I've been playing with KVM on Ubuntu Server 9.10 - I've successfully got machines working however try as I might I cannot get the network bridge to work.

I've been trying this for a very long time and was putting off posting this but I'm getting to the end of my tether and it's holding me up.


I'll try if I may be outlining the steps taken so far followed by the symptoms experienced.

1) The host machine has a br0 interfaced bridged to eth0. The config file in /etc/network/interfaces on the host reads as follows - this seems to work, I have internet access from the host etc.


#### START /etc/network/interfaces on HOST ####

auto eth0
iface eth0 inet manual

auto br0
iface br0 inet static
******* address 88.208.249.44
******* network 88.208.249.0
******* netmask 255.255.252.0

******* gateway 88.208.248.1
******* bridge_ports eth0
******* bridge_stp off
******* bridge_fd 0
******* bridge_maxwait 0

#### END /etc/network/interfaces on HOST ###

2) The virtual machine was created on the host using the following:


virt-install --connect qemu:///system -n nameserver -r 512 --vcpus=1 -f /home/vm_disks/testbox.qcow2 -s 4 -c /home/vm_disks/ubuntu-9.04-server-i386.iso --vnc --noautoconsole --os-type linux --accelerate --network=bridge:br0 --hvm


3) The VM can be connected to at this point it was then configured with the following /etc/network/interfaces

## START /etc/network/interfaces on guest ##

auto eth0
iface eth0 inet static
*** address 88.208.249.45

*** netmask 255.255.252.0
*** network 88.208.248.0
*** broadcast 88.208.251.255
*** gateway 88.208.248.1

## END /etc/network/interfaces on guest ##


Symptoms
----------------
I can now ssh from the host into the guest (from the host) and from the guest to the host, however try as I might I cannot get the guest to access the outside world or the host to pass packets to the guest.


On the host when I do 'ifconfig' I get the following output.

# ifconfig
br0****** Link encap:Ethernet* HWaddr 00:19:99:70:5a:61*
********* inet addr:88.208.249.44* Bcast:88.208.251.255* Mask:255.255.252.0

********* inet6 addr: fe80::219:99ff:fe70:5a61/64 Scope:Link
********* UP BROADCAST RUNNING MULTICAST* MTU:1500* Metric:1
********* RX packets:5863 errors:0 dropped:0 overruns:0 frame:0
********* TX packets:2034 errors:0 dropped:0 overruns:0 carrier:0

********* collisions:0 txqueuelen:0
********* RX bytes:378572 (378.5 KB)* TX bytes:1465549 (1.4 MB)

eth0***** Link encap:Ethernet* HWaddr 00:19:99:70:5a:61*
********* inet6 addr: fe80::219:99ff:fe70:5a61/64 Scope:Link

********* UP BROADCAST RUNNING PROMISC MULTICAST* MTU:1500* Metric:1
********* RX packets:19595 errors:0 dropped:0 overruns:0 frame:0
********* TX packets:4387 errors:0 dropped:0 overruns:0 carrier:0
********* collisions:0 txqueuelen:100

********* RX bytes:1573365 (1.5 MB)* TX bytes:2319893 (2.3 MB)
********* Memory:fc400000-fc420000

lo******* Link encap:Local Loopback*
********* inet addr:127.0.0.1* Mask:255.0.0.0
********* inet6 addr: ::1/128 Scope:Host

********* UP LOOPBACK RUNNING* MTU:16436* Metric:1
********* RX packets:3089 errors:0 dropped:0 overruns:0 frame:0
********* TX packets:3089 errors:0 dropped:0 overruns:0 carrier:0
********* collisions:0 txqueuelen:0

********* RX bytes:1810345 (1.8 MB)* TX bytes:1810345 (1.8 MB)

vnet0**** Link encap:Ethernet* HWaddr ce:59:6f:23:ef:e8*
********* inet6 addr: fe80::cc59:6fff:fe23:efe8/64 Scope:Link
********* UP BROADCAST RUNNING MULTICAST* MTU:1500* Metric:1

********* RX packets:115 errors:0 dropped:0 overruns:0 frame:0
********* TX packets:2295 errors:0 dropped:0 overruns:0 carrier:0
********* collisions:0 txqueuelen:500
********* RX bytes:15107 (15.1 KB)* TX bytes:564732 (564.7 KB)





As you can see, the bridge is working and when I start the VM from virsh# the vnet0 adapter is created, however no tap0 is being created.

The /etc/libvirt/qemu/testbox.xml file is as follows


## Start testbox.xml ###

<domain type='kvm'>
* <name>testbox</name>
* <uuid>1b625323-0a4e-b725-b60d-a66e11472e44</uuid>
* <memory>524288</memory>
* <currentMemory>524288</currentMemory>

* <vcpu>1</vcpu>
* <os>
*** <type arch='x86_64' machine='pc'>hvm</type>
*** <boot dev='hd'/>
* </os>
* <features>
*** <acpi/>

*** <apic/>
*** <pae/>
* </features>
* <clock offset='utc'/>
* <on_poweroff>destroy</on_poweroff>
* <on_reboot>restart</on_reboot>
* <on_crash>restart</on_crash>

* <devices>
*** <emulator>/usr/bin/kvm</emulator>
*** <disk type='file' device='disk'>
***** <source file='/home/vm_disks/testbox.qcow2'/>
***** <target dev='hda' bus='ide'/>

*** </disk>
*** <disk type='file' device='cdrom'>
***** <target dev='hdc' bus='ide'/>
***** <readonly/>
*** </disk>
*** <interface type='bridge'>

***** <mac address='54:52:00:7b:96:f0'/>
***** <source bridge='br0'/>
*** </interface>
*** <serial type='pty'>
***** <target port='0'/>
*** </serial>

*** <console type='pty'>
***** <target port='0'/>
*** </console>
*** <input type='mouse' bus='ps2'/>
*** <graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>

* </devices>
</domain>

## END testbox.xml ##

The MAC address used for that guest VM is unique as it was created by virt-install. I have also tried several VM's to no avail.

As far as I am aware you should not need ipv4 forwarding enabled in the Kernel for bridged networks however despite this it is enabled.


If any of you could shed any light on this issue (since I can't seem to find anyone else with the issue) it would be most appreciated.

Kind Regards,
Jamie.



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