Xen is an exciting and still relatively new technology. This paper describes how to get a bare bones install of xen going with minimal trouble and time

Griffin, Leigh

Ronan, John

Waterford Institute of Technology Repository

Xen Installation and Configuration

WIT Repository

; LOGIN: FEBRUARY 2007 XEN INSTALLATION AND CONFIGURATION 37L E I G H G R I F F I N A N D J O H N R O N A NXen installationand configurationLeigh is a student assistant researcher at the TSSG,Waterford Institute of Technology. He has nowreturned toWIT to complete his studies in the B.Sc.in Applied Computing Degree.lgriffin@tssg.orgJohn Ronan is a senior researcher at the TSSG,Waterford Institute of Technology, and also a radioham (EI7IG).When not experimenting with new net-work technologies, he can be found bouncing AX25packets through LEO satellites.jronan@tssg.orgTH I S GU I D E I S D E S I GN ED TO G E T AXen server up and running in no time. It isa simple copy-and-paste guide whichshould get you through a bare-bonesinstall with minimal trouble and time. Xenis an exciting and still relatively new tech-nology; more information can be obtainedby visiting the Xen homepage athttp://www.xensource.com and also byreading “The Inevitability of Xen,” byCrowcroft et al. [1]. The only limitations youwill find with Xen will derive from yourhardware or your ingenuity.As this guide is aimed at beginners, it is going tocover the installation of Xen from the binarypackages. The binary packages are recommendedfor people who are new to Xen and are uncom-fortable with the range of configuration optionsthat the source install offers.The latest stable release of Xen can be found athttp://www.xensource.com/products/downloads/.The sources are also available from BitTorrentsites, among others; however, we recommendgoing with the official releases to ensure validity,security, and stability [2]. We obtained the 3.0.1binary install file and thus will use this as a refer-ence for the rest of the guide; simply replace 3.0.1with your 3.0.x in the relevant positions in orderto install the software successfully.The Linux Filesystem Hierarchy Standardrecommends placing the source file in the/usr/src folder, and that’s where we will put itand unpack it [3].PrerequisitesWe ran the following commands to install thedependency packages and to remove some outdat-ed and unnecessary packages (using Debian):xen:# apt-get remove exim4 exim4-base lpr nfs-common portmap pidentd pcmcia-cs pppoepppoeconf ppp pppconfigxen:# apt-get install screen ssh debootstrappython python2.3-twisted iproute bridge-utilslibcurl3-devNow that we have the necessary files installed,let’s extract the software from its .tar file and runthe install script:xen:# /usr/src$ cd xen-3.0.1-installxen:# /usr/src/xen-3.0.1-install$ ./install.shxen:# /usr/src/xen-3.0.1-install$ mv /lib/tls /lib/tls.disabledThe last command is necessary to avoid the emulation slowdown problemswith the glibc libraries that are installed by default [2].You should now have the Xen software installed on your computer. To startthe Xen services at boot time, the following commands need to be run:xen:# update-rc.d xend defaults 20 21xen:# update-rc.d xendomains defaults 21 20The final additions to be made are to add the Xen kernel to the bootloaderprogram (Grub). Scroll through the file until you find the line that reads:### BEGIN AUTOMAGIC KERNELS LISTJust above that is the place where we must make our addition to the file.Enter the following text:title Xen 3.0 / XenLinux 2.6.12kernel /boot/xen.gz dom0_mem=64000module /boot/vmlinuz-2.6.12-xen0 root=/dev/hda1 ro console=tty0Note that it is important to make sure that your root is indeed /dev/hda1.If it is not, simply change the value after root= to match it. If you areunsure what your root name is, scroll further down the menu.lst file andyou will see the default kernel and its root value.Reboot the machine; at the boot prompt, Grub will now list Xen 3.0/XenLinux 2.6.12 as the first kernel and boot it automatically. Everythingshould load normally and you will be given your standard login. If themachine does not boot, the following may fix the problem.If the Xen machine executes a hard reboot as it is starting up, the problemrests with