[CentOS-virt] KVM integrated network (user mode) dying after inactivity

[Timo Schöler]

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Hi list,

I searched the web for bug reports regarding this phenomenon I see on
*multiple* machines of a customer, however, I didn't find an exact
fit. So, I'd like to ask here whether anyone else has run into this.

I have multiple CentOS 6 machines running using KVM to virtualize a
bunch of machines on them (LVM-based).

Software releases as following:

[root at fe00 ~]# rpm -qa|egrep '(virt|kvm)'
virt-viewer-0.5.6-8.el6_5.3.x86_64
libvirt-python-0.10.2-29.el6_5.7.x86_64
libvirt-client-0.10.2-29.el6_5.7.x86_64
qemu-kvm-0.12.1.2-2.415.el6_5.8.x86_64
libvirt-0.10.2-29.el6_5.7.x86_64
python-virtinst-0.600.0-18.el6.noarch

[root at fe00 ~]# uname -r
2.6.32-431.17.1.el6.x86_64

The VMs (here: two) have the "default" connection provided by KVM
(heading to the internet) as well as a bridged interface to connect to
a high performance backbone, where sensitive data is kept and
bandwidth is an issue (or better, not :), on a second interface within
the VMs:

[root at fe00 ~]# brctl show
bridge name	bridge id		STP enabled	interfaces
br1		8000.001b21xxxxxx	yes		eth1
							vnet1
							vnet3
virbr0		8000.525400xxxxxx	yes		virbr0-nic
							vnet0
							vnet2

br1 is the interface connected to the backbone, virbr0 KVM's user mode
network.

After some time of inactivity on the virbr0 interface, from *within*
the VMs connection is *lost*. The interface(s) lose their IP; running
dhclient(8) is not of any use.

To get the machine back onto track, ``service libvirtd restart'' has
to be issued: Vanished iptables rules show up again. (This, in
contrast to an Ubuntu document [0], fixes it without shutting the
VM(s) down.) Starting dhclient(8) within the VMs gets connectivity back.

Just out of curiosity I let one of the VMs cause some ICMP traffic now
and then hit the interface -- ``ping -i 10 some.nice.host'' is
sufficient to keep that interface alive.

The virbr0-headed interfaces are almost only being used to get updates
etc -- management and load-balanced services are being served by the
bridged interface.

Is this a bug or a feature?

Best,

Timo

[0] --
https://help.ubuntu.com/community/KVM/Networking#Network_Bridge_Losing_Connectivity

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