[ale] Routing under kernel 4.9

[Alex Carver]

Ok, I've tried every possible thing I could do to get routing to
alternate gateways working correctly and nothing is working short of
putting static routes on any machine that can handle them so I've got to
ask for help again.  Are there any security features in kernel 4.9 that
perhaps weren't present way back in kernel 2.4 that would prevent a
machine operating as the main gateway from issuing a reroute?

If the primary gateway is 10.0.0.1/24 and I add a static route to send
another network to a different machine:
route -add net 10.100.0.0/24 gw 10.0.0.200

What in the kernel would prevent this route from being advertised or
otherwise handed to all clients on 10.0.0.0/24 when they attempt to
respond to a packet coming from 10.100.0.0/24?  I can trace an incoming
packet from 10.100.0.0/24, through the 10.0.0.200 machine all the way to
the 10.0.0.0/24 client, it replies and tries to send back to 10.100.0.0
but the packet heads back towards 10.0.0.1 and never gets redirected
back to 10.0.0.200.

If I add the static route directly to the clients, the connections work.
 The problem is that I can't do that for every client, not all of them
know how to handle routes on their own (not that I really should, this
is the job of a router, no?)

I'm certain it's some feature/setting of the kernel that's new in 4.9
because this worked fine when the router was using the old 2.4 kernel.