the amount of RAM in your machine. You will get no error mes-sage with this problem, and the last thing you will see is a line that says:“Scrubbing free RAM. . . . . . . .”Then the screen will go black and do a hard reboot. The solution is toremove the excess RAM (while still keeping the DIMMs balanced) andreboot the machine. Currently, the binary install can only cope with a max-imum of 3583 MB of RAM. To use more RAM, a source install needs to beperformed and PAE support must be built into the kernel; however, thistopic lies outside the scope of this guide.Creation of DomainsNow that we have the Xen software installed, it is time to get to the cre-ation of the virtual machines. First, we are going to create a storage areafor our virtual machines:xen:# mkdir /virtual && cd /virtualHere we are going to create two directories in which to store and configurethe images:xen:/virtual# mkdir vm_basexen:/virtual# mkdir imagesWe will create a default image and swap image from which our virtualmachines will be derived. Execute the following commands:38 ; LOG I N : VO L . 3 2 , NO . 1xen:/virtual# dd if=/dev/zero of=/virtual/images/vm_base.img bs=1024kcount=xxxxxen:/virtual# dd if=/dev/zero of=/virtual/images/vm_base-swap.imgbs=1024k count=xxxNote that the value that count= specifies is the size the image will be inmegabytes. Simply change it to a value that will suit your needs; only yourmachine capacity is the limit. (See p. 65 for sample file sizes.)Now we need to format the base image to be ext3 so that it can serve asour journaling filesystem. We have chosen ext3 as it is faster than ext2 andhas stronger guarantees for data integrity [4].xen:/virtual# mkfs.ext3 /virtual/images/vm_base.imgAnswer yes to the question prompted regarding the warning about theblock special device. Now we need to configure the swap file to be a swaparea:xen:/virtual# mkswap /virtual/images/vm_base-swap.imgNext it’s time to install the Debian base system to our newly created image.First, though, we need to mount our image:xen:/virtual# mount -o loop /virtual/images/vm_base.img /virtual/vm_baseDebootstrapping the Base ImageWe run the debootstrap command to download all the prerequisite pack-ages, using the following command:xen:/virtual# debootstrap —arch i386 sarge /virtual/vm_base/http://ftp2.de.debian.org/debianNow change root and configure the images apt program to specify how wewant to pull down our software and updates:xen:/virtual# chroot /virtual/vm_basexen:# apt-setupDuring the standard apt setup, you will be asked some basic questionsregarding your location and which mirror you wish to use to speed up theprocess. When this is done, edit the sources.list that comes with apt andchange the word testing to stable wherever it appears in the file. Nowupdate your software repository:xen:# apt-get updateThe next step in the installation process involves setting up the locales foryour region:xen:# apt-get install localeconfChoose the locales to install depending on your country (e.g., en_IEISOxxxx for Ireland or en_US ISOxxxx for the United States).Next, configure the base system using base-config. A menu with variousinstallation options will be presented to you. The important things to con-figure are:1. Users and passwords. This is where you set the default user name,password, and root password. This is an important part as each im-age subsequently created from the base image will have these defaultpasswords, which will need to be changed.2. The time zone.; LOGIN: FEBRUARY 2007 XEN INSTALLATION AND CONFIGURATION 393. Which software to install. When the program prompts for additionalsoftware to be installed we choose “none,” as this is the base image,from which the other virtual machines will later be derived. Each de-rived machine can be customized when it is ready.When you are satisfied with the system, simply hit return and you are fin-ished configuring the base system.There are some small configurations still to be completed. First, removethe hostname from the system. We remove the hostname because deboot-strap copies this from the host machine to the newly created image so bothwill have the same hostname:xen:# rm -f /etc/hostnameNow we need to create our networking interfaces by