The networking fabric of a K3s cluster is governed by the Container Network Interface (CNI), a standardized specification that dictates how network interfaces are configured for Linux application containers. In the K3s ecosystem, the CNI is not merely an optional add-on but a fundamental component that ensures every pod can communicate directly with all other pods across all nodes without the interference of Network Address Translation (NAT). This architecture ensures that the IP address a container perceives as its own is the exact same IP address that other entities in the cluster see when communicating with that container. By removing the need for NAT, K3s simplifies service discovery and maintains a transparent networking model where node agents can also communicate with pods on any node without translation.
The Role of CNI in Kubernetes Orchestration
The Container Network Interface (CNI) serves as the blueprint for how executable plugins are utilized to configure network interfaces for containers. It acts as a standardized layer, allowing Kubernetes to remain agnostic about the specific networking implementation used. This modularity enables K3s to ship with a default solution while allowing users to swap it for others depending on their specific infrastructure requirements.
The CNI implementation in K3s is responsible for several critical functions:
- Pod IP assignment: Every pod is assigned a unique IP address managed by K3s.
- Network interface configuration: The CNI plugin handles the actual creation and plumbing of the network interface within the container's namespace.
- Connectivity management: It ensures that the basic Kubernetes networking requirements—pod-to-pod and node-to-pod communication—are met.
Flannel: The Default K3s Network Fabric
K3s integrates Flannel as its default CNI provider. Flannel is an overlay network that utilizes VXLAN (Virtual Extensible LAN) as its default backend. It was originally developed by CoreOS, and although CoreOS has ceased maintenance, Flannel remains a primary option for Kubernetes networking due to its simplicity and reliability.
In a K3s deployment, Flannel is bundled directly into the distribution and runs as a backend go routine that initializes when K3S starts. This integration eliminates the need for separate installation steps for basic networking.
VXLAN Backend and the VTEP Mechanism
The primary mechanism Flannel employs is the VXLAN tunnel. When Flannel is operational, it creates a network device known as flannel.1, which serves as the VXLAN Tunnel End Point (VTEP). This device allows the cluster to encapsulate Layer 2 Ethernet frames within Layer 3 UDP packets, effectively creating a virtual Layer 2 network on top of an existing Layer 3 infrastructure.
The technical specifications of the flannel.1 device typically include:
- MTU (Maximum Transmission Unit): 8951.
- State: UNKNOWN (typical for virtual interfaces).
- VXLAN ID: 1.
- Port: dstport 8472.
- IP assignment: Often uses a
/32scope global address (e.g.,10.42.0.0/32).
The impact of this design is that it allows K3s to maintain a flat network across multiple physical or virtual hosts regardless of the underlying network topology, as long as the hosts can communicate via UDP.
Advanced CNI Alternatives and Implementation
While Flannel provides essential connectivity, it lacks advanced features such as granular network policies or deep observability. As a cluster grows in complexity or requires stricter security, users may transition to more powerful CNI plugins.
Comparison of CNI Capabilities
The following table outlines the recommended CNI plugins based on specific organizational needs:
| Need | Recommended CNI |
|---|---|
| Enforce network security rules | Calico |
| Deep observability / eBPF | Cilium |
| Multi-cluster support | Cilium, Calico |
| WireGuard encryption | Cilium |
| BGP routing | Calico |
Migrating from Flannel to Calico
To implement a more robust solution like Calico, which allows for the enforcement of pod-to-pod communication rules, the default Flannel installation must be removed. This process requires a full uninstall and a targeted reinstall of K3s.
The migration process involves these steps:
Uninstall K3s using the provided script:
sudo /usr/local/bin/k3s-uninstall.shReinstall K3s while explicitly disabling Flannel and network policies:
curl -sfL https://get.k3s.io | INSTALL_K3S_EXEC="--flannel-backend=none --disable-network-policy" sh -Apply the Tigera operator manifests for Calico:
kubectl apply -f https://raw.githubusercontent.com/projectcalico/calico/v3.26.1/manifests/tigera-operator.yamlInstall the Calico custom resource:
kubectl apply -f https://raw.githubusercontent.com/projectcalico/calico/v3.26.1/manifests/custom-resources.yamlVerify the deployment:
kubectl get pods -n calico-system
Multus CNI and Network Multiplexing
Multus CNI provides a specialized capability: the ability to attach multiple network interfaces to a single pod. Unlike Calico or Cilium, Multus is not a replacement for a CNI plugin; instead, it functions as a CNI multiplexer. This is particularly critical for network-intensive pods that require dataplane acceleration, such as those utilizing SR-IOV (Single Root I/O Virtualization).
