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

From: Davide Baldini
Subject: Re: [Qemu-discuss] Guest fails to reconnect after host network goes down
Date: Thu, 27 Aug 2015 18:06:15 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:31.0) Gecko/20100101 Icedove/31.6.0

With "drop the newtork" I mean that both ping and SSH toward 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, 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.

-- 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.


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 
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?


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 
your provided, the guest is still connected to the bridge on the host and can 
communicate with other guests on the same host.

Once the host network is plugged back in, the bridge should start forwarding 
back to the guest.

A very simple configuration here, with an almost default Debian 7 on both host 
and guest:



    auto lo
    iface lo inet loopback

    auto br0
    iface br0 inet dhcp
        bridge_ports    eth1
        bridge_stp      off
        bridge_maxwait  0
        bridge_fd       0



    auto eth0
    allow-hotplug eth0
    iface eth0 inet static

QEMU is started as:

/usr/local/bin/qemu-system-x86_64 -hda /media/raid1/qemu-109 -m 8G -smp 4 
-netdev bridge,id=br109 -device 
-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.


-- Davide

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