Would like syntax to request a route to gateway IP

[bboreham]

In the (IPv4) case where we have a bridge, the bridge has an IP address we could use as a gateway, but that IP address is not in the subnet we are using for the current ADD, we can make it work by adding a route to the bridge address.

We do this in the Weave Net CNI plugin here: https://github.com/weaveworks/weave/blob/6e3f8a1c7a0ae8c011f7e28e6e06e6164396d03e/plugin/net/cni.go#L157

I would like there to be some way to request this behaviour in the CNI config, either in the spec or as a convention, so we don't have to do it "by magic".

We can't use the existing route syntax because (a) sometimes we need it and sometimes we don't, depending on the subnets and (b) we don't know the gateway address ahead of time: it depends what else has used the network on the same node.

Prompted by containernetworking/plugins#233 which describes a related scenario.

[LionelJouin]

If I understood correctly, the problem is to add a route with a gateway that does not belong to any subnet of the interface being added.

I see 2 ways to solve this problem by modifying the Route spec.

The first one would be to add a route flag property, so onlink could be used. ip route add default via 169.255.100.6 dev net1 onlink, this route could be added even if net1 has no subnet where 169.255.100.6 belongs.

The second one would be to add a route scope property, so link scope could be used.

ip route add 169.255.100.6/32 scope link dev net1
ip route add default via 169.255.100.6 dev net1

An extra route for 169.255.100.6/32 would be needed so 169.255.100.6 can be used as a gateway for the second route.

I created a PR for this: #1068

This configuration would then work (route gateway 169.255.100.160 doesn't belong to 10.0.0.0/24 subnet, but scope 253 (link) allows the route to be created):

{
    "cniVersion": "1.0.0",
    "name": "test-scope",
	"master": "eth0",
	"type": "macvlan",
	"ipam": {
		"addresses": [
			{
				"address": "10.0.0.1/24"
			}
		],
		"routes": [
			{
				"dst": "0.0.0.0/0",
				"gw": "169.255.100.160",
				"scope": 253
			}
		],
		"type": "static"
	}
}

[bboreham]

Thank you. I had forgotten about this issue, and don't use that piece of software any more.
However it seems you know IP routes far better than I do, so thank you for commenting.