» ACL System

Nomad 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. Nomad'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 three major components to the ACL system:

ACL Overview

  • ACL Policies. No permissions are granted by default, making Nomad a default-deny or whitelist system. Policies allow a set of capabilities or actions to be granted or whitelisted. For example, a "readonly" policy might only grant the ability to list and inspect running jobs, but not to submit new ones.

  • ACL Tokens. Requests to Nomad are authenticated 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 to make requests to Nomad. The Secret ID is provided using a request header (X-Nomad-Token) and is used to authenticate the caller. Tokens are either management or client types. The management tokens are effectively "root" in the system, and can perform any operation. The client tokens are associated with one or more ACL policies which grant specific capabilities.

  • Capabilities. Capabilities are the set of actions that can be performed. This includes listing jobs, submitting jobs, querying nodes, etc. A management token is granted all capabilities, while client tokens are granted specific capabilities via ACL Policies. The full set of capabilities is discussed below in the rule specifications.

» ACL Policies

An ACL policy is a named set of rules. Each policy must have a unique name, an optional description, and a rule set. A client ACL token can be associated with multiple policies, and a request is allowed if any of the associated policies grant the capability. Management tokens cannot be associated with policies because they are granted all capabilities.

The special anonymous policy can be defined to grant capabilities to requests which are made anonymously. An anonymous request is a request made to Nomad without the X-Nomad-Token header specified. This can be used to allow anonymous users to list jobs and view their status, while requiring authenticated requests to submit new jobs or modify existing jobs. By default, there is no anonymous policy set meaning all anonymous requests are denied.

» ACL Tokens

ACL tokens are used to authenticate requests and determine if the caller is authorized to perform an action. Each ACL token has a public Accessor ID which is used to identify the token, a Secret ID which is used to make requests to Nomad, and an optional human readable name. All client type tokens are associated with one or more policies, and can perform an action if any associated policy allows it. Tokens can be associated with policies which do not exist, which are the equivalent of granting no capabilities. The management type tokens cannot be associated with policies, but can perform any action.

When ACL tokens are created, they can be optionally marked as Global. This causes them to be created in the authoritative region and replicated to all other regions. Otherwise, tokens are created locally in the region the request was made and not replicated. Local tokens cannot be used for cross-region requests since they are not replicated between regions.

» Capabilities and Scope

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

Policy Scope
namespace Job related operations by namespace
agent Utility operations in the Agent API
node Node-level catalog operations
operator Cluster-level operations in the Operator API
quota Quota specification related operations

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

» Multi-Region Configuration

Nomad supports multi-datacenter and multi-region configurations. A single region is able to service multiple datacenters, and all servers in a region replicate their state between each other. In a multi-region configuration, there is a set of servers per region. Each region operates independently and is loosely coupled to allow jobs to be scheduled in any region and requests to flow transparently to the correct region.

When ACLs are enabled, Nomad depends on an "authoritative region" to act as a single source of truth for ACL policies and global ACL tokens. The authoritative region is configured in the server stanza of agents, and all regions must share a single authoritative source. Any ACL policies or global ACL tokens are created in the authoritative region first. All other regions replicate ACL policies and global ACL tokens to act as local mirrors. This allows policies to be administered centrally, and for enforcement to be local to each region for low latency.

Global ACL tokens are used to allow cross region requests. Standard ACL tokens are created in a single target region and not replicated. This means if a request takes place between regions, global tokens must be used so that both regions will have the token registered.

» Configuring ACLs

ACLs are not enabled by default, and must be enabled. Clients and Servers need to set enabled in the acl stanza. This enables the ACL Policy and ACL Token APIs, as well as endpoint enforcement.

For multi-region configurations, all servers must be configured to use a single authoritative region. The authoritative region is responsible for managing ACL policies and global tokens. Servers in other regions will replicate policies and global tokens to act as a mirror, and must have their replication_token configured.

