CSR attributes and certificate extensions

When Puppet agent nodes request their certificates, the certificate signing request (CSR) usually contains only their certname and the necessary cryptographic information. Agents can also embed additional data in their CSR, useful for policy-based autosigning and for adding new trusted facts.

Embedding additional data into CSRs is useful when:
  • Large numbers of nodes are regularly created and destroyed as part of an elastic scaling system.

  • You are willing to build custom tooling to make certificate autosigning more secure and useful.

It might also be useful in deployments where Puppet is used to deploy private keys or other sensitive information, and you want extra control over nodes that receive this data.

If your deployment doesn’t match one of these descriptions, you might not need this feature.

Timing: When data can be added to CSRs and certificates

When Puppet agent starts the process of requesting a catalog, it checks whether it has a valid signed certificate. If it does not, it generates a key pair, crafts a CSR, and submits it to the certificate authority (CA) Puppet master. For detailed information, see agent/master HTTPS traffic.

For practical purposes, a certificate is locked and immutable as soon as it is signed. For data to persist in the certificate, it has to be added to the CSR before the CA signs the certificate.

This means any desired extra data must be present before Puppet agent attempts to request its catalog for the first time.

Populate any extra data when provisioning the node. If you make an error, see Recovering From Failed Data Embedding below.

Data location and format

Extra data for the CSR is read from the csr_attributes.yaml file in Puppet's confdir. The location of this file can be changed with the csr_attributes configuration setting.

The csr_attributes.yaml file must contain a YAML hash with one or both of the following keys:
  • custom_attributes

  • extension_requests

The value of each key must also be a hash, where:
  • Each key is a valid object identifier (OID) — Puppet-specific OIDs can optionally be referenced by short name instead of by numeric ID.

  • Each value is an object that can be cast to a string — numbers are allowed but arrays are not.

For information about how each hash is used and recommended OIDs for each hash, see the sections below.

Custom attributes (transient CSR data)

Custom attributes are pieces of data that are only embedded in the CSR. The CA can use them when deciding whether to sign the certificate, but they are discarded after that and aren’t transferred to the final certificate.

Default behavior

The puppet cert list command doesn’t display custom attributes for pending CSRs, and basic autosigning (autosign.conf) doesn’t check them before signing.

Configurable behavior

If you use policy-based autosigning your policy executable receives the complete CSR in PEM format. The executable can extract and inspect the custom attributes, and use them to decide whether to sign the certificate.

The simplest method is to embed a pre-shared key of some kind in the custom attributes. A policy executable can compare it to a list of known keys and autosign certificates for any pre-authorized nodes.

A more complex use might be to embed an instance-specific ID and write a policy executable that can check it against a list of your recently requested instances on a public cloud, like EC2 or GCE.

If you use Puppet Server 2.5.0 or higher, you can also sign requests using authorization extensions and the --allow-authorization-extensions flag for puppet cert sign.

Manually checking for custom attributes in CSRs

You can check for custom attributes by using OpenSSL to dump a CSR in pem format to text format, by running this command:

openssl req -noout -text -in <name>.pem

In the output, look for the Attributes section which appears below the Subject Public Key Info block:

    challengePassword        :342thbjkt82094y0uthhor289jnqthpc2290

Recommended OIDs for attributes

Custom attributes can use any public or site-specific OID, with the exception of the OIDs used for core X.509 functionality. This means you can’t re-use existing OIDs for things like subject alternative names.

One useful OID is the challengePassword attribute — 1.2.840.113549.1.9.7. This is a rarely-used corner of X.509 that can easily be repurposed to hold a pre-shared key. The benefit of using this instead of an arbitrary OID is that it appears by name when using OpenSSL to dump the CSR to text; OIDs that openssl req can’t recognize are displayed as numerical strings.

You can also use the Puppet-specific OIDs.

Extension requests (permanent certificate data)

Extension requests are pieces of data that are transferred as extensions to the final certificate, when the CA signs the CSR. They persist as trusted, immutable data, that cannot be altered after the certificate is signed.

They can also be used by the CA when deciding whether or not to sign the certificate.

Default behavior

When signing a certificate, Puppet’s CA tools transfer any extension requests into the final certificate.

You can access certificate extensions in manifests as $trusted[extensions][<EXTENSION OID>].

Any OIDs in the ppRegCertExt range appear using their short names. By default, any other OIDs appear as plain dotted numbers, but you can use the custom_trusted_oid_mapping.yaml file to assign short names to any other OIDs you use at your site. If you do, those OIDs will appear in $trusted as their short names, instead of their full numerical OID.

For more information about $trusted, see facts and special variables.

The visibility of extensions is limited:
  • The puppet cert list command does not display custom attributes for any pending CSRs, and basic autosigning (autosign.conf) doesn’t check them before signing. Either use policy-based autosigning or inspect CSRs manually with the openssl command (see below).

  • The puppet cert print command does display any extensions in a signed certificate, under the X509v3 extensions section.

Puppet’s authorization system (auth.conf) does not use certificate extensions, but Puppet Server’s authorization system, which is based on trapperkeeper-authorization, can use extensions in the ppAuthCertExt OID range, and requires them for requests to write access rules.

