IP traffic from/to VM issue after updating to debian 10.8

[Francesco Piraneo G.]

Hi all,

I have this machine setup:

1. LXC - corpweb - With one NIC address 191.168.20.110/24;
2. VM - nginx-proxy - With two NICs: ens18 with an external IP and ens20 address 192.168.20.106/24. This VM is used as gateway for all VMs and LXCs;

Both VM and LXC are on a proxmox host on a datacenter.

From a machine external to the datacenter, pointing the browser to corpweb where an apache server is running works almost fine, where "almost" mean that the http server answer with a 3-5 seconds delay.

I can ping corpweb LXC from nginx-proxy VM and vice versa.
I can ping 1.1.1.1 from nginx-proxy;
I cannot ping 1.1.1.1 from corpweb LXC and obviously I cannot make any apt update for the LXC.
I cannot ping LXC and VM from host (is this normal?).

Both LXC and VM runs debian 10.8.
Both LXC and VM are on the same bridge called vmbr1 where we have the 192.168.20.xxx; VM for external traffic is on vmbr0.

Everything worked fine until this morning when I updated host, VM, LXC - Debian passed from 10.7 to 10.8!

Here are my configurations

/etc/network/interfaces on nginx-proxy VM

# The primary network interface
allow-hotplug ens18
iface ens18 inet static
    address 82.195.xxx.yyy/29
    gateway 82.195.xxx.zzz
    # dns-* options are implemented by the resolvconf package, if installed
    dns-nameservers 1.1.1.1
    dns-search mydomain.tld

auto ens19
iface ens19 inet static
        address 192.168.10.106/24

auto ens20
iface ens20 inet static
        address 192.168.20.106/24
        post-up   echo 1 > /proc/sys/net/ipv4/ip_forward
        post-up   iptables -t nat -A POSTROUTING -s '192.168.20.0/24' -o eth0 -j MASQUERADE
        post-down iptables -t nat -D POSTROUTING -s '192.168.20.0/24' -o eth0 -j MASQUERADE

NOTE: /proc/sys/net/ipv4/ip_forward is really 1

/etc/network/interfaces on corpweb LXC

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 192.168.20.110/24
        gateway 192.168.20.106

All firewalls are off.

Any help is strongly appreciated!

Francesco