Open source Puppet documentation

An external node classifier (ENC) is a script or application that tells Puppet which classes a node should have. It can replace or work in concert with the node definitions in the main site manifest (site.pp).

Depending on the external data sources you use in your infrastructure, building an external node classifier can be a valuable way to extend Puppet.

External node classifiers

An external node classifier is an executable that Puppet Server or puppet apply can call; it doesn’t have to be written in Ruby. Its only argument is the name of the node to be classified, and it returns a YAML document describing the node.

Inside the ENC, you can reference any data source you want, including PuppetDB. From Puppet’s perspective, the ENC submits a node name and gets back a hash of information.

External node classifiers can co-exist with standard node definitions in site.pp; the classes declared in each source are merged together.

Merging classes from multiple sources

Every node always gets a node object from the configured node terminus. The node object might be empty, or it might contain classes, parameters, and an environment. The node terminus setting, node_terminus, takes effect where the catalog is compiled, on Puppet Server when using an agent-master configuration, and on the node itself when using puppet apply. The default node terminus is plain, which returns an empty node object, leaving node configuration to the main manifest. The exec terminus calls an ENC script to determine what should go in the node object. Every node might also get a  node definition from the main manifest.

When compiling a node's catalog, Puppet includes all of the following:
  • Classes specified in the node object it received from the node terminus.

  • Classes or resources that are in the site manifest but outside any node definitions.

  • Classes or resources in the most specific node definition in site.pp that matches the current node (if site.pp contains any node definitions). The following notes apply:

    • If site.pp contains at least one node definition, it must have a node definition that matches the current node; compilation fails if a match can’t be found.

    • If the node name resembles a dot-separated fully qualified domain name, Puppet makes multiple attempts to match a node definition, removing the right-most part of the name each time. Thus, Puppet would first try agent1.example.com, then agent1.example, then agent1. This behavior isn’t mimicked when calling an ENC, which is invoked only once with the agent’s full node name.

    • If no matching node definition can be found with the node’s name, Puppet tries one last time with a node name of default; most users include a node default {} statement in their site.pp file. This behavior isn’t mimicked when calling an ENC.

Comparing ENCs and node definitions

If you're trying to decide whether to use an ENC or main manifest node definitions (or both), consider the following:

  • The YAML returned by an ENC isn’t an exact equivalent of a node definition in site.pp — it can’t declare individual resources, declare relationships, or do conditional logic. An ENC can only declare classes, assign top-scope variables, and set an environment. So, an ENC is most effective if you’ve done a good job of separating your configurations out into classes and modules.

  • ENCs can set an environment for a node, overriding whatever environment the node requested.

  • Even if you aren’t using node definitions, you can still use site.pp to do things like set global resource defaults.

  • Unlike regular node definitions, where a node can match a less specific definition if an exactly matching definition isn’t found (depending on Puppet’s strict_hostname_checking setting), an ENC is called only once, with the node’s full name.

Connect an ENC

Configure two settings to have Puppet Server connect to an external node classifier.

In the master's puppet.conf file:
  1. Set the node_terminus setting to exec.
  2. Set the external_nodes setting to the path to the ENC executable.
For example:
[master]
  node_terminus = exec
  external_nodes = /usr/local/bin/puppet_node_classifier

ENC output format

An ENC must return either nothing or a YAML hash to standard out. The hash must contain at least one of classes or parameters, or it can contain both. It can also optionally contain an environment key.

ENCs exit with an exit code of 0 when functioning normally, and can exit with a non-zero exit code if you want Puppet to behave as though the requested node was not found.

If an ENC returns nothing or exits with a non-zero exit code, the catalog compilation fails with a “could not find node” error, and the node is unable to retrieve configurations.

For information about the YAML format, see yaml.org.

Classes

If present, the value of classes must be either an array of class names or a hash whose keys are class names. That is, the following are equivalent:
classes:
  - common
  - puppet
  - dns
  - ntp

classes:
  common:
  puppet:
  dns:
  ntp:
If you're specifying parameterized classes, use the hash key syntax, not the array syntax. The value for a parameterized class should be a hash of the class’s parameters and values. Each value can be a string, number, array, or hash. Put string values in quotation marks, because YAML parsers sometimes treat certain unquoted strings (such as on) as Booleans. Non-parameterized classes can have empty values.
classes:
    common:
    puppet:
    ntp:
        ntpserver: 0.pool.ntp.org
    aptsetup:
        additional_apt_repos:
            - deb localrepo.example.com/ubuntu lucid production
            - deb localrepo.example.com/ubuntu lucid vendor

Parameters

If present, the value of the parameters key must be a hash of valid variable names and associated values; these are exposed to the compiler as top-scope variables. Each value can be a string, number, array, or hash.
parameters:
    ntp_servers:
        - 0.pool.ntp.org
        - ntp.example.com
    mail_server: mail.example.com
    iburst: true

Environment

If present, the value of environment must be a string representing the desired environment for this node. This is the only environment used by the node in its requests for catalogs and files.
environment: production

Complete example

---
classes:
    common:
    puppet:
    ntp:
        ntpserver: 0.pool.ntp.org
    aptsetup:
        additional_apt_repos:
            - deb localrepo.example.com/ubuntu lucid production
            - deb localrepo.example.com/ubuntu lucid vendor
parameters:
    ntp_servers:
        - 0.pool.ntp.org
        - ntp.example.com
    mail_server: mail.example.com
    iburst: true
environment: production
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