cert exporter

Common cert-exporter Hardened Image use cases

This guide provides practical examples for using the cert-exporter Hardened Image to monitor x509 certificate expiration in Kubernetes clusters or standalone environments.

Run a cert-exporter container

Watch a certificate directory

docker run -d --name cert-exporter \
    -p 8080:8080 \
    -v /path/to/certs:/certs:ro \
    registry.ghost-prod.alphabravo.io/ghost-base/cert-exporter:<tag> \
    '--include-cert-glob=/certs/*.pem'

With a custom listen address

docker run -d --name cert-exporter \
    -p 9090:9090 \
    -v /path/to/certs:/certs:ro \
    registry.ghost-prod.alphabravo.io/ghost-base/cert-exporter:<tag> \
    '--include-cert-glob=/certs/*.pem' \
    --prometheus-listen-address=:9090

Run with Docker Compose

Monitor a certificate directory

1. Create docker-compose.yaml:

services:
  cert-exporter:
    image: registry.ghost-prod.alphabravo.io/ghost-base/cert-exporter:<tag>
    ports:
      - "8080:8080"
    volumes:
      - /etc/ssl/certs:/certs:ro
    command:
      - --include-cert-glob=/certs/*.pem
    restart: unless-stopped

1. Start the exporter:

docker compose up -d

1. Verify it's running:

curl http://localhost:8080/metrics | grep cert_exporter

Integration with Prometheus

1. Create docker-compose.yaml with Prometheus:

services:
  cert-exporter:
    image: registry.ghost-prod.alphabravo.io/ghost-base/cert-exporter:<tag>
    ports:
      - "8080:8080"
    volumes:
      - /etc/ssl/certs:/certs:ro
    command:
      - --include-cert-glob=/certs/*.pem
    restart: unless-stopped

  prometheus:
    image: prom/prometheus:latest
    ports:
      - "9090:9090"
    volumes:
      - ./prometheus.yml:/etc/prometheus/prometheus.yml:ro
    command:
      - --config.file=/etc/prometheus/prometheus.yml
    depends_on:
      - cert-exporter

1. Create prometheus.yml:

global:
  scrape_interval: 60s

scrape_configs:
  - job_name: 'cert-exporter'
    static_configs:
      - targets: ['cert-exporter:8080']

1. Start the stack:

docker compose up -d

1. Access Prometheus at http://localhost:9090 and query certificate metrics:

# Certificates expiring in less than 30 days
cert_exporter_cert_expires_in_seconds < 86400 * 30

# Seconds until a certificate in a Kubernetes secret expires
cert_exporter_secret_expires_in_seconds

Use cert-exporter in Kubernetes

Use the same container arguments in Kubernetes that you use with docker run. For example, mount certificate files and scan them with:

args:
  - --include-cert-glob=/host-certs/*.crt
ports:
  - name: metrics
    containerPort: 8080
readinessProbe:
  httpGet:
    path: /metrics
    port: metrics

To monitor Kubernetes secrets, grant the service account get, list, and watch permissions for secrets and pass flags such as --secrets-namespace=<namespace> and --secrets-include-glob=*.crt. For Prometheus Operator, configure a ServiceMonitor endpoint with path: /metrics and the metrics port.

Common metrics

The exporter exposes Prometheus metrics for both certificate state and exporter health. Common metrics include:

Command-line options

Common command-line flags:

Image variants

Ghost hardened images come in different variants depending on their intended use.

• Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

  • Run as the nonroot user
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically include dev in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

  • Run as the root user
  • Include a shell and package manager
  • Are used to build or compile applications

• FIPS variants include fips in the variant name and tag. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. For example, usage of MD5 fails in FIPS variants.

Migrate to a Ghost hardened image

To migrate your application to a Ghost hardened image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Ghost hardened image. This and a few other common changes are listed in the following table of migration notes.

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages

   Ghost hardened images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshooting migration

The following are common issues that you may encounter during migration.

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Ghost hardened images is to use Docker Debug to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. To avoid issues, configure your application to listen on port 1025 or higher inside the container, even if you map it to a lower port on the host. For example, docker run -p 80:8080 my-image will work because the port inside the container is 8080, and docker run -p 80:81 my-image won't work because the port inside the container is 81.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Ghost hardened images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Ghost hardened images and update your Dockerfile if necessary.