As máquinas virtuais KVM não funcionarão mais no modo de rede bridge

Question

Alguma coisa quebrou no meu host KVM (Ubuntu 16.04).

Eu tenho algumas máquinas virtuais configuradas para usar br0 para que elas apareçam como máquinas separadas na LAN com seu próprio IP.

Eles obtêm o IP do roteador, via DHCP. Tudo estava bem.

Mas é claro que parou de funcionar e não consigo entender o porquê. br0 ainda está lá, mas as máquinas não podem obter uma concessão de DHCP. O host ainda tem conectividade. Se eu trocar as máquinas para NAT, elas terão acesso à rede. mas não com br0 .

Aqui está /etc/network/interfaces no host:

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

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
# auto enp4s0
# iface enp4s0 inet manual

# VMs bridge
auto br0
iface br0 inet dhcp
        bridge_ports enp4s0
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0

$ ip addr:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enp4s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br0 state UP group default qlen 1000
    link/ether 00:1d:09:0b:87:6d brd ff:ff:ff:ff:ff:ff
    inet6 fe80::21d:9ff:fe0b:876d/64 scope link 
       valid_lft forever preferred_lft forever
3: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:1d:09:0b:87:6d brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.129/24 brd 192.168.1.255 scope global br0
       valid_lft forever preferred_lft forever
    inet6 fe80::21d:9ff:fe0b:876d/64 scope link 
       valid_lft forever preferred_lft forever
4: br-0e57a29cd0f7: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default 
    link/ether 02:42:2b:9a:39:d6 brd ff:ff:ff:ff:ff:ff
    inet 172.18.0.1/16 scope global br-0e57a29cd0f7
       valid_lft forever preferred_lft forever
5: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 02:42:ed:8f:cb:c1 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 scope global docker0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:edff:fe8f:cbc1/64 scope link 
       valid_lft forever preferred_lft forever
7: veth44c7e80@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default 
    link/ether 3e:61:3e:be:b2:8e brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet6 fe80::3c61:3eff:febe:b28e/64 scope link 
       valid_lft forever preferred_lft forever
8: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 52:54:00:41:79:0f brd ff:ff:ff:ff:ff:ff
    inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
       valid_lft forever preferred_lft forever
9: virbr0-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast master virbr0 state DOWN group default qlen 1000
    link/ether 52:54:00:41:79:0f brd ff:ff:ff:ff:ff:ff

$ brctl show

bridge name     bridge id               STP enabled     interfaces
br-0e57a29cd0f7         8000.02422b9a39d6       no
br0             8000.001d090b876d       no              enp4s0
docker0         8000.0242ed8fcbc1       no              veth44c7e80
virbr0          8000.52540041790f       yes             virbr0-nic

lista de rotas $ ip

default via 192.168.1.1 dev br0 
172.17.0.0/16 dev docker0  proto kernel  scope link  src 172.17.0.1 
172.18.0.0/16 dev br-0e57a29cd0f7  proto kernel  scope link  src 172.18.0.1 linkdown 
192.168.1.0/24 dev br0  proto kernel  scope link  src 192.168.1.129 
192.168.122.0/24 dev virbr0  proto kernel  scope link  src 192.168.122.1 linkdown 

brctl show com uma VM iniciada:

bridge name     bridge id               STP enabled     interfaces
br-0e57a29cd0f7         8000.02422b9a39d6       no
br0             8000.001d090b876d       no              enp4s0
                                                        vnet0
docker0         8000.0242ed8fcbc1       no              veth44c7e80
virbr0          8000.52540041790f       yes             virbr0-nic

endereço $ ip show dev vnet0

13: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN group default qlen 1000
    link/ether fe:54:00:81:08:d0 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::fc54:ff:fe81:8d0/64 scope link 
       valid_lft forever preferred_lft forever

Eu não tenho ideia do que fazer neste momento. Eu tentei remover e reinstalar o kvm + qemu, removendo e recriando a bridge, criando uma nova VM na br0 ... Absolutamente nenhuma mudança.