![\includegraphics[]{figs/xenlogo.eps}](img1.png)
| Users' Manual |
| Xen v3.0 |
| Users' Manual |
| Xen v3.0 |
DISCLAIMER: This documentation is always under active development and as such there may be mistakes and omissions -- watch out for these and please report any you find to the developers' mailing list, xen-devel@lists.xensource.com. The latest version is always available on-line. Contributions of material, suggestions and corrections are welcome.
Xen is Copyright ©2002-2005, University of Cambridge, UK, XenSource Inc., IBM Corp., Hewlett-Packard Co., Intel Corp., AMD Inc., and others. All rights reserved.
Xen is an open-source project. Most portions of Xen are licensed for copying under the terms of the GNU General Public License, version 2. Other portions are licensed under the terms of the GNU Lesser General Public License, the Zope Public License 2.0, or under ``BSD-style'' licenses. Please refer to the COPYING file for details.
Xen includes software by Christopher Clark. This software is covered by the following licence:
Copyright (c) 2002, Christopher Clark. All rights reserved.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
=10000 =10000 1.1
Xen is an open-source para-virtualizing virtual machine monitor (VMM), or ``hypervisor'', for the x86 processor architecture. Xen can securely execute multiple virtual machines on a single physical system with close-to-native performance. Xen facilitates enterprise-grade functionality, including:
Usage scenarios for Xen include:
Para-virtualization permits very high performance virtualization, even on architectures like x86 that are traditionally very hard to virtualize.
This approach requires operating systems to be ported to run on Xen. Porting an OS to run on Xen is similar to supporting a new hardware platform, however the process is simplified because the para-virtual machine architecture is very similar to the underlying native hardware. Even though operating system kernels must explicitly support Xen, a key feature is that user space applications and libraries do not require modification.
With hardware CPU virtualization as provided by Intel VT and AMD SVM technology, the ability to run an unmodified guest OS kernel is available. No porting of the OS is required, although some additional driver support is necessary within Xen itself. Unlike traditional full virtualization hypervisors, which suffer a tremendous performance overhead, the combination of Xen and VT or Xen and Pacifica technology complement one another to offer superb performance for para-virtualized guest operating systems and full support for unmodified guests running natively on the processor. Full support for VT and Pacifica chipsets will appear in early 2006.
Paravirtualized Xen support is available for increasingly many operating systems: currently, mature Linux support is available and included in the standard distribution. Other OS ports--including NetBSD, FreeBSD and Solaris x86 v10--are nearing completion.
Xen currently runs on the x86 architecture, requiring a ``P6'' or newer processor (e.g. Pentium Pro, Celeron, Pentium II, Pentium III, Pentium IV, Xeon, AMD Athlon, AMD Duron). Multiprocessor machines are supported, and there is support for HyperThreading (SMT). In addition, ports to IA64 and Power architectures are in progress.
The default 32-bit Xen supports up to 4GB of memory. However Xen 3.0 adds support for Intel's Physical Addressing Extensions (PAE), which enable x86/32 machines to address up to 64 GB of physical memory. Xen 3.0 also supports x86/64 platforms such as Intel EM64T and AMD Opteron which can currently address up to 1TB of physical memory.
Xen offloads most of the hardware support issues to the guest OS running in the Domain 0 management virtual machine. Xen itself contains only the code required to detect and start secondary processors, set up interrupt routing, and perform PCI bus enumeration. Device drivers run within a privileged guest OS rather than within Xen itself. This approach provides compatibility with the majority of device hardware supported by Linux. The default XenLinux build contains support for most server-class network and disk hardware, but you can add support for other hardware by configuring your XenLinux kernel in the normal way.
A Xen system has multiple layers, the lowest and most privileged of which is Xen itself.
Xen may host multiple guest operating systems, each of which is executed within a secure virtual machine. In Xen terminology, a domain. Domains are scheduled by Xen to make effective use of the available physical CPUs. Each guest OS manages its own applications. This management includes the responsibility of scheduling each application within the time allotted to the VM by Xen.
The first domain, domain 0, is created automatically when the system boots and has special management privileges. Domain 0 builds other domains and manages their virtual devices. It also performs administrative tasks such as suspending, resuming and migrating other virtual machines.
Within domain 0, a process called xend runs to manage the system. Xend is responsible for managing virtual machines and providing access to their consoles. Commands are issued to xend over an HTTP interface, via a command-line tool.
Xen was originally developed by the Systems Research Group at the University of Cambridge Computer Laboratory as part of the XenoServers project, funded by the UK-EPSRC.
XenoServers aim to provide a ``public infrastructure for global distributed computing''. Xen plays a key part in that, allowing one to efficiently partition a single machine to enable multiple independent clients to run their operating systems and applications in an environment. This environment provides protection, resource isolation and accounting. The project web page contains further information along with pointers to papers and technical reports: http://www.cl.cam.ac.uk/xeno
Xen has grown into a fully-fledged project in its own right, enabling us to investigate interesting research issues regarding the best techniques for virtualizing resources such as the CPU, memory, disk and network. Project contributors now include XenSource, Intel, IBM, HP, AMD, Novell, RedHat.
Xen was first described in a paper presented at SOSP in 20031.1, and the first public release (1.0) was made that October. Since then, Xen has significantly matured and is now used in production scenarios on many sites.
Xen 3.0.0 offers:
Xen 3.0 features greatly enhanced hardware support, configuration flexibility, usability and a larger complement of supported operating systems. This latest release takes Xen a step closer to being the definitive open source solution for virtualization.
The Xen distribution includes three main components: Xen itself, ports of Linux and NetBSD to run on Xen, and the userspace tools required to manage a Xen-based system. This chapter describes how to install the Xen 3.0 distribution from source. Alternatively, there may be pre-built packages available as part of your operating system distribution.
The following is a full list of prerequisites. Items marked `
' are
required by the xend control tools, and hence required if you want to
run more than one virtual machine; items marked `*' are only required
if you wish to build from source.
Once you have satisfied these prerequisites, you can now install either a binary or source distribution of Xen.
Pre-built tarballs are available for download from the XenSource downloads page:
http://www.xensource.com/downloads/
Once you've downloaded the tarball, simply unpack and install:
# tar zxvf xen-3.0-install.tgz # cd xen-3.0-install # sh ./install.sh
Once you've installed the binaries you need to configure your system as described in Section 2.5.
http://www.xensource.com/downloads/
Once you've downloaded the RPMs, you typically install them via the RPM commands:
# rpm -iv rpmname
See the instructions and the Release Notes for each RPM set referenced at:
http://www.xensource.com/downloads/.
