Background
==========
Imagine you run a container on your machine. Then the routing table
might look like:
default via 10.0.10.1 dev eth0 proto dhcp metric 100
10.0.10.0/28 dev eth0 proto kernel scope link src 10.0.10.5 metric 100
[...]
10.42.0.0/24 via 10.42.0.0 dev flannel.1 onlink
10.42.1.2 dev cali02ad7e68ce1 scope link
10.42.1.3 dev cali8fcecf5aaff scope link
10.42.2.0/24 via 10.42.2.0 dev flannel.1 onlink
10.42.3.0/24 via 10.42.3.0 dev flannel.1 onlink
That is, there are another interfaces with subnets and specific routes.
If nm-cloud-setup now configures rules:
0: from all lookup local
30400: from 10.0.10.5 lookup 30400
32766: from all lookup main
32767: from all lookup default
and
default via 10.0.10.1 dev eth0 table 30400 proto static metric 10
10.0.10.1 dev eth0 table 30400 proto static scope link metric 10
then these other subnets will also be reached via the default route.
This container example is just one case where this is a problem. In
general, if you have specific routes on another interface, then the
default route in the 30400+ table will interfere badly.
The idea of nm-cloud-setup is to automatically configure the network for
secondary IP addresses. When the user has special requirements, then
they should disable nm-cloud-setup and configure whatever they want.
But the container use case is popular and important. It is not something
where the user actively configures the network. This case needs to work better,
out of the box. In general, nm-cloud-setup should work better with the
existing network configuration.
Change
======
Add new routing tables 30200+ with the individual subnets of the
interface:
10.0.10.0/24 dev eth0 table 30200 proto static metric 10
[...]
default via 10.0.10.1 dev eth0 table 30400 proto static metric 10
10.0.10.1 dev eth0 table 30400 proto static scope link metric 10
Also add more important routing rules with priority 30200+, which select
these tables based on the source address:
30200: from 10.0.10.5 lookup 30200
These will do source based routing for the subnets on these
interfaces.
Then, add a rule with priority 30350
30350: lookup main suppress_prefixlength 0
which processes the routes from the main table, but ignores the default
routes. 30350 was chosen, because it's in between the rules 30200+ and
30400+, leaving a range for the user to configure their own rules.
Then, as before, the rules 30400+ again look at the corresponding 30400+
table, to find a default route.
Finally, process the main table again, this time honoring the default
route. That is for packets that have a different source address.
This change means that the source based routing is used for the
subnets that are configured on the interface and for the default route.
Whereas, if there are any more specific routes in the main table, they will
be preferred over the default route.
Apparently Amazon Linux solves this differently, by not configuring a
routing table for addresses on interface "eth0". That might be an
alternative, but it's not clear to me what is special about eth0 to
warrant this treatment. It also would imply that we somehow recognize
this primary interface. In practise that would be doable by selecting
the interface with "iface_idx" zero.
Instead choose this approach. This is remotely similar to what WireGuard does
for configuring the default route ([1]), however WireGuard uses fwmark to match
the packets instead of the source address.
[1] https://www.wireguard.com/netns/#improved-rule-based-routing
The code never set "iface_get_config->cidr_addr", despite
setting "cidr_prefix" and "has_cidr". As a result, cloud-setup
would think that the subnet is "0.0.0.0/$PLEN", and calculate
the gateway as "0.0.0.1".
As a result it would add a default route to table 30400 via 0.0.0.1,
which is obviously wrong.
How to detect the right gateway? Let's try obtain the subnet also via
the meta data. That seems mostly correct, except that we only access
subnet at index 0. What if there are multiple ones? I don't know.
https://bugzilla.redhat.com/show_bug.cgi?id=1912236
It's questionable whether the manual page should explain
exactly what it does.
However, it's a good exercise writing this up (to review
what happens). Also, a manual page that simply says "it configures
the network automatically" without going into how exactly, is
not very useful either.
