Linking more than one subnet with a gateway_ip causes the node not to bring up networking

[Blake Rouse]

If more than one subnet is linked to an interface or interfaces on a node and each subnet has a gateway set curtin will render the networking configuration but it will not be a usable /etc/network/interfaces and the machine will fail to start the networking service.

Two things need to be done to fix this:

1. When more than one gateway_ip is defined for a node we need to make the best possible decision to select the correct gateway_ip. Some of the following should be taken into consideration.

    a. Pick subnets that are managed over subnets that are not managed.
    b. Pick subnets linked to bonds over physical interfaces.
    c. Pick subnets linked to physical interfaces over VLAN interfaces.

2. We will never always be correct every ones network is configured differently and we need to provide a way for the user to state which gateway on the assigned subnets they want to be the default.

[Ante Karamatić (ivoks)]

Think of default gateway as a node property, and not as a network property. So, maybe just have one gateway entry per node?

[Blake Rouse (blake-rouse)]

Ante,

Yeah #2 was where I was describing it as a node property. The user should be able to select which subnet "with a gateway defined on that subnet" and the interface to use as the default gateway.

[Mike Pontillo (mpontillo)]

I see a few use cases here:

(1) Set a Node's default gateway to the default gateway for one of the Subnets for which the Node has an assigned IP address. (#2)
(2) Remove the Node's default gateway (revert to using MAAS's heuristic described in #1 in the bug)
(3) Set the default gateway to an arbitrary IP, which must be on a Subnet attached to the node, but may not be the default gateway for that Subnet; could be a secondary gateway. (not in scope, but could be possible with moderate effort)
(4) Specify arbitrary static routes for a node, such as 10.0.0.0/8 via 192.168.0.1, 0.0.0.0/0 via 10.0.0.1, add "metric" preferences in case interfaces go down, etc. (not in scope)