I have tried forward chains prerouting and postrouting chains.

I don't want to open up port forwarding on my hardware router because the ports are always open. I want iptables to forward the necessary port 29000 to the hardware router when needed.

How do I allow this?

iptables -t nat -A PREROUTING -d Public IP -p tcp -m tcp --dport 29000 -j DNAT --to-destination 192.168.0.12:29000
iptables -t nat -A PREROUTING -d Public IP -p udp -m udp --dport 29000 -j DNAT --to-destination 192.168.0.12:29000
iptables -A FORWARD -d 192.168.0.12/32 -p tcp -m tcp --dport 29000 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -d 192.168.0.12/32 -p udp -m udp --dport 29000 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
iptables -t nat -A POSTROUTING -s 192.168.0.12/32 -p tcp -m tcp --sport 29000 -j SNAT --to-source Public IP
iptables -t nat -A POSTROUTING -s 192.168.0.12/32 -p udp -m udp --sport 29000 -j SNAT --to-source Public IP

I'm sorry, but this doesn't make sense. Is your hardware router not the device with the public IP address?

Yes, but if I forward the ports they are always open even when my linux machine is off. I don't want to port forward like this from the router.

Example, if I launch bittorent the ports are forwarded to the router automatically but are closed when the client is closed. I want iptables to do the same thing but with a specific port.

If your machine is off, nothing is responding to incoming packets for that IP address. That means the router won't be able to forward the packets at all and is forced to drop them, which from the outside looks exactly like a closed or a firewalled port.That means both your router and the bittorrent client support the UPNP protocol. And the port is forwarded FROM the router to the PC, not TO the router.iptables controls the in-built firewall of the Linux kernel. It knows nothing of UPNP.

Thanks, learned something new.