EKS service with internal NLB, but gets timeouts randomly

While deploying both client and server on the EKS cluster and connecting through internal NLB, however, the communication gets timeout randomly.

EKS service with internal NLB, but gets timeouts randomly


While deploying both client and server on their EKS cluster and connecting through internal NLB, however, the communication gets timeout randomly.


In Kubernetes, we can expose the pod to cluster with Kubernetes Service. For sure, we can create a Kubernetes Service of type LoadBalancer, AWS NLB, or CLB is provisioned that load balances network traffic.

Figure source: https://www.docker.com/blog/designing-your-first-application-kubernetes-communication-services-part3/

To manage the workload easily, we might want to deploy both the client and server sides on Kubernetes. However, in the following conditions, you might notice the timeout issue happen randomly.

This use case includes both server-side and client-side in the same EKS cluster, and the server-side must use Kubernetes service with internal NLB - instance target type.

  1. Create a Kubernetes Deployment as server-side. For example, self-host Redis, Nginx server, etc.
  2. Create a Kubernetes Service to expose the server application through the internal NLB.
  3. Create another Kubernetes Deployment to test the application as client-side, and try to connect with the server's service.


In my testing, I launched the Kubernetes cluster with Amazon EKS (1.8.9) with default deployments (such as CoreDNS, AWS CNI Plugin, and kube-proxy). The issue can be reproduced as the steps below:

  1. Deploy a Nginx Deployment and a troubleshooting container. I used the netshoot as troubleshooting pod, which had preinstalled the curl command. Also, you can find the nginx-nlb-service.yaml and netshoot.yaml in the attachments.

    $ kubectl apply -f ./netshoot.yaml
    $ kubectl apply -f ./nginx-nlb-service.yaml


  2. Use the kubctl exec command to attach into troubleshooting(netshoot) container.

    $ kubectl exec -it netshoot -- bash
  3. Access the internal NLB domain name(Nginx's Service) via curl command, the connection will timeout randomly. At the same time, I capture the packets through with tcpdump command in EKS worker node.

    # In the EKS worker node(
    $ sudo tcpdump -i any  -w packets.pcap
    bash-5.1# date
    Mon Mar 22 23:35:13 UTC 2021
    bash-5.1# curl a7487a27201f2434490bada8096adce3-221f6e1d21fadd5b.elb.eu-west-1.amazonaws.com
    ... It will timeout randomly, you might need to try more times.
    $ date
    Mon Mar 22 23:35:32 UTC 2021

Why do we get the timeout from the Internal NLB

Firstly, we must know the NLB introduced the source IP address preservation feature[3]: the original IP addresses and source ports for the incoming connections remain unmodified. When the backend answers a request, the VPC internals capture this packet and forward it to the NLB, which will forward it to its destination.

Therefore, if the worker node is running the client-side, and NLB forward to the same nodes. It will generate a random connection timeout. We can dive into this process:


  1. The troubleshooting container starts from an ephemeral port and sends a SYN packet.

    • src:
    • dst:
  2. The NLB receives the SYN packet and forwards it to the backend EKS worker node with target port 31468, which was registered by Kubernetes Service. NLB modified the destination IP address as Client IP preservation feature. Thus, the EKS worker node receives a SYN packet:

    • src:
    • dst:
  3. Base on the iptables rules(Nginx Service) on the EKS worker node, this SYN packet was forwarded to the Nginx pod.

