Configuring a Custom External Certificate

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Configuring DC/OS Enterprise to use a custom external certificate

External access to a DC/OS Enterprise cluster goes through Admin Router. By default, the certificate presented by Admin Router is signed by the DC/OS Certificate Authority. The default DC/OS CA certificate is not trusted by default, and requires clients to manually configure trust in the CA (e.g. by accepting a pop-up browser dialog). A trusted CA certificate can be provided using custom CA certificates. However, obtaining a suitable CA certificate can be difficult. Custom external certificates allow the cluster administrator to provide an easily obtained non-CA certificate and key that Admin Router will present to external connections.

The benefits of using a custom external certificate for your DC/OS Enterprise cluster include:

  • only needing to obtain a standard certificate instead of an intermediate CA signing certificate, in order to make connections to the cluster trusted by the clients by default (i.e. browser-trusted certificates);
  • using a certificate with different properties for clients connecting to the cluster using different server names.

Contents

Support

  • Custom external certificates are supported in DC/OS Enterprise 2.1 and later.
  • Custom external certificates can be added during initial installation or during an upgrade.
  • Custom external certificates support both RSA and ECC keys.

Glossary

  • Custom external certificate: A certificate in PEM format, that will be used to access the cluster through Admin Router for a provided set of server names. If necessary, it should include all the intermediate CA certificates up to the root CA certificate trusted by the clients. The root CA certificate does not need to be included in the PEM file.

  • Server names: A set of names that Admin Router will match to the custom external certificate’s Canonical Name and Subject Alternative Names.

  • Custom private key: A private key in the PKCS#8 format associated with the custom external certificate.

  • Installation directory: The directory on the bootstrap node where the DC/OS installer resides. It is denoted with $DCOS_INSTALL_DIR in this document.

  • Configuration: The set of the configuration parameters that governs the specific aspects of the installation procedure. The configuration is stored in the DC/OS configuration file.

  • DC/OS configuration file: The file which contains the DC/OS configuration parameters. The DC/OS configuration file is normally called config.yaml and must be present in the $DCOS_INSTALL_DIR/genconf/ directory on the bootstrap node during the installation. It is used by the DC/OS installer.

Requirements

In order to install DC/OS Enterprise with a custom external certificate you will need:

  • A file containing the custom external certificate.
  • A file containing the custom private key.

Specifying locations

The filesystem paths to the custom external certificate and custom private key in the $DCOS_INSTALL_DIR/genconf/ directory on the bootstrap node must be specified in the DC/OS configuration file using, respectively, the external_certificate_path and external_certificate_key_path parameters. The paths must be relative to $DCOS_INSTALL_DIR.

Configuration parameter reference

external_certificate_path

Path to a file (relative to $DCOS_INSTALL_DIR) containing a single X.509 leaf certificate in the OpenSSL PEM format. For example: genconf/external-certificate.crt. If necessary, it should include all the intermediate CA certificates up to the root CA certificate trusted by the clients. It is advised to not to include the final root CA certificate in the PEM file, but is not strictly necessary.

If provided, this is the custom external certificate. If not provided, the DC/OS cluster uses a unique leaf certificate generated during the initial bootstrap phase signed by DC/OS CA and presents it to the clients.

See the external_certificate_validation_disable parameter on the validation that the external certificate undergoes and how it can be overridden.

external_certificate_key_path

Path to a file (relative to $DCOS_INSTALL_DIR) containing the private key corresponding to the custom external certificate, encoded in the OpenSSL (PKCS#8) PEM format. For example: genconf/external-certificate-key.key.

This is highly sensitive data. The configuration processor accesses this file only for configuration validation purposes, and does not copy the data. After successful configuration validation this file needs to be placed out-of-band into the file system of all DC/OS master nodes to the path /var/lib/dcos/pki/tls/private/adminrouter-external.key before most DC/OS systemd units start up. The file must be readable by the root user, and should have 0600 permissions set.

This path is required if external_certificate_path is specified.

external_certificate_servernames

The list of server names that clients can use while accessing the cluster in order to make Admin Router present the custom external certificate to the client. The server names can include:

  • a plain hostname, e.g. host.example.com.
  • an IP address, e.g. 192.0.2.5.
  • a * wildcard, e.g. *.example.com, will match all the subdomains of example.com (e.g. foo.example.com, bar.example.com), but not the domain itself - example.com or deeper subdomains: foo.bar.example.com.
  • a . wildcard, e.g. .example.com, will match the same as * wildcard plus the domain itself (e.g. foo.example.com, bar.example.com, example.com).

The server names cannot include:

  • master.mesos
  • leader.mesos
  • registry.component.thisdcos.directory
  • any IP address returned by the /opt/mesosphere/bin/detect_ip script (created from the ip_detect_contents or ip_detect_filename configuration parameter).

Every server name provided in this option must be present in the custom external certificate’s Canonical Name or Subject Alternative Names.

