I want to forward traffic to one port to another server.

I am doing this in a lab on virtual machines, all IP's will be public addresses in production.

client 192.168.10.100 ->
server 192.168.10.200 : 1900 ->
server2 192.168.10.222 : 1900

Client connects to server on port 1900 which should forward to server2 on port 1900.

This works (on server):
iptables -t nat -I PREROUTING -p tcp -i eth0 -d 192.168.10.200 -dport 1900 -j DNAT --to 192.168.10.222:1900
iptables -t nat -A POSTROUTING -p tcp -o eth0 -j SNAT --to 192.168.10.200

The problem is server2 see's the IP of server and not the client IP.

Thanks

You nedd to remove the second rule this sets all the traffic leaving to 192.168.10.200 which is why the second server see the ip of the first server and not the client.

It doesn't work at all without a POSTROUTING rule, and I tried MASQUERADING and that had the same effect with the server2 getting the wrong ip.

So you want all traffic to pass through 192.168.10.200 and if that is the case then the second server is going to need to see the ip address of the first server in order to route traffic back.

No, only traffic destined to port 1900 should go to the second server, and the second server should see the IP of the client source.

Kindof like on a netgear router or something, you forward a port to a machine inside the network. I'm sure you can do this with linux as many routers use linux.

That router does a DNAT and SNAT just like you are doing now. I haven't found a way at this time to do it any other way. This is requuired to ensure that the client will get the return traffic from the host it sent it to otherwise the client will drop the connection.