» Bootstrapping ACLs

Bootstrapping ACLs on a new cluster requires a few steps, outlined below:

» Enable ACLs on Nomad Servers

The APIs needed to manage policies and tokens are not enabled until ACLs are enabled. To begin, we need to enable the ACLs on the servers. If a multi-region setup is used, the authoritative region should be enabled first. For each server:

  1. Set enabled = true in the acl stanza.
  2. Set authoritative_region in the server stanza.
  3. For servers outside the authoritative region, set replication_token in the acl stanza. Replication tokens should be management type tokens which are either created in the authoritative region, or created as Global tokens.
  4. Restart the Nomad server to pick up the new configuration.

Please take care to restart the servers one at a time, and ensure each server has joined and is operating correctly before restarting another.

» Generate the initial token

Once the ACL system is enabled, we need to generate our initial token. This first token is used to bootstrap the system and care should be taken not to lose it. Once the ACL system is enabled, we use the Bootstrap CLI:

$ nomad acl bootstrap
Accessor ID  = 5b7fd453-d3f7-6814-81dc-fcfe6daedea5
Secret ID    = 9184ec35-65d4-9258-61e3-0c066d0a45c5
Name         = Bootstrap Token
Type         = management
Global       = true
Policies     = n/a
Create Time  = 2017-09-11 17:38:10.999089612 +0000 UTC
Create Index = 7
Modify Index = 7

Once the initial bootstrap is performed, it cannot be performed again unless reset. Make sure to save this AccessorID and SecretID. The bootstrap token is a management type token, meaning it can perform any operation. It should be used to setup the ACL policies and create additional ACL tokens. The bootstrap token can be deleted and is like any other token, so care should be taken to not revoke all management tokens.

» Enable ACLs on Nomad Clients

To enforce client endpoints, we need to enable ACLs on clients as well. This is simpler than servers, and we just need to set enabled = true in the acl stanza. Once configured, we need to restart the client for the change.

» Set an Anonymous Policy (Optional)

The ACL system uses a whitelist or default-deny model. This means by default no permissions are granted. For clients making requests without ACL tokens, we may want to grant some basic level of access. This is done by setting rules on the special "anonymous" policy. This policy is applied to any requests made without a token.

To permit anonymous users to read, we can setup the following policy:

# Store our token secret ID

# Write out the payload
$ cat > payload.json <<EOF
    "Name": "anonymous",
    "Description": "Allow read-only access for anonymous requests",
    "Rules": "
        namespace \"default\" {
            policy = \"read\"
        agent {
            policy = \"read\"
        node {
            policy = \"read\"

# Install the policy
$ curl --request POST \
    --data @payload.json \
    -H "X-Nomad-Token: $NOMAD_TOKEN" \

# Verify anonymous request works
$ curl https://localhost:4646/v1/jobs

» Rule Specification

A core part of the ACL system is the rule language which is used to describe the policy that must be enforced. 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. Policies can contain any number of rules.

Policies typically 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. Deny takes precedence when multiple policies are associated with a token.

Specification in the HCL format looks like:

# Allow read only access to the default namespace
namespace "default" {
    policy = "read"

# Allow writing to the `foo` namespace
namespace "foo" {
    policy = "write"

agent {
    policy = "read"

node {
    policy = "read"

quota {
    policy = "read"

This is equivalent to the following JSON input:

    "namespace": {
        "default": {
            "policy": "read"
        "foo": {
            "policy": "write"
    "agent": {
        "policy": "read"
    "node": {
        "policy": "read"
    "quota": {
        "policy": "read"

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

» Namespace Rules

The namespace policy controls access to a namespace, including the Jobs API, Deployments API, Allocations API, and Evaluations API.

namespace "default" {
    policy = "write"

namespace "sensitive" {
    policy = "read"

Namespace rules are keyed by the namespace name they apply to. When no namespace is specified, the "default" namespace is the one used. For example, the above policy grants write access to the default namespace, and read access to the sensitive namespace. In addition to the coarse grained policy specification, the namespace stanza allows setting a more fine grained list of capabilities. This includes:

  • deny - When multiple policies are associated with a token, deny will take precedence and prevent any capabilities.
  • list-jobs - Allows listing the jobs and seeing coarse grain status.
  • read-job - Allows inspecting a job and seeing fine grain status.
  • submit-job - Allows jobs to be submitted or modified.
  • dispatch-job - Allows jobs to be dispatched
  • read-logs - Allows the logs associated with a job to be viewed.
  • read-fs - Allows the filesystem of allocations associated to be viewed.
  • sentinel-override - Allows soft mandatory policies to be overridden.

