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                     How to use Xen to host  application servers which will have application servers on it. Complete Guide for Newbies. 

To have four application servers which will be running on top of a Xen Server. These servers are having Debian Lenny as their operating system. These four servers will be hosting the applications which are critical to the functioning of organization.We are going to demonstrate how this is possible using Xen and Debian Linux distribution and Apache2.

<centre>Objective</centre>

To have four application servers which will be running on top of a Xen Server. These servers are having Debian Lenny as their operating system. These four servers will be hosting the applications which are critical to the functioning of organization.We are going to demonstrate how this is possible using Xen and Debian Linux distribution and Apache2.

System Description: Dell PowerEdge R710,Debian Lenny (amd64) bit, 500GB Hard disk, 8GB RAM, Quad Core Processor.

Key Terms:Virtualization,Reverse Proxy on Apache2,Xen,Running many servers on same page.

<centre>Who should read this document:</centre>


     People/Organizations who want to have different servers for each application which will be dedicated to those applications without buying a separate hardware for these servers.System administrators who are new to Xen on Linux.Any one who is interested to test virtualization for their applications.Some one who wants to understand how things work for their knowledge. Before you go to read this document familiarize yourself with DNS,Gateway and some basic networking stuff. That will be fine. Those who want to go a level higher they should learn use of IPTABLES on Linux.You do not need to worry about IPABLES if you are not clear with that part.

Table of Contents

   * 1 What is virtualization
   * 2 Test Infrastructure and Network
   * 3 Section A Preparing the base system Dom0
   * 4 Step B Installing Xen on Debian Lenny
   * 5 Step C: Creating LVM based DomU Images on Xen for Debian Lenny
   * 6 Step D:Using the created Virtual Machines i.e. Installed Guest Operating System
   * 7 Step E: Knowing the configuration files
   * 8 Step F:Setting up of Networking on Xenserver and DomU's
   * 9 Reverse proxy using Apache2 on Dom0 and DomU's
   * 10 Check points and possible errors
   * 11 References 


What is virtualization

. Virtualization is the name given to a concept where in you run different many different Operating Systems as Guest.
. Operating systems using a layer which is known as hypervisor. In this document we have used Xen.These Guest Operating Systems can be Windows,Linux,Solaris or any other operating sytems of your choice and you can  always have a different operating system as Host OS. There are commercial applications such as vmware which can be used in place of Xen. KVM,OpenVZ are among the other applications which can be used in place of Xen. Which hypervisor you want to use is totally upon your choice.Discussion of Virtualization is beyond the scope of   this document and this small page about Virtualization should give you a basic insight about Virtualization.If you  want to read more about Virtualization you should search internet.You can read books about Virtualization here are a  few to name.
.  .  . Amazon Link
. http://www.amazon.com/Running-Xen-Hands-Guide-Virtualization/dp/0132349663
. Running Xen: A Hands-On Guide to the Art of Virtualization  by Jeanna N. Matthews; Eli M. Dow; Todd Deshane; Wenjin Hu; Jeremy Bongio; Patrick F.  Wilbur; Brendan Johnsonby Jeanna N. Matthews; Eli M. Dow; Todd Deshane; Wenjin Hu; Jeremy Bongio; Patrick F.   Wilbur;Brendan Johnson
.  . Another is : Xen Virtualization
. http://www.amazon.com/Xen-Virtualization-practical-supporting-hypervisor/dp/1847192483   By:PrabhakarChaganti
. If you are already aware of basics of Virtualization then you may skip this section and move ahead to next section.
.

Test Infrastructure and Network

. We are going to install Debian on Dell PowerEdge R710 Server. We have a network whose configuration is as follows

. Dom0 with IP 192.168.0.100 (This will be our Xen Server) DomU1 with IP 192.168.0.11 DomU2 with IP 192.18.0.12 DomU3 with IP 192.168.0.13 DomU4 with IP 192.168.0.14

 . Subnet for all of the above is 255.255.255.0
  . Gateway to network is 192.168.0.1

Section A: Preparing the base system Dom0

. 1)   Get a copy of Debian amd64 version. This can be obtained from here
. http://cdimage.debian.org/debian-cd/5.0.4/amd64/iso-cd/  download   debian-504-amd64-CD-1.iso
. 2)A firmware upgrade was needed at the time of writing this guide to make LAN card work.
. Firmware update which made R710 lan card work
. http://packages.debian.org/lenny/all/firmware-bnx2/download http://lists.us.dell.com/pipermail/linux-poweredge/2009-April/039119.html
. 3) Install Debian Lenny (amd64) on Dell Poweredge R710.
. Choose  10GB partition to install.We choose 10GB to install base system on it. Choose 30GB for swap space.These specifications are not strictly necessary you can have your own choices depending on your use.
. Rest of the 460GB leave empty creating 4 different partitions. Size of partitions remaining is 160,100,100,100 GB partitions. You can leave them as it is or can create a LVM group which consist of all the  above partition in same volume group.After the base system Debian Lenny has been installed and you have upgraded  firmware and booted the base system,you should update sources.list file.In case you run through some errors in your netwok or LAN before proceeding to the  next section go through the errors section.
. Do following
 xenserver#cat >> /etc/apt/sources.list
 deb http://ftp.us.debian.org/debian/ lenny main
 deb-src http://ftp.us.debian.org/debian/ lenny main
 deb http://security.debian.org/ lenny/updates main contrib
 deb-src http://security.debian.org/ lenny/updates main contrib
 deb http://ftp.us.debian.org/debian/ lenny non-free
 deb http://ftp.us.debian.org/debian/ lenny contrib
. and press enter.Your /etc/apt/sources.list should look like this