This section describes how to obtain, build and install Xen from source.
The Xen source tree is available as either a compressed source tarball or as a clone of our master Mercurial repository.
http://www.xensource.com/downloads/
http://xenbits.xensource.comSee the instructions and the Getting Started Guide referenced at:
http://www.xensource.com/downloads/
The top-level Xen Makefile includes a target ``world'' that will do the following:
After the build has completed you should have a top-level directory called dist/ in which all resulting targets will be placed. Of particular interest are the two XenLinux kernel images, one with a ``-xen0'' extension which contains hardware device drivers and drivers for Xen's virtual devices, and one with a ``-xenU'' extension that just contains the virtual ones. These are found in dist/install/boot/ along with the image for Xen itself and the configuration files used during the build.
To customize the set of kernels built you need to edit the top-level Makefile. Look for the line:
KERNELS ?= linux-2.6-xen0 linux-2.6-xenU
You can edit this line to include any set of operating system kernels which have configurations in the top-level buildconfigs/ directory.
If you wish to build a customized XenLinux kernel (e.g. to support additional devices or enable distribution-required features), you can use the standard Linux configuration mechanisms, specifying that the architecture being built for is xen, e.g:
# cd linux-2.6.12-xen0 # make ARCH=xen xconfig # cd .. # make
You can also copy an existing Linux configuration (.config) into e.g. linux-2.6.12-xen0 and execute:
# make ARCH=xen oldconfig
You may be prompted with some Xen-specific options. We advise accepting the defaults for these options.
Note that the only difference between the two types of Linux kernels that are built is the configuration file used for each. The ``U'' suffixed (unprivileged) versions don't contain any of the physical hardware device drivers, leading to a 30% reduction in size; hence you may prefer these for your non-privileged domains. The ``0'' suffixed privileged versions can be used to boot the system, as well as in driver domains and unprivileged domains.
The files produced by the build process are stored under the dist/install/ directory. To install them in their default locations, do:
# make install
Alternatively, users with special installation requirements may wish to install them manually by copying the files to their appropriate destinations.
The dist/install/boot directory will also contain the config files used for building the XenLinux kernels, and also versions of Xen and XenLinux kernels that contain debug symbols such as (xen-syms-3.0.0 and vmlinux-syms-2.6.12.6-xen0) which are essential for interpreting crash dumps. Retain these files as the developers may wish to see them if you post on the mailing list.
Once you have built and installed the Xen distribution, it is simple to prepare the machine for booting and running Xen.
An entry should be added to grub.conf (often found under /boot/ or /boot/grub/) to allow Xen / XenLinux to boot. This file is sometimes called menu.lst, depending on your distribution. The entry should look something like the following:
title Xen 3.0 / XenLinux 2.6 kernel /boot/xen-3.0.gz dom0_mem=262144 module /boot/vmlinuz-2.6-xen0 root=/dev/sda4 ro console=tty0
The kernel line tells GRUB where to find Xen itself and what boot parameters should be passed to it (in this case, setting the domain 0 memory allocation in kilobytes and the settings for the serial port). For more details on the various Xen boot parameters see Section 10.3.
The module line of the configuration describes the location of the XenLinux kernel that Xen should start and the parameters that should be passed to it. These are standard Linux parameters, identifying the root device and specifying it be initially mounted read only and instructing that console output be sent to the screen. Some distributions such as SuSE do not require the ro parameter.
To use an initrd, add another module line to the configuration, like:
module /boot/my_initrd.gz
When installing a new kernel, it is recommended that you do not delete existing menu options from menu.lst, as you may wish to boot your old Linux kernel in future, particularly if you have problems.
Serial console access allows you to manage, monitor, and interact with your system over a serial console. This can allow access from another nearby system via a null-modem (``LapLink'') cable or remotely via a serial concentrator.
You system's BIOS, bootloader (GRUB), Xen, Linux, and login access must each be individually configured for serial console access. It is not strictly necessary to have each component fully functional, but it can be quite useful.
For general information on serial console configuration under Linux, refer to the ``Remote Serial Console HOWTO'' at The Linux Documentation Project: http://www.tldp.org
Enabling system serial console output neither enables nor disables serial capabilities in GRUB, Xen, or Linux, but may make remote management of your system more convenient by displaying POST and other boot messages over serial port and allowing remote BIOS configuration.
Refer to your hardware vendor's documentation for capabilities and procedures to enable BIOS serial redirection.
Enabling GRUB serial console output neither enables nor disables Xen or Linux serial capabilities, but may made remote management of your system more convenient by displaying GRUB prompts, menus, and actions over serial port and allowing remote GRUB management.
Adding the following two lines to your GRUB configuration file, typically either /boot/grub/menu.lst or /boot/grub/grub.conf depending on your distro, will enable GRUB serial output.
serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1 terminal --timeout=10 serial console
Note that when both the serial port and the local monitor and keyboard are enabled, the text ``Press any key to continue'' will appear at both. Pressing a key on one device will cause GRUB to display to that device. The other device will see no output. If no key is pressed before the timeout period expires, the system will boot to the default GRUB boot entry.
Please refer to the GRUB documentation for further information.
Enabling Xen serial console output neither enables nor disables Linux kernel output or logging in to Linux over serial port. It does however allow you to monitor and log the Xen boot process via serial console and can be very useful in debugging.
In order to configure Xen serial console output, it is necessary to add a boot option to your GRUB config; e.g. replace the previous example kernel line with:
kernel /boot/xen.gz dom0_mem=131072 com1=115200,8n1
This configures Xen to output on COM1 at 115,200 baud, 8 data bits, no parity and 1 stop bit. Modify these parameters for your environment. See Section 10.3 for an explanation of all boot parameters.
One can also configure XenLinux to share the serial console; to achieve this append ``console=ttyS0'' to your module line.
Enabling Linux serial console output at boot neither enables nor disables logging in to Linux over serial port. It does however allow you to monitor and log the Linux boot process via serial console and can be very useful in debugging.