The coarse grained policy dispositions are shorthand for the fine grained capabilities:

  • deny policy - ["deny"]
  • read policy - ["list-jobs", "read-job"]
  • write policy - ["list-jobs", "read-job", "submit-job", "read-logs", "read-fs", "dispatch-job"]

When both the policy short hand and a capabilities list are provided, the capabilities are merged:

# Allow reading jobs and submitting jobs, without allowing access
# to view log output or inspect the filesystem
namespace "default" {
    policy = "read"
    capabilities = ["submit-job"]

» Node Rules

The node policy controls access to the Node API such as listing nodes or triggering a node drain. Node rules are specified for all nodes using the node key:

node {
    policy = "read"

There's only one node policy allowed per rule set, and its value is set to one of the policy dispositions.

» Agent Rules

The agent policy controls access to the utility operations in the Agent API, such as join and leave. Agent rules are specified for all agents using the agent key:

agent {
    policy = "write"

There's only one agent policy allowed per rule set, and its value is set to one of the policy dispositions.

» Operator Rules

The operator policy controls access to the Operator API. Operator rules look like:

operator {
    policy = "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.

» Quota Rules

The quota policy controls access to the quota specification operations in the Quota API, such as quota creation and deletion. Quota rules are specified for all quotas using the quota key:

quota {
    policy = "write"

There's only one quota policy allowed per rule set, and its value is set to one of the policy dispositions.

» Advanced Topics

» Outages and Multi-Region Replication

The ACL system takes some steps to ensure operation during outages. Clients nodes maintain a limited cache of ACL tokens and ACL policies that have recently or frequently been used, associated with a time-to-live (TTL).

When the region servers are unavailable, the clients will automatically ignore the cache TTL, and extend the cache until the outage has recovered. For any policies or tokens that are not cached, they will be treated as missing and denied access until the outage has been resolved.

Nomad servers have all the policies and tokens locally and can continue serving requests even if quorum is lost. The tokens and policies may become stale during this period as data is not actively replicating, but will be automatically fixed when the outage has been resolved.

In a multi-region setup, there is a single authoritative region which is the source of truth for ACL policies and global ACL tokens. All other regions asynchronously replicate from the authoritative region. When replication is interrupted, the existing data is used for request processing and may become stale. When the authoritative region is reachable, replication will resume and repair any inconsistency.

» Resetting ACL Bootstrap

If all management tokens are lost, it is possible to reset the ACL bootstrap so that it can be performed again. First, we need to determine the reset index, this can be done by calling the reset endpoint:

$ nomad acl bootstrap

Error bootstrapping: Unexpected response code: 500 (ACL bootstrap already done (reset index: 7))

Here we can see the reset index. To reset the ACL system, we create the acl-bootstrap-reset file in the data directory of the leader node:

$ echo 7 >> /nomad-data-dir/server/acl-bootstrap-reset

With the reset key setup, we can bootstrap like normal:

$ nomad acl bootstrap
Accessor ID  = 52d3353d-d7b9-d945-0591-1af608732b76
Secret ID    = 4b0a41ca-6d32-1853-e64b-de0d347e4525
Name         = Bootstrap Token
Type         = management
Global       = true
Policies     = n/a
Create Time  = 2017-09-11 18:38:11.929089612 +0000 UTC
Create Index = 11
Modify Index = 11

If we attempt to bootstrap again, we will get a mismatch on the reset index:

$ nomad acl bootstrap

Error bootstrapping: Unexpected response code: 500 (Invalid bootstrap reset index (specified 7, reset index: 11))

This is because the reset file is in place, but with the incorrect index. The reset file can be deleted, but Nomad will not reset the bootstrap until the index is corrected.

» Vault Integration

HashiCorp Vault has a secret backend for generating short-lived Nomad tokens. As Vault has a number of authentication backends, it could provide a workflow where a user or orchestration system authenticates using an pre-existing identity service (LDAP, Okta, Amazon IAM, etc.) in order to obtain a short-lived Nomad token.

