I am trying to configure iptables to do port forwarding for certain request. The scenario is like this:

Login node A can be accessed from outside Compute node B that running the service can be accessed from A but not from outside. I want to set up iptables so that a request for the service on B from outside can be accessed through A.

A has two ethernet ports: Internal eth0, with ip internal_A and External eth1, with ip external_A.

B has 1 ethernet port, internal eth0. let's say its ip is internal_B

The service listens to internal_B:5900. We open external_A:10000 for user access.

I configure my iptables with the following commands:

echo 1> /proc/sys/net/ipv4/ip_forward
/sbin/iptables -P FORWARD ACCEPT
/sbin/iptables -t nat -A POSTROUTING -j MASQUERADE
/sbin/iptables -A PREROUTING -p tcp -t nat --dport 10000 -j DNAT --to-destination interal_B:5900
/sbin/iptables -A OUTPUT     -p tcp -t nat -d external_A --dport 10000 -j DNAT --to-destination internal_B:5900

But it didn't work. Can anybody help me?