To enable Linux output at boot time, add the parameter console=ttyS0 (or ttyS1, ttyS2, etc.) to your kernel GRUB line. Under Xen, this might be:
module /vmlinuz-2.6-xen0 ro root=/dev/VolGroup00/LogVol00 \ console=ttyS0, 115200
to enable output over ttyS0 at 115200 baud.
Logging in to Linux via serial console, under Xen or otherwise, requires specifying a login prompt be started on the serial port. To permit root logins over serial console, the serial port must be added to /etc/securetty.
To automatically start a login prompt over the serial port, add the line:
c:2345:respawn:/sbin/mingetty ttyS0to /etc/inittab. Run init q to force a reload of your inttab and start getty.
To enable root logins, add ttyS0 to /etc/securetty if not already present.
Your distribution may use an alternate getty; options include getty, mgetty and agetty. Consult your distribution's documentation for further information.
Users of the XenLinux 2.6 kernel should disable Thread Local Storage (TLS) (e.g. by doing a mv /lib/tls /lib/tls.disabled) before attempting to boot a XenLinux kernel2.4. You can always reenable TLS by restoring the directory to its original location (i.e. mv /lib/tls.disabled /lib/tls).
The reason for this is that the current TLS implementation uses segmentation in a way that is not permissible under Xen. If TLS is not disabled, an emulation mode is used within Xen which reduces performance substantially. To ensure full performance you should install a `Xen-friendly' (nosegneg) version of the library.
It should now be possible to restart the system and use Xen. Reboot and choose the new Xen option when the Grub screen appears.
What follows should look much like a conventional Linux boot. The first portion of the output comes from Xen itself, supplying low level information about itself and the underlying hardware. The last portion of the output comes from XenLinux.
You may see some error messages during the XenLinux boot. These are not necessarily anything to worry about--they may result from kernel configuration differences between your XenLinux kernel and the one you usually use.
When the boot completes, you should be able to log into your system as usual. If you are unable to log in, you should still be able to reboot with your normal Linux kernel by selecting it at the GRUB prompt.
Booting the system into Xen will bring you up into the privileged management domain, Domain0. At that point you are ready to create guest domains and ``boot'' them using the xm create command.
After installation and configuration is complete, reboot the system and and choose the new Xen option when the Grub screen appears.
What follows should look much like a conventional Linux boot. The first portion of the output comes from Xen itself, supplying low level information about itself and the underlying hardware. The last portion of the output comes from XenLinux.
When the boot completes, you should be able to log into your system as usual. If you are unable to log in, you should still be able to reboot with your normal Linux kernel by selecting it at the GRUB prompt.
The first step in creating a new domain is to prepare a root filesystem for it to boot. Typically, this might be stored in a normal partition, an LVM or other volume manager partition, a disk file or on an NFS server. A simple way to do this is simply to boot from your standard OS install CD and install the distribution into another partition on your hard drive.
To start the xend control daemon, type
# xend start
If you wish the daemon to start automatically, see the instructions in Section 4.1. Once the daemon is running, you can use the xm tool to monitor and maintain the domains running on your system. This chapter provides only a brief tutorial. We provide full details of the xm tool in the next chapter.
Before you can start an additional domain, you must create a configuration file. We provide two example files which you can use as a starting point:
There are also a number of other examples which you may find useful. Copy one of these files and edit it as appropriate. Typical values you may wish to edit include:
You may also want to edit the vif variable in order to choose the MAC address of the virtual ethernet interface yourself. For example:
vif = ['mac=00:16:3E:F6:BB:B3']
If you do not set this variable, xend will automatically generate a
random MAC address from the range 00:16:3E:xx:xx:xx, assigned by IEEE to
XenSource as an OUI (organizationally unique identifier). XenSource
Inc. gives permission for anyone to use addresses randomly allocated
from this range for use by their Xen domains.
For a list of IEEE OUI assignments, see http://standards.ieee.org/regauth/oui/oui.txt
The xm tool provides a variety of commands for managing domains. Use the create command to start new domains. Assuming you've created a configuration file myvmconf based around /etc/xen/xmexample2, to start a domain with virtual machine ID 1 you should type:
# xm create -c myvmconf vmid=1
The -c switch causes xm to turn into the domain's console after creation. The vmid=1 sets the vmid variable used in the myvmconf file.
You should see the console boot messages from the new domain appearing in the terminal in which you typed the command, culminating in a login prompt.
It is possible to have certain domains start automatically at boot time and to have dom0 wait for all running domains to shutdown before it shuts down the system.
To specify a domain is to start at boot-time, place its configuration file (or a link to it) under /etc/xen/auto/.
A Sys-V style init script for Red Hat and LSB-compliant systems is provided and will be automatically copied to /etc/init.d/ during install. You can then enable it in the appropriate way for your distribution.
For instance, on Red Hat:
# chkconfig --add xendomains
By default, this will start the boot-time domains in runlevels 3, 4 and 5.
You can also use the service command to run this script manually, e.g:
# service xendomains start
Starts all the domains with config files under /etc/xen/auto/.
# service xendomains stop
Shuts down all running Xen domains.
This chapter summarizes the management software and tools available.
The Xend node control daemon performs system management functions related to virtual machines. It forms a central point of control of virtualized resources, and must be running in order to start and manage virtual machines. Xend must be run as root because it needs access to privileged system management functions.
An initialization script named /etc/init.d/xend is provided to start Xend at boot time. Use the tool appropriate (i.e. chkconfig) for your Linux distribution to specify the runlevels at which this script should be executed, or manually create symbolic links in the correct runlevel directories.
Xend can be started on the command line as well, and supports the following set of parameters:
# xend start |
start xend, if not already running |
# xend stop |
stop xend if already running |
# xend restart |
restart xend if running, otherwise start it |
# xend status |
indicates xend status by its return code |
A SysV init script called xend is provided to start xend at boot time. make install installs this script in /etc/init.d. To enable it, you have to make symbolic links in the appropriate runlevel directories or use the chkconfig tool, where available. Once xend is running, administration can be done using the xm tool.
As xend runs, events will be logged to /var/log/xend.log and (less frequently) to /var/log/xend-debug.log. These, along with the standard syslog files, are useful when troubleshooting problems.
Xend is written in Python. At startup, it reads its configuration information from the file /etc/xen/xend-config.sxp. The Xen installation places an example xend-config.sxp file in the /etc/xen subdirectory which should work for most installations.
