Resolving Kubernetes Flannel CIDR Overlap With Network IP 10.244.128.11

October 6, 2025 by StackCamp Team 72 views

Hey guys! Ever run into a situation where your Kubernetes cluster's internal network clashes with your external network? It's a tricky spot, especially when you're dealing with overlapping IP ranges. Let's dive into a common scenario: your Kubernetes cluster uses Flannel with the default CIDR 10.244.0.0/16, and you need to connect to an external LDAPS server that happens to be on the same IP range, like 10.244.128.11. Sounds familiar? Don't worry; we've got you covered. This article will walk you through understanding the issue and, more importantly, how to fix it so your cluster can chat happily with your external services.

Understanding the IP Address Overlap Issue

First off, let's break down why this IP address overlap is causing a headache. In the Kubernetes networking world, CIDR (Classless Inter-Domain Routing) blocks are the bread and butter for assigning IP addresses to your pods and services. When you set up a Kubernetes cluster with Flannel, it typically gets a default CIDR like 10.244.0.0/16. This range is like your cluster's private playground, where each pod gets its own IP within this block.

Now, imagine your external LDAPS server, which your applications need to talk to, also lives in the 10.244.0.0/16 range – specifically at 10.244.128.11. This is where the problem kicks in. Your cluster's networking magic (Flannel, in this case) gets confused. When a pod tries to reach 10.244.128.11, it thinks, "Hey, that's one of our own!" and tries to route the traffic internally, never reaching the external LDAPS server. This is a classic case of IP address collision, and it's a common gotcha in networking.

The root cause is that your cluster's internal network (managed by Flannel) and your external network are stepping on each other's toes, IP-wise. This overlap prevents your pods from correctly routing traffic to the external LDAPS server. It's like having two houses on the same street with the same address – the mailman wouldn't know where to deliver the mail! So, the key here is to ensure that your cluster's internal IP range and your external network's IP range are distinct and don't overlap. This might involve reconfiguring your Flannel CIDR or working with your network administrator to adjust the external IP ranges. The goal is to create a clear separation so traffic knows where it's supposed to go.

Diagnosing the Network Conflict

Before we jump into fixing things, let's make sure we've correctly identified the problem. First, verify that your Kubernetes cluster is indeed using Flannel and its default CIDR. You can usually check this in your Kubernetes configuration files or by inspecting the Flannel pod's configuration. Look for the --iface and --kube-subnet-mgr-cidr flags in the Flannel pod's arguments. These will tell you the interface Flannel is using and the CIDR it's managing. Double-check that the CIDR is indeed 10.244.0.0/16 or something similar that overlaps with your external network.

Next, confirm that the external LDAPS server's IP address, in this case, 10.244.128.11, falls within your Flannel CIDR range. A simple ping or traceroute from a pod within your cluster to the LDAPS server can help you understand how traffic is being routed. If the ping fails or the traceroute stays within your cluster's internal network, it's a strong indication of an IP overlap issue. You can exec into a pod and use tools like ping, traceroute, or nslookup to investigate. For example:

kubectl exec -it <pod-name> -- /bin/bash
ping 10.244.128.11
traceroute 10.244.128.11

These commands will help you see if the traffic is even reaching the external server or if it's getting stuck somewhere inside your cluster. Another handy tool is kubectl describe pod <pod-name>, which can give you insights into the pod's network settings and any potential issues. Look for anything related to networking, such as IP addresses, routes, and DNS configurations. These diagnostic steps will give you a clear picture of the network conflict and help you move forward with a solution. By confirming the overlap, you'll know exactly what needs to be adjusted to get your cluster talking to the LDAPS server.

Solutions to Resolve the CIDR Overlap

Okay, so we've confirmed the IP address overlap – now let's get down to brass tacks and fix it! There are a couple of main strategies we can use, and the best one for you will depend on your specific situation and constraints. The primary goal is to make sure your Kubernetes cluster's internal network and your external network have distinct, non-overlapping IP ranges.

