NAT is not replacing internal address

[rustam44]

Hi,

I've install Proxmox VE 5.1-3. I followed the instructions here https://pve.proxmox.com/wiki/Network_Model#Masquerading_.28NAT.29_with_iptables and configured interfaces as follows:

auto lo
iface lo inet loopback

auto enp1s0
iface enp1s0 inet static
    address  x.x.x.33
    netmask  255.255.255.x
    gateway  x.x.x.1

auto vmbr0
iface vmbr0 inet static
    address  192.168.10.45
    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 '192.168.10.0/24' -o enp1s0 -j MASQUERADE
    post-down iptables -t nat -D POSTROUTING -s '192.168.10.0/24' -o enp1s0 -j MASQUERADE

When I try to ping outside IP from guest machine (from IP 192.168.10.10), tcpdump on external host's interface shows that source address of icmp packets is 192.168.10.10, it doesn't replace internal guest's IP with external host's IP.

Any clue on what's the problem?

Edit: I wanted to mention that I had only one physical interface.