Inhaltsverzeichnis

Overview
1. The xen network setup
  1. Overview
  2. Configuration details
Upgrade details

Overview

Updating a debian etch (v4.1) system to lenny (v5.0) also involves an upgrade of xen from v3.0 to v3.2. This can create quite some hassle for specific setups.

The documentation below describes some of the issues, that we ran into, when we upgraded the https://systemausfall.org server.

The xen network setup

Our xen network setup is not very common. Thus it should be helpful to give you a short introduction.

Overview

Network interfaces:

dom0 has one physical network interface: eth0
dummy network interfaces (kernel module dummy) are used for most bridges in dom0
most domU share a common network bridge xenbr0
dom0's eth0 is bridged to only one gateway domU as xenbr1
some domU are connected to the gateway domU via separate bridges (xenbr2 and xenbr3)

Advantages of this setup:

the traffic of all domU hosts flows through the gateway domU:
- effective traffic control is possible
- global routing or tunneling (vpn?) setups are possible
separate networks for different zones of trust

Disadvantages:

complexity

Configuration details

Our network configuration script is linked in /etc/xen/xend-config.sxp:

(network-script custom_separate_bridges.sh)

The above script is located in /etc/xen/scripts/custom_separate_bridges.sh:

dir=$(dirname "$0")

# the common network bridge for all sao hosts
"$dir/network-bridge" "$@" vifnum=0 netdev=dummy0 bridge=xenbr0

# the bridge connected to the internet (only for the gateway)
"$dir/network-bridge" "$@" vifnum=1 netdev=eth0 bridge=xenbr1

# the bridge for the separated sao-network (www-users)
"$dir/network-bridge" "$@" vifnum=2 netdev=dummy1 bridge=xenbr2

# the bridge for the network of public IP addresses (mail server ...)
"$dir/network-bridge" "$@" vifnum=3 netdev=dummy2 bridge=xenbr3

Upgrade details

Disable save/restore for xen

You need to shutdown the domU hosts completely during the upgrade. Thus you may want to disable the following line in /etc/default/xendomains:

#XENDOMAINS_SAVE=/var/lib/xen/save

Additionally you may also want to remove any old saved states of xm machines:

rm /var/lib/xen/save/*

Now the xen hosts will be shutdown instead of being saved during invoke-rc.d xendomains stop. This is what we want during the upgrade. afterwards you may change the behaviour according to your preference.

Update the kernel

Update the kernel link for all domU

To ease the maintenance of the domU kernel during upgrades, we use symlinks in /boot:

thorax:~# ls -l /boot
lrwxrwxrwx 1 root root       35 2008-12-28 05:20 domU-initrd -> /boot/initrd.img-2.6.18-6-xen-amd64
lrwxrwxrwx 1 root root       32 2008-12-28 05:20 domU-kernel -> /boot/vmlinuz-2.6.18-6-xen-amd64
 -- snipped some more details --

Update these links to the new domU kernel:

lrwxrwxrwx 1 root root       35 2008-12-28 05:20 domU-initrd -> /boot/initrd.img-2.6.26-1-xen-amd64
lrwxrwxrwx 1 root root       32 2008-12-28 05:20 domU-kernel -> /boot/vmlinuz-2.6.26-1-xen-amd64

Make sure, that all xen host configuration files (e.g. in /etc/xen/ or /etc/xen/vm/) contain the following lines:

kernel  = '/boot/domU-kernel'
ramdisk = '/boot/domU-initrd'

Update the configuration file of xen-tools (/etc/xen-tools/xen-tools.conf) to use these kernel symlinks by default for new xen hosts:

kernel = /boot/domU-kernel
initrd = /boot/domU-initrd
#kernel      = /boot/vmlinuz-`uname -r`
#initrd      = /boot/initrd.img-`uname -r`

Update the new module directory in all domU

shutdown all xen hosts
mount the system partitions of the xen hosts

copy the new modules directory:

cp -a /lib/modules/`uname -r` $MNT_DIR/lib/modules/

No terminal for ssh login

Behaviour

PTY allocation request failed on channel 0
stdin: is not a tty

Even xm console $HOST_NAME stops before the login prompt.

Workaround

Debian bug #502798 suggests to add the following lines to the configuration file of each domU:

# necessary for ssh login since lenny
# see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=502798
extra   = "console=hvc0"

Additionally you need to install udev in the host: aptitude install udev. Maybe you will need to reset /etc/udev/rules.d/z25_persistent-net.rules, if the network interface does not show up after the next reboot. See the next section below for details.

This allows to login via ssh, but xm console still does not work.

Broken network interfaces in dom0

After the first reboot with the new lenny kernel and xen-utils v3.2, the network interfaces of dom0 were broken.

During bootup error messages like the following appeared:

Ignoring unknown interface xenbr0=xenbr0

This was most probably caused by a problem of the network-bridge script in xen-utils-common. See this post in the xen-users mailing list for details.

Apply the following patch on /etc/xen/scripts/network-bridge to solve this problem:

--- network-bridge.orig 2008-12-28 06:30:54.000000000 +0100
+++ network-bridge      2008-12-28 06:31:08.000000000 +0100
@@ -96,7 +96,7 @@
 }
     
 do_ifup() {
-    if ! ifup $1 ; then
+    if ! ifconfig $1 up ; then
         if [ -n "$addr_pfx" ] ; then
             # use the info from get_ip_info()
             ip addr flush $1

Dummy network interfaces stopped working

The way of using the dummy kernel module changed slightly between 2.6.18 and 2.6.26 (at least in debian).

Previous setup for linux 2.6.26

For 2.6.18 we used something like the following to setup a dummy network interface in /etc/network/interfaces:

auto dummy0
iface dummy0 inet static
        pre-up modprobe -o dummy0 dummy
        post-down modprobe -r -o dummy0 dummy && modprobe -r dummy
        address 10.0.1.5
        netmask 255.255.255.0

Thus we needed to have a separate modprobe -o dummyX dummy call for every single dummy module network interface.

New setup for 2.6.26

In 2.6.26 you need to load the module only once. The number of available dummy network interfaces is determined by the module option numdummies=X. If you omit numdummies, then only one dummy network interface is available.

Thus you can add the dummy module to the /etc/modules file:

dummy numdummies=8

This gives you access to the dummy interfaces dummy0 to dummy7 after bootup.

You may now safely remove all pre-up and post-down hooks for the dummy interfaces in /etc/network/interfaces.

No network interfaces in domU hosts

One of our domU hosts could not access any network interfaces after the upgrade to xen 3.2. The previously used eth0 to eth2 were simply gone.

Maybe this was caused by the new dummy network interface module, that was described in the section above.

The basic problem was probably a change of the MAC addresses of the shared network devices. This invalided the dynamically generated content of /etc/udev/rules.d/z25_persistent-net.rules, which is updated only once, when a new interface appears.

See http://sysadmin.wikia.com/wiki/Network_error_xen for details.

Workaround:

empty the udev MAC assignment file:

echo >/etc/udev/rules.d/z25_persistent-net.rules

restart the domU host
check the order of the new interfaces