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FQDN Templating Guide

What is FQDN Templating?

FQDN templating is a feature that allows to dynamically construct Fully Qualified Domain Names (FQDNs) using a Go templating engine.
Instead of relying solely on annotations or static names, you can use metadata from Kubernetes objects—such as service names, namespaces, and labels—to generate DNS records programmatically and dynamically.

This is useful for:

  • Creating consistent naming conventions across environments.
  • Reducing boilerplate annotations.
  • Supporting multi-tenant or dynamic environments.
  • Migrating from one DNS scheme to another
  • Supporting multiple variants, such as a regional one and then one that doesn’t or similar.

How It Works

ExternalDNS has a flag: --fqdn-template, which defines a Go template for rendering the desired DNS names.

The template uses the following data from the source object (e.g., a Service or Ingress):

Field Description
Name Name of the object (e.g., service)
Namespace Namespace of the object
Labels Map of labels applied to the object
Annotations Map of annotations
TargetName For Service, it’s the service name; for Ingress, the hostname
Endpoint Contains more contextual endpoint info, such as IP/target
Controller Controller type (optional)

Supported Sources

Source Description FQDN Supported
ambassador-host Queries Ambassador Host resources for endpoints.
cloudfoundry Queries Cloud Foundry resources for endpoints.
connector Queries a custom connector source for endpoints.
contour-httpproxy Queries Contour HTTPProxy resources for endpoints.
crd Queries Custom Resource Definitions (CRDs) for endpoints.
empty Uses an empty source, typically for testing or no-op scenarios.
f5-transportserver Queries F5 TransportServer resources for endpoints.
f5-virtualserver Queries F5 VirtualServer resources for endpoints.
fake Uses a fake source for testing purposes.
gateway-grpcroute Queries GRPCRoute resources from the Gateway API.
gateway-httproute Queries HTTPRoute resources from the Gateway API.
gateway-tcproute Queries TCPRoute resources from the Gateway API.
gateway-tlsroute Queries TLSRoute resources from the Gateway API.
gateway-udproute Queries UDPRoute resources from the Gateway API.
gloo-proxy Queries Gloo Proxy resources for endpoints.
ingress Queries Kubernetes Ingress resources for endpoints.
istio-gateway Queries Istio Gateway resources for endpoints.
istio-virtualservice Queries Istio VirtualService resources for endpoints.
kong-tcpingress Queries Kong TCPIngress resources for endpoints.
node Queries Kubernetes Node resources for endpoints.
openshift-route Queries OpenShift Route resources for endpoints.
pod Queries Kubernetes Pod resources for endpoints.
service Queries Kubernetes Service resources for endpoints.
skipper-routegroup Queries Skipper RouteGroup resources for endpoints.
traefik-proxy Queries Traefik Proxy resources for endpoints.

Custom Functions

Function Description
trimPrefix Function from the strings package. Returns string without the provided leading prefix.
replace Function that performs a simple replacement of all old string with new in the source string.
isIPv4 Function that checks if a string is a valid IPv4 address.
isIPv6 Function that checks if a string is a valid IPv6 address (including IPv4-mapped IPv6).

Example Usage

These examples should provide a solid foundation for implementing FQDN templating in your ExternalDNS setup.
If you have specific requirements or encounter issues, feel free to explore the issues or update this guide.

Basic Usage

apiVersion: v1
kind: Service
metadata:
  name: my-service
  namespace: my-namespace
external-dns \
  --provider=aws \
  --source=service \
  --fqdn-template="{{ .Name }}.example.com,{{ .Name }}.{{ .Namespace }}.example.tld"

# This will result in DNS entries like
>route53> my-service.example.com
>route53> my-service.my-namespace.example.tld

With Namespace

---
apiVersion: v1
kind: Service
metadata:
  name: my-service
  namespace: default
---
apiVersion: v1
kind: Service
metadata:
  name: other-service
  namespace: kube-system
args:
  --fqdn-template="{{.Name}}.{{.Namespace}}.example.com"

# This will result in DNS entries like
# route53> my-service.default.example.com
# route53> other-service.kube-system.example.com

Using Labels in Templates

You can also utilize labels in your FQDN templates to create more dynamic DNS entries. Assuming your service has:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  labels:
    environment: staging
args:
  --fqdn-template="{{ .Labels.environment }}.{{ .Name }}.example.com"

# This will result in DNS entries like
# route53> staging.my-service.example.com

Multiple FQDN Templates

ExternalDNS allows specifying multiple FQDN templates, which can be useful when you want to create multiple DNS entries for a single service or ingress.

Be cautious, as this will create multiple DNS records per resource, potentially increasing the number of API calls to your DNS provider.

args:
  --fqdn-template={{.Name}}.example.com,{{.Name}}.svc.example.com

Conditional Templating combined with Annotations processing

In scenarios where you want to conditionally generate FQDNs based on annotations, you can use Go template functions like or to provide defaults.

args:
  - --combine-fqdn-annotation # this is required to combine FQDN templating and annotation processing
  - --fqdn-template={{ or .Annotations.dns "invalid" }}.example.com
  - --exclude-domains=invalid.example.com

Using Annotations for FQDN Templating

This example demonstrates how to use annotations in Kubernetes objects to dynamically generate Fully Qualified Domain Names (FQDNs) using the –fqdn-template flag in ExternalDNS.

