Source routing configuration is not conditional

[Dmitrii Shcherbakov]

As of 2.6 the source routing functionality will help end hosts with using multiple default gateways and routing response traffic out the same interface it has been received on.

https://github.com/maas/maas/commit/442d47053e6f96bf5a94904f16968e9e5e5c965c
https://github.com/maas/maas/commit/45f2632b8164f105eab69baa88ee401cf0f68b56

Example netplan (routing-policy is only added for interfaces that have an address with a subnet that has a default gateway defined in MAAS):
https://paste.ubuntu.com/p/rm9wD949G2/

This functionality is not conditional though, so it is not possible to disable this on a per-interface basis for some hosts.

[Blake Rouse (blake-rouse)]

What is the use-case for this to be configurable?

[Andres Rodriguez (andreserl)]

Hi Dimitrii,

As per the spec and agreements of previous cycle, the ability to disable this per interface was not part of the roadmap.

That said, ti wouldn't seem that having a per interface way of doing this make sense, rather this would make better sense to do it per subnet?

[Dmitrii Shcherbakov (dmitriis)]

Hi {Andres, Blake},

I think doing it on a per-subnet basis is a bit restrictive because there may be different types of hosts attached to the same subnet. For example, a host deployed by MAAS may act as a (software) router.

Other hosts would use it as a default gateway but the router host itself wouldn't need to have a gateway configured for that subnet and it would need to avoid policy rules to be configured on it. The router will of course have rp_filter set to 0 globally on a per-interface basis via some other tooling or cloud-init.

root@OpenWrt:~# sysctl -a | grep rp_filter | head -n2
net.ipv4.conf.all.arp_filter = 0
net.ipv4.conf.all.rp_filter = 0

Example user story: https://serverfault.com/q/816393

The primary use-case for the feature is still end hosts and the default behavior makes sense but I think we have to leave the room for the above as well.

[Blake Rouse (blake-rouse)]

Okay. Thank you for the explanation and use-case.

[Jerzy Husakowski (jhusakowski)]

We moved this feature request to the internal backlog for future consideration.