Comment 4 for bug 1500992

Ah, thanks for confirming. This indeed applies to interfaces which networkd configures; man systemd.network documents this quite clearly.

> Having the bridge setup tool, modify the configuration of all potential outgoing interface to allow forwarding seems completely wrong to me

Full ack. Such scripts should not touch the configuration of interfaces that they didn't set up. This is unexpected, leads to race conditions, and isn't reliable as you wrote.

Setting to wishlist for now as it's currently behaving as documented. I'll raise this upstream and see if they are willing to change the default to "kernel".