The Service object includes an annotation dns.company.com/label with the value my-org-tld-v2. This annotation is used as part of the FQDN template to construct the DNS name.

apiVersion: v1
kind: Service
metadata:
  name: nginx-v2
  namespace: my-namespace
  annotations:
    dns.company.com/label: my-org-tld-v2
spec:
  type: ClusterIP
  clusterIP: None

The –fqdn-template flag is configured to use the annotation value (dns.company.com/label) and append the namespace and a custom domain (company.local) to generate the FQDN.

args:
  --source=service
  --fqdn-template='{{ index .ObjectMeta.Annotations "dns.company.com/label" }}.{{ .Namespace }}.company.local'

# For the given Service object, the resulting FQDN will be:
# route53> my-org-tld-v2.my-namespace.company.local

DNS Scheme Migration

If you’re transitioning from one naming convention to another (e.g., from svc.cluster.local to svc.example.com), –fqdn-template allows you to generate the new records alongside or in place of the old ones — without requiring changes to your Kubernetes manifests.

args:
- --fqdn-template='{{.Name}}.new-dns.example.com'

This helps automate DNS record migration while maintaining service continuity.

Multi-Variant Domain Support

You can also support regional variants or multi-tenant architectures, where the same service is deployed to different regions or environments:

--fqdn-template='{{ .Name }}.{{ .Labels.env }}.{{ .Labels.region }}.example.com, {{ if eq .Labels.env "prod" }}{{ .Name }}.my-company.tld{{ end }}'

With additional context (e.g., annotations), this can produce FQDNs like:

api.prod.us-east-1.example.com
api.my-company.tld

This is helpful in scenarios such as:

  • Blue/green deployments across domains
  • Staging vs. production resolution
  • Multi-cloud or multi-region failover strategies

Tips

  • If --fqdn-template is specified, ExternalDNS ignores any external-dns.alpha.kubernetes.io/hostname annotations.
  • You must still ensure the resulting FQDN is valid and unique.
  • Since Go templates can be error-prone, test your template with simple examples before deploying. Mismatched field names or nil values (e.g., missing labels) will result in errors or skipped entries.

FaQ

Can I specify multiple global FQDN templates?

Yes, you can. Pass in a comma separated list to –fqdn-template. Beware this will double (triple, etc) the amount of DNS entries based on how many services, ingresses and so on you have and will get you faster towards the API request limit of your DNS provider.

Where to find template syntax

FQDN Templating, Helm and improper templating syntax

The user encountered errors due to improper templating syntax:

extraArgs:
  - --fqdn-template={{name}}.uat.example.com

The correct syntax should include a dot prefix: {{ .Name }}.
Additionally, when using Helm’s tpl function, it’s necessary to escape the braces to prevent premature evaluation:

extraArgs:
  - --fqdn-template={{ `{{ .Name }}.uat.example.com` }}

Handling Subdomain-Only Hostnames

In Issue #1872, it was observed that ExternalDNS ignores the --fqdn-template when the ingress host field is set to a subdomain (e.g., foo) without a full domain.
The expectation was that the template would still apply, generating entries like foo.bar.example.com.
This highlights a limitation to be aware of when designing FQDN templates.

⚠ This is currently not supported ! User would expect external-dns to generate a dns record according to the fqdnTemplate
e.g. if the ingress name: foo and host: foo is created while fqdnTemplate={{.Name}}.bar.example.com then a dns record foo.bar.example.com should be created

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: foo
spec:
  rules:
  - host: foo
    http:
      paths:
      - backend:
          serviceName: foo
          servicePort: 80
        path: /

Combining FQDN Template with Annotations

In Issue #3318, a question was raised about the interaction between –fqdn-template and –combine-fqdn-annotation.
The discussion clarified that when both flags are used, ExternalDNS combines the FQDN generated from the template with the annotation value, providing flexibility in DNS name construction.

Using Annotations for Dynamic FQDNs

In Issue #2627, a user aimed to generate DNS entries based on ingress annotations:

args:
  - --fqdn-template={{.Annotations.hostname}}.example.com
  - --combine-fqdn-annotation
  - --domain-filter=example.com

By setting the hostname annotation in the ingress resource, ExternalDNS constructs the FQDN accordingly. This approach allows for dynamic DNS entries without hardcoding hostnames.

Using a Node’s Addresses for FQDNs

args:
  - --fqdn-template="{{range .Status.Addresses}}{{if and (eq .Type \"ExternalIP\") (isIPv4 .Address)}}{{.Address | replace \".\" \"-\"}}{{break}}{{end}}{{end}}.example.com

This is a complex template that iternates through a list of a Node’s Addresses and creates a FQDN with public IPv4 addresses.