SDN

If you have ever applied a Proxmox VE Software-Defined Network configuration and seen this warning pop up in the task view, you are not alone:

WARN: missing 'source /etc/network/interfaces.d/sdn' directive for SDN support!
Created symlink /etc/systemd/system/multi-user.target.wants/dnsmasq@testzone.service -> /lib/systemd/system/dnsmasq@.service.

TASK WARNINGS: 1

It looks harmless, and in most cases it is. But if you want SDN-generated networks to actually come up on the node, that one line is the difference between a pending configuration and a running one. This post explains where the warning comes from in the Proxmox source code, why the fix is a single source directive, and how to verify the whole pipeline works afterwards.