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

[Davide Baldini]

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.

-- 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
>>>>>
>>>>
>>>
>>
>>
>
>