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UDP backend flannel0 interface uses a network address #1049

solarsea opened this Issue Oct 11, 2018 · 0 comments


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solarsea commented Oct 11, 2018

When running in a kubernetes cluster with the UDP backend configured via kubeadm with a pod CIDR (say and a matching flannel manifest with as a network flannel creates a flannel0 interface on the master node using as an IP address. This prevents traffic from other minion nodes' containers back to the master, as it seems that minion hosts (which use 10.244.X.0/16 as their flannel0 interface IP) see the as a network address and drop the packets instead of sending them back.

Manually adding a secondary IP address on the flannel0 interface on the master node and trying a ping through that results in traffic getting back to the master node.

Expected Behavior

Connectivity between containers running on the minion nodes and the master's flannel0 interface should be possible.

Current Behavior

Packets get dropped due to flannel0 using a network address as an ip address on the master node.

Possible Solution

I guess that starting from the first available host should work.

Steps to Reproduce (for bugs)

  • create a cluster with two nodes using kubeadm and a flannel manifest
  • run a daemonset with a pause container so that it starts on each node
  • try to ping a container on the minion node from the master node


We're running local k8s clusters with flannel for container networking

Your Environment

  • Flannel version: 0.10.0
  • Backend used (e.g. vxlan or udp): udp
  • Etcd version: whatever comes with k8s
  • Kubernetes version (if used): 1.11
  • Operating System and version: vmware photon os 2.
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