Configuring CoreDNS

Configuring CoreDNS Specifications

1. Log in to the CCE console and click the cluster name to access the cluster console.
2. In the navigation pane, choose Add-ons. Then, click Edit under CoreDNS.
3. In the Specifications area, configure coredns specifications.
4. Change the number of replicas, CPU quotas, and memory quotas as needed to adjust the domain name resolution QPS provided by CoreDNS.

   

5. Click OK.

Properly Configuring the Stub Domain for DNS

1. Log in to the CCE console and click the cluster name to access the cluster console.
2. In the navigation pane, choose Add-ons. Then, click Edit under CoreDNS.
3. Add a stub domain in the Parameters area. The format is a key-value pair. The key is a DNS suffix domain name, and the value is a DNS IP address or a group of DNS IP addresses, for example, consul.local -- 10.150.0.1.
   Figure 1 Adding a stub domain
   
   Corefile:

   .:5353 {
       bind {$POD_IP}
       cache 30 {
           servfail 5s
       }
       errors
       health {$POD_IP}:8080
       kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           fallthrough in-addr.arpa ip6.arpa
       }
       loadbalance round_robin
       prometheus {$POD_IP}:9153
       forward . /etc/resolv.conf {
           policy random
       }
       reload
       ready {$POD_IP}:8081
   }
   consul.local:5353 {
       bind {$POD_IP}
       errors
       cache 30
       forward . 10.150.0.1
   }

4. Click OK.
5. Choose ConfigMaps and Secrets in the navigation pane, select the kube-system namespace, and view the ConfigMap data of coredns to check whether the update is successful.

Properly Configuring the Host

To specify hosts for a specific domain name, you can use the hosts add-on. An example is as follows:

1. Log in to the CCE console and click the cluster name to access the cluster console.
2. In the navigation pane, choose Add-ons. Then, click Edit under CoreDNS.
3. Edit extended parameters in Parameters and add the following content to the plugins field:

   {
     "configBlock": "192.168.1.1 www.example.com\nfallthrough",
     "name": "hosts"
   }

   

   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 add-on of CoreDNS. 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/.

   Figure 2 Modifying the CoreDNS hosts configuration
   
   Corefile:

   .:5353 {
       bind {$POD_IP}
       hosts {
         192.168.1.1 www.example.com
         fallthrough
       }
       cache 30
       errors
       health {$POD_IP}:8080
       kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           fallthrough in-addr.arpa ip6.arpa
       }
       loadbalance round_robin
       prometheus {$POD_IP}:9153
       forward . /etc/resolv.conf {
           policy random
       }
       reload
       ready {$POD_IP}:8081
   }

4. Click OK.
5. Choose ConfigMaps and Secrets in the navigation pane, select the kube-system namespace, and view the ConfigMap data of coredns to check whether the update is successful.

Configuring the Default Protocol Between the forward Plug-in and the Upstream DNS Service

1. The NodeLocal DNSCache uses TCP to communicate with the CoreDNS. The CoreDNS communicates with the upstream DNS server based on the protocol used by the request source. By default, external domain name resolution requests from service containers pass through NodeLocal DNSCache and CoreDNS in sequence, and finally request the DNS server in the VPC using TCP.
2. However, the cloud server does not support TCP. To use NodeLocal DNSCache, modify the CoreDNS configuration so that UDP is preferentially used to communicate with the upstream DNS server, preventing resolution exceptions. You are advised to use the following method to modify the CoreDNS configuration file:

   The forward plug-in is used to set the upstream Nameservers DNS server. The following parameters are included:

   prefer_udp: Even if a request is received through TCP, UDP must be used first.

   If you want CoreDNS to preferentially use UDP to communicate with upstream systems, set the protocol in the forward plug-in to prefer_udp. For details about the forward plug-in, see https://coredns.io/plugins/forward/.

   1. Log in to the CCE console and click the cluster name to access the cluster console.
   2. In the navigation pane, choose Add-ons. Then, click Edit under CoreDNS.
   3. Edit the advanced configuration under Parameters and modify the following content in the plugins field:

      {
          "configBlock": "prefer_udp",
          "name": "forward",
          "parameters": ". /etc/resolv.conf"
      }

      Corefile:

      Corefile: |-
        .:5353 {
            bind {$POD_IP}
            cache 30 {
                servfail 5s
            }
            errors
            health {$POD_IP}:8080
            kubernetes cluster.local in-addr.arpa ip6.arpa {
                pods insecure
                fallthrough in-addr.arpa ip6.arpa
            }
            loadbalance round_robin
            prometheus {$POD_IP}:9153
            forward . /etc/resolv.conf {
                prefer_udp
            }
            reload
            ready {$POD_IP}:8081
        }

Configuring IPv6 Resolution Properly

If the IPv6 kernel module is not disabled on the Kubernetes cluster host machine, the container initiates IPv4 and IPv6 resolution at the same time by default when requesting the coredns add-on. Generally, only IPv4 addresses are used. Therefore, if you only configure DOMAIN in IPv4 address, the coredns add-on forwards the request to the upstream DNS server for resolution because the local configuration cannot be found. As a result, the DNS resolution request of the container slows down.

CoreDNS provides the template plug-in. After being configured, CoreDNS can immediately return an empty response to all IPv6 requests to prevent the requests from being forwarded to the upstream DNS.

1. Log in to the CCE console and click the cluster name to access the cluster console.
2. In the navigation pane, choose Add-ons. Then, click Edit under CoreDNS.
3. Edit extended parameters in Parameters and add the following content to the plugins field.
   • AAAA indicates an IPv6 resolution request. If NXDOMAIN is returned in the rcode control response, meaning that no resolution result is returned.

   For details about the template plug-in, visit https://github.com/coredns/coredns/tree/master/plugin/template.

   {
     "configBlock": "rcode NXDOMAIN",
     "name": "template",
     "parameters": "ANY AAAA"
   }

4. Click OK.
5. In the navigation pane, choose ConfigMaps and Secrets. In the kube-system namespace, view the coredns configuration data to check whether the update is successful.

   

   Corresponding Corefile content:

   .: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 {$POD_IP}:9153
       forward . /etc/resolv.conf {
           policy random
       }
       reload
       template ANY AAAA {
           rcode NXDOMAIN
       }
       ready {$POD_IP}:8081
   }

Properly Configuring Cache Policies

If you configure CoreDNS with an upstream DNS server, you can implement a cache policy that enables CoreDNS to use the expired local cache when it is unable to access the upstream DNS server.

1. Log in to the CCE console and click the cluster name to access the cluster console.
2. In the navigation pane, choose Add-ons. Then, click Edit under CoreDNS.
3. Edit extended parameters in Parameters and modify the cache content in the plugins field. For details about how to configure the cache, see https://coredns.io/plugins/cache/.

   {
       "configBlock": "servfail 5s\nserve_stale 60s immediate",
       "name": "cache",
       "parameters": 30
   }

   

4. Click OK.
5. In the navigation pane, choose ConfigMaps and Secrets. Select the kube-system namespace, view the data of the ConfigMap named coredns to check whether the update is successful.

   Corresponding Corefile content:

   .:5353 {
       bind {$POD_IP}
       cache 30 {
           servfail 5s
           serve_stale 60s immediate
       }
       errors
       health {$POD_IP}:8080
       kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           fallthrough in-addr.arpa ip6.arpa
       }
       loadbalance round_robin
       prometheus {$POD_IP}:9153
       forward . /etc/resolv.conf {
           policy random
       }
       reload
       ready {$POD_IP}:8081
   }