  deb http://ftp.us.debian.org/debian/ lenny main
  deb-src http://ftp.us.debian.org/debian/ lenny main
  deb http://security.debian.org/ lenny/updates main contrib
  deb-src http://security.debian.org/ lenny/updates main contrib
  deb http://ftp.us.debian.org/debian/ lenny non-free
  deb http://ftp.us.debian.org/debian/ lenny contrib
. Note: After you have installed DomU's the above mirrors can also be used onDomU's
. 4) Update your network settings i.e. IP, gateway,DNS,netmask etc.
. vi /etc/network/interfaces
  1. The loopback network interface auto lo iface lo inet loopback
  2. The primary network interface
 #allow-hotplug eth0
  1. iface eth0 inet dhcp

auto eth0 iface eth0 inet static address 192.168.0.100 netmask 255.255.255.0 network 192.168.0.0 broadcast 192.168.0.255 gateway 192.168.0.1

. Then restart your network:

/etc/init.d/networking restart

Then edit /etc/hosts.

. 
vi /etc/hosts
 127.0.0.1       localhost.localdomain           localhost
 192.168.0.100  xenserver.lan.ernet.in           xenserver
. Modify the settings for your network replace the domain names with what ever is applicable for you.
. Edit /etc/resolv.conf and add some nameservers:

vi /etc/resolv.conf

nameserver 192.168.1.24 nameserver 192.168.1.25

where 192.168.1.24 and 192.168.1.25 are DNS servers.

They should contain entries which are relevant for you.

. 5) apt-get update
. . 6)Do apt-get install ssh vim-full

Step B Installing Xen on Debian Lenny

. There are two ways to install Xen.
. a) Compile Xen from sources.This one you can read here
 . ["http://mightydreams.blogspot.com/2010/04/compiling-xen-from-sources-on-debian.html"]
 . b) Install a precompiled binary for your architecture.
 .

We have chosen the second approach.

Step 1) Installing Xen


apt-get install xen-hypervisor-3.2-1-amd64 xen-linux-system-2.6.26-2-xen-amd64

xen-utils-3.2-1 xenstore-utils    xenwatch     xen-shell    xen-tools

To check what these packages contain you can check

. http://packages.debian.org/lenny/xen-hypervisor-3.2-1-amd64
. http://packages.debian.org/lenny/xen-linux-system-2.6.26-2-xen-amd64
. http://packages.debian.org/lenny/xen-utils-3.2-1
. http://packages.debian.org/lenny/xenstore-utils
. http://packages.debian.org/lenny/xenwatch
. http://packages.debian.org/lenny/xen-shell
. http://packages.debian.org/lenny/xen-tools
. While installing above note the messages some of them download the xen-utils package so you need not to re
. install them again. Before you proceed to check that which package is installed you can do so by  dpkg -s <package name>

Step2) open /etc/xen/xend-config.sxp

vi /etc/xen/xend-config.sxp

. We have used a bridged network setup if you want to use NAT or routed setup based on IPTABLES then there will be  a different setup.
. Step 3) uncomment the line
 (network-script network-bridge)
. and comment out the line

{{{ (network-script network-dummy) }}}

. Also make sure that the line (vif-script vif-bridge) is enabled:
. The above file should appear some thing like this
{{{

[...] (network-script network-bridge) [...]

  1. (network-script network-dummy)
[...]

(vif-script vif-bridge)

[...]
}}}
 . Then reboot the
 . system: reboot
 . and check
 . check  uname -r and your new Xen kernel should show up:
 . {{{

xenserver:~# uname -r 2.6.26-2-xen-amd64

     xenserver:~# }}}
  . you can do an xm info an output like some thing as follows should come up

{{{

 xenserver:~# xm info
   host                      :  xenserver
   release                   : 2.6.26-2-xen-amd64
   version                   :  #1 SMP Wed Mar 10 00:29:48 UTC 2010
   machine                   :  x86_64
   nr_cpus                   : 8
   nr_nodes                  : 1
   cores_per_socket          : 4
   threads_per_core          : 2
   cpu_mhz                   : 2261
   hw_caps                   :bfebfbff:28100800:00000000:00000140:009ce3bd:00000000:00000001
   total_memory              : 8182
   free_memory               : 6
   node_to_cpu               : node0:0-7
   xen_major                 : 3
   xen_minor                 : 2
   xen_extra                 : -1
   xen_caps                  : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32  hvm-3.0-x86_32p hvm-3.0-x86_64
   xen_scheduler             : credit
   xen_pagesize              : 4096
   platform_params           : virt_start=0xffff800000000000
   xen_changeset             : unavailable
   cc_compiler               : gcc version 4.3.1 (Debian 4.3.1-2)
   cc_compile_by             : waldi
   cc_compile_domain         : debian.org
   cc_compile_date           : Sat Jun 28 09:32:18 UTC 2008
   xend_config_format        : 4

}}}

. = Step C: Creating LVM based DomU Images on Xen for Debian Lenny =
. Before we go on to proceed with LVM based images we will check for the LVM setup.You can skip this section if you have already set a LVM based setup and go to Creation ofLVM based images.
. Check your hard disk structure