    • src:
    • dst:
      # Kubernetes Service will update the following iptables rules in every node.
      $ sudo iptables-save | grep "service-nginx-demo"
      -A KUBE-NODEPORTS -p tcp -m comment --comment "nginx-demo/service-nginx-demo:" -m tcp --dport 31919 -j KUBE-MARK-MASQ
      -A KUBE-NODEPORTS -p tcp -m comment --comment "nginx-demo/service-nginx-demo:" -m tcp --dport 31919 -j KUBE-SVC-7D7VEXWNCKBQRZ7W
      -A KUBE-SEP-2IF7DICDPRGPK5UI -s -m comment --comment "nginx-demo/service-nginx-demo:" -j KUBE-MARK-MASQ
      -A KUBE-SEP-2IF7DICDPRGPK5UI -p tcp -m comment --comment "nginx-demo/service-nginx-demo:" -m tcp -j DNAT --to-destination
      -A KUBE-SEP-2V4THTSJVFKRX4LC -s -m comment --comment "nginx-demo/service-nginx-demo:" -j KUBE-MARK-MASQ
      -A KUBE-SEP-2V4THTSJVFKRX4LC -p tcp -m comment --comment "nginx-demo/service-nginx-demo:" -m tcp -j DNAT --to-destination
      -A KUBE-SEP-73EDW25F4C3YFWYZ -s -m comment --comment "nginx-demo/service-nginx-demo:" -j KUBE-MARK-MASQ
      -A KUBE-SEP-73EDW25F4C3YFWYZ -p tcp -m comment --comment "nginx-demo/service-nginx-demo:" -m tcp -j DNAT --to-destination
      -A KUBE-SERVICES -d -p tcp -m comment --comment "nginx-demo/service-nginx-demo: cluster IP" -m tcp --dport 80 -j KUBE-SVC-7D7VEXWNCKBQRZ7W
      -A KUBE-SVC-7D7VEXWNCKBQRZ7W -m comment --comment "nginx-demo/service-nginx-demo:" -m statistic --mode random --probability 0.33333333349 -j KUBE-SEP-2IF7DICDPRGPK5UI
      -A KUBE-SVC-7D7VEXWNCKBQRZ7W -m comment --comment "nginx-demo/service-nginx-demo:" -m statistic --mode random --probability 0.50000000000 -j KUBE-SEP-73EDW25F4C3YFWYZ
      -A KUBE-SVC-7D7VEXWNCKBQRZ7W -m comment --comment "nginx-demo/service-nginx-demo:" -j KUBE-SEP-2V4THTSJVFKRX4LC
  4. Nginx pod replies the SYN-ACK packet back to the EKS worker node. We can view the tcp.stream eq 9 for step 3 and step4.


  5. VPC CNI plugin maintains the route table for binding the ENI with pod. Base on the route table in Node 1, it will reply the SYN-ACK packet for the SYN packet for troubleshooting pod in step 2.

    # Node 1 - routing table
    $ route -n
    Kernel IP routing table
    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface         UG    0      0        0 eth0 UH    0      0        0 eth0   U     0      0        0 eth0 UH    0      0        0 eni708fb089496
  6. The troubleshooting pod notices this SYN-ACK connection is an abnormal connection, and send RST packet to close the connection to

  7. The RST packet is forwarded to the Nginx pod through iptables rules(Nginx server) as well. Also, the troubleshooting container will send both TCP_RETRANSMISSION and RST several times until it times out.


  8. The initiating socket(src:, dst: still expects the SYN-ACK from the NLB However, the troubleshooting pod did not receive the SYN-ACK, so the troubleshooting pod will send several TCP RETRANSMISSION - SYN until it times out.



If you don't care about keeping the source IP address, we can suggest that you can consider using the NLB IP mode[4].

      name: my-service
        service.beta.kubernetes.io/aws-load-balancer-type: "nlb-ip"

2021-05-21 Update

The NLB adds support to configure client IP preservation[5] with instance mode, which is supported in version v2.2.0[6]. Thus, we can use the following annotations for the client IP preservation with instance mode.

  • Note: Please use the annotation service.beta.kubernetes.io/aws-load-balancer-type: "external" to ignore in-tree cloud-controller-manager[7] to create an in-tree NLB service.
    service.beta.kubernetes.io/aws-load-balancer-type: "external"
    service.beta.kubernetes.io/aws-load-balancer-nlb-target-type: instance
    service.beta.kubernetes.io/aws-load-balancer-target-group-attributes: "preserve_client_ip.enabled=false"


  1. Connecting Applications with Services - https://kubernetes.io/docs/concepts/services-networking/connect-applications-service/
  2. Network load balancing on Amazon EKS - https://docs.aws.amazon.com/eks/latest/userguide/load-balancing.html
  3. Target groups for your Network Load Balancers - Client IP preservationhttps://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#client-ip-preservation
  4. https://kubernetes-sigs.github.io/aws-load-balancer-controller/latest/guide/service/nlb_ip_mode/#nlb-ip-mode
  5. https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#client-ip-preservation
  6. AWS Load Balancer Controller - v2.2.0 - https://github.com/kubernetes-sigs/aws-load-balancer-controller/releases/tag/v2.2.0
  7. AWS Load Balancer Controller - Network Load Balancer - https://kubernetes-sigs.github.io/aws-load-balancer-controller/v2.2/guide/service/nlb/#configuration