Public bug reported:

System:
-----------
Dell R410 Dual processor 2.4Ghz w/16G RAM
Distributor ID: Ubuntu
Description:    Ubuntu 12.04 LTS
Release:        12.04
Codename:       precise

Setup:
---------
We're running 3 KVM guests, all Ubuntu 12.04 LTS using bridged networking. 

>From the host:
# cat /etc/network/interfaces
auto br0
iface br0 inet static
        address 212.XX.239.98
        netmask 255.255.255.240
        gateway 212.XX.239.97
        bridge_ports eth0
        bridge_fd 9
        bridge_hello 2
        bridge_maxage 12
        bridge_stp off

# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr d4:ae:52:84:2d:5a  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:11278363 errors:0 dropped:3128 overruns:0 frame:0
          TX packets:14437384 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4115980743 (4.1 GB)  TX bytes:5451961979 (5.4 GB)
          Interrupt:36 Memory:da000000-da012800 

# ifconfig br0
br0       Link encap:Ethernet  HWaddr d4:ae:52:84:2d:5a  
          inet addr:212.XX.239.98  Bcast:212.XX.239.111  Mask:255.255.255.240
          inet6 addr: fe80::d6ae:52ff:fe84:2d5a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1720861 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1708622 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:210152198 (210.1 MB)  TX bytes:300858508 (300.8 MB)

# brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.d4ae52842d5a       no              eth0

I have no default network configured to autostart in libvirt as we're using 
bridged networking:
# virsh net-list --all
Name                 State      Autostart
-----------------------------------------
default              inactive   no        

# arp
Address                  HWtype  HWaddress           Flags Mask            Iface
mailer03.xxxx.com       ether   52:54:00:82:5f:0f   C                     br0
mailer01.xxxx.com       ether   52:54:00:d2:f7:31   C                     br0
mailer02.xxxx.com       ether   52:54:00:d3:8f:91   C                     br0
dxi-gw2.xxxx.com        ether   00:1a:30:2a:b1:c0   C                     br0

>From one of the guests:
<domain type='kvm' id='4'>
  <name>mailer01</name>
  <uuid>d41d1355-84e8-ae23-e84e-227bc0231b97</uuid>
  <memory>2097152</memory>
  <currentMemory>2097152</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='pc-1.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/dev/mapper/vg_main-mailer01--root'/>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/dev/mapper/vg_main-mailer01--swap'/>
      <target dev='hdb' bus='ide'/>
      <alias name='ide0-0-1'/>
      <address type='drive' controller='0' bus='0' unit='1'/>
    </disk>
    <controller type='ide' index='0'>
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' 
function='0x1'/>
    </controller>
    <interface type='bridge'>
      <mac address='52:54:00:d2:f7:31'/>
      <source bridge='br0'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' 
function='0x0'/>
    </interface>
    <serial type='pty'>
      <source path='/dev/pts/0'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/0'>
      <source path='/dev/pts/0'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='5900' autoport='yes' listen='127.0.0.1'>
      <listen type='address' address='127.0.0.1'/>
    </graphics>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' 
function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' 
function='0x0'/>
    </memballoon>
  </devices>
  <seclabel type='dynamic' model='apparmor' relabel='yes'>
    <label>libvirt-d41d1355-84e8-ae23-e84e-227bc0231b97</label>
    <imagelabel>libvirt-d41d1355-84e8-ae23-e84e-227bc0231b97</imagelabel>
  </seclabel>
</domain>

>From within the guest:
# cat /etc/network/interfaces 
# The primary network interface
auto eth0
iface eth0 inet static
        address 212.XX.239.100
        netmask 255.255.255.240 
        network 212.XX.239.96
        broadcast 212.XX.239.111
        gateway 212.XX.239.97

# ifconfig
eth0      Link encap:Ethernet  HWaddr 52:54:00:d2:f7:31  
          inet addr:212.XX.239.100  Bcast:212.XX.239.111  Mask:255.255.255.240
          inet6 addr: fe80::5054:ff:fed2:f731/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:5631830 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6683416 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:2027322829 (2.0 GB)  TX bytes:2076698690 (2.0 GB)


A commandline which starts the KVM guest:
/usr/bin/kvm -S -M pc-1.0 -enable-kvm -m 2048 -smp 
1,sockets=1,cores=1,threads=1 -name mailer01 -uuid 
d41d1355-84e8-ae23-e84e-227bc0231b97 -nodefconfig -nodefaults -chardev 
socket,id=charmonitor,path=/var/lib/libvirt/qemu/mailer01.monitor,server,nowait 
-mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown 
-drive 
file=/dev/mapper/vg_main-mailer01--root,if=none,id=drive-ide0-0-0,format=raw 
-device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 
-drive 
file=/dev/mapper/vg_main-mailer01--swap,if=none,id=drive-ide0-0-1,format=raw 
-device ide-drive,bus=ide.0,unit=1,drive=drive-ide0-0-1,id=ide0-0-1 -netdev 
tap,fd=18,id=hostnet0 -device 
virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:d2:f7:31,bus=pci.0,addr=0x3 
-chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 
-usb -vnc 127.0.0.1:0 -vga cirrus -device 
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4

Problem:
------------
Periodically (at least once a day), one or more of the guests lose network 
connectivity.  Ping responds with 'host unreachable', even from the dom host. 
Logging in via the serial console shows no problems: eth0 is up, can ping the 
local host, but no outside connectivity.  Restart the network 
(/etc/init.d/networking restart) does nothing. Reboot the machine and it comes 
alive again.

I've verified there's no arp games going on on the primary host (the arp
tables remain the same before - when it had connectivity - and after -
when it doesn't.

This is a critical issue affecting production services on the latest LTS
release of Ubuntu. It's similar to an issue which was 'resolved' in
10.04 but appears to have risen its ugly head again.

** Affects: libvirt (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: kvm networking

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https://bugs.launchpad.net/bugs/997978

Title:
  KVM images lose connectivity with bridged network

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