Configurable behavior

If you use policy-based autosigning, your policy executable receives the complete CSR in pem format. The executable can extract and inspect the extension requests, and use them when deciding whether to sign the certificate.

Manually checking for extensions in CSRs and certificates

You can check for extension requests in a CSR by running the OpenSSL command to dump a CSR in pem format to text format:

openssl req -noout -text -in <name>.pem

In the output, look for a section called Requested Extensions, which appears below the Subject Public Key Info and Attributes blocks:

Requested Extensions:
Note: Every extension is preceded by any combination of two characters (.$ and .. in the example above) that contain ASN.1 encoding information. Because OpenSSL is unaware of Puppet’s custom extensions OIDs, it’s unable to properly display the values.

Any Puppet-specific OIDs (see below) appear as numeric strings when using OpenSSL.

You can check for extensions in a signed certificate by running puppet cert print <name>. In the output, look for the X509v3 extensions section. Any of the Puppet-specific registered OIDs appear as their descriptive names:

X509v3 extensions:
    Netscape Comment:
        Puppet Ruby/OpenSSL Internal Certificate
    X509v3 Subject Key Identifier:
    Puppet Node UUID:
    X509v3 Extended Key Usage: critical
        TLS Web Server Authentication, TLS Web Client Authentication
    X509v3 Basic Constraints: critical
    Puppet Node Preshared Key:
    X509v3 Key Usage: critical
        Digital Signature, Key Encipherment
    Puppet Node Image Name:

Recommended OIDs for extensions

Extension request OIDs must be under the ppRegCertExt (, ppPrivCertExt (, or ppAuthCertExt ( OID arcs.

Puppet provides several registered OIDs (under ppRegCertExt) for the most common kinds of extension information, a private OID range (ppPrivCertExt) for site-specific extension information, and an OID range for safe authorization to Puppet Server (ppAuthCertExt).

There are several benefits to using the registered OIDs:
  • You can reference them in the csr_attributes.yaml file with their short names instead of their numeric IDs.

  • You can access them in $trusted[extensions] with their short names instead of their numeric IDs.

  • When using Puppet tools to print certificate info, they will appear using their descriptive names instead of their numeric IDs.

The private range is available for any information you want to embed into a certificate that isn’t widely used already. It is completely unregulated, and its contents are expected to be different in every Puppet deployment.

You can use the custom_trusted_oid_mapping.yaml file to set short names for any private extension OIDs you use. Note that this only enables the short names in the $trusted[extensions] hash.

Puppet-specific registered IDs


The ppRegCertExt OID range contains the following OIDs:

Numeric ID Short name Descriptive name pp_uuid Puppet node UUID pp_instance_id Puppet node instance ID pp_image_name Puppet node image name pp_preshared_key Puppet node preshared key pp_cost_center Puppet node cost center name pp_product Puppet node product name pp_project Puppet node project name pp_application Puppet node application name pp_service Puppet node service name pp_employee Puppet node employee name pp_created_by Puppet node created_by tag pp_environment Puppet node environment name pp_role Puppet node role name pp_software_version Puppet node software version pp_department Puppet node department name pp_cluster Puppet node cluster name pp_provisioner Puppet node provisioner name pp_region Puppet node region name pp_datacenter Puppet node datacenter name pp_zone Puppet node zone name pp_network Puppet node network name pp_securitypolicy Puppet node security policy name pp_cloudplatform Puppet node cloud platform name pp_apptier Puppet node application tier pp_hostname Puppet node hostname


The ppAuthCertExt OID range contains the following OIDs:
Numeric ID Short name Descriptive name pp_authorization Certificate extension authorization pp_auth_role Puppet node role name for authorization

AWS attributes and extensions population example

To populate the csr_attributes.yaml file when you provision a node, use an automated script such as cloud-init.

For example, when provisioning a new node from the AWS EC2 dashboard, enter the following script into the Configure Instance Details —> Advanced Details section:

if [ ! -d /etc/puppetlabs/puppet ]; then
   mkdir /etc/puppetlabs/puppet
cat > /etc/puppetlabs/puppet/csr_attributes.yaml << YAML
    1.2.840.113549.1.9.7: mySuperAwesomePassword
    pp_instance_id: $(curl -s
    pp_image_name:  $(curl -s

Assuming your image has the erb binary available, this populates the attributes file with the AWS instance ID, image name, and a pre-shared key to use with policy-based autosigning.


Recovering from failed data embedding

When testing this feature for the first time, you might not embed the right information in a CSR, or certificate, and might want to start over for your test nodes. This is not really a problem once your provisioning system is changed to populate the data, but it can easily happen when doing things manually.

To start over, do the following.

On the test node:
  • Turn off Puppet agent, if it’s running.

  • Check whether a CSR is present in $ssldir/certificate_requests/<name>.pem. If it exists, delete it.

  • Check whether a certificate is present in $ssldir/certs/<name>.pem. If it exists, delete it.

On the CA Puppet master:
  • Check whether a signed certificate exists. Use puppet cert list --all to see the complete list. If it exists, revoke and delete it with puppet cert clean <name>.

  • Check whether a CSR for the node exists in $ssldir/ca/requests/<name>.pem. If it exists, delete it.

After you’ve done that, you can start over.