Kubernetes pods can't ping each other using ClusterIP - kubernetes

Question

I'm trying to ping the kube-dns service from a dnstools pod using the cluster IP assigned to the kube-dns service. The ping request times out. From the same dnstools pod, I tried to curl the kube-dns service using the exposed port, but that timed out as well.
Following is the output of kubectl get pods --all-namespaces -o wide
NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE
default pod/busybox 1/1 Running 62 2d14h 192.168.1.37 kubenode <none>
default pod/dnstools 1/1 Running 0 2d13h 192.168.1.45 kubenode <none>
default pod/nginx-deploy-7c45b84548-ckqzb 1/1 Running 0 6d11h 192.168.1.5 kubenode <none>
default pod/nginx-deploy-7c45b84548-vl4kh 1/1 Running 0 6d11h 192.168.1.4 kubenode <none>
dmi pod/elastic-deploy-5d7c85b8c-btptq 1/1 Running 0 2d14h 192.168.1.39 kubenode <none>
kube-system pod/calico-node-68lc7 2/2 Running 0 6d11h 10.62.194.5 kubenode <none>
kube-system pod/calico-node-9c2jz 2/2 Running 0 6d12h 10.62.194.4 kubemaster <none>
kube-system pod/coredns-5c98db65d4-5nprd 1/1 Running 0 6d12h 192.168.0.2 kubemaster <none>
kube-system pod/coredns-5c98db65d4-5vw95 1/1 Running 0 6d12h 192.168.0.3 kubemaster <none>
kube-system pod/etcd-kubemaster 1/1 Running 0 6d12h 10.62.194.4 kubemaster <none>
kube-system pod/kube-apiserver-kubemaster 1/1 Running 0 6d12h 10.62.194.4 kubemaster <none>
kube-system pod/kube-controller-manager-kubemaster 1/1 Running 1 6d12h 10.62.194.4 kubemaster <none>
kube-system pod/kube-proxy-9hcgv 1/1 Running 0 6d11h 10.62.194.5 kubenode <none>
kube-system pod/kube-proxy-bxw9s 1/1 Running 0 6d12h 10.62.194.4 kubemaster <none>
kube-system pod/kube-scheduler-kubemaster 1/1 Running 1 6d12h 10.62.194.4 kubemaster <none>
kube-system pod/tiller-deploy-767d9b9584-5k95j 1/1 Running 0 3d9h 192.168.1.8 kubenode <none>
nginx-ingress pod/nginx-ingress-66wts 1/1 Running 0 5d17h 192.168.1.6 kubenode <none>
In the above output, why do some pods have an IP assigned in the 192.168.0.0/24 subnet whereas others have an IP that is equal to the IP address of my node/master? (10.62.194.4 is the IP of my master, 10.62.194.5 is the IP of my node)
This is the config.yml I used to initialize the cluster using kubeadm init --config=config.yml
apiServer:
certSANs:
- 10.62.194.4
extraArgs:
authorization-mode: Node,RBAC
timeoutForControlPlane: 4m0s
apiVersion: kubeadm.k8s.io/v1beta2
certificatesDir: /etc/kubernetes/pki
clusterName: dev-cluster
controllerManager: {}
dns:
type: CoreDNS
etcd:
local:
dataDir: /var/lib/etcd
imageRepository: k8s.gcr.io
kind: ClusterConfiguration
kubernetesVersion: v1.15.1
networking:
dnsDomain: cluster.local
podSubnet: 192.168.0.0/16
serviceSubnet: 10.96.0.0/12
scheduler: {}
Result of kubectl get svc --all-namespaces -o wide
NAMESPACE NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
default service/kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 6d12h <none>
default service/nginx-deploy ClusterIP 10.97.5.194 <none> 80/TCP 5d17h run=nginx
dmi service/elasticsearch ClusterIP 10.107.84.159 <none> 9200/TCP,9300/TCP 2d14h app=dmi,component=elasticse
dmi service/metric-server ClusterIP 10.106.117.2 <none> 8098/TCP 2d14h app=dmi,component=metric-se
kube-system service/calico-typha ClusterIP 10.97.201.232 <none> 5473/TCP 6d12h k8s-app=calico-typha
kube-system service/kube-dns ClusterIP 10.96.0.10 <none> 53/UDP,53/TCP,9153/TCP 6d12h k8s-app=kube-dns
kube-system service/tiller-deploy ClusterIP 10.98.133.94 <none> 44134/TCP 3d9h app=helm,name=tiller
The command I ran was kubectl exec -ti dnstools -- curl 10.96.0.10:53
EDIT:
I raised this question because I got this error when trying to resolve service names from within the cluster. I was under the impression that I got this error because I cannot ping the DNS server from a pod.
Output of kubectl exec -ti dnstools -- nslookup kubernetes.default
;; connection timed out; no servers could be reached
command terminated with exit code 1
Output of kubectl exec dnstools cat /etc/resolv.conf
nameserver 10.96.0.10
search default.svc.cluster.local svc.cluster.local cluster.local reddog.microsoft.com
options ndots:5
Result of kubectl get ep kube-dns --namespace=kube-system
NAME ENDPOINTS AGE
kube-dns 192.168.0.2:53,192.168.0.3:53,192.168.0.2:53 + 3 more... 6d13h
EDIT:
Ping-ing the CoreDNS pod directly using its Pod IP times out as well:
/ # ping 192.168.0.2
PING 192.168.0.2 (192.168.0.2): 56 data bytes
^C
--- 192.168.0.2 ping statistics ---
24 packets transmitted, 0 packets received, 100% packet loss
EDIT:
I think something has gone wrong when I was setting up the cluster. Below are the steps I took when setting up the cluster:
Edit host files on master and worker to include the IP's and hostnames of the nodes
Disabled swap using swapoff -a and disabled swap permanantly by editing /etc/fstab
Install docker prerequisites using apt-get install apt-transport-https ca-certificates curl software-properties-common -y
Added Docker GPG key using curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add -
Added Docker repo using add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
Install Docker using apt-get update -y; -get install docker-ce -y
Install Kubernetes prerequisites using curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key add -
Added Kubernetes repo using echo 'deb http://apt.kubernetes.io/ kubernetes-xenial main' | sudo tee /etc/apt/sources.list.d/kubernetes.list
Update repo and install Kubernetes components using apt-get update -y; apt-get install kubelet kubeadm kubectl -y
Configure master node:
kubeadm init --apiserver-advertise-address=10.62.194.4 --apiserver-cert-extra-sans=10.62.194.4 --pod-network-cidr=192.168.0.0/16
Copy Kube config to $HOME: mkdir -p $HOME/.kube; sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config; sudo chown $(id -u):$(id -g) $HOME/.kube/config
Installed Calico using kubectl apply -f https://docs.projectcalico.org/v3.1/getting-started/kubernetes/installation/hosted/rbac-kdd.yaml; kubectl apply -f https://docs.projectcalico.org/v3.1/getting-started/kubernetes/installation/hosted/kubernetes-datastore/calico-networking/1.7/calico.yaml
On node:
On the node I did the kubeadm join command using the command printed out from kubeadm token create --print-join-command on the master