See the example configuration file xend-debug.sxp and the section 5 man page xend-config.sxp for a full list of parameters and more detailed information. Some of the most important parameters are discussed below.
An HTTP interface and a Unix domain socket API are available to communicate with Xend. This allows remote users to pass commands to the daemon. By default, Xend does not start an HTTP server. It does start a Unix domain socket management server, as the low level utility xm requires it. For support of cross-machine migration, Xend can start a relocation server. This support is not enabled by default for security reasons.
Note: the example xend configuration file modifies the defaults and starts up Xend as an HTTP server as well as a relocation server.
From the file:
#(xend-http-server no) (xend-http-server yes) #(xend-unix-server yes) #(xend-relocation-server no) (xend-relocation-server yes)
Comment or uncomment lines in that file to disable or enable features that you require.
Connections from remote hosts are disabled by default:
# Address xend should listen on for HTTP connections, if xend-http-server is # set. # Specifying 'localhost' prevents remote connections. # Specifying the empty string '' (the default) allows all connections. #(xend-address '') (xend-address localhost)
It is recommended that if migration support is not needed, the xend-relocation-server parameter value be changed to ``no'' or commented out.
The xm tool is the primary tool for managing Xen from the console. The general format of an xm command line is:
# xm command [switches] [arguments] [variables]
The available switches and arguments are dependent on the command chosen. The variables may be set using declarations of the form variable=value and command line declarations override any of the values in the configuration file being used, including the standard variables described above and any custom variables (for instance, the xmdefconfig file uses a vmid variable).
For online help for the commands available, type:
# xm help
This will list the most commonly used commands. The full list can be obtained
using xm help --long. You can also type xm help <command>
for more information on a given command.
One useful command is # xm list which lists all domains running in rows
of the following format:
The meaning of each field is as follows:
The xm list command also supports a long output format when the -l switch is used. This outputs the full details of the running domains in xend's SXP configuration format.
You can get access to the console of a particular domain using
the # xm console command (e.g. # xm console myVM).
The following contains the syntax of the domain configuration files and description of how to further specify networking, driver domain and general scheduling behavior.
Xen configuration files contain the following standard variables. Unless otherwise stated, configuration items should be enclosed in quotes: see the configuration scripts in /etc/xen/ for concrete examples.
vif = [ 'mac=00:16:3E:00:00:11, bridge=xen-br0',
'bridge=xen-br1' ]
to assign a MAC address and bridge to the first interface and assign
a different bridge to the second interface, leaving xend to choose
the MAC address. The settings that may be overridden in this way are
type, mac, bridge, ip, script, backend, and vifname.
disk = [ 'phy:hda1,sda1,r' ]
exports physical device /dev/hda1 to the domain as
/dev/sda1 with read-only access. Exporting a disk read-write
which is currently mounted is dangerous - if you are certain
you wish to do this, you can specify w! as the mode.
Additional fields are documented in the example configuration files (e.g. to configure virtual TPM functionality).
For additional flexibility, it is also possible to include Python scripting commands in configuration files. An example of this is the xmexample2 file, which uses Python code to handle the vmid variable.
For many users, the default installation should work ``out of the box''. More complicated network setups, for instance with multiple Ethernet interfaces and/or existing bridging setups will require some special configuration.
The purpose of this section is to describe the mechanisms provided by xend to allow a flexible configuration for Xen's virtual networking.
Each domain network interface is connected to a virtual network interface in dom0 by a point to point link (effectively a ``virtual crossover cable''). These devices are named vif<domid>.<vifid> (e.g. vif1.0 for the first interface in domain 1, vif3.1 for the second interface in domain 3).
Traffic on these virtual interfaces is handled in domain 0 using standard Linux mechanisms for bridging, routing, rate limiting, etc. Xend calls on two shell scripts to perform initial configuration of the network and configuration of new virtual interfaces. By default, these scripts configure a single bridge for all the virtual interfaces. Arbitrary routing / bridging configurations can be configured by customizing the scripts, as described in the following section.
Xen's virtual networking is configured by two shell scripts (by default network-bridge and vif-bridge). These are called automatically by xend when certain events occur, with arguments to the scripts providing further contextual information. These scripts are found by default in /etc/xen/scripts. The names and locations of the scripts can be configured in /etc/xen/xend-config.sxp.
Other example scripts are available (network-route and vif-route, network-nat and vif-nat). For more complex network setups (e.g. where routing is required or integrate with existing bridges) these scripts may be replaced with customized variants for your site's preferred configuration.
Individual PCI devices can be assigned to a given domain to allow that domain direct access to the PCI hardware. To use this functionality, ensure that the PCI Backend is compiled in to a privileged domain (e.g. domain 0) and that the domains which will be assigned PCI devices have the PCI Frontend compiled in. In XenLinux, the PCI Backend is available under the Xen configuration section while the PCI Frontend is under the architecture-specific "Bus Options" section. You may compile both the backend and the frontend into the same kernel; they will not affect each other.
The PCI devices you wish to assign to unprivileged domains must be "hidden"
from your backend domain (usually domain 0) so that it does not load a driver
for them. Use the pciback.hide kernel parameter which is specified on
the kernel command-line and is configurable through GRUB (see
Section 2.5). Note that devices are not really hidden from the
backend domain. The PCI Backend ensures that no other device driver loads
for those devices. PCI devices are identified by hexadecimal
slot/funciton numbers (on Linux, use lspci to determine slot/funciton
numbers of your devices) and can be specified with or without the PCI domain:
An example kernel command-line which hides two PCI devices might be:
To configure a domU to receive a PCI device:
(device (pci
(dev (domain 0x0)(bus 0x3)(slot 0x1a)(func 0x1)
(dev (domain 0x0)(bus 0x1)(slot 0x5)(func 0x0)
)
There are a number of security concerns associated with PCI Driver Domains that you can read about in Section 9.2.
Storage can be made available to virtual machines in a number of different ways. This chapter covers some possible configurations.
The most straightforward method is to export a physical block device (a hard drive or partition) from dom0 directly to the guest domain as a virtual block device (VBD).
Storage may also be exported from a filesystem image or a partitioned filesystem image as a file-backed VBD.
Finally, standard network storage protocols such as NBD, iSCSI, NFS, etc., can be used to provide storage to virtual machines.
