prom-label-proxy

packaged by Chainguard

Chainguard Container for prom-label-proxy

Minimalist image for prom-label-proxy, which enforces a tenant label in PromQL queries so callers of a shared Prometheus, Thanos, or Alertmanager only see their own data

Chainguard Containers are regularly-updated, secure-by-default container images.

Download this Container Image

For those with access, this container image is available on cgr.dev:

docker pull cgr.dev/ORGANIZATION/prom-label-proxy:latest

Be sure to replace the ORGANIZATION placeholder with the name used for your organization's private repository within the Chainguard Registry.

Compatibility Notes

Chainguard's prom-label-proxy image is comparable to the upstream prom-label-proxy image and is designed to be a drop-in replacement for it. It runs as UID 65534 (nobody) and uses the same entrypoint path, /bin/prom-label-proxy, as the upstream image. The one difference to be aware of is that this image does not include a shell or package manager.

prom-label-proxy has no --version flag. Upstream does not link the version package into the binary, so use the image tag to identify the release you are running.

Prerequisites

The proxy does not store data of its own; it needs an upstream to forward to. That upstream is a Prometheus, Thanos, or Alertmanager API, and the series it serves must carry the label you intend to enforce. Refer to the upstream documentation for the endpoints supported against each upstream.

Getting Started

The proxy requires two flags: --upstream, the API it forwards to, and --label, the label it enforces on every query. Start by confirming the image runs and listing the available flags:

docker run --rm cgr.dev/ORGANIZATION/prom-label-proxy:latest --help

To see it enforce a label, run it in front of a Prometheus instance on a shared Docker network:

docker network create promnet
docker run -d --name prometheus --network promnet cgr.dev/ORGANIZATION/prometheus:latest
docker run -d --name prom-label-proxy --network promnet -p 8080:8080 \
  cgr.dev/ORGANIZATION/prom-label-proxy:latest \
  --insecure-listen-address=0.0.0.0:8080 \
  --upstream=http://prometheus:9090 \
  --label=namespace

Confirm the proxy is serving:

curl -s http://localhost:8080/healthz

{"ok":true}

A query must now name the tenant it is asking about. The proxy rewrites the PromQL expression so it is scoped to that tenant before forwarding it upstream:

curl -sG --data-urlencode 'query=up' --data-urlencode 'namespace=production' \
  http://localhost:8080/api/v1/query

A query that names no tenant is rejected rather than forwarded, which is what makes the proxy safe to expose to callers you do not trust:

curl -s -o /dev/null -w '%{http_code}\n' -G --data-urlencode 'query=up' \
  http://localhost:8080/api/v1/query

400

Configuration

The tenant value can come from one of three mutually exclusive sources: a query parameter (--query-param, the default, named after --label), an HTTP header (--header-name), or a fixed value baked into the process (--label-value). Header-based extraction is usually the right choice in front of an authenticating proxy, because the tenant is then set by infrastructure the caller cannot influence rather than by a parameter the caller supplies.

A hardened multi-tenant configuration looks like this:

docker run -d --name prom-label-proxy --network promnet -p 8080:8080 \
  cgr.dev/ORGANIZATION/prom-label-proxy:latest \
  --insecure-listen-address=0.0.0.0:8080 \
  --internal-listen-address=127.0.0.1:8081 \
  --upstream=http://prometheus:9090 \
  --label=namespace \
  --header-name=X-Namespace \
  --error-on-replace

--error-on-replace is the flag that matters most for isolation. Without it, a query that already contains a matcher for the enforced label has that matcher rewritten; with it, the request is rejected instead, so a caller cannot probe another tenant and receive silently altered results. --internal-listen-address moves the proxy's own metrics and pprof endpoints onto a separate port, keeping them off the listener that serves tenants.

When the upstream is reached over TLS, supply the trust anchor with --upstream-ca-cert, and add --upstream-client-cert and --upstream-client-key if the upstream requires mutual TLS.

To deploy with the upstream Helm chart, point it at this image with a values file:

cat > values.yaml <<EOF
image:
  repository: cgr.dev/ORGANIZATION/prom-label-proxy
  tag: latest
config:
  upstream: http://prometheus.monitoring.svc:9090
  label: namespace
EOF

helm install prom-label-proxy prometheus-community/prom-label-proxy --values values.yaml

Documentation and Resources

• prom-label-proxy upstream repository
• prom-label-proxy Helm chart
• Prometheus HTTP API reference
• Chainguard Academy

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