Proxmox NAT - /etc/network/interfaces

interfaces

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# vmbr0: Bridging. Make sure to use only MAC adresses that were assigned to you.
auto vmbr0
iface vmbr0 inet static
    address 91.121.221.193
    netmask 255.255.255.0
    network 91.121.221.0
    broadcast 91.121.221.255
    gateway 91.121.221.254
    bridge_ports eth0
    bridge_stp off
    bridge_fd 0

# for Routing
auto vmbr1
iface vmbr1 inet manual
    post-up /etc/pve/kvm-networking.sh
    bridge_ports dummy0
    bridge_stp off
    bridge_fd 0

auto vmbr2
iface vmbr2 inet static
    address 10.21.21.254
    netmask 255.255.255.0
    bridge_ports none
    bridge_stp off
    bridge_fd 0
    post-up echo 1 > /proc/sys/net/ipv4/ip_forward
    post-up iptables -t nat -A POSTROUTING -s '10.21.21.0/24' -o vmbr0 -j MASQUERADE
    post-down iptables -t nat -D POSTROUTING -s '10.21.21.0/24' -o vmbr0 -j MASQUERADE
    post-up iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport 2222 -j DNAT --to 10.21.21.5:22
    post-down iptables -t nat -D PREROUTING -i vmbr0 -p tcp --dport 2222 -j DNAT --to 10.21.21.5:22