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.

Solution

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.

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 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.
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.

   The corresponding Corefile content is as follows:

   .: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
   }

Modifying the CoreDNS Hosts Configuration File

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

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 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 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/.

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.

   The corresponding Corefile content is as follows:

   .: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
   }

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. For example, the request for accessing the example.com domain name is redirected to the example.default.svc.cluster.local domain name, that is, the example service in the default namespace.

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 add the following content to the plugins field:

   {
      "name": "rewrite",
      "parameters": "name example.com example.default.svc.cluster.local"
   }

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.

   The corresponding Corefile content is as follows:

   .:5353 {
       bind {$POD_IP}
       rewrite name example.com example.default.svc.cluster.local
       cache 30
       errors
       health {$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
   }

Using CoreDNS to Cascade Self-Built DNS

By default, CoreDNS uses the /etc/resolv.conf file of the node for resolution. You can also change the resolution address to that of the external 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 the advanced configuration under Parameters and modify the following content in the plugins field:

   {
       "configBlock": "policy random",
       "name": "forward",
       "parameters": ". 192.168.1.1"
   }

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.

   The corresponding Corefile content is as follows:

   .: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 {$POD_IP}:9153
       forward . 192.168.1.1 {
           policy random
       }
       reload
       ready {$POD_IP}:8081
   }