One of the simplest configurations is to directly export individual partitions from domain 0 to other domains. To achieve this use the phy: specifier in your domain configuration file. For example a line like
disk = ['phy:hda3,sda1,w']
specifies that the partition /dev/hda3 in domain 0 should be
exported read-write to the new domain as /dev/sda1; one could
equally well export it as /dev/hda or /dev/sdb5 should
one wish.
In addition to local disks and partitions, it is possible to export any device that Linux considers to be ``a disk'' in the same manner. For example, if you have iSCSI disks or GNBD volumes imported into domain 0 you can export these to other domains using the phy: disk syntax. E.g.:
disk = ['phy:vg/lvm1,sda2,w']
Block devices should typically only be shared between domains in a read-only fashion otherwise the Linux kernel's file systems will get very confused as the file system structure may change underneath them (having the same ext3 partition mounted rw twice is a sure fire way to cause irreparable damage)! Xend will attempt to prevent you from doing this by checking that the device is not mounted read-write in domain 0, and hasn't already been exported read-write to another domain. If you want read-write sharing, export the directory to other domains via NFS from domain 0 (or use a cluster file system such as GFS or ocfs2).
It is also possible to use a file in Domain 0 as the primary storage for a virtual machine. As well as being convenient, this also has the advantage that the virtual block device will be sparse -- space will only really be allocated as parts of the file are used. So if a virtual machine uses only half of its disk space then the file really takes up half of the size allocated.
For example, to create a 2GB sparse file-backed virtual block device (actually only consumes 1KB of disk):
# dd if=/dev/zero of=vm1disk bs=1k seek=2048k count=1
Make a file system in the disk file:
# mkfs -t ext3 vm1disk
(when the tool asks for confirmation, answer `y')
Populate the file system e.g. by copying from the current root:
# mount -o loop vm1disk /mnt
# cp -ax /{root,dev,var,etc,usr,bin,sbin,lib} /mnt
# mkdir /mnt/{proc,sys,home,tmp}
Tailor the file system by editing /etc/fstab, /etc/hostname, etc. Don't forget to edit the files in the mounted file system, instead of your domain 0 filesystem, e.g. you would edit /mnt/etc/fstab instead of /etc/fstab. For this example put /dev/sda1 to root in fstab.
Now unmount (this is important!):
# umount /mnt
In the configuration file set:
disk = ['file:/full/path/to/vm1disk,sda1,w']
As the virtual machine writes to its `disk', the sparse file will be filled in and consume more space up to the original 2GB.
Note that file-backed VBDs may not be appropriate for backing I/O-intensive domains. File-backed VBDs are known to experience substantial slowdowns under heavy I/O workloads, due to the I/O handling by the loopback block device used to support file-backed VBDs in dom0. Better I/O performance can be achieved by using either LVM-backed VBDs (Section 6.3) or physical devices as VBDs (Section 6.1).
Linux supports a maximum of eight file-backed VBDs across all domains by default. This limit can be statically increased by using the max_loop module parameter if CONFIG_BLK_DEV_LOOP is compiled as a module in the dom0 kernel, or by using the max_loop=n boot option if CONFIG_BLK_DEV_LOOP is compiled directly into the dom0 kernel.
A particularly appealing solution is to use LVM volumes as backing for domain file-systems since this allows dynamic growing/shrinking of volumes as well as snapshot and other features.
To initialize a partition to support LVM volumes:
# pvcreate /dev/sda10
Create a volume group named `vg' on the physical partition:
# vgcreate vg /dev/sda10
Create a logical volume of size 4GB named `myvmdisk1':
# lvcreate -L4096M -n myvmdisk1 vg
You should now see that you have a /dev/vg/myvmdisk1 Make a filesystem, mount it and populate it, e.g.:
# mkfs -t ext3 /dev/vg/myvmdisk1 # mount /dev/vg/myvmdisk1 /mnt # cp -ax / /mnt # umount /mnt
Now configure your VM with the following disk configuration:
disk = [ 'phy:vg/myvmdisk1,sda1,w' ]
LVM enables you to grow the size of logical volumes, but you'll need to resize the corresponding file system to make use of the new space. Some file systems (e.g. ext3) now support online resize. See the LVM manuals for more details.
You can also use LVM for creating copy-on-write (CoW) clones of LVM volumes (known as writable persistent snapshots in LVM terminology). This facility is new in Linux 2.6.8, so isn't as stable as one might hope. In particular, using lots of CoW LVM disks consumes a lot of dom0 memory, and error conditions such as running out of disk space are not handled well. Hopefully this will improve in future.
To create two copy-on-write clones of the above file system you would use the following commands:
# lvcreate -s -L1024M -n myclonedisk1 /dev/vg/myvmdisk1 # lvcreate -s -L1024M -n myclonedisk2 /dev/vg/myvmdisk1
Each of these can grow to have 1GB of differences from the master volume. You can grow the amount of space for storing the differences using the lvextend command, e.g.:
# lvextend +100M /dev/vg/myclonedisk1
Don't let the `differences volume' ever fill up otherwise LVM gets rather confused. It may be possible to automate the growing process by using dmsetup wait to spot the volume getting full and then issue an lvextend.
In principle, it is possible to continue writing to the volume that has been cloned (the changes will not be visible to the clones), but we wouldn't recommend this: have the cloned volume as a `pristine' file system install that isn't mounted directly by any of the virtual machines.
First, populate a root filesystem in a directory on the server machine. This can be on a distinct physical machine, or simply run within a virtual machine on the same node.
Now configure the NFS server to export this filesystem over the network by adding a line to /etc/exports, for instance:
/export/vm1root 1.2.3.4/24 (rw,sync,no_root_squash)
Finally, configure the domain to use NFS root. In addition to the normal variables, you should make sure to set the following values in the domain's configuration file:
root = '/dev/nfs' nfs_server = '2.3.4.5' # substitute IP address of server nfs_root = '/path/to/root' # path to root FS on the server
The domain will need network access at boot time, so either statically configure an IP address using the config variables ip, netmask, gateway, hostname; or enable DHCP (dhcp='dhcp').
Note that the Linux NFS root implementation is known to have stability problems under high load (this is not a Xen-specific problem), so this configuration may not be appropriate for critical servers.
Xen allows a domain's virtual CPU(s) to be associated with one or more host CPUs. This can be used to allocate real resources among one or more guests, or to make optimal use of processor resources when utilizing dual-core, hyperthreading, or other advanced CPU technologies.
