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Advanced features

The features that are not required for basic usage, but are needed for certain environments are described in this section.

Zone cache

The zone cache is enabled by setting a value for the environment variable ZONE_CACHE_TTL greater than zero. This parameter allows the webhook to download once the list of zones and keep using it for the given number of seconds. When set to zero (default value) the zone cache is disabled and the zones will be reloaded every time the webhook is called by ExternalDNS.

Hetzner labels

Note

This feature is not available when bulk mode is activated.

Hetzner labels are supported since version 0.8.0 as provider-specific annotations.

The labels are set with an annotation prefixed with: external-dns.alpha.kubernetes.io/webhook-hetzner-label-.

For example, if we want to set these labels:

Label Value
it.env production
department education

The annotation syntax will be:

  external-dns.alpha.kubernetes.io/webhook-hetzner-label-it.env: production
  external-dns.alpha.kubernetes.io/webhook-hetzner-label-department: education

This kind of label:

Label Value
prefix/label value

requires an escape sequence for the slash part. By default this will be: --slash--, so the label will be written as:

  external-dns.alpha.kubernetes.io/webhook-hetzner-label-prefix--slash--label: value

This can be changed using the SLASH_ESC_SEQ environment variable.

Bulk mode

This mode is activated by setting the BULK_MODE environment variable to true. It works by exporting the zonefile, editing it and then uploading the modified version. It is meant to be used in environments with a high number of record changes per zone and a relatively long interval between the updates, a combination that could cause the exhaustion of the permitted API calls.

Danger

Beware that this method of updating the records is potentially destructive and subject to "race conditions" if manual edits are applied while the zone is being updated. Theoretically, unsupported records won't be affected, but this method is to be considered HIGHLY EXPERIMENTAL, and bugs are likely to be found.

It comes with some limitations.

  1. Hetzner labels are not supported, as there is no way to import them in the zonefile. Record comments are unsupported as well.
  2. All the records must be not protected as they will all be overwritten during the import operation, including the SOA. This is why the bulk mode should be used with care.
  3. The SOA serial number is updated on each import, but the logic of the serial number only accepts the standard 10-digits serial number and will refuse to update it if the serial of the day is 99. Most configurations will be OK with this limitation.

  4. If the zones managed by this webhook are also manipuilated by other software the following situation, although unlikely, could happen:

    1. the webhook downloads the zonefile for a zone
    2. the other software manipulates some record on the same zone
    3. the webhook uploads the zonefile, missing the changes applied by the other software
    4. since the upload rewrites the zonefile, those changes are now lost.

Please check the Zone file import section of the Hetzner documentation for more details.