For evaluation purposes, a Vault server in "dev" mode can be used.

$ vault server -dev
==> Vault server configuration:

                     Cgo: disabled
         Cluster Address:
              Listener 1: tcp (addr: "", cluster address: "", tls: "disabled")
               Log Level: info
                   Mlock: supported: false, enabled: false
        Redirect Address:
                 Storage: inmem
                 Version: Vault v0.8.3
             Version Sha: a393b20cb6d96c73e52eb5af776c892b8107a45d

==> WARNING: Dev mode is enabled!

In this mode, Vault is completely in-memory and unsealed.
Vault is configured to only have a single unseal key. The root
token has already been authenticated with the CLI, so you can
immediately begin using the Vault CLI.

The only step you need to take is to set the following
environment variables:

    export VAULT_ADDR=''

The unseal key and root token are reproduced below in case you
want to seal/unseal the Vault or play with authentication.

Unseal Key: YzFfPgnLl9R1f6bLU7tGqi/PIDhDaAV/tlNDMV5Rrq0=
Root Token: f84b587e-5882-bba1-a3f0-d1a3d90ca105

» Pre-requisites

  • Nomad ACL system bootstrapped.
  • A management token (the bootstrap token can be used, but for production systems it's recommended to have a separate token)
  • A set of policies created in Nomad
  • An unsealed Vault server (Vault running in dev mode is unsealed automatically upon startup)
    • Vault must be version 0.9.3 or later to have the Nomad plugin

» Configuration

Mount the nomad secret backend in Vault:

$ vault mount nomad
Successfully mounted 'nomad' at 'nomad'!

Configure access with Nomad's address and management token:

$ vault write nomad/config/access \
    address= \
Success! Data written to: nomad/config/access

Vault secret backends have the concept of roles, which are configuration units that group one or more Vault policies to a potential identity attribute, (e.g. LDAP Group membership). The name of the role is specified on the path, while the mapping to policies is done by naming them in a comma separated list, for example:

$ vault write nomad/role/role-name policies=policyone,policytwo
Success! Data written to: nomad/role/role-name

Similarly, to create management tokens, or global tokens:

$ vault write nomad/role/role-name type=management global=true
Success! Data written to: nomad/role/role-name

Create a Vault policy to allow different identities to get tokens associated with a particular role:

$ echo 'path "nomad/creds/role-name" {
  capabilities = ["read"]
}' | vault policy write nomad-user-policy -
Policy 'nomad-user-policy' written.

If you have an existing authentication backend (like LDAP), follow the relevant instructions to create a role available on the Authentication backends page. Otherwise, for testing purposes, a Vault token can be generated associated with the policy:

$ vault token create -policy=nomad-user-policy
Key             Value
---             -----
token           deedfa83-99b5-34a1-278d-e8fb76809a5b
token_accessor  fd185371-7d80-8011-4f45-1bb3af2c2733
token_duration  768h0m0s
token_renewable true
token_policies  [default nomad-user-policy]

Finally obtain a Nomad Token using the existing Vault Token:

$ vault read nomad/creds/role-name
Key             Value
---             -----
lease_id        nomad/creds/role-name/6fb22e25-0cd1-b4c9-494e-aba330c317b9
lease_duration  768h0m0s
lease_renewable true
accessor_id     10b8fb49-7024-2126-8683-ab355b581db2
secret_id       8898d19c-e5b3-35e4-649e-4153d63fbea9

Verify that the token is created correctly in Nomad, looking it up by its accessor:

$ nomad acl token info 10b8fb49-7024-2126-8683-ab355b581db2
Accessor ID  = 10b8fb49-7024-2126-8683-ab355b581db2
Secret ID    = 8898d19c-e5b3-35e4-649e-4153d63fbea9
Name         = Vault test root 1507307164169530060
Type         = management
Global       = true
Policies     = n/a
Create Time  = 2017-10-06 16:26:04.170633207 +0000 UTC
Create Index = 228
Modify Index = 228

Any user or process with access to Vault can now obtain short lived Nomad Tokens in order to carry out operations, thus centralising the access to Nomad tokens.