Xen enumerates physical CPUs in a `depth first' fashion. For a system with both hyperthreading and multiple cores, this would be all the hyperthreads on a given core, then all the cores on a given socket, and then all sockets. I.e. if you had a two socket, dual core, hyperthreaded Xeon the CPU order would be:
| socket0 | socket1 | ||||||
| core0 | core1 | core0 | core1 | ||||
| ht0 | ht1 | ht0 | ht1 | ht0 | ht1 | ht0 | ht1 |
| #0 | #1 | #2 | #3 | #4 | #5 | #6 | #7 |
Having multiple vcpus belonging to the same domain mapped to the same physical CPU is very likely to lead to poor performance. It's better to use `vcpus-set' to hot-unplug one of the vcpus and ensure the others are pinned on different CPUs.
If you are running IO intensive tasks, its typically better to dedicate either a hyperthread or whole core to running domain 0, and hence pin other domains so that they can't use CPU 0. If your workload is mostly compute intensive, you may want to pin vcpus such that all physical CPU threads are available for guest domains.
The administrator of a Xen system may suspend a virtual machine's current state into a disk file in domain 0, allowing it to be resumed at a later time.
For example you can suspend a domain called ``VM1'' to disk using the command:
# xm save VM1 VM1.chk
This will stop the domain named ``VM1'' and save its current state into a file called VM1.chk.
To resume execution of this domain, use the xm restore command:
# xm restore VM1.chk
This will restore the state of the domain and resume its execution. The domain will carry on as before and the console may be reconnected using the xm console command, as described earlier.
Migration is used to transfer a domain between physical hosts. There are two varieties: regular and live migration. The former moves a virtual machine from one host to another by pausing it, copying its memory contents, and then resuming it on the destination. The latter performs the same logical functionality but without needing to pause the domain for the duration. In general when performing live migration the domain continues its usual activities and--from the user's perspective--the migration should be imperceptible.
To perform a live migration, both hosts must be running Xen / xend and the destination host must have sufficient resources (e.g. memory capacity) to accommodate the domain after the move. Furthermore we currently require both source and destination machines to be on the same L2 subnet.
Currently, there is no support for providing automatic remote access to filesystems stored on local disk when a domain is migrated. Administrators should choose an appropriate storage solution (i.e. SAN, NAS, etc.) to ensure that domain filesystems are also available on their destination node. GNBD is a good method for exporting a volume from one machine to another. iSCSI can do a similar job, but is more complex to set up.
When a domain migrates, it's MAC and IP address move with it, thus it is only possible to migrate VMs within the same layer-2 network and IP subnet. If the destination node is on a different subnet, the administrator would need to manually configure a suitable etherip or IP tunnel in the domain 0 of the remote node.
A domain may be migrated using the xm migrate command. To live migrate a domain to another machine, we would use the command:
# xm migrate --live mydomain destination.ournetwork.com
Without the -live flag, xend simply stops the domain and copies the memory image over to the new node and restarts it. Since domains can have large allocations this can be quite time consuming, even on a Gigabit network. With the -live flag xend attempts to keep the domain running while the migration is in progress, resulting in typical down times of just 60-300ms.
For now it will be necessary to reconnect to the domain's console on the new machine using the xm console command. If a migrated domain has any open network connections then they will be preserved, so SSH connections do not have this limitation.
This chapter describes how to secure a Xen system. It describes a number of scenarios and provides a corresponding set of best practices. It begins with a section devoted to understanding the security implications of a Xen system.
When deploying a Xen system, one must be sure to secure the management domain (Domain-0) as much as possible. If the management domain is compromised, all other domains are also vulnerable. The following are a set of best practices for Domain-0:
Driver domains address a range of security problems that exist regarding the use of device drivers and hardware. On many operating systems in common use today, device drivers run within the kernel with the same privileges as the kernel. Few or no mechanisms exist to protect the integrity of the kernel from a misbehaving (read "buggy") or malicious device driver. Driver domains exist to aid in isolating a device driver within its own virtual machine where it cannot affect the stability and integrity of other domains. If a driver crashes, the driver domain can be restarted rather than have the entire machine crash (and restart) with it. Drivers written by unknown or untrusted third-parties can be confined to an isolated space. Driver domains thus address a number of security and stability issues with device drivers.
However, due to limitations in current hardware, a number of security concerns remain that need to be considered when setting up driver domains (it should be noted that the following list is not intended to be exhaustive).
In this scenario, each node has two network cards in the cluster. One network card is connected to the outside world and one network card is a physically isolated management network specifically for Xen instances to use.
As long as all of the management partitions are trusted equally, this is the most secure scenario. No additional configuration is needed other than forcing Xend to bind to the management interface for relocation.
In this scenario, each node has only one network card but the entire cluster sits behind a firewall. This firewall should do at least the following:
The following iptables rules can be used on each node to prevent migrations to that node from outside the subnet assuming the main firewall does not do this for you:
# this command disables all access to the Xen relocation
# port:
iptables -A INPUT -p tcp --destination-port 8002 -j REJECT
# this command enables Xen relocations only from the specific
# subnet:
iptables -I INPUT -p tcp -{}-source 192.168.1.1/8 \
--destination-port 8002 -j ACCEPT
Migration on an untrusted subnet is not safe in current versions of Xen. It may be possible to perform migrations through a secure tunnel via an VPN or SSH. The only safe option in the absence of a secure tunnel is to disable migration completely. The easiest way to do this is with iptables:
# this command disables all access to the Xen relocation port
iptables -A INPUT -p tcp -{}-destination-port 8002 -j REJECT
This chapter describes the build- and boot-time options which may be used to tailor your Xen system.
Top-level configuration is achieved by editing one of two files: Config.mk and Makefile.
The former allows the overall build target architecture to be specified. You will typically not need to modify this unless you are cross-compiling or if you wish to build a PAE-enabled Xen system. Additional configuration options are documented in the Config.mk file.
The top-level Makefile is chiefly used to customize the set of kernels built. Look for the line:
KERNELS ?= linux-2.6-xen0 linux-2.6-xenU
Allowable options here are any kernels which have a corresponding build configuration file in the buildconfigs/ directory.
Xen provides a number of build-time options which should be set as environment variables or passed on make's command-line.
These options are used to configure Xen's behaviour at runtime. They should be appended to Xen's command line, either manually or by editing grub.conf.
