Pod in CrashLoopBackOff state in Kubernetes? It may not be what you think!

The "CrashLoopBackOff" error in Kubernetes can be misleading. It doesn't necessarily mean your application is broken. Often, it can be traced back to connection issues—particularly timeouts or failures due to misconfigured network policies (NetPols).

If your pod is repeatedly crashing, it's worth checking if a network policy is causing the problem. Network policies in Kubernetes can sometimes prevent pods from reaching critical services they need, causing them to fail and restart in an endless loop.

Why is This Happening?

Network Policies in Kubernetes act like gatekeepers for your pods, determining which traffic is allowed. While they help secure your cluster, they can be too strict or misconfigured, leading to unintended consequences. This often results in situations where pods can't reach essential services, causing a cascade of failures, like CrashLoopBackOffs.

A pod might be configured to access another service at startup. If that connection is blocked due to a restrictive network policy, the pod will fail, restart, and try again—resulting in a CrashLoopBackOff.

What Does CrashLoopBackOff Look Like?

When a pod is stuck in CrashLoopBackOff, you might see log messages like:

Warning  BackOff  1m (x4 over 2m)  kubelet  Back-off restarting failed containerWarning Failed  1m  kubelet  Error: CrashLoopBackOff

When describing the pod with kubectl describe pod <pod-name>, you might see:

State: WaitingReason: CrashLoopBackOff

The logs of the container itself may show timeout or connection errors, such as:

Node.js 

Error: connect ETIMEDOUT at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1125:14)

Java

java.net.SocketTimeoutException: connect timed out

Python

socket.timeout: timed out

Go

dial tcp: i/o timeout

How to Fix CrashLoopBackOff Due to Network Policies

1. Check Existing Network Policies: Make sure the server pod isn’t covered by a policy that’s too restrictive or missing rules that allow incoming traffic from your client.
   
   

2. Test Connectivity using kubectl exec and netcat

   To verify if the connection is being blocked by a network policy, you can use kubectl exec to test the connection directly from the source pod:
   
   

   kubectl exec -it my_source_pod -- nc -zv my_svc 8080

   
   

   • Replace my_source_pod with the name of the pod acting as the source.

   • Replace my_svc with the name of the destination service.

   • Replace 8080 with the destination service port.

   If no shell is available in the container (distroless image) or netcat is not installed, you can use kubectl debug to achieve the same:
   
   

   kubectl debug my_source_pod -it --image=ubuntu --target=my_source_container apt-get update && apt-get install -y netcat 

   
   

   And run the command:

   nc -zv my_svc 8080

   
   

3. Review and Adjust Network Policies

Example of a problematic policy:

This policy protects the server pod but doesn’t have a rule allowing traffic from the client—hence the CrashLoopBackOff.

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: restrict-server
spec:
  podSelector:
    matchLabels:
      app: server
  policyTypes:
  - Ingress
  ingress:
  - from:
      - podSelector: {}

Once you have created a network policy for a target service, each source client that needs to access that destination service also needs a matching network policy, that is the trick here!

Corrected policy:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-client-to-server
spec:
  podSelector:
    matchLabels:
      app: server
  policyTypes:
  - Ingress
  ingress:
  - from:
      - podSelector:
          matchLabels:
            app: client

Monitor traffic using tcpdump

To monitor the network traffic and see if packets are being blocked, you can use tcpdump on the host node. This is useful for catching traffic at the kube-proxy level and diagnosing issues:

1. Run an ephemeral container that shares the host network and has access to the host filesystem:
   

   kubectl debug node/<node-name> -it --image=ubuntu

   
   

2. Update the package sources and install tcpdump and iproute2:

   apt-get update && apt-get install tcpdump iproute2 -y

   
   

3. Use tcpdump to monitor traffic from the source pod IP to the destination service:

tcpdump -i any src <source-pod-ip> and dst <destination-ip> and port 8080

• Replace <source-pod-ip> with the IP address of the source pod.

• Replace <destination-ip> with the IP address of the destination service.

• Replace 8080 with the relevant port.

  
  
  
  Making life easier with Otterize
  
  

  Let’s be real—keeping network policies up to date can be a huge headache. Luckily, Otterize’s Intents Operator and Network Mapper are here to save the day.
  

  
  Otterize can automatically create and manage network policies that evolve as your app does, so you don’t have to worry about unexpected connection issues or spend hours troubleshooting them.
  
  Here’s a quick tutorial on how to deploy Otterize and start creating network policies, and a longer and detailed guide on mastering cloud-native packets flows in your cluster.
  
  

Ready to make network policies a breeze?

Stop stressing over network policy details. Let Otterize handle the heavy lifting with ClientIntents, and get back to focusing on your app’s real business.

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