Incorrect behavior when a routing table contains a route for destination cidr 0.0.0.0/0

[cgoIT]

Hi,

I maybe found an issue. I'm using localstack and I first thought it is an error on the localstack side (see localstack/localstack#7915). But after a little debugging I suspect the problem is in moto.

I have a routing table with the following content:

{
  "RouteTables": [
    {
      "Associations": [
        {
          "Main": false,
          "RouteTableAssociationId": "rtbassoc-7be6934d",
          "RouteTableId": "rtb-8f86de50",
          "SubnetId": "subnet-3ba9d3c0",
          "AssociationState": {
            "State": "associated"
          }
        }
      ],
      "RouteTableId": "rtb-8f86de50",
      "Routes": [
        {
          "DestinationCidrBlock": "10.1.0.0/16",
          "GatewayId": "local",
          "Origin": "CreateRouteTable",
          "State": "active"
        },
        {
          "DestinationCidrBlock": "0.0.0.0/0",
          "GatewayId": "igw-e1f33d15",
          "Origin": "CreateRoute",
          "State": "active"
        }
      ],
      "Tags": [],
      "VpcId": "vpc-f8d94257",
      "OwnerId": "000000000000"
    }
  ]
}

I try to add a route with the destination cidr 10.2.0.0/16 to this routing table. When I try to add the route I get an error "creating Route in Route Table (rtb-050b9d8a) with destination (10.2.0.0/16): RouteAlreadyExists: The route identified by 10.2.0.0/16 already exists".

During debugging I saw, the source of this error themes to be the if clause in

moto/moto/ec2/models/route_tables.py

Line 517 in 8ec9118

if not route.local and ip_v4_network.overlaps(

which was introduced with commit c3f0606 (see issue #5187). The method call for the ip_v4_network = 10.2.0.0/16 with the route.destination_cidr_block = 0.0.0.0/0 returns True.

Maybe (I'm not a python expert) it would be correct to add another statement to the if clause:

Current clause:

if not route.local and ip_v4_network.overlaps(
                ipaddress.IPv4Network(str(route.destination_cidr_block))
            )

New clause:

if not route.local and and not route.destination_cidr_block == "0.0.0.0/0" and ip_v4_network.overlaps(
                ipaddress.IPv4Network(str(route.destination_cidr_block))
            )

[bblommers]

Hi @cgoIT, thanks for raising this. I just did some cursory testing in AWS, and it looks like they explicitly allow overlaps - only direct matches throws this error.

[bblommers]

This should now be fixed as of moto >= 4.1.6.dev17

[cgoIT]

This should now be fixed as of moto >= 4.1.6.dev17

Thx a lot