There is a router 192.168.1.1 , the Internet comes to it.

On the router, the 6666 port is tried on the server with ubuntu (192.168.1.2 , the eth0 interface). There is also a second interface ETH1 , which is connected to the server (IP server address in this network 10.10.10.2 , the main gateway is 10/10/10.1 ). Through this locator, a computer is available in the 3rd network 192.168.100.1:6666 .

Task: Configure IPTables Ubuntu so that users via the Internet by port 6666 could reach 192.168.100.1 .
Forwarding included.

I can’t understand how to properly register the following logic (if it is generally correct):

1. Redirect packages from one Eth0 on another eth1 if they answer 192.168.1.2:6666 .
2. Redirect packets with 10.10.10.2 ETH1 on the IP address available through this locomotive 192.168.100.1:6666 .

Answer 1, Authority 100%

(on the ubuntu server) specify the route to 192.168.100.1 :

ip Route Add 192.168.100.1 Via 10/10/10.1

Change the destination in the packages on 192.168.100.1 :

iptables --table nat --insert prerouting \
  --Proto TCP --DPORT 6666 --JUMP DNAT --TO-Destination 192.168.100.1:6666
IPTABLES --Table Nat --insert Prerouting \
  --Proto UDP --Dport 6666 --Jump Dnat --to-Destination 192.168.100.1:6666

If the router>192.168.0.1 does not have masquerade, then:

iptables --table nat --insert postruting --jump Masquerade

must, in theory, earn.