Understanding Kubernetes networking, pods with same ip - kubernetes

Question

I checked the pods in the kube-system namespace and noticed that some pods share the same ip address.The pods that share the same ip address appear to be on the same node.
In the Kubernetes documenatation it said that "Evert pod gets its own ip address." (https://kubernetes.io/docs/concepts/cluster-administration/networking/). I'm confused as to how same ip for some pods came about.

Answer 1

This was reported in issue 51322 and can depend on the network plugin you are using.
The issue was seen when using the basic kubenet network plugin on Linux.
Sometime, a reset/reboot can help
I suspect nodes have been configured with overlapped podCIDRs for such cases.
The pod CIDR could be checked by kubectl get node -o jsonpath='{.items[*].spec.podCIDR}'

Answer 2

Please check the Kubernetes manifests of the pods that have the same IP address as their node. If they have the parameter 'hostNetwork' set to be true, then this is not an issue.

Answer 3

master-node after logging in using PuTTY
worker-node01 after logging in using PuTTY
It clearly shows a separate CIDR for weave network. So it depends on the network plug-in. And some cases will override the pod specification CIDR provided during initialization.
After re-deploying across the new node - worker-node02

Answer 4

Yes. I have checked my 2 node clusters created using kubeadm on VMs running on AWS.
In the manifest files for static Pods hostNetwork=true is set.
Pods are:
-rw------- 1 root root 2100 Feb 4 16:48 etcd.yaml
-rw------- 1 root root 3669 Feb 4 16:48 kube-apiserver.yaml
-rw------- 1 root root 3346 Feb 4 16:48 kube-controller-manager.yaml
-rw------- 1 root root 1385 Feb 4 16:48 kube-scheduler.yaml
I have checked with weave and flannel.
All other pods getting IP, which was set during cluster initialization by kubeadm:
kubeadm init --pod-network-cidr=10.244.0.0/16
ubuntu#master-node:~$ kubectl get all -o wide --all-namespaces
NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
default pod/my-nginx-deployment-5976fbfd94-2n2ff 1/1 Running 0 20m 10.244.1.17 worker-node01
default pod/my-nginx-deployment-5976fbfd94-4sghq 1/1 Running 0 20m 10.244.1.12 worker-node01
default pod/my-nginx-deployment-5976fbfd94-57lfp 1/1 Running 0 20m 10.244.1.14 worker-node01
default pod/my-nginx-deployment-5976fbfd94-77nrr 1/1 Running 0 20m 10.244.1.18 worker-node01
default pod/my-nginx-deployment-5976fbfd94-m7qbn 1/1 Running 0 20m 10.244.1.15 worker-node01
default pod/my-nginx-deployment-5976fbfd94-nsxvm 1/1 Running 0 20m 10.244.1.19 worker-node01
default pod/my-nginx-deployment-5976fbfd94-r5hr6 1/1 Running 0 20m 10.244.1.16 worker-node01
default pod/my-nginx-deployment-5976fbfd94-whtcg 1/1 Running 0 20m 10.244.1.13 worker-node01
kube-system pod/coredns-f9fd979d6-nghhz 1/1 Running 0 63m 10.244.0.3 master-node
kube-system pod/coredns-f9fd979d6-pdbrx 1/1 Running 0 63m 10.244.0.2 master-node
kube-system pod/etcd-master-node 1/1 Running 0 63m 172.31.8.115 master-node
kube-system pod/kube-apiserver-master-node 1/1 Running 0 63m 172.31.8.115 master-node
kube-system pod/kube-controller-manager-master-node 1/1 Running 0 63m 172.31.8.115 master-node
kube-system pod/kube-proxy-8k9s4 1/1 Running 0 63m 172.31.8.115 master-node
kube-system pod/kube-proxy-ln6gb 1/1 Running 0 37m 172.31.3.75 worker-node01
kube-system pod/kube-scheduler-master-node 1/1 Running 0 63m 172.31.8.115 master-node
kube-system pod/weave-net-jc92w 2/2 Running 1 24m 172.31.8.115 master-node
kube-system pod/weave-net-l9rg2 2/2 Running 1 24m 172.31.3.75 worker-node01
NAMESPACE NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
default service/kubernetes ClusterIP 10.96.0.1 443/TCP 63m
kube-system service/kube-dns ClusterIP 10.96.0.10 53/UDP,53/TCP,9153/TCP 63m k8s-app=kube-dns
NAMESPACE NAME DESIRED CURRENT READY UP-TO-DATE AVAILABLE NODE SELECTOR AGE CONTAINERS IMAGES SELECTOR
kube-system daemonset.apps/kube-proxy 2 2 2 2 2 kubernetes.io/os=linux 63m kube-proxy k8s.gcr.io/kube-proxy:v1.19.16 k8s-app=kube-proxy
kube-system daemonset.apps/weave-net 2 2 2 2 2 24m weave,weave-npc ghcr.io/weaveworks/launcher/weave-kube:2.8.1,ghcr.io/weaveworks/launcher/weave-npc:2.8.1 name=weave-net
NAMESPACE NAME READY UP-TO-DATE AVAILABLE AGE CONTAINERS IMAGES SELECTOR
default deployment.apps/my-nginx-deployment 8/8 8 8 20m nginx nginx app=my-nginx-deployment
kube-system deployment.apps/coredns 2/2 2 2 63m coredns k8s.gcr.io/coredns:1.7.0 k8s-app=kube-dns
NAMESPACE NAME DESIRED CURRENT READY AGE CONTAINERS IMAGES SELECTOR
default replicaset.apps/my-nginx-deployment-5976fbfd94 8 8 8 20m nginx nginx app=my-nginx-deployment,pod-template-hash=5976fbfd94
kube-system replicaset.apps/coredns-f9fd979d6 2 2 2 63m coredns k8s.gcr.io/coredns:1.7.0 k8s-app=kube-dns,pod-template-hash=f9fd979d6
ubuntu#master-node:~$
I will add another worker node and check.
Note: I was testing with a one master and 3 worker node cluster, where pods were getting IP from some other CIDR 10.38 and 10.39. I am not sure, but the way steps are followed matters. I could not fix that cluster.