{{{

           xenserver#fdisk      -l
       Device Boot      Start         End      Blocks              Id      System
       /dev/sda1               1        1216     9767488+          83      Linux
       /dev/sda2            1217       60801   478616512+           5        Extended
       /dev/sda5            1217        4863    29294496           82      Linux swap / Solaris
       /dev/sda6            4864       17021    97659103+          83      Linux
       /dev/sda7           17022       29179    97659103+          83      Linux
       /dev/sda8           29180       41337    97659103+          83      Linux
       /dev/sda9           41338       60801   156344548+          83      Linux

}}}

. First you need to  update some packages.
{{{
apt-get install lvm2
}}}
. The above system configuration has 4 hard disks partitions
 . {{{
               /dev/sda6
               /dev/sda7
               /dev/sda8
               /dev/sda9

}}} {{{

               xenserver#pvcreate /dev/sda6 /dev/sda7 /dev/sda8 /dev/sda9
               Physical volume "/dev/sda6" successfully created
               Physical volume "/dev/sda7" successfully created
               Physical volume "/dev/sda8" successfully created
               Physical volume "/dev/sda9" successfully created

}}}

. Check point if you did not made 4 partitions while installing you need to do some home work.
. You can run pvdisplay to learn about the current state of your physical volumes.
. {{{
                 xenserver:~# pvdisplay
                               --- Physical volume ---
                 PV Name               /dev/sda6
                 VG Name
                 PV Size               93.13 GB / not usable 2.22 MB
                 Allocatable           yes
                 PE Size (KByte)       4096
                 Total PE              23842
                 Free PE               128
                 Allocated PE          23714
                 PV UUID               l8lIGR-246n-js5p-bb6y-w2zI-XCdy-r6H2CY
                                 --- Physical volume ---
                 PV Name               /dev/sda7
                 VG Name
                 PV Size               93.13 GB / not usable 2.22 MB
                 Allocatable           yes (but full)
                 PE Size (KByte)       4096
                 Total PE              23842
                 Free PE               0
                 Allocated PE          23842
                 PV UUID               m6eqKv-ud3V-TJP2-p9E3-k4FK-3SxM-ZSzKrz
                                 --- Physical volume ---
                 PV Name               /dev/sda8
                 VG Name
                 PV Size               93.13 GB / not usable 2.22 MB
                 Allocatable           yes (but full)
                 PE Size (KByte)       4096
                 Total PE              23842
                 Free PE               0
                 Allocated PE          23842
                 PV UUID               DGGrt0-Bxhn-JmYc-lq2O-acw8-DAt3-KaPC7P
                                 --- Physical volume ---
                 PV Name               /dev/sda9
                 VG Name
                 PV Size               149.10 GB / not usable 228.50 KB
                 Allocatable           yes (but full)
                 PE Size (KByte)       4096
                 Total PE              38170
                 Free PE               0
                 Allocated PE          38170
                 PV UUID               6ushwj-9Rse-cGGu-RXDk-2vTe-uSbw-6N5R42

}}}

 . Now we will create our volume group virtualization and add /dev/sda6 - /dev/sda7-    /dev/sda8  /dev/sda9

{{{

   xenserver:~# vgcreate virtualization /dev/sda6 /dev/sda7 /dev/sda8 /dev/sda9
         Volume group "virtualization" successfully created
         xenserver:~# vgscan
         Reading all physical volumes.  This may take a while...
         Found volume group "virtualization" using metadata type lvm2

}}}

.        You do not need to create LVM here. LVM will be created by the xen-create-image 
 . it will create a swap and root partition which will be an LVM image.

Creation of LVM based images

. {{{
   vi /etc/xen-tools/xen-tools.conf
               [........]
                lvm = virtualization
               [.......]
               [...]
               dist   = lenny     # Default distribution to install.
               [...]
               gateway   = 192.168.0.100
                #(your gateway will be same as your Dom0 for DomU's)
               netmask   = 255.255.255.0
               broadcast = 192.168.0.255
               [.........]
               size   = 150Gb      # Disk image size.
               memory = 2048Mb    # Memory size
               swap   = 4096Mb    # Swap size
               # noswap = 1      # Don't use swap at all for the new system.
               fs     = ext3     # use the EXT3 filesystem for the disk image.
               dist   = lenny    # Default distribution to install.
               image  = sparse   # Specify sparse vs. full disk images.
                       [........]
               # Uncomment the following line if you wish to interactively setup
               # a new root password for images.
               #
                passwd = 1
                [..........]
               mirror = http://ftp2.de.debian.org/debian/
                       [...........]
               serial_device = hvc0
               [...]
               disk_device = xvda #default

