Support for NAT Gateways with AWS

[jon-shanks]

Currently there is a limitation on being able to use Nat gateways with resiliency i.e. across multiple AZ's. The AWS architecture is:

Compute Subnet (eu-west-1 AZ A) -> RT_A -> NG_A -> DF_RT -> IGW
Compute Subnet (eu-west-1 AZ B) -> RT_B -> NG_B -> DF_RT -> IGW
Compute Subnet (eu-west-1 AZ C) -> RT_C -> NG_C -> DF_RT -> IGW

RT = Route table
NG = Nat gateway
DF = Default
IGW = Internet Gateway

A to C = is the availability zones

The issue is that flannel at the moment only supports adding the route tables to a specified route table

    cfg := struct {
            RouteTableID string
    }{}

At the moment this is a string if undefined it finds the route table associated with the instance and it will add it there. In this instance, it means that routes will only be populated in one route table i.e. RT_A or RT_B etc, hence any traffic from RT_B to RT_A won't know how to route, we would therefore need to populate it as an array of route table ID's.

I am happy to do a PR for this. Without this, traffic can't route accordingly when you are using NAT gateways or decide to use more than one route table for your cluster (for whatever reason).

[stevedbashton]

Hi Jon,
I'd be happy to test, as it fits a use case I have.
Cheers, steve

[tomdee]

Closing and tracking this just under #193