How to fix upstream connect error or disconnect/reset before headers in Kubernetes

# Check listening ports in the pod
kubectl exec -it <pod-name> -- netstat -tlnp
# or
kubectl exec -it <pod-name> -- ss -tlnp

# For Node.js
ENV HOST=0.0.0.0

# In application code
server.listen(8080, '0.0.0.0')

# Get service selector
kubectl get service my-service -o yaml | grep -A3 selector

# Check pod labels
kubectl get pods --show-labels

# Verify endpoints exist
kubectl get endpoints my-service
kubectl get endpointslices -l kubernetes.io/service-name=my-service

# Service selector
spec:
  selector:
    app: my-app  # Must match pod labels

# Pod labels
metadata:
  labels:
    app: my-app  # Must match service selector

# Check pod status
kubectl get pods -o wide

# Look for probe failures
kubectl describe pod <pod-name> | grep -A5 "Readiness"

# Check events for crash reasons
kubectl get events --field-selector involvedObject.name=<pod-name>

readinessProbe:
  httpGet:
    path: /health
    port: 8080
  initialDelaySeconds: 10
  periodSeconds: 5
  failureThreshold: 3

# Check container port
kubectl get pod <pod-name> -o jsonpath='{.spec.containers[*].ports}'

# Check service ports
kubectl get service my-service -o yaml | grep -A5 ports

# Test connectivity directly
kubectl port-forward <pod-name> 8080:8080
curl http://localhost:8080/health

# Container
ports:
- containerPort: 8080  # App listens here

# Service
ports:
- port: 80             # Service exposes this
  targetPort: 8080     # Routes to container port

# NGINX Ingress logs
kubectl logs -n ingress-nginx deployment/ingress-nginx-controller | grep "upstream"

# Istio sidecar logs
kubectl logs <pod-name> -c istio-proxy | grep "upstream"

# Envoy admin interface (if available)
kubectl port-forward <pod-name> 19000:19000
# Visit http://localhost:19000/config_dump

apiVersion: networking.istio.io/v1beta1
kind: VirtualService
metadata:
  name: my-service
spec:
  hosts:
  - my-service
  http:
  - timeout: 30s
    retries:
      attempts: 3
      perTryTimeout: 10s
      retryOn: "5xx,reset,connect-failure"
    route:
    - destination:
        host: my-service
        port:
          number: 8080

annotations:
  nginx.ingress.kubernetes.io/proxy-connect-timeout: "30"
  nginx.ingress.kubernetes.io/proxy-read-timeout: "30"