}}}

. Here mirror is pointing to a debian repository which is used for installation.If you use a  loop mounted CDROM then you can replace it with the location of mounted directory. I have  mentioned both the methods below you can choose any.
Do not forget to chage serial_device = hvc0 and disk_device =xvda as shown above.
 . What ever you write above will be replaced by anything that you will specify on command     line.
  .  Most  Imp:   Specify IP of gateway as the IP of your Dom0
We do  not need to make or compile a DomU kernel.We can use default debian kernels on   the installation media.  Default  installation OS type above in file was etch where as we are using lenny so replace it by that. Size above 160GB can vary  depending upon availability of  hard disk space you have.
 . The dir line specifies where the virtual machine images will be stored. '
dist specifies the distribution to be installed in the virtual machines (Debian Lenny) (there's a comment in the file
 . that explains what distributions are currently supported).
The passwd = 1 line makes that you can specify a root password when you create a new    guest domain. Make sure that you  uncomment the lvm line and fill in the name of your     volume group (virtualization is the volumegroup here). At the same  time make sure that the dir line is commented out.
 . It is very important that you add the line serial_device = hvc0 (not xvc0 or tty1)otherwise   the console of your
 . virtual machines might not work properly.
Method 1 (If you do not want to go for internet one you may skip it and go to Method2) Using internet based repository
{{{
       xenserver#xen-create-image --hostname=domu1 --size=160GB --swap=4096Mb --ip=192.168.0.11 --memory=2048 --arch=amd64 --role=udev --force --install-method=debootstrap

}}}

 . What ever you specify on the command line here will over write the
 . /etc/xen-tools/xen-tools.conf
You may specify a different internet mirror above. Note –role-udev above do not miss it. Method 2 (You can skip it and go to section image ) Using CD as your local repository
 . http://cdimage.debian.org/debian-cd/5.0.4/amd64/iso-cd/
 . If you have the CD image burnt on to a CD  do the following insert Debian CD in CDROM

{{{ dd if=/dev/cdrom of=/home/xenserver/debian.iso bs=1024}}}

. After a few minutes you will get a message saying that the CD has been copied.Same
. command will work for DVD also.You  need to check your DVD rom in your /dev  .
 . Now make a directory named local in your home directory i.e.(the name local is not strictly required  you may choose
 . what ever you wish it)
{{{
       mkdir /home/xenserver/local
       mount   -o loop -t iso9660 /home/xenserver/debian.iso   /home/xenserver/local
  }}}
 . You can navigate to directory local and see  the contents of CDROM in local directory.
{{{
      xen-create-image --hostname=domu1 --size=150GB --swap=4096Mb --ip=192.168.0.11 --memory=2048 --arch=amd64
          --role=udev --force   --install-method=debootstrap --install-source=/home/xenserver/local

}}}

 . Image
  . The xen-create-image command will now create the domu1 virtual machine for us. This will take a few minutes. The output should be similar to this one:
{{{xen-create-image --hostname=domu1 --size=150GB --swap=4096Mb --ip=192.168.0.11 --memory=2048 --arch=amd64 --role=udev --force  --install-method=debootstrap --install-source=/home/xenserver/local }}}

{{{ General Information


Hostname  : domu1 Distribution  : lenny Partitions  : swap4096Mb (swap)

                 /150Gb   (ext3)

Image type  : sparse Memory size  : 2096Mb Kernel path  : /boot/vmlinuz-2.6.26-2-xen-amd64 Initrd path  : /boot/initrd.img-2.6.26-2-xen-amd64 Networking Information


IP Address 1  : 192.168.0.11 Netmask  : 255.255.255.0 Broadcast  : 192.168.0.255 Gateway  : 192.168.0.100 Creating partition image: Done Creating swap on Done Creating partition image: Done Creating ext3 filesystem on Done Installation method: debootstrap Done Running hooks Done Role: udev

       File: /etc/xen-tools/role.d/udev

Role script completed. Creating Xen configuration file Done Setting up root password Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully All done Logfile produced at:

        /var/log/xen-tools/domu1.log

xenserver:~# }}}

. There should now be a domu1 configuration file - /etc/xen/domu1.cfg. If you have a dual-core or quad-core CPU and want the virtual machine to use all CPU    cores, please add the line vcpus =
 . '2' or vcpus = '4' to the configuration file If you are wondering that we have not created Logical Volumes then just notice the outputs    of above commands which have
 . created separate root and swap partitions.You can confirm it   by command lvdisplay
{{{
            --- Logical volume ---
         LV Name                /dev/virtualization/domu1-swap
         VG Name                virtualization
         LV UUID                jN8OjS-kngH-v5Na-NYBJ-Oy3H-QQyL-M0z8zk
         LV Write Access        read/write
         LV Status              available
         # open                 1
         LV Size                4.00 GB
         Current LE             1024
         Segments               1
         Allocation             inherit
         Read ahead sectors     auto
         - currently set to     256
         Block device           254:0
                         --- Logical volume ---
         LV Name                /dev/virtualization/domu1-disk
         VG Name                virtualization
         LV UUID                LEx5WN-Dnkc-pjqJ-WyIJ-6W7n-jTHo-8dDPre
         LV Write Access        read/write
         LV Status              available
         # open                 1
         LV Size                150.00 GB
         Current LE             38400
         Segments               2
         Allocation             inherit
         Read ahead sectors     auto
         - currently set to     256
         Block device           254:1

