How to change Proxmox listening port

… the easy way:

iptables -A INPUT -p tcp --dport NEW_PORT -j ACCEPT
iptables -A INPUT -p udp --dport NEW_PORT -j ACCEPT
iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport NEW_PORT -j REDIRECT --to-port 8006
iptables -t nat -A PREROUTING -i vmbr0 -p udp --dport NEW_PORT -j REDIRECT --to-port 8006