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Containerized CoreDNS on Container Optimized OS

This module manages a cloud-config configuration that starts a containerized CoreDNS service on Container Optimized OS, using the official image.

The resulting cloud-config can be customized in a number of ways:

a custom CoreDNS configuration can be set using the coredns_config variable
additional files (eg for hosts or zone files) can be passed in via the files variable
a completely custom cloud-config can be passed in via the cloud_config variable, and additional template variables can be passed in via config_variables

The default instance configuration inserts iptables rules to allow traffic on the DNS TCP and UDP ports, and the 8080 port for the optional HTTP health check that can be enabled via the CoreDNS health plugin.

Logging and monitoring are enabled via the Google Cloud Logging agent configured for the instance via the google-logging-enabled metadata property, and the Node Problem Detector service is started by default on boot.

The module renders the generated cloud config in the cloud_config output, to be used in instances or instance templates via the user-data metadata.

For convenience during development or for simple use cases, the module can optionally manage a single instance via the test_instance variable. If the instance is not needed the instance*tf files can be safely removed. Refer to the top-level README for more details on the included instance.

Examples

Default CoreDNS configuration

This example will create a cloud-config that uses the module's defaults, creating a simple DNS forwarder.

module "cos-coredns" {
  source           = "./fabric/modules/cloud-config-container/coredns"
}

# use it as metadata in a compute instance or template
module "vm-coredns" {
  source = "./fabric/modules/compute-vm"
  metadata = {
    user-data              = module.cos-coredns.cloud_config
    google-logging-enabled = true
  }
}

Custom CoreDNS configuration

This example will create a cloud-config using a custom CoreDNS configuration, that leverages the CoreDNS hosts plugin to serve a single zone via an included hosts format file.

module "cos-coredns" {
  source           = "./fabric/modules/cloud-config-container/coredns"
  coredns_config = "./fabric/modules/cloud-config-container/coredns/Corefile-hosts"
  files = {
    "/etc/coredns/example.hosts" = {
      content     = "127.0.0.2 foo.example.org foo"
      owner       = null
      permissions = "0644"
    }
}

CoreDNS instance

This example shows how to create the single instance optionally managed by the module, providing all required attributes in the test_instance variable. The instance is purposefully kept simple and should only be used in development, or when designing infrastructures.

module "cos-coredns" {
  source           = "./fabric/modules/cloud-config-container/coredns"
  test_instance = {
    project_id = "my-project"
    zone       = "europe-west1-b"
    name       = "cos-coredns"
    type       = "f1-micro"
    network    = "default"
    subnetwork = "https://www.googleapis.com/compute/v1/projects/my-project/regions/europe-west1/subnetworks/my-subnet"
  }
}

Variables

name	description	type	default
cloud_config	Cloud config template path. If null default will be used.	`string`	`null`
config_variables	Additional variables used to render the cloud-config and CoreDNS templates.	`map(any)`	`{}`
coredns_config	CoreDNS configuration path, if null default will be used.	`string`	`null`
file_defaults	Default owner and permissions for files.	`object({…})`	`{…}`
files	Map of extra files to create on the instance, path as key. Owner and permissions will use defaults if null.	`map(object({…}))`	`{}`
test_instance	Test/development instance attributes, leave null to skip creation.	`object({…})`	`null`
test_instance_defaults	Test/development instance defaults used for optional configuration. If image is null, COS stable will be used.	`object({…})`	`{…}`

Outputs

name	description	sensitive
cloud_config	Rendered cloud-config file to be passed as user-data instance metadata.
test_instance	Optional test instance name and address.

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