[libvirt-users] KVM NAT stops from working

Hi. First of all thank you for the work you are doing with libvirt. I am not sure this is the right place to ask, I'd appreciate if you can give me any hint or directions. I have several similar KVM Linux boxes and one of them has a really strange behavior with the KVM NAT: It just suddenly stops from working. This is a Linux Ubuntu Server 19.04 with - libvirt-bin 4.0.0 - qemu-kvm 1:2.11 Everything works fine and then suddenly the virtual machines can't reach outside. If I run a tcpdump in the host I see the NAT isn't working. When the server just boots I can see the packets with the server address going out: x.y.z.w.49138 > 8.8.8.8.53 Then, it may be some hours or days later, instead the server address I see the internal domains address: 192.168.122.33.19132 > 8.8.8.8.53 ^^^^^^^^^^^^^^ I try to restart the iptables but it won't help. Any hints ? Thank you very much