Troubleshooting Network

Network issues comes up frequently for new installations of Kubernetes or increasing Kubernetes load. This chapter introduces various network problems and troubleshooting method when using kubernetes.


Kubernetes "IP-per-pod" model solves 4 distinct networking problems:

  • Highly-coupled container-to-container communications: this is solved by pods and localhost communications.
  • Pod-to-Pod communications: this is solved by CNI network plugin
  • Pod-to-Service communications: this is solved by services
  • External-to-Service communications: this is solved by services

And these are exactly the direction of what we should do when encountering network issues. Reasons include:

  • CNI network plugin configure error
    • CIDR conflicts with existing ones
    • using protocols not supported by underlying network (e.g. multicast may be disabled for clusters on public cloud)
    • IP forward is not enabled
      • sysctl net.ipv4.ip_forward
      • sysctl net.bridge.bridge-nf-call-iptables
  • Missing route tables
    • default kubenet plugin requires a network route for each podCIDR to node IP
    • kube-controller-manager should configure the route table for all nodes, but if something is wrong (e.g. not authorized, exceed quota, etc), route may be missing
  • Forbidden by security groups or firewall rules
    • iptables not managed by kubernetes may forbid kubernetes network connections
    • security groups on public cloud may forbid kubernetes network connections
    • ACL on switches or routers may also forbid kubernetes network connections

Flannel Pods stuck in Init:CrashLoopBackOff

When using Flannel network plugin, it is very easy to install for a fresh setup

kubectl apply -f

However, after a while, Flannel Pods may be stuck in Init:CrashLoopBackOff state and also result in not able to create other pods (because network ready is a requirement).

$ kubectl -n kube-system get pod
NAME                            READY     STATUS                  RESTARTS   AGE
kube-flannel-ds-ckfdc           0/1       Init:CrashLoopBackOff   4          2m
kube-flannel-ds-jpp96           0/1       Init:CrashLoopBackOff   4          2m

Check logs of Pod kube-flannel-ds-jpp96

$ kubectl -n kube-system logs kube-flannel-ds-jpp96 -c install-cni
cp: can't create '/etc/cni/net.d/10-flannel.conflist': Permission denied

This issue is usually caused by SELinux, close SELinux should solve the problem. There are two ways to do this:

  • Set SELINUX=disabled in file /etc/selinux/config (persistent even after reboot)
  • Execute command setenforce 0 (not persistent after reboot)

DNS not work

If your docker version is above 1.13+, then docker would change default iptables FORWARD policy to DROP (at each restart). This change may cause kube-dns not reaching upstream DNS servers. A solution is run iptables -P FORWARD ACCEPT on each nodes, e.g.

echo "ExecStartPost=/sbin/iptables -P FORWARD ACCEPT" >> /etc/systemd/system/docker.service.d/exec_start.conf
systemctl daemon-reload
systemctl restart docker

Or if you are using flannel or weave network plugin, upgrades to latest version could also solve the problem.

There is also possible that kube-dns is not running normally.

$ kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
NAME                    READY     STATUS    RESTARTS   AGE
kube-dns-v19-ezo1y      3/3       Running   0           1h

If kube-dns pods are in CrashLoopBackOff state, refer to Troubleshooting kube-dns/dashboard CrashLoopBackOff for troubleshooting kube-dns problem.

Or else, check whether kube-dns service and endpoints are normal:

$ kubectl get svc kube-dns --namespace=kube-system
NAME          CLUSTER-IP     EXTERNAL-IP   PORT(S)             AGE
kube-dns      <none>        53/UDP,53/TCP        1h

$ kubectl get ep kube-dns --namespace=kube-system
NAME       ENDPOINTS                       AGE
kube-dns,    1h

If kube-dns service doens't exist, or its endpoints are empty, then you should recreate kube-dns service, e.g.

apiVersion: v1
kind: Service
  name: kube-dns
  namespace: kube-system
    k8s-app: kube-dns "true" "KubeDNS"
    k8s-app: kube-dns
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
    protocol: TCP

Service not accessible within Pods

The first step is checking whether endpoints have been created automatically for the service

kubectl get endpoints <service-name>

If you got an empty result, there is possible your service's label selector is wrong. Confirm it as follows:

# Query Service LabelSelector
kubectl get svc <service-name> -o jsonpath='{.spec.selector}'
# Get Pods matching the LabelSelector and check whether they are running
kubectl get pods -l key1=value1,key2=value2

If all of above steps are still ok, confirm further by

  • checking whether Pod containerPort is same with Service containerPort
  • checking whether podIP:containerPort is working

Further, there are also other reasons could also cause service problems. Reasons include:

  • container is not listening to specified containerPort (check pod description again)
  • CNI plugin error or network route error
  • kube-proxy is not running or iptables rules are not configured correctly

Normally, following iptables should be created for a service named hostnames:

