Add-on Management

CoreDNS

The CoreDNS add-on provides the internal domain name resolution service for your other workloads. Do not delete or upgrade this workload; otherwise, the internal domain name resolution service will become unavailable.

Installing an Add-on

1. Log in to the CCI console. In the navigation pane, choose Add-ons > Add-on Marketplace. Then, click on the card of the add-on you want to install.
   Figure 1 CoreDNS add-on
   

2. Select a version from the Add-on Version drop-down list, and click Submit.
   1. When installing CoreDNS v2.5.9 or later, you must configure the following parameters:
      • Stub Domain: A DNS server that resolves user-defined domain names. The stub domain contains the suffix of the DNS domain name followed by one or more DNS IP addresses. For example, acme.local -- 1.2.3.4,6.7.8.9 means that DNS requests with the .acme.local suffix are forwarded to a DNS listening at 1.2.3.4,6.7.8.9.
      • Upstream DNS Server: A DNS server that resolves all domain names except intra-cluster service domain names and user-defined domain names. The value can be one or more DNS IP addresses, for example, 8.8.8.8,8.8.4.4.
   2. When installing CoreDNS v2.5.10 or later, you can also configure the following parameter:
      • Log Output: You can select the types of domain name resolution logs to be printed based on service requirements, for example, Success log and Error log. For details, see Configuring Log Output Options for CoreDNS.

   After you complete the installation, the installed add-on is available at Add-ons > Add-on Instances.

   Figure 2 CoreDNS installed
   

Configuring Stub Domains for CoreDNS

As a cluster administrator, you can modify the ConfigMap for the CoreDNS Corefile to change how service discovery works. You can configure stub domains for CoreDNS using the proxy plug-in.

Assume that you are a cluster administrator and you have a Consul DNS server located at 10.150.0.1 and all Consul domain names have the suffix .consul.local. To configure this Consul DNS server in CoreDNS, you need to write the following information in the CoreDNS ConfigMap:

consul.local:5353 {
        errors
        cache 30
        proxy . 10.150.0.1
    }

ConfigMap after modification:

apiVersion: v1
data:
  Corefile: |-
    .:5353 {
        cache 30
        errors
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          upstream /etc/resolv.conf
          fallthrough in-addr.arpa ip6.arpa
        }
        loadbalance round_robin
        prometheus 0.0.0.0:9153
        proxy . /etc/resolv.conf
        reload
    }

    consul.local:5353 {
        errors
        cache 30
        proxy . 10.150.0.1
    }
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system

Configuring Log Output Options for CoreDNS

CoreDNS uses the log plug-in to print the domain name resolution logs to standard output. You can configure Log Output to define the log content to be output, and view the resolution logs on the AOM console. If there is a large number of domain name resolution requests, frequent log printing may affect the CoreDNS performance.

The log plug-in supports three log output options: Success log, Denial log and Error log.

Figure 3 Configuring options

The backend configuration format is as follows:

log [NAMES...] [FORMAT] {
    class CLASSES...
}

CLASSES indicates the classes of responses that should be logged. It is a list separated by spaces.

The log output options include:

• Success log:

  If this option is selected, the success response parameter is added to the CLASSES list of the log plug-in, and CoreDNS prints the logs that are successfully resolved to the standard output.

• Denial log:

  If this option is selected, the denial response parameter is added to the CLASSES list of the log plug-in, and CoreDNS prints the logs that fail to be resolved. For example, NXDOMAIN or nodata response (the name exists but the type does not exist) will be printed to the standard output.

• Error log:

  If this option is selected, the error response parameter is added to the CLASSES list of the log plug-in, and CoreDNS prints logs about resolution errors to the standard output, for example, SERVFAIL, NOTIMP, and REFUSED. This helps detect problems such as DNS server unavailability in a timely manner.

• Deselect all:

  If none of the preceding options is selected, the log plug-in is disabled.

  

  Disabling the log plug-in takes effect only for the resolution records of CoreDNS. The logs of the CoreDNS service process are still displayed, which are small in volume and do not affect performance.

log . {
    class success denial
}

The ConfigMap corresponding to the created CoreDNS is as follows:

apiVersion: v1
data:
  Corefile: |-
    .:5353 {
        cache 30
        errors
        log . {
          classes success denial
        }
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          upstream /etc/resolv.conf
          fallthrough in-addr.arpa ip6.arpa
        }
        loadbalance round_robin
        prometheus 0.0.0.0:9153
        proxy . /etc/resolv.conf
        reload
    }
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system

Viewing Resolution Logs

After configuring the log plug-in, you can view resolution logs on the AOM console.

1. Log in to the CCI console. In the navigation pane, choose Add-ons > Add-on Instances. Select CoreDNS on the right to display the CoreDNS page.
   Figure 4 Add-on instances
   

2. Click CoreDNS Deployment in the resource list to go to the pod list.
   Figure 5 CoreDNS deployment
   

3. Click View Logs in the Operation column in Pod List to access the AOM console to view the CoreDNS logs.
   Figure 6 Pod list
   

How Does Domain Name Resolution Work in Kubernetes?

DNS policies can be set on a per-pod basis. Kubernetes supports four types of DNS policies: Default, ClusterFirst, ClusterFirstWithHostNet, and None. For details, see DNS for Services and Pods. These policies are specified in the dnsPolicy field in the pod-specific.

• Default: Pods inherit the name resolution configuration from the node that runs the pods. The custom upstream DNS server and the stub domain cannot be used together with this policy.
• ClusterFirst: Any DNS query that does not match the configured cluster domain suffix, such as www.kubernetes.io, is forwarded to the upstream name server inherited from the node. Cluster administrators may have extra stub domains and upstream DNS servers configured.
• ClusterFirstWithHostNet: For pods running with hostNetwork, set its DNS policy ClusterFirstWithHostNet.
• None: It allows a pod to ignore DNS settings from the Kubernetes environment. All DNS settings are supposed to be provided using the dnsPolicy field in the pod-specific.

• Clusters of Kubernetes v1.10 and later support Default, ClusterFirst, ClusterFirstWithHostNet, and None. Clusters earlier than Kubernetes v1.10 support only Default, ClusterFirst, and ClusterFirstWithHostNet.
• Default is not the default DNS policy. If dnsPolicy is not explicitly specified, ClusterFirst is used.

Routing

• Without stub domain configurations: Any query that does not match the configured cluster domain suffix, such as www.kubernetes.io, is forwarded to the upstream DNS server inherited from the node.
• With stub domain configurations: If you have configured stub domains and upstream DNS servers, DNS queries are routed according to the following flow:
  1. The query is first sent to the DNS caching layer in CoreDNS.
  2. From the caching layer, the suffix of the request is examined and then forwarded to the appropriate DNS, based on the following cases:
     • Names with the cluster suffix, for example, .cluster.local: The request is sent to CoreDNS.
     • Names with the stub domain suffix, for example, .acme.local: The request is sent to the configured custom DNS resolver, listening for example at 1.2.3.4.
     • Names that do not match the suffix (for example, widget.com): The request is forwarded to the upstream DNS.
  Figure 7 Routing
  

Follow-Up Operations

After the add-on is installed, you can perform the following operations on the add-on.

CoreDNS Release History