I was making some research about how K8s resolves the services using the clusterIP services and how CNIs like WeaveNet or how service meshes like Istio provide additional features to this functionality. However, I'm new on the topic and I'd like to share here what I've found to see if somebody can expand and correct my points:
Istiod has a service registry. This service registry is filled with the entries coming from K8s services clusterIPs (which in turn is the service registry of K8s) and other possible external services defined with Kind: ServiceEntry (see seciton 5.5 of book istio in action) This service registry is then mixed with more information about virtualservices and destination rules. These new/added K8s kinds are CRDs from Istio. They are what give the features of L7 load balancing that allow to distribute traffic by HTTP headers or URI path.
Without Istio, K8s has different (3) ways to implement the clusterIPs services concept. This services provide load balancing at L4. https://kubernetes.io/docs/concepts/services-networking/service/ The most extended one nowadays is the iptables proxy mode. The iptables of the Linux machine are populated in bases of what theh kube-proxy provides. Kube-proxy gets those data from the kube-apiserver and (problably the core-dns). The kube-apisever will in turn consult the etcd database to know about the k8s clusterIP services. The entry of the iptables is populated with a the clusterIP->pod IP with only one pod IP out of the many pod that a deployment behind the clusterIP could be. Any piece of code/application inside of the container could make calls directy to the kube-apiserver if using the correct authentication and get the pod address but that would be not practic
K8s can use CNIs (container network interfaces). One example of this would be Weavenet. https://www.weave.works/docs/net/latest/overview/ Wevenet creates a new layer 2 network using Linux kernel features. One daemon sets up this L2 network and manages the routing between machines and there are various ways to attach machines to the network. In this network the containers can be exposed to the outside world. Weavenet implements a micro DNS server at each node. You simply name containers and the routing just can work without the use of services, including the load balancing across multiple continers with the same name.
