Add support to configure policy based routing

[Giridhar Jayavelu]

When there are multiple interfaces with their own default gateways, this could result in asymmetric routing. In order to solve this, policy based routing would have to be configured.

- There is already support to configure routes in cloud-init, but it misses ability to specify routing table.
- Need ability to create routing table entries in /etc/iproute2/rt_tables
- Configure routing table lookup for a given network.

Here is a sample configuration:
echo "100 eth0" >> /etc/iproute2/rt_tables
echo "101 eth1" >> /etc/iproute2/rt_tables

auto eth0
iface eth0 inet static
    address 10.172.142.37/24
    dns-nameservers 10.172.40.1
    gateway 10.172.142.253
    up ip rule add from 10.172.142.0/24 lookup eth0
    up ip route add 10.172.142.0/24 dev eth0 table eth0
    up ip route add default via 10.172.142.253 table eth0

auto eth1
iface eth1 inet static
    address 10.172.144.56/24
    dns-nameservers 10.172.40.1
    gateway 10.172.144.253
    up ip rule add from 10.172.144.0/24 lookup eth1
    up ip route add 10.172.144.0/24 dev eth1 table eth1
    up ip route add default via 10.172.144.253 table eth1

[Ryan Harper (raharper)]

Hi,

Thanks for filing the bug. This is a valid scenario.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3298