ExternalDNS - Hetzner Webhook

⚠️ This software is experimental. ⚠️

ExternalDNS is a Kubernetes add-on for automatically DNS records for Kubernetes services using different providers. By default, Kubernetes manages DNS records internally, but ExternalDNS takes this functionality a step further by delegating the management of DNS records to an external DNS provider such as this one. This webhook allows you to manage your Hetzner domains inside your kubernetes cluster.

ℹ️ If you are upgrading to 0.7.x from 0.6.x read the Upgrading from previous versions section.

Requirements

An API token for the account managing your domains is required for this webhook to work properly.

This webhook can be used in conjunction with ExternalDNS v0.14.0 or higher, configured for using the webhook interface. Some examples for a working configuration are shown in the next section.

Kubernetes Deployment

The Hetzner webhook is provided as a regular Open Container Initiative (OCI) image released in the GitHub container registry. The deployment can be performed in every way Kubernetes supports.

Here are provided examples using the External DNS chart and the Bitnami chart.

In either case, a secret that stores the Hetzner API key is required:

kubectl create secret generic hetzner-credentials --from-literal=api-key='<EXAMPLE_PLEASE_REPLACE>' -n external-dns

Using the ExternalDNS chart

Skip this step if you already have the ExternalDNS repository added:

helm repo add external-dns https://kubernetes-sigs.github.io/external-dns/

Update your helm chart repositories:

helm repo update

You can then create the helm values file, for example external-dns-hetzner-values.yaml:

namespace: external-dns
policy: sync
provider:
  name: webhook
  webhook:
    image:
      repository: ghcr.io/mconfalonieri/external-dns-hetzner-webhook
      tag: v0.7.0
    env:
      - name: HETZNER_API_KEY
        valueFrom:
          secretKeyRef:
            name: hetzner-credentials
            key: api-key
    livenessProbe:
      httpGet:
        path: /health
        port: http-wh-metrics
      initialDelaySeconds: 10
      timeoutSeconds: 5
    readinessProbe:
      httpGet:
        path: /ready
        port: http-wh-metrics
      initialDelaySeconds: 10
      timeoutSeconds: 5

extraArgs:
  - "--txt-prefix=reg-%{record_type}-"

And then:

# install external-dns with helm
helm install external-dns-hetzner external-dns/external-dns -f external-dns-hetzner-values.yaml --version 0.15.0 -n external-dns

Using the Bitnami chart

Skip this step if you already have the Bitnami repository added:

helm repo add bitnami https://charts.bitnami.com/bitnami

Update your helm chart repositories:

helm repo update

You can then create the helm values file, for example external-dns-hetzner-values.yaml:

provider: webhook
policy: sync
extraArgs:
  webhook-provider-url: http://localhost:8888
  txt-prefix: "reg-%{record_type}-"

sidecars:
  - name: hetzner-webhook
    image: ghcr.io/mconfalonieri/external-dns-hetzner-webhook:v0.7.0
    ports:
      - containerPort: 8888
        name: webhook
      - containerPort: 8080
        name: http-wh-metrics
    livenessProbe:
      httpGet:
        path: /health
        port: http-wh-metrics
      initialDelaySeconds: 10
      timeoutSeconds: 5
    readinessProbe:
      httpGet:
        path: /ready
        port: http-wh-metrics
      initialDelaySeconds: 10
      timeoutSeconds: 5
    env:
      - name: HETZNER_API_KEY
        valueFrom:
          secretKeyRef:
            name: hetzner-credentials
            key: api-key

And then:

# install external-dns with helm
helm install external-dns-hetzner bitnami/external-dns -f external-dns-hetzner-values.yaml -n external-dns

Upgrading from previous versions

0.6.x to 0.7.x

The configuration for previous versions are still compatible, but consider that some warnings will be emitted if HEALTH_HOST and HEALTH_PORT are set. The changes to be aware of are:

• HEALTH_HOST is deprecated in favor of METRICS_HOST;

• HEALTH_PORT is deprecated in favor of METRICS_PORT;

• the previous health/public socket is now called "metrics socket" in conformity to ExternalDNS terminology, and now supports some additional endpoints:

  • /metrics and
  • /healthz;

  their description can be found in the Metrics socket section.

Environment variables

The following environment variables can be used for configuring the application.

Hetzner DNS API calls configuration

These variables control the behavior of the webhook when interacting with Hetzner DNS API.

Variable	Description	Notes
HETZNER_API_KEY	Hetzner API token	Mandatory
BATCH_SIZE	Number of zones per call	Default: 100, max: 100
DEFAULT_TTL	Default record TTL	Default: 7200

Test and debug

These environment variables are useful for testing and debugging purposes.