In addition, the following options may be specified on the Xen command line. Since domain 0 shares responsibility for booting the platform, Xen will automatically propagate these options to its command line. These options are taken from Linux's command-line syntax with unchanged semantics.
In addition to the standard Linux kernel boot options, we support:
| `xencons=off': disable virtual console |
| `xencons=tty': attach console to /dev/tty1 (tty0 at boot-time) |
| `xencons=ttyS': attach console to /dev/ttyS0 |
If you have questions that are not answered by this manual, the sources of information listed below may be of interest to you. Note that bug reports, suggestions and contributions related to the software (or the documentation) should be sent to the Xen developers' mailing list (address below).
For developers interested in porting operating systems to Xen, the Xen Interface Manual is distributed in the docs/ directory of the Xen source distribution.
The official Xen web site can be found at:
http://www.xensource.com
This contains links to the latest versions of all online documentation, including the latest version of the FAQ.
Information regarding Xen is also available at the Xen Wiki at
http://wiki.xensource.com/xenwiki/The Xen project uses Bugzilla as its bug tracking system. You'll find the Xen Bugzilla at http://bugzilla.xensource.com/bugzilla/.
There are several mailing lists that are used to discuss Xen related topics. The most widely relevant are listed below. An official page of mailing lists and subscription information can be found at
http://lists.xensource.com/
Xen supports guest domains running unmodified Guest operating systems using Virtualization Technology (VT) available on recent Intel Processors. More information about the Intel Virtualization Technology implementing Virtual Machine Extensions (VMX) in the processor is available on the Intel website at
http://www.intel.com/technology/computing/vptech
The following packages need to be installed in order to build Xen with VT support. Some Linux distributions do not provide these packages by default.
| Package | Description |
| dev86 | The dev86 package provides an assembler and linker for real mode 80x86 instructions. You need to have this package installed in order to build the BIOS code which runs in (virtual) real mode.
If the dev86 package is not available on the x86_64 distribution, you can install the i386 version of it. The dev86 rpm package for various distributions can be found at http://www.rpmfind.net/linux/rpm2html/search.php?query=dev86&submit=Search |
| LibVNCServer | The unmodified guest's VGA display, keyboard, and mouse are virtualized using the vncserver library provided by this package. You can get the sources of libvncserver from http://sourceforge.net/projects/libvncserver. Build and install the sources on the build system to get the libvncserver library. The 0.8pre version of libvncserver is currently working well with Xen. |
| SDL-devel, SDL | Simple DirectMedia Layer (SDL) is another way of virtualizing the unmodified guest console. It provides an X window for the guest console.
If the SDL and SDL-devel packages are not installed by default on the build system, they can be obtained from http://www.rpmfind.net/linux/rpm2html/search.php?query=SDL&submit=Search , http://www.rpmfind.net/linux/rpm2html/search.php?query=SDL-devel&submit=Search |
The Xen installation includes a sample configuration file, /etc/xen/xmexample.vmx. There are comments describing all the options. In addition to the common options that are the same as those for paravirtualized guest configurations, VMX guest configurations have the following settings:
| Parameter | Description |
| kernel | The VMX firmware loader, /usr/lib/xen/boot/vmxloader |
| builder | The domain build function. The VMX domain uses the vmx builder. |
| acpi | Enable VMX guest ACPI, default=0 (disabled) |
| apic | Enable VMX guest APIC, default=0 (disabled) |
| vif | Optionally defines MAC address and/or bridge for the network interfaces. Random MACs are assigned if not given. type=ioemu means ioemu is used to virtualize the VMX NIC. If no type is specified, vbd is used, as with paravirtualized guests. |
| disk | Defines the disk devices you want the domain to have access to, and what you want them accessible as. If using a physical device as the VMX guest's disk, each disk entry is of the form
phy:UNAME,ioemu:DEV,MODE, where UNAME is the device, DEV is the device name the domain will see, and MODE is r for read-only, w for read-write. ioemu means the disk will use ioemu to virtualize the VMX disk. If not adding ioemu, it uses vbd like paravirtualized guests. If using disk image file, its form should be like file:FILEPATH,ioemu:DEV,MODE If using more than one disk, there should be a comma between each disk entry. For example: disk = ['file:/var/images/image1.img,ioemu:hda,w', 'file:/var/images/image2.img,ioemu:hdb,w'] |
| cdrom | Disk image for CD-ROM. The default is /dev/cdrom for Domain0. Inside the VMX domain, the CD-ROM will available as device /dev/hdc. The entry can also point to an ISO file. |
| boot | Boot from floppy (a), hard disk (c) or CD-ROM (d). For example, to boot from CD-ROM, the entry should be:
boot='d' |
| device_model | The device emulation tool for VMX guests. This parameter should not be changed. |
| sdl | Enable SDL library for graphics, default = 0 (disabled) |
| vnc | Enable VNC library for graphics, default = 1 (enabled) |
| vncviewer | Enable spawning of the vncviewer (only valid when vnc=1), default = 1 (enabled)
If vnc=1 and vncviewer=0, user can use vncviewer to manually connect VMX from remote. For example: vncviewer domain0_IP_address:VMX_domain_id |
| ne2000 | Enable ne2000, default = 0 (disabled; use pcnet) |
| serial | Enable redirection of VMX serial output to pty device |
| localtime | Set the real time clock to local time [default=0, that is, set to UTC]. |
| enable-audio | Enable audio support. This is under development. |
| full-screen | Start in full screen. This is under development. |
| nographic | Another way to redirect serial output. If enabled, no 'sdl' or 'vnc' can work. Not recommended. |
# dd if=/dev/zero of=hd.img bs=1M count=1 seek=1023
Install Xen and create VMX with the original image file with booting from CD-ROM. Then it is just like a normal Linux OS installation. The VMX configuration file should have these two entries before creating:
cdrom='/dev/cdrom' boot='d'
If this method does not succeed, you can choose the following method of copying an installed Linux OS into an image file.
# losetup /dev/loop0 hd.img
# fdisk -b 512 -C 4096 -H 16 -S 32 /dev/loop0
press 'n' to add new partition
press 'p' to choose primary partition
press '1' to set partition number
press "Enter" keys to choose default value of "First Cylinder" parameter.
press "Enter" keys to choose default value of "Last Cylinder" parameter.
press 'w' to write partition table and exit
# losetup -d /dev/loop0
The losetup option -o 16384 skips the partition table in the image file. It is the number of sectors times 512. We need /dev/loop because grub is expecting a disk device name, where name represents the entire disk and name1 represents the first partition.