editing/etc/network/interfaces:auto loiface lo inet loopbackaddress 127.0.0.1netmask 255.0.0.0Next we edit the fstab file; it must end up looking exactly like the follow-ing in order to represent the internal structure of the virtual image, itsmountpoints, and its filesystem types:/dev/hda1 / ext3 defaults 1 2/dev/hda2 none swap sw 0 0/dev/pts devpts gid=5,mode=620 0 0none /dev/shm tmpfs defaults 0 0These values will map to the configuration file values for the root andswap later on in the configuration of the virtual domains themselves.Our last configuration option sees the creation of the hosts file:127.0.0.1 localhost.localdomain localhost# The following lines are desirable for IPv6 capable hosts::1 ip6-localhost ip6-loopbackfe00::0 ip6-localnetff00::0 ip6-mcastprefixff02::1 ip6-allnodesff02::2 ip6-allroutersff02::3 ip6-allhostsNow we leave the chroot environment with exit. All that is left is for us tocopy the kernel modules to our virtual machine and unmount the image.xen:/virtual/vm_base# cp -dpR /lib/modules/2.6.12.6-xenU\/virtual/vm_base/lib/modules/xen:/virtual/vm_base# mv /virtual/vm_base/lib/tls /virtual\/vm_base/lib/tls.disabledxen:/virtual/vm_base# umount /virtual/vm_baseThe base image is now complete.Creation of Virtual DomainsNow that we have a base image to work off of, it is time to go and makesome virtual machines. We will do this by copying the base image like so:xen:/virtual/vm_base# cp -pf /virtual/images/vm_base.img\/virtual/images/vm01.img40 ; LOG I N : VO L . 3 2 , NO . 1xen:/virtual/vm_base# cp -pf /virtual/images/vm_base-swap.img\/virtual/images/vm01-swap.imgNow we create a configuration file for this new domain. Xen is located in/etc/xen, so that is the place where we will leave the configuration files,because the Xen software automatically scans this directory for the match-ing file. We use /etc/xen/myfirstdomain.sxp as the name for our firstdomain.Here is a copy of the domain that we created:name=”myfirstdomain”kernel=”/boot/vmlinuz-2.6.12.6-xenU”root=”/dev/hda1”memory=64disk=[‘file:/virtual/images/vm01.img,hda1,w’,’file:/virtual/images\/vm01-swap.img,hda2,w’]# networkvif=[ ‘’ ]dhcp=”off”ip=”10.0.0.50”netmask=”255.0.0.0”gateway=”10.0.0.254”hostname=”myfirstdomain.yourdomain.org”extra=”3”The ip addresses should match ranges within your organization’s network.It is simply a case of sorting the networking out more then anything else.We set DHCP to be off in our instance, but if your network requires DHCPto be on as a means of dealing with addresses it’s simply a matter of chang-ing “off” to “on.”The root value is the value that we set earlier in the /etc/fstab file. Themappings between the swap and root values that we set earlier in the/etc/fstab file are also evident in the specification of the disk value.Note that the memory value is the amount of RAM in megabytes that youare going to give to this domain. In this case, our domain is going toreceive 64 MB of RAM to work with.Now to start the machine, you need to be logged in as root.Once in as root, you have access to the Xen software. If you type in xmhelp, you will get a listing of the available commands and how to use them.To create a domain we will execute the following command:xen:# xm create -c myfirstdomain.sxpThere is no need to specify the exact path to the myfirstdomain file, as theXen software automatically looks in /etc/xen for a file matching the config-uration file you are using. If you placed the configuration file elsewhere,simply insert the complete path to the file.The -c flag is used to ask for a console for the domain you have justlaunched. If all goes ok, you should see the machine booting up and even-tually you will get to the login prompt. If you get an error saying thedomain failed to balloon, it is an error associated with allocating too littlememory to the virtual machine. You have not allocated enough RAM toallow domU to boot successfully. You will have to use the xm destroymyfirstdomain to stop the domain, then edit the configuration file to allo-cate more memory to your domain, and use the create command to launchthe domain.; LOGIN: FEBRUARY 2007 XEN INSTALLATION AND CONFIGURATION 41Log in with the default username and password that you specified in theconfiguration