Re: [Qemu-discuss] Guest fails to reconnect after host network goes down

[Vlad Yasevich]

On 08/27/2015 12:13 PM, Davide Baldini wrote:
> With "drop the newtork" I mean that both ping and SSH toward any of the 
> guests throw a
> "Destination Host Unreachable".
> 
> When the network drops, running `ifconfig` from inside the guests keeps 
> showing the same,
> correct data as before the drop, like IP address, MAC and netmask. But after 
> the ethernet
> cable is unplugged and re-plugged causing a drop, the guests may no longer be 
> reached via
> SSH nor ping-ed, and vice versa, the guests may not reach any virtual or 
> physical host in
> the network, yielding a "Destination Host Unreachable".
> 
> A ping toward "localhost" from inside the guests works correctly, as well as 
> SSH.
> 
> My network configuration has nothing peculiar; I have no NAT or firewall 
> configured
> anywhere in my network, except for a few unrelated port forwards set on the 
> home ADSL
> modem/router, but they do not apply since all accesses to my guest machines 
> are made via
> their local IP addresses (ie 192.168.1.100), and so those NAT rules are 
> bypassed. In this
> consumer-grade modem/router, NAT rules are applied only to packets coming 
> from internet
> and not to packets coming from the local network, and I'm connecting to the 
> guests from
> inside the local network.
> 
> I'm not involved at all with settings pertaining layer 2 or layer X of the 
> network. The
> only peculiar setting I made is the "br0" bridge interface, as shown in the 
> first post of
> this thread.
>

Hmm..  You could always run tcpdump on the tap device to see if the bridge is 
forwarding
packets.  If you seen them there, you could also try running tcpdump on the 
guest to
check if you see the same packets.

I just looked and Debian7 is very old and has 3.2 kernel.  Any chance to try 
with
something a bit more modern?

-vlad

> -- Davide
> 
> On 08/24/2015 08:29 PM, Tony Su wrote:
>> Consider whether you're talking about layer 2  or layer 3 network
>> connectivity (test).
>> If this is a Workgroup, consider whether you have a Browse Master issue.
>> Consider whether your issue is a name resolution issue.
>> I assume the connection is bridged? Try NAT if it doesn't make a diff to you.
>>
>> Tony
>>
>> On Mon, Aug 24, 2015 at 7:29 AM, Vlad Yasevich
> <address@hidden> wrote:
>>> On 08/23/2015 07:31 AM, Davide Baldini wrote:
>>>> Thanks Vlad,
>>>>
>>> .. snip..
>>>>
>>>> As far as I notice, there are no differences shown by `ifconfig`.
>>>> The fact is, that all guests drop the network after a cable is unplugged 
>>>> and then
>>>> replugged, while the host's network keeps functioning.
>>>
>>> What do you mean by "drop network"?   Is the link going down in the guest?  
>>> Is
>>> the guest losing IP address?  Is the guest simply stops being reachable?
>>>
>>> Are there any firewall rules on the host?
>>>
>>> Thanks
>>> -vlad
>>>
>>>>
>>>> Where else can I look?
>>>>
>>>> -- Davide
>>>>
>>>> On 08/19/2015 08:02 PM, Vlad Yasevich wrote:
>>>>> On 08/16/2015 12:55 PM, Davide Baldini wrote:
>>>>>> When I unplug the ethernet cable from the physical host, guest fails to 
>>>>>> reconnect; host
>>>>>> does reconnect.
>>>>>
>>>>> What do you mean by 'reconnect'?  Unplugging the cable for the nic on the 
>>>>> physical host
>>>>> does not really disconnect the guest from the network.  According to the 
>>>>> configuration
>>>>> your provided, the guest is still connected to the bridge on the host and 
>>>>> can still
>>>>> communicate with other guests on the same host.
>>>>>
>>>>> Once the host network is plugged back in, the bridge should start 
>>>>> forwarding packets
>>>>> back to the guest.
>>>>>
>>>>>>
>>>>>> A very simple configuration here, with an almost default Debian 7 on 
>>>>>> both host and
> guest:
>>>>>>
>>>>>> -- ON HOST:
>>>>>>
>>>>>> /etc/network/interfaces
>>>>>>
>>>>>>     auto lo
>>>>>>     iface lo inet loopback
>>>>>>
>>>>>>     auto br0
>>>>>>     iface br0 inet dhcp
>>>>>>         bridge_ports    eth1
>>>>>>         bridge_stp      off
>>>>>>         bridge_maxwait  0
>>>>>>         bridge_fd       0
>>>>>>
>>>>>>
>>>>>> -- ON GUEST:
>>>>>>
>>>>>> /etc/network/interfaces
>>>>>>
>>>>>>     auto eth0
>>>>>>     allow-hotplug eth0
>>>>>>     iface eth0 inet static
>>>>>>         address 192.168.1.109
>>>>>>         netmask 255.255.255.0
>>>>>>         network 192.168.1.0
>>>>>>         broadcast 192.168.1.255
>>>>>>         gateway 192.168.1.1
>>>>>>         dns-nameservers 192.168.1.1
>>>>>>
>>>>>>
>>>>>> QEMU is started as:
>>>>>>
>>>>>> /usr/local/bin/qemu-system-x86_64 -hda /media/raid1/qemu-109 -m 8G -smp 
>>>>>> 4 -enable-kvm
>>>>>> -netdev bridge,id=br109 -device
> virtio-net-pci,netdev=br109,id=nic0,mac=00:00:00:00:01:09
>>>>>> -k it -daemonize
>>>>>>
>>>>>> QEMU emulator version 2.3.0
>>>>>>
>>>>>> Nothing appears on dmesg. Any hints?
>>>>>
>>>>> Make sure that the bridge on the host is formed correctly after the cable 
>>>>> has been
> plugged
>>>>> back in.
>>>>>
>>>>> -vlad
>>>>>
>>>>>>
>>>>>> -- Davide
>>>>>>
>>>>>
>>>>
>>>
>>>
>>
>>
> 
> 
> 
>