1. Change the Flannel CIDR

One of the most straightforward solutions is to change the Flannel CIDR to a different IP range that doesn't conflict with your external network. This means reconfiguring Flannel to use a new CIDR block, like 10.245.0.0/16 or 192.168.0.0/16. When choosing a new CIDR, make sure it's a private IP range (10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16) and that it doesn't clash with any other networks your cluster needs to communicate with. Changing the Flannel CIDR involves editing the Flannel DaemonSet or deployment configuration. You'll need to update the --kube-subnet-mgr-cidr flag with your new CIDR value. For example, if you're using a ConfigMap to configure Flannel, you might need to edit the kube-flannel-cfg ConfigMap in the kube-system namespace. Here’s a quick rundown of the steps:

Edit the Flannel ConfigMap:

kubectl edit configmap kube-flannel-cfg -n kube-system

Change the net-conf.json data:

In the data section, find the net-conf.json key and modify the Network field to your new CIDR. For example:
```
data:
  net-conf.json: |-
    {
      "Network": "10.245.0.0/16",
      "Backend": {
        "Type": "vxlan"
      }
    }
```
Restart Flannel pods:

To apply the changes, you'll need to restart the Flannel pods. You can do this by deleting the Flannel DaemonSet, and Kubernetes will automatically recreate them:
```
kubectl delete daemonset kube-flannel-ds -n kube-system
```

Keep in mind that changing the CIDR will require restarting your pods, as their IP addresses will need to be updated. This means a brief downtime for your applications, so plan accordingly. It's also wise to drain your nodes before restarting Flannel to minimize disruption. Remember to thoroughly test the connectivity after making this change to ensure everything is working as expected.

2. Network Segmentation and Routing

Another approach, especially useful in more complex network environments, is to use network segmentation and routing. This involves creating distinct network segments for your Kubernetes cluster and your external network, and then setting up routing rules to allow traffic between them. This can be achieved using technologies like Virtual LANs (VLANs) or Virtual Private Networks (VPNs). The idea is to keep your Kubernetes cluster's network isolated while still enabling communication with external services like your LDAPS server.

For instance, you could put your Kubernetes nodes on a separate VLAN from your LDAPS server and then configure a router to handle traffic between the VLANs. This ensures that the IP ranges don't overlap and that traffic is correctly routed. If you're using a cloud provider like AWS, Azure, or GCP, you can leverage their networking services, such as VPC peering or VPN gateways, to create these network connections. For example, in AWS, you might use VPC peering to connect your Kubernetes cluster's VPC with the VPC where your LDAPS server resides. In Azure, you could use Virtual Network peering or a VPN gateway to achieve a similar setup. The specifics of setting up network segmentation and routing will vary depending on your infrastructure and networking setup. You'll likely need to work with your network administrator to configure the appropriate routing rules and firewall settings. This approach often involves more initial setup and configuration, but it can provide a more robust and scalable solution for managing network traffic in the long run. It also gives you more control over the traffic flow and security between your cluster and external services.

3. Using Network Policies

Network Policies in Kubernetes are like firewalls for your pods. They let you define rules that control the traffic flow between pods and between pods and external networks. While network policies don't directly solve the IP address overlap, they can help you manage and restrict traffic to minimize the impact of the conflict. For example, you can create a network policy that explicitly allows traffic from your pods to the LDAPS server's IP address (10.244.128.11) while denying other traffic within the overlapping range. This can help prevent accidental connections to other services within the 10.244.0.0/16 range that might conflict with your cluster's internal network.

To implement network policies, you'll first need a network policy controller, such as Calico or Cilium. These controllers enforce the policies you define. Once you have a controller set up, you can create NetworkPolicy resources in Kubernetes. Here's a basic example of a network policy that allows traffic to the LDAPS server:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-ldaps
  namespace: <your-namespace>
spec:
  podSelector:
    matchLabels:
      app: <your-app-label>
  egress:
  - to:
    - ipBlock:
        cidr: 10.244.128.11/32
  policyTypes:
  - Egress

This policy allows pods with the label app: <your-app-label> to make outbound connections to 10.244.128.11. You can create more complex policies to control ingress and egress traffic based on pod labels, namespaces, and IP address ranges. Network policies are a powerful tool for enhancing the security and control of your Kubernetes networking. They add an extra layer of defense against misconfigured or malicious traffic. However, keep in mind that network policies are not a substitute for resolving the IP address overlap issue. They are more of a complementary measure to manage traffic and minimize the impact of the conflict.

