newly-created policy. We can now create tokens and assign it this policy.

$ consul acl token create -description "Agent Token" -policy-name "agent-token"

AccessorID:   499ab022-27f2-acb8-4e05-5a01fff3b1d1
SecretID:     da666809-98ca-0e94-a99c-893c4bf5f9eb
Description:  Agent Token
Local:        false
Create Time:  2018-10-19 14:23:40.816899 -0400 EDT
Policies:
   fcd68580-c566-2bd2-891f-336eadc02357 - agent-token

We can now add this to our Consul server configuration and restart the servers once more to apply it:

{
  "primary_datacenter": "dc1",
  "acl" : {
    "enabled" : true,
    "default_policy" : "deny",
    "down_policy" : "extend-cache",
    "tokens" : {
      "master" : "b1gs33cr3t",
      "agent" : "fcd68580-c566-2bd2-891f-336eadc02357"
    }
  }
}

In Consul 0.9.1 and later you can also introduce the agent token using an API, so it doesn't need to be set in the configuration file:

$ consul acl set-agent-token agent fcd68580-c566-2bd2-891f-336eadc02357

ACL token "agent" set successfully

With that ACL agent token set, the servers will be able to sync themselves with the catalog:

2017/07/08 23:42:59 [INFO] agent: Synced node info

See the ACL Agent Token section for more details.

» Enable ACLs on the Consul Clients

Since ACL enforcement also occurs on the Consul clients, we need to also restart them with a configuration file that enables ACLs:

{
  "primary_datacenter": "dc1",
  "acl" : {
    "enabled" : true,
    "default_policy" : "deny",
    "down_policy" : "extend-cache",
    "tokens" : {
      "agent" : "fcd68580-c566-2bd2-891f-336eadc02357"
    }
  }
}

Similar to the previous example, in Consul 0.9.1 and later you can also introduce the agent token using an API, so it doesn't need to be set in the configuration file:

$ consul acl set-agent-token agent "fcd68580-c566-2bd2-891f-336eadc02357"

ACL token "agent" set successfully

We used the same ACL agent token that we created for the servers, which will work since it was not specific to any node or set of service prefixes. In a more locked-down environment it is recommended that each client get an ACL agent token with node write privileges for just its own node name, and service read privileges for just the service prefixes expected to be registered on that client.

Anti-entropy syncing requires the ACL agent token to have service read privileges for all services that may be registered with the agent, so generally an empty service prefix can be used, as shown in the example.

Clients will report similar permission denied errors until they are restarted with an ACL agent token.

» Configure the Anonymous Token (Optional)

At this point ACLs are bootstrapped with ACL agent tokens configured, but there are no other policies set up. Even basic operations like consul members will be restricted by the ACL default policy of "deny":

$ consul members

We don't get an error since the ACL has filtered what we see, and we aren't allowed to see any nodes by default.

If we supply the token we created above we will be able to see a listing of nodes because it has write privileges to an empty node prefix, meaning it has access to all nodes:

$ CONSUL_HTTP_TOKEN=fcd68580-c566-2bd2-891f-336eadc02357 consul members
Node    Address         Status  Type    Build     Protocol  DC
node-1  127.0.0.1:8301  alive   server  0.9.0dev  2         dc1
node-2  127.0.0.2:8301  alive   client  0.9.0dev  2         dc1

It's pretty common in many environments to allow listing of all nodes, even without a token. The policies associated with the special anonymous token can be updated to configure Consul's behavior when no token is supplied. The anonymous token is managed like any other ACL token, except that anonymous is used for the ID. In this example we will give the anonymous token read privileges for all nodes:

$ consul acl policy create -name 'list-all-nodes' -rules 'node_prefix "" { policy = "read" }'

ID:           e96d0a33-28b4-d0dd-9b3f-08301700ac72
Name:         list-all-nodes
Description:
Datacenters:
Rules:
node_prefix "" { policy = "read" }

$ consul acl token update -id 00000000-0000-0000-0000-000000000002 -policy-name list-all-nodes -description "Anonymous Token - Can List Nodes"

