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 configure a DNS server for a specific domain name and perform this operation on the console.
• Using the CoreDNS Hosts plug-in to configure resolution for any domain name: You can add any record set for a specific domain name, 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 ClusterIP Services: Perform CNAME resolution on a domain name in the cluster and redirect it to another domain name in the cluster. This is like giving an alias to the Kubernetes Service, and you do not have to get the IP address of the record set beforehand.
• Using the CoreDNS Forward plug-in to set the on-premises DNS as the upstream DNS: Use the on-premises DNS as the external DNS server. You can manage a large number of records on the on-premises DNS, without modifying 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.
   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.

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 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.

Adding the CoreDNS Rewrite Configuration to Point the Domain Name to ClusterIP Services

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 extended parameters in Parameters and add the following content to the plugins field:

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

   Figure 3 Adding the CoreDNS rewrite configuration to point the domain name to ClusterIP Services
   
   Corefile:

   .: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
   }

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.

Using CoreDNS to Cascade On-premises DNS

If the domain name is not in the Kubernetes domain, CoreDNS will use the /etc/resolv.conf file of the node for resolution by default. 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 extended parameters in Parameters and modify the following content in the plugins field:

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

   Figure 4 Using CoreDNS to cascade on-premises DNS
   
   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 {$POD_IP}:9153
       forward . 192.168.1.1 {
           policy random
       }
       reload
       ready {$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.