}}}

 . Now you have one DomU1 up same process you can apply to create other instances.
=Step D:Using the created Virtual Machines i.e. Installed  Guest Operating Systems=
 . To be able to use the newly created guest i.e. Virtual machine whose name is domu
  . xm create /etc/xen/domu1.cfg
 and then run
xenserver#xm console domu1 The output should look like this:
{{{
       xenserver:~#xm list
       Name                                          ID  Mem   VCPUs     State  Time(s)
       Domain-0                                     0   8192    8                r----- 398.2
       domu1                                         1   2048    4                -b---- 2.8}}}
To shut down domu1, do this: xm shutdown domu1
 . For more commands you can use xm help.
 . Now suppose you have 4 different DomU's installed
 {{{
    xenserver:~# xm list
               Name         ID   Mem VCPUs      State   Time(s)
               Domain-0      0   875     8     r-----    361.4
               domu1         7  3072     4     -b----     32.9
               domu2         9  1024     4     -b----     32.0
               domu3         8  1024     4     -b----     34.0
               domu4         6  2048     4     -b----     32.2
}}}
If you want domu1 to start automatically at the next boot of the system, then do this: ln -s /etc/xen/domu1.cfg /etc/xen/auto

Step E: Knowing the configuration files

. A sample configuration file for domU looks like this

{{{

  1. Configuration file for the Xen instance vignettes, created
  2. by xen-tools 3.9 on Fri Apr 9 09:01:18 2010.
  3. Kernel + memory size

kernel = '/boot/vmlinuz-2.6.26-2-xen-amd64' ramdisk = '/boot/initrd.img-2.6.26-2-xen-amd64' memory = '2048'

  1. Disk device(s).

root = '/dev/xvda2 ro' disk = [

                 'phy:/dev/virtualization/domu1-swap,xvda1,w',
                 'phy:/dev/virtualization/domu1-disk,xvda2,w',
             ]
  1. Hostname

name = 'domu1'

  1. Networking

vif = [ 'ip=192.168.0.11,mac=00:16:3E:66:03:E1' ]

  1. Behaviour

on_poweroff = 'destroy' on_reboot = 'restart' on_crash = 'restart' vcpus = '4' ~

       If you have a dual-core or quad-core CPU and want the virtual machine to use all CPU    cores, please add the line vcpus = '2' or vcpus = '4' to the configuration file

}}}

. A list of virtual machines that were created with the xen-create-image command can be found by  xen-list-images

Step F:Setting up of Networking on Xenserver and DomU's

. When you install 4 DomU's with Ips on them it is as good as bringing 4 hosts on network.
 . In our case
  . We have following

{{{

         IP of Domu1:192.168.0.11                    Lets call it as a
         IP of Domu2:192.168.0.12                    Lets call it as b
         IP of Domu3:192.168.0.13                    Lets Call it as c
         IP of Domu4:192.168.0.14                    Lets Call it as d
               Ip of Dom0 :192.168.0.100  (Will behave as Gateway for DomU's)
               Lets call Dom0 as A
        Gateway for the network is 192.168.0.1
                       We will call it as G.

}}}

. So our network structure is some thing like that
 . DomU1            DomU2                DomU3               DomU4
192.168.0.11    192.168.0.12        192.168.0.13          192.168.0.14
 . a                   b                  c                      d
  . Dom0
  . 192.168.0.100                                      [XenServer]
  . IP Forwarding Enabled on Dom0
   . Figure1
  . We have to enable Ip forwarding.For the hosts to appear on network.
   . xenserver:~# echo 1 > /proc/sys/net/ipv4/ip_forward
  . also in /etc/sysctl.conf uncomment net.ipv4.ip_forward=1
 . Now your hosts should be appearing on network.Do not have any other host have the same IP as DomU's otherwise IP
collision will occur.So suppose you have a machine C  the structure is now like this
{{{
     A(Xen Server)
               192.168.0.100
                     a
               192.168.0.11
               (DomU1)
                     b
               192.168.0.12
               (DomU2)
                    c
               192.168.0.13                                                     G (Gateway)
               (DomU3)                                                          192.168.0.1
                    d
               192.168.0.14
                 (DomU4)
                  C
               192.168.0.41
           Some machine on network

}}}

 . Note : Inspite of the fact that that a,b,c,d are DomU's they are available on network as
  . actual machines. This is what you should have achieved by now.
   . Figure 2
 When you login to any of the DomU you should set the gateway as Dom0.
 . DNS can be pointing to actual DNS of your network since with IP Forwarding enabled the DomU's will be able to access the  proxy and rest of the network.
  . Now since you have enabled IP Forwarding so you should be able to ping from any other machine to you DomU's to  make sure the things are working as expected you ping from C to a you should get a reply.
We are at  C and we can check
 . ping   192.168.0.11  some out put like following will come

{{{

          PING 192.168.0.11 (192.168.0.11) 56(84) bytes of data.
       64 bytes from 192.168.0.11: icmp_seq=1 ttl=64 time=3.32 ms
       64 bytes from 192.168.0.11: icmp_seq=2 ttl=64 time=0.045 ms
       ^C
       --- 192.168.0.11 ping statistics ---
       2 packets transmitted, 2 received, 0% packet loss, time 1008ms
       rtt min/avg/max/mdev = 0.045/1.685/3.326/1.641 ms

}}}

. So that means that our host that is DomU1 is accessible from C and in turn from the rest of     the network. Similary  ssh and other things should work. If it does not happen then check if you have        enabled ip forwarding. You do NOT need to enable IP FORWARDING on DomU's 
 . IP Forwarding should be enabled only on Dom0.
Xen has a default network managment which will create a bridge whose name will be  same as your interface name.If you want to read more about it look at the following  Wiki page.
http://wiki.xensource.com/xenwiki/XenNetworking

