[CentOS-virt] Disappearing TAP interface

James Roman james_roman at ssaihq.com
Mon Sep 8 15:58:53 UTC 2008

I have a CentOS 5.2 Xen host, running several guest domains. Each guest
domain has a single interface assigned to it. I am using both the
network-bridge and vif-bridge scripts. 

On my only windows guest, the tap interface connected to the bridge
keeps getting disabled. If I check "brctl show" the tap0 interface is
not present, and there will be several message log entries indicating
that the interface tap0 is being disabled. This means that the guest
becomes invisible. I can not open a VNC console or connect to it from
the network. From the hosts log files, it appears that the OS keeps
running, even though it it is unreachable. (this is a little hard to
guage, because the times in the eventlog keep jumping around.) The only
way I have been able to restore a connection is to destroy the domain
and re-create it. I can not find much in the various Xen logs that
identifies what triggers the tap interface to be disabled. 

I have several questions. 

Is there any way to manually rebuild the tap interface to get access to
the host again (either through VNC or network)? Maybe through the virsh
command or calling the vif-bridge or qemu-ifup command directly?

Is there another way that I could use to maintain a connection to the

Does anyone have any idea what causes the tap interface to go down?

--qemu-dm log----------------------------------------------------

domid: 1
qemu: the number of cpus is 2
*** image /local/domain/0/backend/vbd/1/768 format raw driver 0x80e7940
*** image /local/domain/0/backend/vbd/1/5632 format raw driver 0x80e7940
*** image /local/domain/0/backend/vbd/1/832 format raw driver 0x80e7940
Watching /local/domain/1/logdirty/next-active
xs_read(): vncpasswd get
error. /vm/e2a537ca-5ef5-e87d-6505-855529af3ea4/vncpasswd.
char device redirected to /dev/pts/1
qemu_map_cache_init nr_buckets = 4000
shared page at pfn 3ffff
buffered io page at pfn 3fffd
xs_read(/vm/e2a537ca-5ef5-e87d-6505-855529af3ea4/rtc/timeoffset): read
I/O request not ready: 0, ptr: 0, port: 0, data: 0, count: 0, size: 0
I/O request not ready: 0, ptr: 0, port: 0, data: 0, count: 0, size: 0
Triggered log-dirty buffer switch
rtc/timeoffset): write error
rtc/timeoffset): write error


name = "vm-002_win"
uuid = "e2a537ca5ef5e87d6505855529af3ea4"
maxmem = 2048
memory = 1024
vcpus = 2
builder = "hvm"
kernel = "/usr/lib/xen/boot/hvmloader"
boot = "c"
pae = 1
acpi = 1
apic = 1
on_poweroff = "destroy"
on_reboot = "restart"
on_crash = "restart"
device_model = "/usr/lib/xen/bin/qemu-dm"
sdl = 0
vnc = 1
vncunused = 1
keymap = "en-us"
disk = [ "file:/var/lib/xen/images/vm-002_win.img,hda,w",
"phy:/dev/VolGroup00/LVssavmWin001Data,hdb,w" ]
vif =
[ "ip=,mac=00:16:3e:0d:7f:be,bridge=xenbr0,type=ioemu" ]
serial = "pty"

James D. Roman
IT Network Administration

Terranet Inc.On contract to:
Science Systems and Applications, Inc.

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