Fixed wireguard MTU and added windows iface func

[rbrtbnfgl]

Description

• Fixed MTU for flannel wireguard interfaces to be the same of external interface.
• Added GetInterfaceBySpecificIPRouting on windows

Todos

• Tests
• Documentation
• Release note

Release Note

None required

[sclem]

I believe these MTU changes are incorrect. The wireguard MTU needs to account for the packet overhead from the host interface. This makes the flannel-wg interface set MTU to 1500. (Default is 1420). I am using k3s with --flannel-backend=wireguard-native and the latest update (v1.23.8+k3s2) broke my network connectivity between nodes.

[rbrtbnfgl]

It shouldn't break it because the MTU of the NIC of the POD is rightly measured here https://github.com/flannel-io/flannel/blob/master/backend/wireguard/wireguard_network.go#L72 with the overhead so the packets are generated by the PODs with the right size even if the MTU of flannel-wg is higher.

[sclem]

Here is some more information:

I have a vps agent node and a master node at home. Both have gateway interfaces with mtu 1500. If I do an iperf test on the cni0 interface when using wireguard-native with flannel-wg mtu at 1500, I get terrible speeds (0-50kbps, sometimes zero) between nodes. Dropping the mtu to 1420 and it restores near native speeds. I am not an expert, but I do not think the wireguard interface MTU can be the same as the host mtu.

[rbrtbnfgl]

Ok so you are launching the iperf on the machine and not from a pod. The MTU is fixed because it was designed to transport the traffic of the pods and with the previous setting it wasn't working in case of MTU greater then 1500. I can add the fixed overhead also for flannel-wg.

[sclem]

Yes I think that will do it! For more context, the behavior I specifically was seeing over wireguard-native with mtu 1500:

Create a librespeed deployment and metallb loadbalancer (lan ip) service on my cluster. Set nodeSelector to the cloud vps node. This way I can have a gui speed test between my cloud node and my private lan, over the wireguard-native tunnel. I could download, but my upload speed dropped to zero. That's what prompted me to track this down.

[rbrtbnfgl]

#1620 should fix it