prometheus config reloader

For the following examples, replace <tag> with the image variant you want to run. To confirm the correct namespace and repository name of the mirrored repository, select View in repository.

Start a Prometheus Config Reloader instance

Run the following command to see the binary's flags and defaults:

$ docker run --rm registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag> --help

Use --version to print the binary version:

$ docker run --rm registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag> --version

To run the reloader against a config file, mount the file into the container and pass --config-file and --reload-url:

$ docker run -d --name config-reloader \
  -v /path/to/prometheus.yml:/etc/prometheus/prometheus.yml:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag> \
  --config-file=/etc/prometheus/prometheus.yml \
  --reload-url=http://prometheus:9090/-/reload

The hostname in --reload-url must resolve on the network where the container runs. In the example above, prometheus should be a container name or service name reachable on a shared Docker network (for example, via docker network create and --network, or as a service in Docker Compose or Kubernetes). If the target is not reachable, the reloader logs a DNS or connection error and retries on the next tick; it does not exit.

The reloader watches the config file and any directories passed with --watched-dir (may be repeated). When it detects a change, it waits --delay-interval (default 1s) and then issues the configured reload. The full watch cycle runs every --watch-interval (default 3m0s).

Common Prometheus Config Reloader use cases

Reload Prometheus on config change

The defaults of prometheus-config-reloader target a Prometheus instance on localhost:9090. When Prometheus and the reloader share a network namespace (for example, running both in the same Docker network with aliases, or as sidecar containers in a single pod), the reloader POSTs to http://127.0.0.1:9090/-/reload on any detected change without any --reload-url override.

Prometheus must be started with --web.enable-lifecycle for the /-/reload endpoint to accept POST requests. Without this flag, the reloader receives 404 Not Found and retries on the next tick.

Reload Alertmanager on config change

To use the reloader with Alertmanager, override --reload-url to point at the Alertmanager reload endpoint:

$ docker run -d --name alertmanager-reloader \
  -v /path/to/alertmanager.yml:/etc/alertmanager/alertmanager.yml:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag> \
  --config-file=/etc/alertmanager/alertmanager.yml \
  --reload-url=http://alertmanager:9093/-/reload

Reload via signal instead of HTTP

When the target process's HTTP reload endpoint is disabled (for example, a Prometheus started without --web.enable-lifecycle), use --reload-method=signal. The reloader sends SIGHUP to a process named in --process-executable-name:

$ docker run -d --name config-reloader \
  --pid=container:<target-container> \
  --network=container:<target-container> \
  --user 65532:65532 \
  -v /path/to/config.yml:/etc/prometheus/prometheus.yml:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag> \
  --config-file=/etc/prometheus/prometheus.yml \
  --reload-method=signal \
  --process-executable-name=prometheus

Signal reload has three strict requirements that differ from HTTP reload:

• Shared PID namespace (--pid=container:... in Docker, automatic for containers in the same Kubernetes pod). Without it, the reloader can't see the target process to signal it.
• Shared network namespace (--network=container:... in Docker, also automatic in a Kubernetes pod). Signal mode still performs an HTTP pre-reload check against --runtimeinfo-url (default http://127.0.0.1:9090/api/v1/status/runtimeinfo) and a post-reload confirmation, so the target's HTTP port must be reachable.
• Matching UIDs. The reloader runs as UID 65532 and can only signal processes owned by the same UID. If the target runs as a different non-root user (for example, upstream Prometheus defaults to UID 65534), the reloader gets failed to send SIGHUP to pid N: operation not permitted. Run the target with --user 65532:65532 or configure a matching securityContext.runAsUser: 65532 in Kubernetes.

Use with the Prometheus Operator

The Prometheus Operator deploys prometheus-config-reloader as a sidecar automatically in the pods it manages. To tell the operator to use a specific reloader image (for example, the Ghost hardened image), pass --prometheus-config-reloader to the operator at startup:

--prometheus-config-reloader=registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag>

For FIPS-compliant clusters, use a tag that includes fips, for example registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<version>-debian13-fips.

The operator then references this image in the Prometheus and Alertmanager CRD pod specs it creates. This flag only takes effect when the operator runs inside a Kubernetes cluster with access to the API server — it cannot be tested standalone with docker run.

Enable metrics

The reloader's own metrics endpoint is disabled by default. To enable it, set --listen-address:

$ docker run -d --name config-reloader -p 8080:8080 \
  -v /path/to/prometheus.yml:/etc/prometheus/prometheus.yml:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag> \
  --config-file=/etc/prometheus/prometheus.yml \
  --reload-url=http://prometheus:9090/-/reload \
  --listen-address=:8080

Reloader metrics are then available at http://localhost:8080/metrics in Prometheus exposition format. Key counters include:

• reloader_config_apply_operations_total — number of config-apply cycles triggered by detected file changes (not individual HTTP POST attempts)
• reloader_config_apply_operations_failed_total — apply cycles that exhausted all retries without success
• prometheus_config_reloader_build_info — gauge labeled with the reloader's version, revision, branch, and Go version for fleet inventory

Non-hardened images vs. Ghost hardened images

Key differences

Why no shell or package manager?

Ghost hardened images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened image contains only the prometheus-config-reloader binary and its required libraries — no shell, no coreutils, no package manager, no editors. Common debugging methods for applications built with Ghost hardened images include:

• Docker Debug to attach to containers
• Docker's Image Mount feature to mount debugging tools

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. For example:

$ docker debug config-reloader

Or mount debugging tools with the Image Mount feature:

$ docker run --rm -it --pid container:config-reloader \
  --mount=type=image,source=registry.ghost-prod.alphabravo.io/ghost-base/busybox:1,destination=/dbg,ro \
  --entrypoint /dbg/bin/sh \
  registry.ghost-prod.alphabravo.io/ghost-base/prometheus-config-reloader:<tag>

For operational visibility without attaching a debugger, enable the reloader's metrics endpoint with --listen-address (see Enable metrics).

Image variants

Ghost hardened images come in different variants depending on their intended use. Image variants are identified by their tag.

• 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 tag 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. They come in both runtime and build-time variants. 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.

To view the image variants and get more information about them, select the Tags tab for this repository, and then select a tag.

For debugging minimal runtime containers without a shell, you can use Docker Debug to attach to running containers.

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.

For the reloader specifically, most operational debugging can be done through logs and the optional metrics endpoint. Use --log-level=debug for verbose reload activity, and enable --listen-address=:8080 to expose reload counters at /metrics.

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.

The reloader reads the file passed to --config-file and any directories passed to --watched-dir. These must be readable by UID 65532. If --config-envsubst-file is set, the output path must be writable by the same UID.

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.

The reloader does not listen on any ports by default. When --listen-address is set for metrics, use a port above 1024, for example :8080.

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.