Comment 2 for bug 1500992

Oh, then maybe it's only on networkd managed interfaces.

The case I saw was on Mike MacCracken's machine where he had a basic dhcp config for eth0 which would then lead networkd to set /proc/sys/net/ipv4/conf/eth0/forwarding to off and IIRC same happened to /proc/sys/net/ipv6/conf/eth0/forwarding

So to clarify, I'd expect that unless I explictly turn off IPForward in networkd's config that it doesn't go and alter the interface state and instead leaves it at the default value set by the kernel.

This means that we can then set /proc/sys/net/ipv4/ip_forward to 1 and get the expected behavior (which is what most scripts, firewalls, ... do).