Troubleshooting Network Connectivity Problems in Kubernetes

Kubernetes networking is a complex and multifaceted domain, and troubleshooting network connectivity issues can be challenging. This blog post will delve into common network connectivity problems in Kubernetes and provide detailed steps for diagnosing and resolving these issues. We will cover various tools and techniques that can help Platform Engineering teams effectively troubleshoot and manage their Kubernetes clusters.

Common Network Connectivity Issues

1. DNS Resolution Issues

DNS resolution is crucial for service discovery in Kubernetes. Issues with DNS resolution can prevent pods from communicating with each other and with services outside the cluster.

Symptoms

Pods cannot resolve service names.
Applications fail to connect to services.

Diagnosis

Check DNS Service Status Ensure the kube-dns service is running:

 kubectl get svc kube-dns --namespace=kube-system

Example output:

 NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)         AGE
 kube-dns   ClusterIP   10.36.0.10   <none>        53/UDP,53/TCP   3m51s

Inspect DNS Logs Check the logs of the kube-dns service for any errors:
```
 kubectl logs --namespace=kube-system -l k8s-app=kube-dns
```
Verify Cluster CIDR Ensure the DNS service is using the correct cluster CIDR:
```
 kubectl cluster-info dump | grep --cluster-cidr
```
Example output:
```
 --cluster-cidr=10.32.0.0/14
```
Check CNI Plugin Configuration Verify that the CNI plugin is correctly configured and initialized:
```
 kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
```
If the pods are not running, check the CNI plugin logs for errors.

Resolution

Restart the kube-dns service if it is not running.
Update the CNI plugin configuration if it is misconfigured.
Consult the CNI plugin documentation for specific troubleshooting steps.

2. Ingress Controller Issues

Ingress controllers manage incoming HTTP requests and route them to appropriate services. Issues with ingress controllers can prevent external access to applications.

Symptoms

External requests to the ingress controller fail.
Applications are not accessible from outside the cluster.

Diagnosis

Check Ingress Controller Status Ensure the ingress controller is running:

 kubectl get pods --namespace=ingress-nginx

Example output:

 NAME                              READY   STATUS    RESTARTS   AGE
 ingress-nginx-controller-...      1/1     Running   0          10m

Inspect Ingress Controller Logs Check the logs of the ingress controller for any errors:
```
 kubectl logs --namespace=ingress-nginx ingress-nginx-controller-...
```

Verify Ingress Configuration Ensure the ingress resource is correctly configured:

 kubectl get ingress --namespace=ingress-nginx

Example output:

 NAME            HOSTS   ADDRESS   PORTS   AGE
 ingress-nginx   *       <pending>   80      10m

Resolution

Restart the ingress controller if it is not running.
Update the ingress configuration if it is misconfigured.
Consult the ingress controller documentation for specific troubleshooting steps.

3. Pod CIDR Conflicts

Pod CIDR conflicts can occur when multiple pods have overlapping IP addresses, causing network connectivity issues.

Symptoms

Pods cannot communicate with each other.
Network traffic is not forwarded correctly.

Diagnosis

Check Pod CIDR Configuration Ensure that the pod CIDR ranges do not overlap:

 kubectl get nodes -o jsonpath='{.items[*].spec.podCIDR}'

Example output:

 ["10.244.0.0/24", "10.244.1.0/24"]

Inspect Network Policies Check network policies for any conflicts:
```
 kubectl get networkpolicies --all-namespaces
```

Resolution

Update the pod CIDR ranges to avoid conflicts.
Review and update network policies to ensure they are correctly configured.

4. Firewall Rules Blocking Overlay Network Traffic

Firewall rules can block overlay network traffic, causing pods to lose connectivity.

Symptoms

Pods cannot communicate with each other.
Network traffic is not forwarded correctly.

Diagnosis

Check Firewall Rules Ensure that firewall rules are not blocking overlay network traffic:

 iptables -n -L -v

Example output:

 Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
   pkts bytes target     prot opt in     out     source               destination
      0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
      0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0
      0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0
      0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate NEW
      0     0 REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0            reject-with icmp-host-prohibited

Use iperf to Test Network Traffic Use iperf to test network traffic between pods:
```
 iperf -s -p 8472 -u
```
On the client side:
```
 iperf -c 172.28.128.103 -u -p 8472 -b 1K
```

Resolution

Update firewall rules to allow overlay network traffic.
Consult the firewall documentation for specific configuration steps.

5. CNI Plugin Not Initialized

The CNI plugin not being initialized can cause network connectivity issues for pods.

Symptoms

Pods cannot communicate with each other.
Network traffic is not forwarded correctly.

Diagnosis

Check CNI Plugin Status Ensure the CNI plugin is initialized:

 kubectl get pods --namespace=kube-system -l k8s-app=kube-dns

Example output:

 NAME                              READY   STATUS    RESTARTS   AGE
 kube-dns-...                      1/1     Running   0          10m

Inspect CNI Plugin Logs Check the logs of the CNI plugin for any errors:
```
 kubectl logs --namespace=kube-system -l k8s-app=kube-dns
```

Resolution

Restart the CNI plugin if it is not initialized.
Update the CNI plugin configuration if it is misconfigured.
Consult the CNI plugin documentation for specific troubleshooting steps.

Conclusion

Troubleshooting network connectivity issues in Kubernetes requires a systematic approach, involving the use of various tools and techniques. By understanding the common issues and their symptoms, Platform Engineering teams can effectively diagnose and resolve network connectivity problems, ensuring the smooth operation of their Kubernetes clusters.