of the base system. It is a good idea now to change thedefault password. This is a major security issue, as each domain is createdfrom the same base system and thus has the same username and pass-words!With your domain you should be able to ping the master Xen server, otherxen domains floated, and other hosts on the same network as yours. Itshould also be possible to ssh into the domain.When you are finished with your domain and wish to exit it, you can do ashutdown as normal, which will send you back to the original Xen domainfrom where you came, or if you wish to leave it running and wish to returnto Xen, simply hold down CTRL + ]. This will take you back to Xen. If yourun xm list you should see your domains that are successfully floated, in-cluding information such as how much memory they are allocated andtheir domain name. To get a console to one of them simply run xm consolemyfirstdomain where myfirstdomain is the name of the domain we specifiedwithin the configuration file and is the name that appears in the list ofdomains we see when xm list is run.If you wish to create more domains it is simply a matter of copying thebase image:xen:# cp -pf /virtual/images/vm_base.img /virtual/images/vm0X.imgxen:# cp -pf /virtual/images/vm_base-swap.img /virtual/images\/vm0X-swap.imgThe vm0X just needs to be changed to a new unique number or name.A corresponding config file needs to be created in /etc/xen, which refer-ences the newly created image file in its disk= parameter.If you wish to have your domains started automatically at startup, a linkmust be created in the auto folder that Xen scans as the system boots. Thiscan be achieved by doing this:xen:# ln -s /etc/xen/myfirstdomain.sxp /etc/xen/autoRestart the machine and see if the domains come up successfully.Extra ConfigurationThe final configuration that must be done in order to create more thanthree domains may need to be performed now.Each virtual image and its swap area run on a loop each. The default num-ber of loops is 7. If you attempt to float a fourth or fifth domain you willget this error:“Error: Device 769 (vbd) could not be connected. Backend device not found.”This means that we can only create at most three domains with this setup(as each requires two loops to run). So we need to do some editing to vitalfiles. Again, ensure a backup has been made in case things go wrong.We need to edit the modules configuration file /etc/modules.conf and addthese options anywhere in the file:options loop max_loop=64rmmod loopmodprobe loopOnce done, if you are running devfs, the new loops should have been auto-matically created.42 ; LOG I N : VO L . 3 2 , NO . 1If you still only see seven values for loop, you need to edit /dev/MAKEDEVand recompile it to make the changes take place. This is a very big file; youneed to scroll down until you see the following:loop)for part in 0 1 2 3 4 5 6 7domakedev loop$part b 7 $part $diskdone;;This needs to be changed to:loop)for part in `seq 0 63`domakedev loop$part b 7 $part $diskdone;;Then recompile by running makedev loop.Verify in /dev that there are now 64 loops created, which is enough for 32machines to be created. If you need more, change the 63 to a number youdesire.When this is done, restart the machine and everything should be workingfine.RESOURCES AND LINKS[1] J. Crowcroft et al., “The Inevitability of Xen,” ;login:, 30, no. 4 (2005):10–13. Available at http://www.usenix.org/publications/login/2005-08/pdfs/crowcroft.pdf.[2] Xen User’s Manual. Available at http://www.cl.cam.ac.uk/Research/SRG/netos/xen/readmes/user/.[3] Filesystem Hierarchy Standard Group, R. Russell et al., eds., File SystemHierarchy Standard, 2004. Available at http://www.pathname.com/fhs/pub/fhs-2.3.pdf.[4] M.K. Johnson, “Red Hat’s New Journaling File System: ext3,” 2001.Available at http://www.redhat.com/support/wpapers/redhat/ext3/#advantages.[5] F. Timme, “The Perfect Xen 3.0 Setup for Debian,” 2006. Available athttp://www.howtoforge.com/perfect_setup_xen3_debian.Acknowledgment: This work is partially supported by the IST ENABLE project under theEuropean Commission’s 6th framework program.; LOGIN: FEBRUARY 2007 XEN INSTALLATION AND CONFIGURATION 43

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