{{{

       xenserver:~# brctl show
       bridge name     bridge id               STP enabled     interfaces
       eth2            8000.0026b9824238       no              peth2
                                                               vif6.0
                                                               vif7.0
                                                               vif8.0
                                                               vif9.0

}}}

. We have default ethernet as eth2 ,it can be different on your network settings.
 . If  some thing on your LAN card does not work check  /etc/xen/xend-config.sxp
Here is a sample note  netdev=eth2 in below output These are my entries
 . vi /etc/xen/xend-config.sxp

{{{

            # Your default ethernet device is used as the outgoing interface, by default.
            # To use a different one (e.g. eth1) use
                #
                # (network-script 'network-bridge netdev=eth1')
                   (network-script 'network-bridge netdev=eth2')

}}}

. If    (network-script 'network-bridge netdev=eth2') is commented or missing some thing
then uncomment and put the name netdev=<name of your eth interfac> as above.
 . and there is one  more entry you should check that is (vif-script vif-bridge) vi /etc/xen/xend-config.sxp

{{{

               # If you are using only one bridge, the vif-bridge script will discover that,
               # so there is no need to specify it explicitly.
               #
               (vif-script vif-bridge)
               If  (vif-script vif-bridge) is commented uncomment it.

}}}

. So by now your all 4 hosts must be up and running on your network and you should be able to access these DomU's  from  any other machine on your network.

Reverse proxy using Apache2 on Dom0 and DomU's

Section 1: Setting up of Apache2 on Dom0 and DomU
. Before you proceed to this section  and do as mentioned make sure that your guest Operating Systems or DomU's are able  to access your LAN.
 . We are doing this with Dom0 which has a public IP or say internet access without  any proxy.
. To make apt-get work with proxy
 . mention following lines in .bashrc on individual DomU's
. {{{
       export http_proxy=http://IP of your proxy:Port on which proxy runs
       export ftp_proxy=http://IP of your proxy:Port on which proxy runs
        }}}
 . Now  its time for work aptitude install mysql-server mysql-client You will be asked to provide a password for the MySQL root user - this  password is valid for the user root@localhost
 New password for the MySQL "root" user:    <-- yourrootsqlpassword Repeat password for the MySQL "root" user: <-- yourrootsqlpassword
  . Apache2 is available as a Debian package, therefore we can install it like this:
 {{{aptitude install apache2}}} Now direct your browser to http://192.168.0.100, and you should see the Apache2 placeholder page (It works!): {{{aptitude install php5 libapache2-mod-php5}}} Repeat all these steps  for respective DomU's  so that you install apache2 on all DomU's and Dom0.
  . From your LAN you can check
http://IP of Dom0/
 . I created Dom0 on IP 192.168.0.100
Now open a browser and go to following url
http://192.168.0.100
 . you should get a message
 It Works!! and same for all DomU's and Dom0 check
in browser and go to following urls
 . http://IP of DomU1/             http://192.168.0.11/ http://IP of DomU2/             http://192.168.0.12/ http://IP of DomU3/             http://192.168.0.13/ http://IP of DomU4/             http://192.168.0.14/ you should get a message It Works!!  Replace the IP of DomU with what ever you have in your network. =Section 2:  Setting up for access to internet for application servers (Reverse Proxy)=
  . Now go to Dom0 and open your command prompt

{{{

       a2enmod proxy
       a2enmod proxy_http

}}}

. Now reload Apache: /etc/init.d/apache2 force-reload vi /etc/apache2/apache2.conf

{{{

       [...]
       # Include the virtual host configurations:
       Include /etc/apache2/sites-enabled/
       <Directory /var/www/>
           Options +Includes -Indexes
           AllowOverride None
           AllowOverride Indexes AuthConfig Limit FileInfo
           Order allow,deny
           Allow from all
           <Files ~ "^\.ht">
           Deny from all
           </Files>
       </Directory>
       [...]

}}}

. Right before this section, we add the following lines:

{{{

       NameVirtualHost *
       <VirtualHost *>
         ServerName xenserver.lan.in
               DocumentRoot /var/www
         ProxyRequests Off
              <Proxy *>
        Order deny,allow
              Allow from all
        </Proxy>
            ProxyPass /domu1  http://192.168.0.11/
            ProxyPass /domu2  http://192.168.0.12/
            ProxyPass /domu3  http://192.168.0.13/
            ProxyPass /domu4  http://192.168.0.14/
            ProxyPassReverse /domu1  http://192.168.0.11/
            ProxyPassReverse /domu2  http://192.168.0.12/
            ProxyPassReverse /domu3  http://192.168.0.13/
            ProxyPassReverse /domu4  http://192.168.0.14/
       </VirtualHost>

}}}

. Now you should be able to access DomUs from Internet as well.If you have a public IP on       Dom0.
 . Suppose you have a domain name www.myxenserver.com So you should be able to access
  . http://www.myxenserver.com/domu1
  . http://www.myxenserver.com/domu2
  . http://www.myxenserver.com/domu3
  . http://www.myxenserver.com/domu4
. . .         Then restart Apache: /etc/init.d/apache2 restart

