Here's my environment, I have k8s cluster and some physical machines outside k8s cluster. Now I create a pod in k8s, and this pod will act like a master to create some processes in these physical machines outside k8s cluster. And I need to establish rpc connection between the k8s pod and these external processes. I don't want to use k8s service here. So what kind of other approach I can use to connect a pod in k8s from external world.
You would need to set up your CNI networking in such a way that pod IPs are routable from outside the cluster. How you do this depends on you CNI plugin and your existing network design. You could also use a VPN into the cluster network in some cases.