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As discussed here, using unicast ip as secondary on loopback is now supported, but I have problem making it work with latest k3s and default flannel. The ultimate goal is to have a single node k3s cluster without external network. Since loopback is always online, I am trying to leverage it.
On a single node Debian 12, I assigned 10.10.10.10 as secondary ip to loopback interface:
ip address add 10.10.10.10/32 dev lo
ip route add default via 10.10.10.10 dev lo metric 1000
I also added below to /etc/network/interfaces so that the secondary IP and backup dummy default route can be added after system reboot:
auto lo:0
iface lo:0 inet static
address 10.10.10.10/32
up ip route add default via 10.10.10.10 dev $IFACE metric 1000 || true
I then installll latest k3s with default flannel cni:
curl -sfL https://get.k3s.io | sh -s - --write-kubeconfig-mode 644 --disable-network-policy --flannel-backend=host-gw --cluster-cidr=10.200.0.0/16 --service-cidr=10.201.0.0/16 --disable traefik --flannel-iface=lo --advertise-address=10.10.10.10 --node-ip=10.10.10.10
The flannel fails to work. sudo journalctl -u k3s.service gives the error:
Jun 29 00:12:17 debian-arm64 k3s[922]: E0629 00:12:17.448967 922 log.go:32] "RunPodSandbox from runtime service failed" err="rpc error: code = Unknown desc = failed to setup network for sandbox \"8c019ef97d66ec0550d7bd84b411c1a3dc7168e50ca6da0c4702f6c31bd104b1\": plugin type=\"flannel\" failed (add): failed to create bridge \"cni0\": could not add \"cni0\": invalid argument"
Jun 29 00:12:17 debian-arm64 k3s[922]: E0629 00:12:17.449000 922 kuberuntime_sandbox.go:72] "Failed to create sandbox for pod" err="rpc error: code = Unknown desc = failed to setup network for sandbox \"8c019ef97d66ec0550d7bd84b411c1a3dc7168e50ca6da0c4702f6c31bd104b1\": plugin type=\"flannel\" failed (add): failed to create bridge \"cni0\": could not add \"cni0\": invalid argument" pod="kube-system/metrics-server-6f4c6675d5-fndcm"
Jun 29 00:12:17 debian-arm64 k3s[922]: E0629 00:12:17.449012 922 kuberuntime_manager.go:1237] "CreatePodSandbox for pod failed" err="rpc error: code = Unknown desc = failed to setup network for sandbox \"8c019ef97d66ec0550d7bd84b411c1a3dc7168e50ca6da0c4702f6c31bd104b1\": plugin type=\"flannel\" failed (add): failed to create bridge \"cni0\": could not add \"cni0\": invalid argument" pod="kube-system/metrics-server-6f4c6675d5-fndcm"
BTW, the same setup works if I use k3s with cilium cni.
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