Check points and possible errors

. The first time I booted the system Lan Card driver was not recognized. I got a error firmware: requesting bnx2-09-4.0.5.fw  it is available on following page
 . http://packages.debian.org/lenny/all/firmware-bnx2/download Reboot then. If you run into any sort of problem on network check if your IP is conflicting with any other Ip on network ,ping the gateway,DNS.
 . . Open a terminal and type
 . ping  <IP of Gateway>
 . ping  <IP of DNS>
  . May be network card does not seems to be up then check  as root
 . . ifconfig -a
 . the lan card which you configured might be not the default eth0 it might be
eth1 or eth2 or eth3 like that. You need to check the LAN cable in your rack.
 . . Try different permutations to see that when you ping some machine on your network you
  . get a response.I was running out of space when I was creating DomU's. It may happen with you also.
 . Before you choose to install separate OS as DomU's calculte the amount of RAM they will               be consuming and swap space that you will be allocating them. When Dom's are running they require a certain amount of memory to be allocated to them  suppose you have 8 GB RAM and you decide to give 2GB Ram to each of the virtual host you            create i.e. DomU then you wont be left with memory for Dom0. We planned to give 875 Mb memory to the Dom0 so we were left with  8192-875=7317Mb of Ram which we planned to give to the DomU's.If you notice the output of xm list you can see it

{{{

    xenserver:~# xm list
               Name         ID   Mem VCPUs      State   Time(s)
               Domain-0      0   875     8     r-----    361.4
               domu1         7  3072     4     -b----     32.9
               domu2         9  1024     4     -b----     32.0
               domu3         8  1024     4     -b----     34.0
               domu4         6  2048     4     -b----     32.2

}}}

. Where domu1,domu2,domu3,domu4 are different dom's running.

References

Here are a few references that I went through in this process you may like to check them. If you want to install Xen by compiling a Xenified Debian Kernel then you should read:

