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Using ExternalDNS to automate DNS setup

ExternalDNS is a tool that synchronizes exposed Kubernetes Services and Ingresses with DNS providers.

This doc explains how to set up ExternalDNS within a Knative cluster using Google Cloud DNS to automate the process of publishing the Knative domain.

Prerequisite

A Google Kubernetes Engine cluster with Cloud DNS scope. You can create a GKE cluster with Cloud DNS scope by entering the following command:
```
gcloud container clusters create "external-dns" \
    --scopes "https://www.googleapis.com/auth/ndev.clouddns.readwrite"
```
Knative Serving installed on your cluster.
A public domain that will be used in Knative.
Knative configured to use your custom domain.

kubectl edit cm config-domain --namespace knative-serving

This command opens your default text editor and allows you to edit the config map.

apiVersion: v1
data:
  example.com: ""
kind: ConfigMap
[...]

Edit the file to replace example.com with the domain you'd like to use and save your changes. In this example, we use domain external-dns-test.my-org.do for all routes:

apiVersion: v1
data:
  external-dns-test.my-org.do: ""
kind: ConfigMap
[...]

Setup steps

This guide uses Google Cloud Platform as an example to show how to set up ExternalDNS. You can find detailed instructions for other cloud providers in the ExternalDNS documentation.

Choose a DNS provider

Skip this step if you already have a DNS provider for your domain.

Here is a list of DNS providers supported by ExternalDNS. Choose a DNS provider from the list.

Create a DNS zone for managing DNS records

Skip this step if you already have a zone for managing the DNS records of your custom domain.

A DNS zone which will contain the managed DNS records needs to be created. Assume your custom domain is external-dns-test.my-org.do.

Use the following command to create a DNS zone with Google Cloud DNS:

gcloud dns managed-zones create "external-dns-zone" \
    --dns-name "external-dns-test.my-org.do." \
    --description "Automatically managed zone by kubernetes.io/external-dns"

Make a note of the nameservers that were assigned to your new zone.

gcloud dns record-sets list \
    --zone "external-dns-zone" \
    --name "external-dns-test.my-org.do." \
    --type NS

You should see output similar to the following:

NAME                             TYPE  TTL    DATA
external-dns-test.my-org.do.  NS    21600  ns-cloud-e1.googledomains.com.,ns-cloud-e2.googledomains.com.,ns-cloud-e3.googledomains.com.,ns-cloud-e4.googledomains.com.

In this case, the DNS nameservers are ns-cloud-{e1-e4}.googledomains.com. Yours could differ slightly, e.g. {a1-a4}, {b1-b4} etc.

If this zone has the parent zone, you need to add NS records of this zone into the parent zone so that this zone can be found from the parent. Assuming the parent zone is my-org-do and the parent domain is my-org.do, and the parent zone is also hosted at Google Cloud DNS, you can follow these steps to add the NS records of this zone into the parent zone:

gcloud dns record-sets transaction start --zone "my-org-do"
gcloud dns record-sets transaction add ns-cloud-e{1..4}.googledomains.com. \
    --name "external-dns-test.my-org.do." --ttl 300 --type NS --zone "my-org-do"
gcloud dns record-sets transaction execute --zone "my-org-do"

Deploy ExternalDNS

Use the following command to apply the manifest to install ExternalDNS

cat <<EOF | kubectl apply --filename -
<the-content-of-manifest-with-custom-domain-filter>
EOF

Note that you need to set the argument domain-filter to your custom domain.

You should see ExternalDNS is installed by running:

kubectl get deployment external-dns

Configuring Knative Gateway service

In order to publish the Knative Gateway service, the annotation external-dns.alpha.kubernetes.io/hostname: '*.external-dns-test.my-org.do' needs to be added into Knative gateway service:

kubectl edit svc knative-ingressgateway --namespace istio-system

This command opens your default text editor and allows you to add the annotation to knative-ingressgateway service. After you've added your annotation, your file may look similar to this:

apiVersion: v1
kind: Service
metadata:
  annotations:
    external-dns.alpha.kubernetes.io/hostname: '*.external-dns-test.my-org.do'
    ...

Verify ExternalDNS works

After roughly two minutes, check that a corresponding DNS record for your service was created.

gcloud dns record-sets list     --zone "external-dns-zone"     --name "*.external-dns-test.my-org.do."

You should see output similar to:

NAME                            TYPE  TTL  DATA
*.external-dns-test.my-org.do.  A     300  35.231.248.30
*.external-dns-test.my-org.do.  TXT   300  "heritage=external-dns,external-dns/owner=my-identifier,external-dns/resource=service/istio-system/knative-ingressgateway"

Verify domain has been published

You can check if the domain has been published to the Internet be entering the following command:

host test.external-dns-test.my-org.do

You should see the below result after the domain is published:

test.external-dns-test.my-org.do has address 35.231.248.30

Note: The process of publishing the domain to the Internet can take several minutes.

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