Using CoreDNS for Custom Domain Name Resolution

Challenges

When using CCE, you may need to resolve custom internal domain names in the following scenarios:

• In the legacy code, a fixed domain name is configured for calling other internal services. If the system decides to use Kubernetes Services, the code refactoring workload could be heavy.
• A service is created outside the cluster. Data in the cluster needs to be sent to the service through a fixed domain name.

Solutions

There are several CoreDNS-based solutions for custom domain name resolution:

• Configuring the Stub Domain for CoreDNS: You can add it on the console, which is easy to operate.
• Using the CoreDNS Hosts plug-in to configure resolution for any domain name: You can add any record set, which is similar to adding a record set in the local /etc/hosts file.
• Using the CoreDNS Rewrite plug-in to point a domain name to a service in the cluster: A nickname is assigned to the Kubernetes Service. You do not need to know the IP address of the resolution record in advance.
• Using the CoreDNS Forward plug-in to set the self-built DNS as the upstream DNS: The self-built DNS can manage a large number of resolution records. You do not need to modify the CoreDNS configuration when adding or deleting records.

Precautions

Improper modification on CoreDNS configuration may cause domain name resolution failures in the cluster. Perform tests before and after the modification.

Configuring the Stub Domain for CoreDNS

Cluster administrators can modify the ConfigMap for the CoreDNS Corefile to change how service discovery works. They can configure stub domains for CoreDNS using the proxy plug-in.

Assume that a cluster administrator has a Consul DNS server located at 10.150.0.1 and all Consul domain names have the suffix .consul.local.

1. Log in to the CCE console.
2. In the navigation pane on the left, choose Add-ons. On the Add-on Instance tab page, click coredns.
3. Choose the Parameters tab page and click Edit to add a stub domain.
   Figure 1 Configuring the stub domain
   

4. Click OK.

Modifying the CoreDNS Hosts Configuration File

1. Use kubectl to connect to the cluster.
2. Modify the CoreDNS configuration file and add the custom domain name to the hosts file.

   Point www.example.com to 192.168.1.1. When CoreDNS resolves www.example.com, 192.168.1.1 is returned.

   

   The fallthrough field must be configured. fallthrough indicates that when the domain name to be resolved cannot be found in the hosts file, the resolution task is transferred to the next CoreDNS plug-in. If fallthrough is not specified, the task ends and the domain name resolution stops. As a result, the domain name resolution in the cluster fails.

   For details about how to configure the hosts file, visit https://coredns.io/plugins/hosts/.

   $ kubectl edit configmap coredns -n kube-system
   apiVersion: v1
   data:
     Corefile: |-
       .:5353 {
           bind {$POD_IP}
           cache 30
           errors
           health {$POD_IP}:8080
           kubernetes cluster.local in-addr.arpa ip6.arpa {
             pods insecure
             fallthrough in-addr.arpa ip6.arpa
           }
           hosts {
             192.168.1.1 www.example.com
             fallthrough
           }
           loadbalance round_robin
           prometheus {$POD_IP}:9153
           forward . /etc/resolv.conf
           reload
       }
   kind: ConfigMap
   metadata:
     creationTimestamp: "2021-08-23T13:27:28Z"
     labels:
       app: coredns
       k8s-app: coredns
       kubernetes.io/cluster-service: "true"
       kubernetes.io/name: CoreDNS
       release: cceaddon-coredns
     name: coredns
     namespace: kube-system
     resourceVersion: "460"
     selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
     uid: be64aaad-1629-441f-8a40-a3efc0db9fa9

   After modifying the hosts file in CoreDNS, you do not need to configure the hosts file in each pod.

Adding the CoreDNS Rewrite Configuration to Point the Domain Name to Services in the Cluster

Use the Rewrite plug-in of CoreDNS to resolve a specified domain name to the domain name of a Service.

1. Use kubectl to connect to the cluster.
2. Modify the CoreDNS configuration file to point example.com to the example service in the default namespace.

   $ kubectl edit configmap coredns -n kube-system
   apiVersion: v1
   data:
     Corefile: |-
       .:5353 {
           bind {$POD_IP}
           cache 30
           errors
           health {$POD_IP}:8080
           kubernetes cluster.local in-addr.arpa ip6.arpa {
             pods insecure
             fallthrough in-addr.arpa ip6.arpa
           }
           rewrite name example.com example.default.svc.cluster.local
           loadbalance round_robin
           prometheus {$POD_IP}:9153
           forward . /etc/resolv.conf
           reload
       }
   kind: ConfigMap
   metadata:
     creationTimestamp: "2021-08-23T13:27:28Z"
     labels:
       app: coredns
       k8s-app: coredns
       kubernetes.io/cluster-service: "true"
       kubernetes.io/name: CoreDNS
       release: cceaddon-coredns
     name: coredns
     namespace: kube-system
     resourceVersion: "460"
     selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
     uid: be64aaad-1629-441f-8a40-a3efc0db9fa9

Using CoreDNS to Cascade Self-Built DNS

1. Use kubectl to connect to the cluster.
2. Modify the CoreDNS configuration file and change /etc/resolv.conf following forward to the IP address of the external DNS server.

   $ kubectl edit configmap coredns -n kube-system
   apiVersion: v1
   data:
     Corefile: |-
       .:5353 {
           bind {$POD_IP}
           cache 30
           errors
           health {$POD_IP}:8080
           kubernetes cluster.local in-addr.arpa ip6.arpa {
             pods insecure
             fallthrough in-addr.arpa ip6.arpa
           }
           loadbalance round_robin
           prometheus {$POD_IP}:9153
           forward . 192.168.1.1
           reload
       }
   kind: ConfigMap
   metadata:
     creationTimestamp: "2021-08-23T13:27:28Z"
     labels:
       app: coredns
       k8s-app: coredns
       kubernetes.io/cluster-service: "true"
       kubernetes.io/name: CoreDNS
       release: cceaddon-coredns
     name: coredns
     namespace: kube-system
     resourceVersion: "460"
     selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
     uid: be64aaad-1629-441f-8a40-a3efc0db9fa9