Container Service for Kubernetes:DNS overview

Unmanaged CoreDNS

CoreDNS is a component in a Kubernetes cluster that handles DNS resolution. It can resolve both internal service domain names and external domain names. The CoreDNS project is hosted by the Cloud Native Computing Foundation (CNCF). For more information, see CoreDNS: DNS and Service Discovery.

Following the open-source Kubernetes standard, ACK clusters install CoreDNS as the default DNS server. CoreDNS is deployed as a Deployment in the kube-system namespace. kube-dns is the ClusterIP-type Service for CoreDNS. For information about how to configure CoreDNS, see Configure Unmanaged CoreDNS.

In the following example, a Pod in the default namespace attempts to access database-svc and goes through the following process:

1. The pod checks the /etc/resolv.conf configuration file and determines that the IP address of the DNS server is 172.0.XX.XX. This is the ClusterIP of kube-dns. An example of the configuration file is as follows.

   nameserver 172.0.XX.XX                                              # Defines the IP address of the DNS server.
   search default.svc.cluster.local svc.cluster.local cluster.local    # Sets the search suffix rules for domain names. The more search paths are configured, the more lookup attempts are made.
   options ndots:5                                                     # Defines options for the domain name resolution configuration file. Multiple key-value pairs are supported.

2. A pod sends a DNS query to kube-dns, using the values in the search field as suffixes to sequentially resolve the following names:

   1. database-svc.default.svc.cluster.local (A Service in the same namespace as the pod)

   2. database-svc.svc.cluster.local (A Service in another namespace)

   3. database-svc.cluster.local (cluster-scoped domain name)

   4. database-svc (external domain name)

3. The CoreDNS pod returns the result, which is the IP address 172.4.XX.XX.

4. The pod uses this IP to access the target service database-svc.

5. If the request is for an external domain name, such as a VPC-internal domain name or a public domain name, the CoreDNS pod forwards the query to the upstream DNS server.

   Important

   Do not modify the DNS server configuration on the node because CoreDNS inherits the upstream DNS server address from the node on which it runs.

   CoreDNS configuration is stored in the coredns ConfigMap in the kube-system namespace. In the default configuration, the forward plugin points to /etc/resolv.conf. This means that for upstream DNS queries, CoreDNS uses the DNS server addresses from its container's /etc/resolv.conf file. The /etc/resolv.conf file in the CoreDNS container is, by default, the same as the /etc/resolv.conf file on its host node, which uses the default DNS server addresses 100.100.2.136 and 100.100.2.138. If you modify the DNS server configuration on the node, CoreDNS will use incorrect upstream DNS addresses, leading to resolution failures or unexpected results.

       .:53 {
           ...
           forward . /etc/resolv.conf {
             prefer_udp
           }
           ...
       }

Managed CoreDNS

Managed CoreDNS works on the same principle as Unmanaged CoreDNS but is fully managed by ACK. It does not consume cluster resources and requires no O&M from you. It also autoscales based on the load. For more information, see Managed CoreDNS performance.

Managed CoreDNS is supported in ACK managed clusters with auto mode enabled. To use an ACK managed cluster with auto mode enabled, see Create an ACK managed cluster with auto mode enabled.