Token updated successfully.
AccessorID:   00000000-0000-0000-0000-000000000002
SecretID:     anonymous
Description:  Anonymous Token - Can List Nodes
Local:        false
Create Time:  0001-01-01 00:00:00 +0000 UTC
Hash:         ee4638968d9061647ac8c3c99e9d37bfdd2af4d1eaa07a7b5f80af0389460948
Create Index: 5
Modify Index: 38
Policies:
   e96d0a33-28b4-d0dd-9b3f-08301700ac72 - list-all-nodes

The anonymous token is implicitly used if no token is supplied, so now we can run consul members without supplying a token and we will be able to see the nodes:

$ consul members
Node    Address         Status  Type    Build     Protocol  DC
node-1  127.0.0.1:8301  alive   server  0.9.0dev  2         dc1
node-2  127.0.0.2:8301  alive   client  0.9.0dev  2         dc1

The anonymous token is also used for DNS lookups since there's no way to pass a token as part of a DNS request. Here's an example lookup for the "consul" service:

$ dig @127.0.0.1 -p 8600 consul.service.consul

; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 -p 8600 consul.service.consul
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 9648
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;consul.service.consul.         IN      A

;; AUTHORITY SECTION:
consul.                 0       IN      SOA     ns.consul. postmaster.consul. 1499584110 3600 600 86400 0

;; Query time: 2 msec
;; SERVER: 127.0.0.1#8600(127.0.0.1)
;; WHEN: Sun Jul  9 00:08:30 2017
;; MSG SIZE  rcvd: 89

Now we get an NXDOMAIN error because the anonymous token doesn't have access to the "consul" service. Let's add that to the anonymous token's policy:

$ consul acl policy create -name 'service-consul-read' -rules 'service "consul" { policy = "read" }'
ID:           3c93f536-5748-2163-bb66-088d517273ba
Name:         service-consul-read
Description:
Datacenters:
Rules:
service "consul" { policy = "read" }

$ consul acl token update -id 00000000-0000-0000-0000-000000000002 --merge-policies -description "Anonymous Token - Can List Nodes" -policy-name service-consul-read
Token updated successfully.
AccessorID:   00000000-0000-0000-0000-000000000002
SecretID:     anonymous
Description:  Anonymous Token - Can List Nodes
Local:        false
Create Time:  0001-01-01 00:00:00 +0000 UTC
Hash:         2c641c4f73158ef6d62f6467c68d751fccd4db9df99b235373e25934f9bbd939
Create Index: 5
Modify Index: 43
Policies:
   e96d0a33-28b4-d0dd-9b3f-08301700ac72 - list-all-nodes
   3c93f536-5748-2163-bb66-088d517273ba - service-consul-read

With that new policy in place, the DNS lookup will succeed:

$ dig @127.0.0.1 -p 8600 consul.service.consul

; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 -p 8600 consul.service.consul
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46006
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;consul.service.consul.         IN      A

;; ANSWER SECTION:
consul.service.consul.  0       IN      A       127.0.0.1

;; Query time: 0 msec
;; SERVER: 127.0.0.1#8600(127.0.0.1)
;; WHEN: Sun Jul  9 00:11:14 2017
;; MSG SIZE  rcvd: 55

The next section shows an alternative to the anonymous token.

» Set Agent-Specific Default Tokens (Optional)

An alternative to the anonymous token is the acl.tokens.default configuration item. When a request is made to a particular Consul agent and no token is supplied, the acl.tokens.default will be used for the token, instead of being left empty which would normally invoke the anonymous token.

In Consul 0.9.1 and later, the agent ACL tokens can be introduced or updated via the /v1/agent/token API.

This behaves very similarly to the anonymous token, but can be configured differently on each agent, if desired. For example, this allows more fine grained control of what DNS requests a given agent can service, or can give the agent read access to some key-value store prefixes by default.

If using acl.tokens.default, then it's likely the anonymous token will have a more restrictive policy than shown in the examples here.

» Create Tokens for UI Use (Optional)

If you utilize the Consul UI with a restrictive ACL policy, as above, the UI will not function fully using the anonymous ACL token. It is recommended that a UI-specific ACL token is used, which can be set in the UI during the web browser session to authenticate the interface.