Answer 1

The kubernetes system pods get assigned the host ip since they provide low level services that are not dependant on an overlay network (or in case of calico even provide the overlay network). They have the ip of the node where they run.
A common pod uses the overlay network and gets assigned an ip from the calico range, not from the metal node they run on.
You can't access DNS (port 53) with HTTP using curl. You can use dig to query a DNS resolver.
A service IP is not reachable by ping since it is a virtual IP just used as a routing handle for the iptables rules setup by kube-proxy, therefore a TCP connection works, but ICMP not.
You can ping a pod IP though, since it is assigned from the overlay network.

Answer 2

You should check on the same namespace
Currently, you are in default namespace and curl to other kube-system namespace.
You should check in the same namespace, I think it works.

Answer 3

On some cases the local host that Elasticsearch publishes is not routable/accessible from other hosts. On these cases you will have to configure network.publish_host in the yml config file, in order for Elasticsearch to use and publish the right address.
Try configuring network.publish_host to the right public address.
See more here:
https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-network.html#advanced-network-settings

Answer 4

note that control plane components like api server, etcd that runs on master node are bound to host network. and hence you see the ip address of the master server.
On the other hand, the apps that you deployed are going to get the ips from the pod subnet range. those vary from cluster node ip's
Try below steps to test dns working or not
deploy nginx.yaml
apiVersion: v1
kind: Service
metadata:
name: nginx
labels:
app: nginx
spec:
ports:
- port: 80
name: web
clusterIP: None
selector:
app: nginx
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: web
labels:
app: nginx
spec:
serviceName: "nginx"
replicas: 2
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: k8s.gcr.io/nginx-slim:0.8
ports:
- containerPort: 80
name: web
volumeMounts:
- name: www
mountPath: /usr/share/nginx/html
volumes:
- name: www
emptyDir:
kuebctl create -f nginx.yaml
master $ kubectl get po
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 1m
web-1 1/1 Running 0 1m
master $ kubectl get svc
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 35m
nginx ClusterIP None <none> 80/TCP 2m
master $ kubectl run -i --tty --image busybox:1.28 dns-test --restart=Never --rm
If you don't see a command prompt, try pressing enter.
/ # nslookup nginx
Server: 10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
Name: nginx
Address 1: 10.40.0.1 web-0.nginx.default.svc.cluster.local
Address 2: 10.40.0.2 web-1.nginx.default.svc.cluster.local
/ #
/ # nslookup web-0.nginx
Server: 10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-0.nginx
Address 1: 10.40.0.1 web-0.nginx.default.svc.cluster.local
/ # nslookup web-0.nginx.default.svc.cluster.local
Server: 10.96.0.10
Address 1: 10.96.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-0.nginx.default.svc.cluster.local
Address 1: 10.40.0.1 web-0.nginx.default.svc.cluster.local