Feat/include snat cidrs

[tanguyfalconnet]

What type of PR is this?
feature

Which issue does this PR fix:
#1224

What does this PR do / Why do we need it:
#1224

If an issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:

Testing done on this change:
Testing done on v1.6.1 (backport)

Automation added to e2e:

Will this break upgrades or downgrades. Has updating a running cluster been tested?:

Does this change require updates to the CNI daemonset config files to work?:

Does this PR introduce any user-facing change?:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[tanguyfalconnet]

@totahuanocotl I saw your PR #520, could you explain me how to handle rpc ? I'm missing something...

[totahuanocotl]

@totahuanocotl I saw your PR #520, could you explain me how to handle rpc ? I'm missing something...

It's been a year since that PR, and I haven't worked on the project since but I'll try to do my best.
If you haven't read the cni proposal I would recommend it, as it very descriptive of what the plugin is doing.

You will find how the cni plugin interacts with the ipam daemonset via grpc.

The ipam daemonset sets up the host networking on startup, in our case we were interested in the iptables rules

1. Creates ipamd
2. Initializes the node and
3. Sets up the host network
4. iptables mostly

The plugin, on the other hand is what interacts with kubelet, and this will setup the ip routes in the host, which would be the NS for the pod, if it requires it. This interaction is the one done via grpc to the agent. Our interest was in the add command, since it is the one that sets up everything.

1. The plugin is requested to Add a network
2. Which it delegates to the agent via grpc
3. The handler does it business, and because we were interested in excluding specific CIDRS, we had to provide those CIDRs back along with the VPC ones so that,
4. Using those CIDRS the plugin could setup the NS
5. Which would set up the ip rules for those vpc CIDRs on the host

I hope this is enough to give you more clarity of how it fits together.

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