Variable	Description	Notes
DRY_RUN	If set, changes won't be applied	Default: false
HETZNER_DEBUG	Enables debugging messages	Default: false

Socket configuration

These variables control the sockets that this application listens to.

Variable	Description	Notes
WEBHOOK_HOST	Webhook hostname or IP address	Default: localhost
WEBHOOK_PORT	Webhook port	Default: 8888
METRICS_HOST	Metrics hostname	Default: 0.0.0.0
METRICS_PORT	Metrics port	Default: 8080
READ_TIMEOUT	Sockets' read timeout in ms	Default: 60000
WRITE_TIMEOUT	Sockets' write timeout in ms	Default: 60000

Please notice that the following variables were deprecated:

Variable	Description
HEALTH_HOST	Metrics hostname (deprecated)
HEALTH_PORT	Metrics port (deprecated)

Domain filtering

Additional environment variables for domain filtering. When used, this webhook will be able to work only on domains matching the filter.

Environment variable	Description
DOMAIN_FILTER	Filtered domains
EXCLUDE_DOMAIN_FILTER	Excluded domains
REGEXP_DOMAIN_FILTER	Regex for filtered domains
REGEXP_DOMAIN_FILTER_EXCLUSION	Regex for excluded domains

If the REGEXP_DOMAIN_FILTER is set, the following variables will be used to build the filter:

• REGEXP_DOMAIN_FILTER
• REGEXP_DOMAIN_FILTER_EXCLUSION

otherwise, the filter will be built using:

• DOMAIN_FILTER
• EXCLUDE_DOMAIN_FILTER

Endpoints

This process exposes several endpoints, that will be available through these sockets:

Socket name	Socket address
Webhook	WEBHOOK_HOST:WEBHOOK_PORT
Metrics	METRICS_HOST:METRICS_PORT

The environment variables controlling the socket addresses are not meant to be changed, under normal circumstances, for the reasons explained in Tweaking the configuration. The endpoints expected by ExternalDNS are marked with *.

Webhook socket

All these endpoints are required by ExternalDNS.

Endpoint	Purpose
/	Initialization and DomainFilter negotiations
/record	Get and apply records
/adjustendpoints	Adjust endpoints before submission

Metrics socket

ExternalDNS doesn't have functional requirements for this endpoint, but some of them are recommended. In this table those endpoints are marked with *.

Endpoint	*	Purpose
/health		Implements the liveness probe
/ready		Implements the readiness probe
/healthz	*	Implements a combined liveness and readiness probe
/metrics	*	Exposes the available metrics

Please check the Exposed metrics section for more information.

Tweaking the configuration

While tweaking the configuration, there are some points to take into consideration:

• if WEBHOOK_HOST and METRICS_HOST are set to the same address/hostname or one of them is set to 0.0.0.0 remember to use different ports. Please note that it highly recommendend for WEBHOOK_HOST to be localhost, as any address reachable from outside the pod might be a security issue; besides this, changing these would likely need more tweaks than just setting the environment variables. The default settings are compatible with the ExternalDNS assumptions;

• if your records don't get deleted when applications are uninstalled, you might want to verify the policy in use for ExternalDNS: if it's upsert-only no deletion will occur. It must be set to sync for deletions to be processed. Please check that external-dns-hetzner-values.yaml include:

  policy: sync

• the --txt-prefix parameter should really include: %{record_type}, as any other value will cause a weird duplication of database records. Change the value provided in the sample configuration only if you really know what are you doing.

Exposed metrics

The following metrics related to the API calls towards Hetzner are available for scraping.

Name	Type	Labels	Description
successful_api_calls_total	Counter	action	The number of successful Hetzner API calls
failed_api_calls_total	Counter	action	The number of Hetzner API calls that returned an error
filtered_out_zones	Gauge	none	The number of zones excluded by the domain filter
skipped_records	Gauge	zone	The number of skipped records per domain
api_delay_hist	Histogram	action	Histogram of the delay (ms) when calling the Hetzner API

The label action can assume one of the following values, depending on the Hetzner API endpoint called:

• get_zones
• get_records
• create_record
• delete_record
• update_record

The label zone can assume one of the zone names as its value.

Please notice that in some cases an update request from ExternalDNS will be transformed into a delete_record and subsequent create_record calls by this webhook.

Development

The basic development tasks are provided by make. Run make help to see the available targets.

Credits

This Webhook was forked and modified from the IONOS Webhook to work with Hetzner. It also contains huge parts from DrBu7cher's Hetzner provider.

Contributors

Name	Contribution
DerQue	local CNAME fix
sschaeffner	build configuration for arm64