$ iptables-save | grep hostnames
-A KUBE-SEP-57KPRZ3JQVENLNBR -s -m comment --comment "default/hostnames:" -j MARK --set-xmark 0x00004000/0x00004000
-A KUBE-SEP-57KPRZ3JQVENLNBR -p tcp -m comment --comment "default/hostnames:" -m tcp -j DNAT --to-destination
-A KUBE-SEP-WNBA2IHDGP2BOBGZ -s -m comment --comment "default/hostnames:" -j MARK --set-xmark 0x00004000/0x00004000
-A KUBE-SEP-WNBA2IHDGP2BOBGZ -p tcp -m comment --comment "default/hostnames:" -m tcp -j DNAT --to-destination
-A KUBE-SEP-X3P2623AGDH6CDF3 -s -m comment --comment "default/hostnames:" -j MARK --set-xmark 0x00004000/0x00004000
-A KUBE-SEP-X3P2623AGDH6CDF3 -p tcp -m comment --comment "default/hostnames:" -m tcp -j DNAT --to-destination
-A KUBE-SERVICES -d -p tcp -m comment --comment "default/hostnames: cluster IP" -m tcp --dport 80 -j KUBE-SVC-NWV5X2332I4OT4T3
-A KUBE-SVC-NWV5X2332I4OT4T3 -m comment --comment "default/hostnames:" -m statistic --mode random --probability 0.33332999982 -j KUBE-SEP-WNBA2IHDGP2BOBGZ
-A KUBE-SVC-NWV5X2332I4OT4T3 -m comment --comment "default/hostnames:" -m statistic --mode random --probability 0.50000000000 -j KUBE-SEP-X3P2623AGDH6CDF3
-A KUBE-SVC-NWV5X2332I4OT4T3 -m comment --comment "default/hostnames:" -j KUBE-SEP-57KPRZ3JQVENLNBR

There should be 1 rule in KUBE-SERVICES, 1 or 2 rules per endpoint in KUBE-SVC-(hash) (depending on SessionAffinity), one KUBE-SEP-(hash) chain per endpoint, and a few rules in each KUBE-SEP-(hash) chain. The exact rules will vary based on your exact config (including node-ports and load-balancers).

Pod cannot reach itself via Service IP

This can happen when the network is not properly configured for “hairpin” traffic, usually when kube-proxy is running in iptables mode and Pods are connected with bridge network.

Kubelet exposes a --hairpin-mode option, which should be configured as promiscuous-bridge or hairpin-veth instead of none (default is promiscuous-bridge).

Confirm hairpin-veth is working by:

$ for intf in /sys/devices/virtual/net/cbr0/brif/*; do cat $intf/hairpin_mode; done

Confirm promiscuous-bridge is working by:

$ ifconfig cbr0 |grep PROMISC

Can't access Kubernetes API

Many addons and containers need to access Kubernetes API for various data (e.g. kube-dns and operator containers). If such errors happened, then confirm whether Kubernetes API is accessible within Pods first:

$ kubectl run curl  --image=appropriate/curl -i -t  --restart=Never --command -- sh
If you don't see a command prompt, try pressing enter.
/ #
/ # KUBE_TOKEN=$(cat /var/run/secrets/
/ # curl -sSk -H "Authorization: Bearer $KUBE_TOKEN" https://$KUBERNETES_SERVICE_HOST:$KUBERNETES_SERVICE_PORT/api/v1/namespaces/default/pods
  "kind": "PodList",
  "apiVersion": "v1",
  "metadata": {
    "selfLink": "/api/v1/namespaces/default/pods",
    "resourceVersion": "2285"
  "items": [

If timeout error is reported, then confirm whether kubernetes service and its endpoints are normal or not:

$ kubectl get service kubernetes
kubernetes   ClusterIP    <none>        443/TCP   25m
$ kubectl get endpoints kubernetes
NAME         ENDPOINTS          AGE
kubernetes   25m

If both are still OK, then it's probably kube-apiserver is not start or it is blocked by firewall. Check kube-apiserver status and its logs

# Check kube-apiserver status
kubectl -n kube-system get pod -l component=kube-apiserver

# Get kube-apiserver logs
PODNAME=$(kubectl -n kube-system get pod -l component=kube-apiserver -o jsonpath='{.items[0]}')
kubectl -n kube-system logs $PODNAME --tail 100

But if 403 - Forbidden error is reported, then it's probably kube-apiserver is configured with RBAC. And your container's serviceAccount is not authorized to resources. For such case, you should create proper RoleBindings and ClusterRoleBindings, e.g. to make CoreDNS container run, you need

# 1. service account
apiVersion: v1
kind: ServiceAccount
  name: coredns
  namespace: kube-system
  labels: "true" Reconcile
# 2. cluster role
kind: ClusterRole
  labels: rbac-defaults Reconcile
  name: system:coredns
- apiGroups:
  - ""
  - endpoints
  - services
  - pods
  - namespaces
  - list
  - watch
# 3. cluster role binding
kind: ClusterRoleBinding
  annotations: "true"
  labels: rbac-defaults EnsureExists
  name: system:coredns
  kind: ClusterRole
  name: system:coredns
- kind: ServiceAccount
  name: coredns
  namespace: kube-system
# 4. use created service account
apiVersion: extensions/v1beta1
kind: Deployment
  name: coredns
  namespace: kube-system
    k8s-app: coredns "true" Reconcile "CoreDNS"
  replicas: 2
      k8s-app: coredns
        k8s-app: coredns
      serviceAccountName: coredns


© Pengfei Ni all right reserved,powered by GitbookUpdated at 2018-04-23 09:14:48

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