$ consul acl policy create -name "ui-policy" \
                           -description "Necessary permissions for UI functionality"
                           -rules 'key "" { policy = "write" } node "" { policy = "read" } service "" { policy = "read" }'

ID:           9cb99b2b-3c20-81d4-a7c0-9ffdc2fbf08a
Name:         ui-policy
Description:  Necessary permissions for UI functionality
Datacenters:
Rules:
key "" { policy = "write" } node "" { policy = "read" } service "" { policy = "read" }

$ consul acl token create -description "UI Token" -policy-name "ui-policy"

AccessorID:   56e605cf-a6f9-5f9d-5c08-a0e1323cf016
SecretID:     117842b6-6208-446a-0d1e-daf93854857d
Description:  UI Token
Local:        false
Create Time:  2018-10-19 14:55:44.254063 -0400 EDT
Policies:
   9cb99b2b-3c20-81d4-a7c0-9ffdc2fbf08a - ui-policy

The token can then be set on the "settings" page of the UI.

» Next Steps

The examples above configure a basic ACL environment with the ability to see all nodes by default, and limited access to just the "consul" service. The ACL API can be used to create tokens for applications specific to their intended use, and to create more specific ACL agent tokens for each agent's expected role.

» Rule Specification

A core part of the ACL system is the rule language which is used to describe the policy that must be enforced. There are two types of rules: prefix based rules and exact matching rules. The rules is composed of a resource, a segment (for some resource areas) and a policy disposition. The general structure of a rule is:

<resource> "<segment>" {
  policy = "<policy disposition>"
}

Segmented resource areas allow operators to more finely control access to those resources. Note that not all resource areas are segmented such as the keyring, operator and acl resources. For those rules they would look like:

<resource> = "<policy disposition>"

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

When using 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. Exact matching rules will only apply to the exact resource specified.

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_prefix "" {
  policy = "read"
}
key_prefix "foo/" {
  policy = "write"
}
key_prefix "foo/private/" {
  policy = "deny"
}

key "foo/bar/secret" {
  policy = "deny"
}

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

This is equivalent to the following JSON input:

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

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

Here's a sample request using the HCL form:

$ curl \
    --request PUT \
    --data \
'{
  "Name": "my-app-policy",
  "Rules": "key \"\" { policy = \"read\" } key \"foo/\" { policy = \"write\" } key \"foo/private/\" { policy = \"deny\" } operator = \"read\""
}' http://127.0.0.1:8500/v1/acl/policy?token=<token with ACL "write">

Here's an equivalent request using the JSON form:

$ curl \
    --request PUT \
    --data \
'{
  "Name": "my-app-policy",
  "Rules": "{\"key\":{\"\":{\"policy\":\"read\"},\"foo/\":{\"policy\":\"write\"},\"foo/private\":{\"policy\":\"deny\"}},\"operator\":\"read\"}"
}' http://127.0.0.1:8500/v1/acl/policy?token=<management token>

On success, the Policy is returned:

{
    "CreateIndex": 7,
    "Hash": "UMG6QEbV40Gs7Cgi6l/ZjYWUwRS0pIxxusFKyKOt8qI=",
    "ID": "5f423562-aca1-53c3-e121-cb0eb2ea1cd3",
    "ModifyIndex": 7,
    "Name": "my-app-policy",
    "Rules": "key \"\" { policy = \"read\" } key \"foo/\" { policy = \"write\" } key \"foo/private/\" { policy = \"deny\" } operator = \"read\""
}

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 Authorization Bearer token header, or Consul's CLI commands via the token argument, or the CONSUL_HTTP_TOKEN environment variable.

» ACL Rules

The acl resource controls access to ACL operations in the ACL API.

ACL rules look like this:

acl = "write"

There is only one acl rule allowed per policy and its value is set to one of the policy dispositions. In the example above ACLs may be read or written including discovering any token's secret ID. Snapshotting also requires acl = "write" permissions due to the fact that all the token secrets are contained within the snapshot.