4. NAT (Network Address Translation)

NAT, or Network Address Translation, is a technique that can help you map internal IP addresses to external IP addresses. In the context of your Kubernetes cluster, you can use NAT to translate the traffic from your pods to a different IP address when it's destined for the external LDAPS server. This way, the traffic doesn't appear to be coming from the overlapping IP range, and the external server can correctly identify and respond to the requests.

There are a couple of ways to implement NAT in Kubernetes. One common approach is to use a service of type LoadBalancer or NodePort along with some iptables rules on your nodes. You can create a service that forwards traffic to the LDAPS server, and then use iptables to rewrite the source IP address of the traffic coming from your pods. This essentially makes the traffic appear to originate from the node's IP address rather than the pod's IP address.

Another option is to use a dedicated NAT gateway or a proxy server within your cluster. This gives you more control over the NAT process and can be useful in more complex network setups. For example, you can set up a proxy server like HAProxy or Nginx and configure it to perform NAT for traffic to the LDAPS server. Here's a simplified example of how you might set up NAT using iptables:

Create a service for the LDAPS server:

apiVersion: v1
kind: Service
metadata:
  name: ldaps-nat-service
  namespace: <your-namespace>

spec: type: ExternalName externalName: 10.244.128.11 # LDAPS server IP ports: - protocol: TCP port: 636 # LDAPS port targetPort: 636 ```

Add iptables rules on your nodes:
```
iptables -t nat -A POSTROUTING -d 10.244.128.11 -j MASQUERADE
```
This rule tells iptables to masquerade traffic destined for 10.244.128.11, effectively using the node's IP address as the source.

NAT can be a powerful workaround for IP address overlaps, but it's important to understand its implications. NAT can make troubleshooting more complex, as the source IP addresses seen by the external server will be different from the pod IP addresses. It's also crucial to ensure that your NAT configuration is secure and doesn't introduce any new vulnerabilities. While NAT can be a quick fix, it's often a good idea to consider more permanent solutions like changing the CIDR or using network segmentation for long-term network stability.

Step-by-Step Guide to Changing Flannel CIDR

Since changing the Flannel CIDR is one of the most common and effective solutions, let's walk through a detailed step-by-step guide. This will help you make the change smoothly and minimize disruptions to your cluster. Before you start, make sure you have kubectl installed and configured to access your Kubernetes cluster. It's also a good idea to back up your Flannel configuration files so you can easily revert if something goes wrong.

Step 1: Choose a New CIDR

First, you'll need to pick a new CIDR that doesn't overlap with your external network or any other internal networks. As mentioned earlier, stick to private IP ranges (10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16) and make sure it's a range that's large enough to accommodate your current and future pod IP address needs. A /16 CIDR (e.g., 10.245.0.0/16) is often a good starting point for small to medium-sized clusters. For larger clusters, you might need a /12 or /8. Write down your new CIDR; you'll need it in the next steps.

Step 2: Edit the Flannel ConfigMap

Flannel's configuration is typically stored in a ConfigMap in the kube-system namespace. You'll need to edit this ConfigMap to update the CIDR. Run the following command to open the ConfigMap in your default text editor:

kubectl edit configmap kube-flannel-cfg -n kube-system

This will open the ConfigMap in your editor. Look for the data section, and within that, find the net-conf.json key. The value of this key is a JSON string that contains Flannel's network configuration. You'll need to modify the Network field to your new CIDR. The net-conf.json should look something like this:

{
  "Network": "10.244.0.0/16",
  "Backend": {
    "Type": "vxlan"
  }
}

Change the `