var (
	// The default dns opt strings.
	defaultDNSOptions = []string{"ndots:5"}
)
…
func (c *Configurer) generateSearchesForDNSClusterFirst(hostSearch []string, pod *v1.Pod) []string {
	if c.ClusterDomain == "" {
		return hostSearch
	}

	nsSvcDomain := fmt.Sprintf("%s.svc.%s", pod.Namespace, c.ClusterDomain)
	svcDomain := fmt.Sprintf("svc.%s", c.ClusterDomain)
	clusterSearch := []string{nsSvcDomain, svcDomain, c.ClusterDomain}

	return omitDuplicates(append(clusterSearch, hostSearch...))
}
…
	switch dnsType {
	case podDNSNone:
		// DNSNone should use empty DNS settings as the base.
		dnsConfig = &runtimeapi.DNSConfig{}
	case podDNSCluster:
		if len(c.clusterDNS) != 0 {
			// For a pod with DNSClusterFirst policy, the cluster DNS server is
			// the only nameserver configured for the pod. The cluster DNS server
			// itself will forward queries to other nameservers that is configured
			// to use, in case the cluster DNS server cannot resolve the DNS query
			// itself.
			dnsConfig.Servers = []string{}
			for _, ip := range c.clusterDNS {
				dnsConfig.Servers = append(dnsConfig.Servers, ip.String())
			}
			dnsConfig.Searches = c.generateSearchesForDNSClusterFirst(dnsConfig.Searches, pod)
			dnsConfig.Options = defaultDNSOptions
			break
		}

Warm-up