Proxmox VM with NAT using Host Server

Create a new network bridge:
Create a new bridge interface (e.g., vmbr1) that is not linked to any physical network interface. You can do this via the Proxmox web interface or by editing /etc/network/interfaces.

Add IP forwarding:
Add a line to enable IP forwarding in the host’s /etc/network/interfaces file for the new bridge:

post-up echo 1 > /proc/sys/net/ipv4/ip_forward.

Configure iptables:
Add an iptables rule to the new bridge’s post-up script. This rule will perform Network Address Translation (NAT) by masquerading traffic from your private VM subnet through the host’s primary network bridge (vmbr0).

post-up iptables -t nat -A POSTROUTING -s '192.168.199.0/24' -o vmbr0 -j MASQUERADE

You should also add the corresponding post-down rule to remove the rule when the interface is brought down.

Reload the network configuration:
After saving the changes, reload the network configuration. You may need to reboot the host for all changes to take effect.systemctl restart networking

ifreload -a