Is that the setting you mean? How to reproduce the disabling of forwarding? Apparently it's more than just "start networkd" (I don't have any actual *.network config on this machine). Or do you mean "all interfaces that have a config for networkd" instead of "all interfaces"? It seems reasonable that networkd puts the devices it configures itself into a defined state. You wouldn't use it to manage tun0 for LXC, as LXC itself already sets that up?
Can you please be more specific what exactly happens here?
$ cat /proc/sys/ net/ipv4/ conf/tun0/ forwarding net/ipv4/ conf/tun0/ forwarding
1
$ sudo systemctl start systemd-networkd
$ cat /proc/sys/
1
Is that the setting you mean? How to reproduce the disabling of forwarding? Apparently it's more than just "start networkd" (I don't have any actual *.network config on this machine). Or do you mean "all interfaces that have a config for networkd" instead of "all interfaces"? It seems reasonable that networkd puts the devices it configures itself into a defined state. You wouldn't use it to manage tun0 for LXC, as LXC itself already sets that up?