Kubernetes supports 2 primary modes of finding a Service - environment variables and DNS.
When a Pod is run on a Node, the kubelet adds a set of environment variables for each active Service. It supports both Docker links compatible variables (see makeLinkVariables) and simpler {SVCNAME}_SERVICE_HOST and {SVCNAME}_SERVICE_PORT variables, where the Service name is upper-cased and dashes are converted to underscores.
从1.13起，k8s内置了CoreDNS
CoreDNS: DNS and Service Discovery
If you have a Service called "my-service" in a Kubernetes Namespace called "my-ns", a DNS record for "my-service.my-ns" is created. Pods which exist in the "my-ns" Namespace should be able to find it by simply doing a name lookup for "my-service". Pods which exist in other **Namespaces**must qualify the name as "my-service.my-ns". The result of these name lookups is the cluster IP.
Kubernetes also supports DNS SRV (service) records for named ports. If the "my-service.my-ns" **Service**has a port named "http" with protocol TCP, you can do a DNS SRV query for "_http._tcp.my-service.my-ns" to discover the port number for "http".