Add support to configure policy based routing

This bug was originally filed in Launchpad as [LP: #1807297](https://bugs.launchpad.net/cloud-init/+bug/1807297)

Launchpad details

affected_projects = []

assignee = holmanb

assignee_name = Brett Holman

date_closed = None

date_created = 2018-12-07T01:39:53.561170+00:00

date_fix_committed = None

date_fix_released = None

id = 1807297

importance = high

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1807297

milestone = None

owner = gjayavelu

owner_name = Giridhar Jayavelu

private = False

status = triaged

submitter = gjayavelu

submitter_name = Giridhar Jayavelu

tags = []

duplicates = []

_Launchpad user **Giridhar Jayavelu(gjayavelu)** wrote on 2018-12-07T01:39:53.561170+00:00_

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