tcp forwarding with a local source isn't working

Here's one I can't quite wrap my head around.

I got tcp port forwarding working from machine 192.168.0.100 to machine 192.168.0.200 with two rules:

iptables -t nat -A PREROUTING -d 192.168.0.100 -p tcp --dport 22 \
-j DNAT --to 192.168.0.200
iptables -t nat -A POSTROUTING -d 192.168.0.200 -p tcp --dport 22 \
-j SNAT --to 192.168.0.100

I realized that I needed the second rule because the hosts were on the same network, and without it, replies from .200 would go straight to the source.

This works for all machines *except* 192.168.0.100. I wanted to connect from .100 to .100 on the port, and have it forwarded to .200. First, I realized that I needed a rule on OUTPUT, because locally generated packets don't traverse PREROUTE. So for testing, I flushed all the rules, and started over with:

iptables -t nat -A OUTPUT -d 192.168.0.100 --dport 22 \
-j DNAT --to 192.168.0.200

Now, I'm only trying to get this one case working:

(from machine 192.168.0.100:) ssh 192.168.0.100

and I'd like it to connect to 192.168.0.200. I'm not sure why it isn't.

I've also tried the above rule with a second SNAT rule, which doesn't help.

From what I understand, this should be the case:
(1) The packet starts as

    SOURCE: 192.168.0.100:port_a (some random port)
    DEST:   192.168.0.100:22
(2) While traversing the OUTPUT chain in the NAT table, it's changed:
    SOURCE: 192.168.0.100:port_a
    DEST:   192.168.0.200:22
(3) The packet is sent out
(4) Host 192.168.0.200 sees it and sends the reply
    SOURCE: 192.168.0.200:22
    DEST:   192.168.0.100:port_a
(5) The packet arrives, and is un-snat'd:
    SOURCE: 192.168.0.100:22
    DEST:   192.168.0.100:port_a

(6) The local process sees a reply from the local machine, and accepts it.

What's actually happening is that it's getting as far as (4), and the reply comes in, but the local process doesn't accept it. I'm guessing this is because it wasn't un-snat'd correctly, or I'm doing something wrong.

I've also tried a few permutations of putting 0 in /proc/sys/net/ipv4/conf/eth0/rp_filter, in case something weird was happening there.

I would be grateful if anyone had any insight into why this doesn't work, what I'm doing wrong, or how to forward a tcp port from machine A to machine B and have it work when the packets originate from machine A itself.

Thanks,

----------------------------------
Frank Wallingford
[email protected]

Local port forwarding/nat was broken in earlier kernels, don't quite remember which. I think it was fixed either in 2.4.18 or 2.4.19.