The Multus Dependency Model
Multus cannot function as a standalone CNI. It requires at least one conventional CNI plugin to satisfy the baseline Kubernetes cluster network requirements. This primary CNI serves as the default for Multus and provides the primary interface for all pods. In a standard K3s installation, Flannel fulfills this role.
CNI Binary Paths and Versioning
The location of CNI binaries has evolved in K3s to improve stability during upgrades.
October 2024 and Newer: K3s utilizes a fixed CNI binary path. This applies to versions
v1.28.15+k3s1,v1.29.10+k3s1,v1.30.6+k3s1, andv1.31.2+k3s1. The binaries are located at:
/var/lib/rancher/k3s/data/cniPre-October 2024: CNI binaries were stored in a hashed directory:
$DATA_DIR/data/$HASH/bin
The transition to a fixed path resolved a significant pain point where CNI plugins had to be redeployed every time K3s was upgraded because the hash in the path would change. For users on older releases, the recommended path for Multus deployment is /var/lib/rancher/k3s/data/current/bin/, though redeployment remains necessary after upgrades.
IP Address Management (IPAM) in Multus
When using Multus to create additional interfaces, an IP Address Manager (IPAM) plugin is required to assign IP addresses to those extra interfaces. Multiple IPAMs can be combined as needed.
Available IPAM Plugins
K3s supports several IPAM options for Multus:
- host-local: This plugin allocates IP addresses from a set of ranges and stores the state locally on the host's filesystem. This ensures IP uniqueness on a single host. However, because the state is local, it is not recommended for multi-node clusters. This plugin requires no additional deployment.
- Whereabouts: An IPAM designed for cluster-wide IP management.
- Multus DHCP daemon: An IPAM that leverages DHCP for address assignment.
For these plugins to function, the binaries must be installed in /var/lib/rancher/k3s/data/cni/ and the configuration must be placed in /var/lib/rancher/k3s/agent/etc/cni/net.d.
Pod Networking Execution Flow
The process of establishing a network for a pod involves a coordinated chain of events between the Kubelet, the Container Runtime Interface (CRI), and the CNI.
The CRI and Containerd Interaction
When a pod is scheduled on a node, the following sequence occurs:
- Kubelet initiates a call to the container runtime via the CRI plugin.
- The CRI plugin invokes a function call to containerd.
- Containerd starts the
containerd-shim.
The containerd-shim is a critical component that allows for daemonless containers. It acts as the parent process of the container, facilitating the creation of the pod sandbox ID and the pod network namespace.
CNI Configuration Management
K3S takes responsibility for managing the CNI plugin configuration file and the final generated network configuration file. This ensures that the networking state remains consistent across the cluster and that the la lier-level configuration matches the requirements of the selected CNI plugin.
Analysis of CNI Integration and Evolution
The evolution of CNI in K3s reflects a shift from simple, bundled solutions toward a more flexible, enterprise-ready architecture. The initial reliance on Flannel allowed K3s to provide an "out-of-the-box" experience, which is the primary value proposition of the distribution. By bundling Flannel as a go routine, K3s eliminates the overhead of managing separate networking pods for the majority of users.
However, the introduction of fixed binary paths in the October 2024 releases signals a maturation of the platform. By decoupling the CNI binary location from the release hash, K3s has enabled a more sustainable lifecycle for third-party plugins like Multus. This is a critical improvement for edge computing and telco-grade deployments where SR-IOV and multiple network interfaces are mandatory.
The architectural choice to support Multus as a multiplexer rather than a replacement highlights the layered nature of Kubernetes networking. The primary CNI (Flannel) handles the "control plane" of the network—ensuring basic connectivity—while Multus handles the "data plane" requirements for high-performance applications.
Furthermore, the ability to swap Flannel for Calico or Cilium demonstrates that K3s is not locked into a specific networking philosophy. The transition from VXLAN-based overlay networks (Flannel) to BGP-routed or eBPF-powered networks (Calico/Cilium) allows K3s to scale from a single-node edge device to a massive multi-cluster environment.