K8S CoreDNS Fall Through

What is fallthrough?

fallthrough is a directive used in CoreDNS configurations to allow queries to continue to the next plugin if no matching answer is found. Essentially, it helps in handling unresolved DNS queries by passing them to subsequent plugins in the CoreDNS chain.

How it Works:

When a DNS query is received, CoreDNS processes it through a series of plugins as specified in the CoreDNS configuration file. If one plugin cannot resolve the query, it may pass the query to the next plugin if the fallthrough directive is used.

Example Use Case:

Consider the following CoreDNS configuration using the hosts plugin with the fallthrough directive:

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  appname.override: |
    hosts {
      10.0.0.115 appname.domain.net
      fallthrough
    }    

What This Configuration Does:

1. Hosts Plugin: The hosts plugin is used to define static DNS entries.

   • 10.0.0.115 appname.domain.net: Maps the IP address 10.0.0.115 to the domain appname.domain.net.

2. Fallthrough Directive: The fallthrough directive allows the query to proceed to the next plugin if no matching entry is found in the hosts plugin.

   • If a query for appname.domain.net is received, the hosts plugin will resolve it to 10.0.0.115.
   • If a query for any other domain is received (not defined in the hosts plugin), it will fall through to the next plugin in the CoreDNS configuration for further resolution.

Benefits of Using fallthrough:

• Enhanced Flexibility: Allows combining multiple DNS sources for query resolution.
• Redundancy: Ensures that unresolved queries are not dropped but are passed to the next available resolver.
• Custom Configurations: Facilitates custom DNS setups where certain domains are resolved internally, while others are handled by external DNS servers.

Real-World Scenario:

Imagine you have a Kubernetes cluster where certain internal services are resolved using static IP addresses, but you also want to allow external domain resolution. You can configure CoreDNS with the hosts plugin for internal services and fallthrough to external DNS servers.

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        hosts {
            10.0.0.115 appname.domain.net
            fallthrough
        }
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }    

In this example:

• The hosts plugin resolves appname.domain.net to 10.0.0.115.
• The forward plugin handles queries not resolved by the hosts plugin, forwarding them to the external DNS servers specified in /etc/resolv.conf.

Using fallthrough, you can create a robust and flexible DNS setup in your Kubernetes cluster, ensuring seamless resolution of both internal and external domains.