This option is required if external_certificate_path is specified.

external_certificate_validation_disable

The custom external certificate undergoes validation before being taken into consideration. Namely:

  • the restrictions for server names mentioned for external_certificate_servernames parameter
  • certificate key must be of type RSA or ECC
  • minimum size of an RSA key is 2048
  • minimum size of the ECC key is 256
  • the ECC’s key curve must be one of SECP256R1 or SECP384R1,
  • key usage extension must be defined
  • the validity period of the certificate must not start in the future
  • the validity period of the certificate must not end sooner than in 365 days
  • the basic constraints are not required, but if they are defined, they should not have the CA flag set
  • key usage extension must have the digital signature and key encipherment extensions set
  • extended key usage extension must be present and tls server authentication flag must be set

Additionally, if the certificate chain is present in the external certificate’s PEM, the chain itself must:

  • start with the leaf certificate and be followed by intermediate CA’s, each one signing its predecessor and being signed by its successor
  • each intermediate CA cert must be a valid CA cert (expiration, etc…)

If these restrictions are too strict, they can be overridden by setting the external_certificate_validation_disable parameter to true.

Installing DC/OS Enterprise with a custom external certificate

Prerequisites

  • The installation of DC/OS Enterprise via the Installer has been prepared according to the corresponding documentation, up to the section Install DC/OS of that documentation.

  • The configuration parameters external_certificate_path, external_certificate_key_path and external_certificate_servernames are specified in the DC/OS configuration file $DCOS_INSTALL_DIR/genconf/config.yaml and point to the relevant locations in the file system. Example of commands issued on the bootstrap node:

cd $DCOS_INSTALL_DIR
cat genconf/config.yaml
[...]
external_certificate_path: genconf/external-certificate.crt
external_certificate_key_path: genconf/external-certificate-key.key
external_certificate_servernames:
  - foo.example.com
  - bar.example.com
  - .example.org
  - *.example.net
[...]

Copy custom external certificate and private key to the bootstrap node

The custom external certificate and the custom private key must be put in the $DCOS_INSTALL_DIR/genconf/ directory on the bootstrap node:

cd $DCOS_INSTALL_DIR
ls genconf/
external-certificate.crt
external-certificate-key.key

Manually copy the custom private key to all master nodes

For security reasons, the installer will not copy the private key from the bootstrap node to the master nodes. The custom private key must be distributed securely to every DC/OS master node before starting the installation.

The filesystem path for the custom private key file must be /var/lib/dcos/pki/tls/private/adminrouter-external.key. The file containing the private key adminrouter-external.key corresponding to the custom external certificate must be owned by the root Unix user and have 0600 permissions set. It is also recommended to ensure that the directory /var/lib/dcos/pki/tls/private is owned by root and has 0700 permissions.

If you copy the private key file over the network onto the master nodes, the network channel must be adequately protected.

For example, this could be achieved by running the following commands as root on each master node (W.X.Y.Z indicates the IP address of the bootstrap node):

mkdir -p /var/lib/dcos/pki/tls/private/
chown root /var/lib/dcos/pki/tls/private/
chmod 700 /var/lib/dcos/pki/tls/private/
scp centos@W.X.Y.Z:$DCOS_INSTALL_DIR/genconf/external-certificate-key.key /var/lib/dcos/pki/tls/private/
chown root /var/lib/dcos/pki/tls/private/adminrouter-external.key
chmod 600 /var/lib/dcos/pki/tls/private/adminrouter-external.key

Installation

Proceed with the installation as described in the documentation of the Installer. Note that the current working directory when executing dcos_generate_config.ee.sh must be the $DCOS_INSTALL_DIR directory.

If you are changing from a DC/OS CA issued certificate to a custom external certificate or vice versa, note that your clients may need to trust both CA’s during the upgrade.

Verify installation

To verify that the DC/OS Enterprise cluster was installed correctly with the custom external certificate, initiate a TLS connection to Admin Router which will present the custom external certificate for the configured server names. In order to do this, specify a server name in the request that will match one of the entries in external_certificate_servernames.

Provided that you have the CA root certificate for the signed external certificate, the following command can be executed:

openssl s_client -connect <public_ip_master_node_X>:443 -showcerts -servername <one of configured server names> -verifyCAfile <path to the CA file that signed external certificate/external certificate intermediate chain>  | grep -e "s:" -e "i:" -e "return code:" | grep -e "s:" -e "i:" -e "return code:"

The output should look like the following:

depth=4 C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Root CA
verify return:1
depth=3 C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 0
verify return:1
depth=2 C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 1
verify return:1
depth=1 C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 2
verify return:1
depth=0 C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Leaf Certificate
verify return:1
 0 s:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Leaf Certificate
   i:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 2
 1 s:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 2
   i:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 1
 2 s:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 1
   i:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 0
 3 s:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Intermediate CA 0
   i:C = US, ST = CA, L = San Francisco, O = "Mesosphere, Inc.", CN = Root CA
    Verify return code: 0 (ok)