Container Compute Service:Configure custom parameters for managed CoreDNS

Prerequisites

Before you begin, ensure that you have:

• Managed CoreDNS plug-in installed, version 1.9.3.20 or later. For installation instructions, see Manage components.

• A kubectl client connected to your cluster. For connection instructions, see Obtain the kubeconfig file of a cluster and use kubectl to connect to the cluster.

How it works

CustomDNSConfig is a Kubernetes custom resource definition (CRD) that lets you configure DNS behavior per zone without editing the CoreDNS Corefile directly. When you apply a CustomDNSConfig CR, the managed CoreDNS controller reads it, generates the corresponding Corefile configuration, and reloads CoreDNS automatically.

The CR supports two configuration options per zone:

• `forward` — Route DNS queries for a zone to one or more upstream DNS servers.

• `hosts` — Map domain names to static IP addresses within a zone.

The following example shows the full CR structure:

apiVersion: networking.alibabacloud.com/v1beta1
kind: CustomDNSConfig
metadata:
  name: default
  namespace: default
spec:
  zones:
  - name: example.com
    forward:
      protocolStrategy: ""
      transportConfig: {}
      upstreams:
      - xxx.xxx.xxx.xxx     # IP address
      - xxx.xxx.xxx.xxx:53  # IP:port
    hosts:
    - hostName: "a.example.com"
      ipAddress: xxx.xxx.xxx.xxx

Add custom DNS zones

Use this approach to route queries for specific domains to dedicated upstream DNS servers, or to add static host entries.

1. Create a file named default.yaml with the following content:

   apiVersion: networking.alibabacloud.com/v1beta1
   kind: CustomDNSConfig
   metadata:
     name: default
   spec:
     zones:
     - name: example.com
       forward:
         upstreams:
         - 100.100.2.136  # Upstream DNS servers for example.com
         - 100.100.2.138
     - name: foo.com
       hosts:
       - hostName: "a.foo.com"  # Static IP mappings for foo.com
         ipAddress: 192.168.0.251
       - hostName: "b.foo.com"
         ipAddress: 192.168.0.252

   This configuration defines two zones:

   • `example.com` — Queries are forwarded to 100.100.2.136 and 100.100.2.138.

     Note: 100.100.2.136 and 100.100.2.138 are the default internal DNS resolution service addresses. For more information, see Endpoints.

   • `foo.com` — a.foo.com resolves to 192.168.0.251 and b.foo.com resolves to 192.168.0.252.

2. Apply the CR:

   kubectl apply -f default.yaml

3. Verify the Corefile was generated:

   kubectl get customdnsconfig default -o yaml | grep corefile -A 35 -B 1

   The output should look similar to:

    status:
     corefile: |
       example.com:53 {
           prometheus :9153
           forward .  100.100.2.136 100.100.2.138 {
             policy random
             prefer_udp
           }
   ...
       }
       foo.com:53 {
           prometheus :9153
           hosts {
             192.168.0.251    a.foo.com
             192.168.0.252    b.foo.com
             fallthrough
           }
           forward .  /etc/resolv.conf {
             policy random
             prefer_udp
           }
   ...
       }
   ...
   --
     corefileHash: 41f7be21cf3022c305091665ed33b1e5
     lastTransitionTime: "2024-09-13T09:07:37Z"
     phase: GenerateSuccess

   A phase: GenerateSuccess status confirms the CR was processed and the Corefile was updated.

Modify the default zone

To override the upstream DNS servers used for all cluster queries (the "." zone), configure a zone named ".".

1. Create a file named default.yaml with the following content:

   apiVersion: networking.alibabacloud.com/v1beta1
   kind: CustomDNSConfig
   metadata:
     name: default
   spec:
     zones:
     - name: .
       forward:
         upstreams:
         - 100.100.2.136  # Custom upstream DNS servers for the default zone
         - 100.100.2.138

2. Apply the CR:

   kubectl apply -f default.yaml

3. Verify the Corefile was generated:

   kubectl get customdnsconfig default -o yaml | grep corefile -A 35 -B 1

   The output should look similar to:

    status:
     corefile: |
       .:53 {
           errors
           health {
             lameduck 20s
           }
           ready
           kubeapi {
             kubeconfig /etc/kubernetes/config/managed-coredns.conf
           }
           k8s_event {
             level error warning
           }
   ...
           prometheus :9153
           forward .  100.100.2.136 100.100.2.138 {
             policy random
             prefer_udp
           }
   ...
       }
     corefileHash: 847bf69cc4c97cee965945f45d17c661
     lastTransitionTime: "2024-09-13T09:54:22Z"
     phase: GenerateSuccess

   After you create or update a CustomDNSConfig CR, CoreDNS performs a configuration reload. This takes approximately 20 seconds by default (controlled by the lameduck parameter in the Corefile).

Map a domain to multiple IP addresses

A single hostName entry can map to multiple IP addresses. CoreDNS returns all addresses and the client selects one:

hosts:
  - hostName: "a.example.com"
    ipAddress: 10.0.0.123
  - hostName: "a.example.com"
    ipAddress: 10.0.0.124

Troubleshooting

apiVersion: networking.alibabacloud.com/v1beta1
kind: CustomDNSConfig
metadata:
  name: test
spec:
  zones:
  - name: example.com
    forward:
      upstreams:
      - 100.100.2.138
  - name: foo.com
    hosts:
    - hostName: "ah.foo.com"
      ipAddress: 1.1.xx.251
    - hostName: "aha.foo.com"
      ipAddress: 1.1.xx.252

kubectl get customdnsconfig

NAME      PHASE             VERSION                            AGE
default   GenerateSuccess   847bf69cc4c97cee96xxxxxxxxxxx      89m
test      NotSupported                                         9s

View sync events

The managed CoreDNS controller syncs events to the default namespace. To view recent events:

kubectl get events

A successful sync produces output similar to:

LAST SEEN   TYPE     REASON                  OBJECT                           MESSAGE
45m         Normal   CustomDNSConfigSyncOk   customdnsconfig/default          custom dns config sync to coredns configmap success