Cilium vs Flannel

Cilium is an open-source, cloud-native system for providing, securing, and observing network connectivity between workloads using eBPF. Its networking capabilities include CNI, Layer 4 load balancing, Cluster Mesh, kube-proxy replacement, BGP, service mesh, Gateway API, IPv6, host firewall, and ingress. Network policies can enforce identity-aware rules from L3 through L7, including DNS- and HTTP-aware filtering. Security features include runtime protection and transparent encryption of managed host and endpoint traffic using IPsec, WireGuard, or ztunnel; ztunnel support is marked beta. For visibility, Cilium lists service maps, metrics and tracing export, network flow logs, and protocol visibility. Hubble, built on Cilium and eBPF, provides visibility into service communication and behavior. Documentation describes Prometheus and Grafana integrations and Kubernetes support for services including GKE, AKS, EKS, OpenShift, RKE, k3s, and Alibaba ACK. Its container image requires AMD64 or AArch64 hosts running Linux kernel 5.10 or later, or an equivalent kernel such as 4.18 on RHEL 8.10. For environments above 500 nodes, the quick installation guide directs readers to large-environment guidance or specific datapath modes.

Flannel is a free network fabric for containers designed for Kubernetes. A small flanneld agent on each host allocates a subnet lease from a configured address space, then forwards packets between hosts using a selected backend. Documented choices include VXLAN, host-gw, WireGuard and UDP, with VXLAN recommended. WireGuard encapsulates and encrypts packets using the in-kernel WireGuard implementation. Network configuration and subnet allocations can be stored in the Kubernetes API or directly in etcd. The project documents Kubernetes and Docker deployments, with Linux and Windows build instructions. Flannel does not enforce network policies itself, though its Helm chart can deploy a Kubernetes SIGs network policy controller alongside it. It runs as a privileged daemonset with access to the host network, so the security policy advises least privilege and keeping it updated. Security fixes are maintained for the latest release only, and the backend documentation says not to change a selected backend at runtime.