DNS Configuration

Every Kubernetes cluster has a built-in DNS add-on (Kube-DNS or CoreDNS) to provide domain name resolution for workloads in the cluster. When handling a high concurrency of DNS queries, Kube-DNS/CoreDNS may encounter a performance bottleneck, that is, it may fail occasionally to fulfill DNS queries. There are cases when Kubernetes workloads initiate unnecessary DNS queries. This makes DNS overloaded if there are many concurrent DNS queries. Tuning DNS configuration for workloads will reduce the risks of DNS query failures to some extent.

For more information about DNS, see CoreDNS.

DNS Configuration Items

Run the cat /etc/resolv.conf command on a Linux node or container to view the DNS resolver configuration file. The following is an example DNS resolver configuration of a container in a Kubernetes cluster:
nameserver 10.247.x.x
search default.svc.cluster.local svc.cluster.local cluster.local
options ndots:5
Configuration Options
  • nameserver: an IP address list of a name server that the resolver will query. If this parameter is set to 10.247.x.x, the resolver will query the kube-dns/CoreDNS. If this parameter is set to another IP address, the resolver will query a cloud or on-premises DNS server.
  • search: a search list for host-name lookup. When a domain name cannot be resolved, DNS queries will be attempted combining the domain name with each domain in the search list in turn until a match is found or all domains in the search list are tried. For CCE clusters, the search list is currently limited to three domains per container. When a nonexistent domain name is being resolved, eight DNS queries will be initiated because each domain name (including those in the search list) will be queried twice, one for IPv4 and the other for IPv6.
  • options: options that allow certain internal resolver variables to be modified. Common options include timeout and ndots.

    The value ndots:5 means that if a domain name has fewer than 5 dots (.), DNS queries will be attempted by combining the domain name with each domain in the search list in turn. If no match is found after all the domains in the search list are tried, the domain name is then used for DNS query. If the domain name has 5 or more than 5 dots, it will be tried first for DNS query. In case that the domain name cannot be resolved, DNS queries will be attempted by combining the domain name with each domain in the search list in turn.

    For example, the domain name www.***.com has only two dots (smaller than the value of ndots), and therefore the sequence of DNS queries is as follows: www.***.com.default.svc.cluster.local, www.***.com.svc.cluster.local, www.***.com.cluster.local, and www.***.com. This means that at least seven DNS queries will be initiated before the domain name is resolved into an IP address. It is clear that when many unnecessary DNS queries will be initiated to access an external domain name. There is room for improvement in workload's DNS configuration.

For more information about configuration options in the resolver configuration file used by Linux operating systems, visit http://man7.org/linux/man-pages/man5/resolv.conf.5.html.

Configuring DNS for a Workload Using the Console

Kubernetes provides DNS-related configuration options for applications. The use of application's DNS configuration can effectively reduce unnecessary DNS queries in certain scenarios and improve service concurrency. The following procedure uses an Nginx application as an example to describe how to add DNS configurations for a workload on the console.

  1. Log in to the CCE console, access the cluster console, select Workloads in the navigation pane, and click Create Workload in the upper right corner.
  2. Configure basic information about the workload. For details, see Creating a Workload.
  3. In the Advanced Settings area, click the DNS tab and set the following parameters as required:

    • DNS Policy: The DNS policies provided on the console correspond to the dnsPolicy field in the YAML file. For details, see Table 1.
      • Supplement defaults: corresponds to dnsPolicy=ClusterFirst. Containers can resolve both the cluster-internal domain names registered by a Service and the external domain names exposed to public networks.
      • Replace defaults: corresponds to dnsPolicy=None. You must configure IP Address and Search Domain. Containers only use the user-defined IP address and search domain configurations for domain name resolution.
      • Inherit defaults: corresponds to dnsPolicy=Default. Containers use the domain name resolution configuration from the node that pods run on and cannot resolve the cluster-internal domain names.
    • Optional Objects: The options parameters in the dnsConfig field. Each object may have a name property (required) and a value property (optional). After setting the properties, click confirm to add.
      • timeout: Timeout interval, in seconds.
      • ndots: Number of dots (.) that must be present in a domain name. If a domain name has dots fewer than this value, the operating system will look up the name in the search domain. If not, the name is a fully qualified domain name (FQDN) and will be tried first as an absolute name.
    • IP Address: nameservers in the dnsConfig. You can configure the domain name server for the custom domain name. The value is one or a group of DNS IP addresses.
    • Search Domain: searches in the dnsConfig. A list of DNS search domains for hostname lookup in the pod. This property is optional. When specified, the provided list will be merged into the search domain names generated from the chosen DNS policy in dnsPolicy. Duplicate domain names are removed.

  4. Click Create Workload.

Configuring DNS Using the Workload YAML

When creating a workload using a YAML file, you can configure the DNS settings in the YAML. The following is an example for an Nginx application:
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
  namespace: default
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: container-1
          image: nginx:latest
          imagePullPolicy: IfNotPresent
      imagePullSecrets:
        - name: default-secret
      dnsPolicy: None
      dnsConfig:
        options:
          - name: ndots
            value: '5'
          - name: timeout
            value: '3'
        nameservers:
          - 10.2.3.4
        searches:
          - my.dns.search.suffix

Configuration Examples

The following example describes how to configure DNS for workloads.

Parent topic: DNS