# lomount -t ext3 -diskimage hd.img -partition 1 /mnt/guest
# cp -ax /var/guestos/{root,dev,var,etc,usr,bin,sbin,lib} /mnt/guest
# mkdir /mnt/guest/{proc,sys,home,tmp}
# vim /mnt/guest/etc/fstab
/dev/hda1 / ext3 defaults 1 1
none /dev/pts devpts gid=5,mode=620 0 0
none /dev/shm tmpfs defaults 0 0
none /proc proc defaults 0 0
none /sys sysfs efaults 0 0
Now, the guest OS image hd.img is ready. You can also reference http://free.oszoo.org for quickstart images. But make sure to install the boot loader.
disk = [ 'file:/var/images/guest.img,ioemu:hda,w' ]
replacing guest.img with the name of the guest OS image file you just made.
# xend start
# xm create /etc/xen/vmxguest.vmx
In the default configuration, VNC is on and SDL is off. Therefore VNC windows will open when VMX guests are created. If you want to use SDL to create VMX guests, set sdl=1 in your VMX configuration file. You can also turn off VNC by setting vnc=0.
poweroff
in the VMX guest's console first to prevent data loss. Then execute the command
xm destroy vmx_guest_id
at the Domain0 console.
Ctrl+Alt+2 switches from guest VGA window to the control window. Typing help shows the control commands help. For example, 'q' is the command to destroy the VMX guest.
Ctrl+Alt+1 switches back to VMX guest's VGA.
Ctrl+Alt+3 switches to serial port output. It captures serial output from the VMX guest. It works only if the VMX guest was configured to use the serial port.
Xen optionally supports virtual networking for domains using vnets. These emulate private LANs that domains can use. Domains on the same vnet can be hosted on the same machine or on separate machines, and the vnets remain connected if domains are migrated. Ethernet traffic on a vnet is tunneled inside IP packets on the physical network. A vnet is a virtual network and addressing within it need have no relation to addressing on the underlying physical network. Separate vnets, or vnets and the physical network, can be connected using domains with more than one network interface and enabling IP forwarding or bridging in the usual way.
Vnet support is included in xm and xend:
# xm vnet-create <config>creates a vnet using the configuration in the file
<config>.
When a vnet is created its configuration is stored by xend and the vnet persists until it is
deleted using
# xm vnet-delete <vnetid>The vnets xend knows about are listed by
# xm vnet-listMore vnet management commands are available using the vn tool included in the vnet distribution.
The format of a vnet configuration file is
(vnet (id <vnetid>)
(bridge <bridge>)
(vnetif <vnet interface>)
(security <level>))
White space is not significant. The parameters are:
<vnetid>: vnet id, the 128-bit vnet identifier. This can be given
as 8 4-digit hex numbers separated by colons, or in short form as a single 4-digit hex number.
The short form is the same as the long form with the first 7 fields zero.
Vnet ids must be non-zero and id 1 is reserved.
<bridge>: the name of a bridge interface to create for the vnet. Domains
are connected to the vnet by connecting their virtual interfaces to the bridge.
Bridge names are limited to 14 characters by the kernel.
<vnetif>: the name of the virtual interface onto the vnet (optional). The
interface encapsulates and decapsulates vnet traffic for the network and is attached
to the vnet bridge. Interface names are limited to 14 characters by the kernel.
<level>: security level for the vnet (optional). The level may be one of
none: no security (default). Vnet traffic is in clear on the network.
auth: authentication. Vnet traffic is authenticated using IPSEC
ESP with hmac96.
conf: confidentiality. Vnet traffic is authenticated and encrypted
using IPSEC ESP with hmac96 and AES-128.
(vnet (id 97) (bridge vnet97) (vnetif vnif97)
(security none))
Then xm vnet-create vnet97.sxp will define a vnet with id 97 and no security.
The bridge for the vnet is called vnet97 and the virtual interface for it is vnif97.
To add an interface on a domain to this vnet set its bridge to vnet97
in its configuration. In Python:
vif="bridge=vnet97"In sxp:
(dev (vif (mac aa:00:00:01:02:03) (bridge vnet97)))Once the domain is started you should see its interface in the output of brctl show under the ports for vnet97.
To get best performance it is a good idea to reduce the MTU of a domain's interface onto a vnet to 1400. For example using ifconfig eth0 mtu 1400 or putting MTU=1400 in ifcfg-eth0. You may also have to change or remove cached config files for eth0 under /etc/sysconfig/networking. Vnets work anyway, but performance can be reduced by IP fragmentation caused by the vnet encapsulation exceeding the hardware MTU.
(network-script network-vnet)This script insmods the module and calls the network-bridge script.
The vnet code is not compiled and installed by default. To compile the code and install on the current system use make install in the root of the vnet source tree, tools/vnet. It is also possible to install to an installation directory using make dist. See the Makefile in the source for details.
The vnet module creates vnet interfaces vnif0002, vnif0003 and vnif0004 by default. You can test that vnets are working by configuring IP addresses on these interfaces and trying to ping them across the network. For example, using machines hostA and hostB:
hostA# ifconfig vnif0004 10.0.0.100 up hostB# ifconfig vnif0004 10.0.0.101 up hostB# ping 10.0.0.100
The vnet implementation uses IP multicast to discover vnet interfaces, so all machines hosting vnets must be reachable by multicast. Network switches are often configured not to forward multicast packets, so this often means that all machines using a vnet must be on the same LAN segment, unless you configure vnet forwarding.
You can test multicast coverage by pinging the vnet multicast address:
# ping -b 224.10.0.1You should see replies from all machines with the vnet module running. You can see if vnet packets are being sent or received by dumping traffic on the vnet UDP port:
# tcpdump udp port 1798
If multicast is not being forwaded between machines you can configure multicast forwarding using vn. Suppose we have machines hostA on 10.10.0.100 and hostB on 10.11.0.100 and that multicast is not forwarded between them. We use vn to configure each machine to forward to the other:
hostA# vn peer-add hostB hostB# vn peer-add hostAMulticast forwarding needs to be used carefully - you must avoid creating forwarding loops. Typically only one machine on a subnet needs to be configured to forward, as it will forward multicasts received from other machines on the subnet.