How do I route a packet to use localhost as a gateway?

I'm trying to test a gateway I wrote(see What's the easiest way to test a gateway? for context). Due to an issue I'd rather not get into, the gateway and "sender" have to be on the same machine. I have a receiver(let's say 9.9.9.9) which the gateway is able to reach.

So I'll run an application ./sendStuff 开发者_开发百科9.9.9.9 which will send some packets to that IP address.

The problem is: how do I get the packets destined for 9.9.9.9 to go to the gateway on localhost? I've tried:

sudo route add -host 9.9.9.9 gw 127.0.0.1 lo

sudo route add -host 9.9.9.9 gw <machine's external IP address> eth0

but neither of those pass packets through the gateway. I've verified that the correct IPs are present in sudo route. What can I do?

Per request, here is the route table, after running the second command(IP addresses changed to match the question. x.y.z.t is the IP of the machine I'm running this on):

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
9.9.9.9         x.y.z.t         255.255.255.255 UGH   0      0        0 eth0
x.y.z.0         0.0.0.0         255.255.254.0   U     0      0        0 eth0
0.0.0.0         <gateway addr>  0.0.0.0         UG    100    0        0 eth0

127.0.0.1 is probably picking up the packets, then forwarding them on their merry way if ipv4 packet forwarding is enabled. If it's not enabled, it will drop them.

If you are trying to forward packets destined to 9.9.9.9 to 127.0.0.1, look into iptables.

Edit: try this:

iptables -t nat -A OUTPUT -d 9.9.9.9 -j DNAT --to-destination 127.0.0.1

That should redirect all traffic to 9.9.9.9 to localhost instead.