» Agent Rules

The agent and agent_prefix resources control 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 or node_prefix and service or service_prefix policies instead.

Agent rules look like this:

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

agent_prefix"" {
  policy = "read"
}

Agent rules are keyed by the node name 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 reqired 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 and event_prefix resources control access to event operations in the Event API, such as firing events and listing events.

Event rules look like this:

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

Event rules are segmented by the event name they apply to. In the example above, the rules allow read-only access to any event, and firing of the "deploy" event.

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 and key_prefix resources control access to key/value store operations in the KV API. Key rules look like this:

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

Key rules are segmented by the key name they apply to. In the example above, the rules allow read-only access to any key name with the empty prefix rule, allow read-write access to the "foo" key, and deny access to the "bar" key.

» List Policy for Keys

Consul 1.0 introduces a new list policy for keys that is only enforced when opted in via the boolean config param "acl.enable_key_list_policy". list controls access to recursively list entries and keys, and enables more fine grained policies. With "acl.enable_key_list_policy", recursive reads via the KV API with an invalid token result in a 403. Example:

key_prefix "" {
 policy = "deny"
}

key_prefix "bar" {
 policy = "list"
}

key_prefix "baz" {
 policy = "read"
}

In the example above, the rules allow reading the key "baz", and only allow recursive reads on the prefix "bar".

A token with write access on a prefix also has list access. A token with list access on a prefix also has read access on all its suffixes.

» Sentinel Integration

Consul Enterprise supports additional optional fields for key write policies for Sentinel integration. An example key rule with a Sentinel code policy looks like this:

key "foo" {
  policy = "write"
  sentinel {
      code = <<EOF
        import "strings\
        main = rule { strings.has_suffix(value, "bar") }
EOF
      enforcementlevel = "hard-mandatory"
  }
}

For more detailed documentation, see the Consul Sentinel Guide.

» Keyring Rules

The keyring resource 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 and node_prefix resources 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_prefix "" {
  policy = "read"
}
node "app" {
  policy = "write"
}
node "admin" {
  policy = "deny"
}

Node rules are segmented by the node name they apply to. In the example above, the rules allow read-only access to any node name with the empty prefix, allow read-write access to the "app" node, and deny all access to the "admin" node.

Agents need to be configured with an acl.tokens.agent 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.default 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.tokens.default 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.

In addition to ACLs, in Consul 0.9.0 and later, the agent must be configured with enable_script_checks set to true in order to enable script checks.

» Operator Rules

The operator resource 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 rule 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 and query_prefix resources control access to create, update, and delete prepared queries in the Prepared Query API. Executing queries is subject to node/node_prefix and service/service_prefix policies, as will be explained below.

Query rules look like this:

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

Query rules are segmented by the query name they apply to. In the example above, the rules allow read-only access to any query name with the empty prefix, and allow read-write access to the query named "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 and query_prefix ACL resources. Clients are required to have an ACL token with permissions on to access that query name. 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 token with management privileges 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 token with management privileges 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 token with management privileges is required to list any queries. The client token's query ACL policy is used to determine which queries they can see. Only tokens with management privileges 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 and service_prefix resources control service-level registration and read access to the Catalog API and service discovery with the Health API.

Service rules look like this:

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

Service rules are segmented by the service name they apply to. In the example above, the rules allow read-only access to any service name with the empty prefix, allow read-write access to the "app" service, and deny all access to the "admin" service.

Consul's DNS interface is affected by restrictions on service rules. If the acl.tokens.default 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.tokens.default 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. Note: all tokens passed to an agent are persisted on local disk to allow recovery from restarts. See -data-dir flag documentation for notes on securing access.

In addition to ACLs, in Consul 0.9.0 and later, the agent must be configured with enable_script_checks or enable_local_script_checks set to true in order to enable script checks.

» Session Rules

The session and session_prefix resources controls access to Session API operations.

Session rules look like this:

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

Session rules are segmented by the node name they apply to. In the example above, the rules allow read-only access to sessions on node name with the empty prefix, allow creating sessions on the node named "app", and deny all access to any sessions on the "admin" node.