Allow to specify interface to flannel

[uablrek]

Is your feature request related to a problem? Please describe.

There are options to flannel to specify the interface used for vxlan. The default is to use the interface of the default route. But if the default route has multiple hops;

# ip ro
default 
        nexthop via 192.168.1.201 dev eth1 weight 1 
        nexthop via 192.168.1.202 dev eth1 weight 1 

Then k3s fails with;

FATA[2019-03-24T12:28:35.562033903Z] flannel exited: failed to get default interface: Found default route but could not determine interface

Another case is of course if the default route points to an interface where vxlan between nodes is unwanted or impossible.

Describe the solution you'd like

I would like a command line option (or similar) that allows specification of the interface to flannel. Or perhaps even better to allow any extra option to be passed to flannel.

Describe alternatives you've considered

To disable the included flannel CNI plugin and use an own CNI plugin. But that removes some of the desired "easy-to-use" feature of k3s.

Additional context

The problem happened when setting up k3s on xcluster. It now works fine with just one hop on the dfault route. Please check the k3s ovl for more info.

IMHO this is a really simple way to try k3s in a cluster and may be suitable for e2e tests. But I am biased since I am the author of xcluster.

[aaliddell]

#250 should cover this if merged

[uablrek]

I can confirm that with PR #250 applied my problem is solved and k3s works with a multi-dest default route.
I close this issue since it is a duplicate.