Lets say we would like to forward traffic inside our Proxmox hypervisor to serve our application or get inside the running virtual machine over ssh. For this purpose we would need hypervisor's iptables firewall to route traffic from certain outer por to the inside one.
Example command for it will look like this:
iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport <outside_port> -j DNAT --to <internal_IP_address>:<internal_port>
This assumes that we have vmbr0 on our Proxmox node bridged to the physical network card.
To stop making port forwarding in this case, command will look like this:
iptables -t nat -D PREROUTING -i vmbr0 -p tcp --dport <outside_port> -j DNAT --to <internal_IP_address>:<internal_port>
To make these port forwarding settings permanents just add these instructions to the /etc/network/interfaces file of your Proxmox node under the vmbr1 section.
post-up iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport <outside_port> -j DNAT --to <internal_IP_address>:<internal_port>
post-down iptables -t nat -D PREROUTING -i vmbr0 -p tcp --dport <outside_port> -j DNAT --to <internal_IP_address>:<internal_port>
After saving the file make sure port forwarding will persist by reloading the hypervisor.