* http://mightydreams.blogspot.com/2010/04/compiling-xen-from-sources-on- debian.html
* http://www.intel.com/technology/virtualization/index.htm
* http://wiki.xensource.com/xenwiki/HVM_Compatible_Motherboards
* LVM Based Set up for Xen
 * http://www.howtoforge.com/using-xen-with-lvm-based-vms-instead-of-image-based-vms-      debian-etch
* Networking on Xen
 * http://wiki.xensource.com/xenwiki/XenNetworking
* Reverse Proxy on Apache2
 * http://www.howtoforge.com/apache_reverse_proxy_ispconfig
* Great tutorial about Reverse Proxy of Apache2.
 * http://www.apachetutor.org/admin/reverseproxies
* Apache module mod_proxy
 * http://httpd.apache.org/docs/1.3/mod/mod_proxy.html
* Apache2 Debian DNS Caveats,Apache2 as a proxy server
 * http://httpd.apache.org/docs/1.3/dns-caveats.html
 * http://www.debianhelp.co.uk/virtualhosts.htm
* Setting up IPTABLES Dom0 Debian Xen
 * http://permalink.gmane.org/gmane.linux.hardware.dell.poweredge/38423
* Some improvement in Xen FAQ
 * http://wiki.xensource.com/xenwiki/XenFaq
 * Squid set up http://www.freeos.com/articles/2516/
* Novell article for NAT etc on Xen
 * http://www.novell.com/communities/node/4094/xen-network-bridges-explained-      with-   troubleshooting-notes
* Some good information is here
 * http://lists.xensource.com/archives/html/xen-users/2006-02/msg00030.html
 * http://www.howtoforge.com/perfect_xen_setup_debian_ubuntu_p6
 * http://lists.us.dell.com/pipermail/linux-poweredge/2010-April/041936.html
 * http://wiki.xensource.com/xenwiki/Dom0
 * http://wiki.debian.org/Xen#Dom0.28host.29
 * http://wiki.xensource.com/xenwiki/XenDom0Kernels
 * http://wiki.xensource.com/xenwiki/XenParavirtOps
 * http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/snapshot_command.html
* Remote Snapshot
 * http://librenix.com/?inode=12272
 * http://community.citrix.com/display/ocb/2009/08/31/The+Xen+Cloud+Project
 * http://www.google.co.in/search?hl=en&client=firefox-a&rls=org.mozilla:en-US:official&hs=9o&q=how+to+forge+compile+Dom0+kernel+ubuntu+8.04&start=1 0&sa=N
* Installing Xen On An Ubuntu 7.10 (Gutsy Gibbon) Server From The Ubuntu Repositories - Page 2
 * http://www.howtoforge.com/ubuntu-7.10-server-install-xen-from-ubuntu-repositories-p2
 * http://deshantm.livejournal.com/29959.html?thread=24327
 * http://www.howtoforge.com/installing-xen-3.3-with-kernel-2.6.27-on-ubuntu-8.10-x86_64-p2
 * http://howtoforge.com/ubuntu-8.04-server-install-xen-from-ubuntu-repositories-p2
 * http://www.howtoforge.org/installing-xen-3.3-with-kernel-2.6.27-on-ubuntu-      8.10-   x86_64
 * http://ubuntuforums.org/showthread.php?p=8403877#post8403877
 * http://www.infohit.net/blog/post/deploying-domu-centos-5-with-xen-on-   ubuntu-904-     jaunty-jakalope.html
 * https://help.ubuntu.com/community/Xen
* CentOS DomU on Ubuntu Hardy
 * http://www.howtoforge.com/create-centos5.2-domu-on-ubuntu-hardy-dom0
 * http://www.linuxinstead.com/blog/?p=63#more-63
 * http://www.xen.org/support/ http://wiki.debian.org/Xen
 * http://wiki.xensource.com/xenwiki/XenDom0Kernels/
 * http://ganto.no-ip.org/?cat=20
* Then some more
 * http://librenix.com/?page=Xen
 * http://www.gamelinux.org/?cat=4
* Installing Xen 3.0 upon Debian Unstable, with a custom Kernel
 * http://www.debian-administration.org/articles/320
* Some more help
 * http://cwshep.blogspot.com/2008/12/howto-ubuntu-intrepid-ibex-810-xen-  dom0.html
* Re: Debian testing + xen 3.2.1 + dom0 kernel 2.6.18 + clvm 2.x ->       impossible ?
 * http://lists.debian.org/debian-user/2008/09/msg01745.html
 * http://gnutolinux.wordpress.com/2009/02/17/debian-50lennyreleased/
* Has got mentioned using ATA over ethernet
 * http://www.unix-tutorials.com/search.php?       act=search&term=How+To+Compile+A+Kernel+-+Debian+Etch&page=25 Installing a Kernel and Generating a Menu List
 * http://library.linode.com/advanced/pv-grub-howto
* Fedora Core 8 as DomU
 * http://www.ilande.co.uk/index.php?/categories/2-Blog
* An interesting article related to Xen http://www.debian-administration.org/articles/505
 * http://pdg86.wordpress.com/2009/09/16/howto-compile-2-6-31-kernel-in-slackware-13-0/
* Compile a Dom0 kernel the Debian way http://lists-archives.org/debian-user/3481939-how-to-compile-a-xen-dom0-kernel-the-     debian-way.html
 * http://swik.net/Debian+virtualization
* http://readlist.com/lists/lists.xensource.com/xen-users/4/20551.html
* This is a very good link to study http://librenix.com/?find=Boris%20Derzhavets
 * http://lists.zerezo.com/debian-user/msg3494236.html
* Some tutorials on this page
 * http://www.unix-tutorials.com/search.php?       act=search&term=Debian+Kernel+Compile+Howto+(Kernel+2.6)&page=29 Building custom kernel packages
 * http://wiki.debian.org/HowToRebuildAnOfficialDebianKernelPackage
* They discuss here about closing the 2.6.18 tree
 * http://lists.xensource.com/archives/html/xen-devel/2009-06/msg00241.html
* This is the discussion going on about Virtualization where they have talked relevant things     about patches etc http://lists.xensource.com/archives/html/xen-devel/2009-06/msg00127.html
 * http://en.opensuse.org/Xen_Full_Virtualization_Example
 * http://www.cl.cam.ac.uk/research/srg/netos/xen/documentation.html
 * http://lists.xensource.com/archives/html/xen-users/2006-07/msg00100.html
 * http://wiki.xensource.com/xenwiki/HVM_Compatible_Processors
* Intel http://wiki.xensource.com/xenwiki/IntelVT
* A step by step guide and some relevant discussion
 * http://lists.xensource.com/archives/html/xen-users/2006-05/msg00568.html
* Xen Full Virtualization examples
 * OpenSuse http://en.opensuse.org/Xen_Full_Virtualization_Example
* Remote VNC Solutions http://help.punt.nl/?id=421411&r=1
* Some Faq about Xen http://wiki.xensource.com/xenwiki/XenFaq#head-  31bd14b4c509f5e23c4f82250273ac642364a060 Xen Related entire thing from Cambridge
 * http://www.cl.cam.ac.uk/research/srg/netos/xen/readmes/interface/interface.html
* FaQ's about mercurial
 * http://mercurial.selenic.com/wiki/FAQ#General_Questions
 * Some information regarding Mercurial http://www.cl.cam.ac.uk/research/srg/netos/xen/readmes/hg-cheatsheet.txt
 * http://www.intel.com/technology/virtualization/index.htm
 * http://wiki.xensource.com/xenwiki/HVM_Compatible_Motherboards
* All the commands for virtualization
 * http://centos.org/docs/5/html/5.2/Virtualization/chap-Virtualization-   Managing_guests_with_virsh.html
* Stress testing of Virtualization Server is mentioned here
 * http://centos.org/docs/5/html/5.2/Virtualization/part-Virtualization-Installation_Procedures.html
* Guest and Host OS compatibility list here
 * http://centos.org/docs/5/html/5.2/Virtualization/chap-Virtualization-   Compatibility_of_host_and_guest_combinations.html
* Blktap driver documentation
 * http://lxr.xensource.com/lxr/source/tools/blktap/
 * http://wiki.centos.org/HowTos/Xen/MoveNative2DomU
 * http://wiki.xensource.com/xenwiki/Dom0
 * http://wiki.debian.org/Xen#Dom0.28host.29
 * http://wiki.xensource.com/xenwiki/XenDom0Kernels
 * http://wiki.xensource.com/xenwiki/XenParavirtOps
 * http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/snapshot_command.html
* Remote Snapshot
 * http://librenix.com/?inode=12272
 * http://community.citrix.com/display/ocb/2009/08/31/The+Xen+Cloud+Project
 * http://www.google.co.in/search?hl=en&client=firefox-a&rls=org.mozilla:en-S:official&hs=9o&q=how+to+forge+compile+Dom0+kernel+ubuntu+8.04&start=10&sa=N
* Installing Xen On An Ubuntu 7.10 (Gutsy Gibbon) Server From The Ubuntu Repositories -   Page 2
 * http://www.howtoforge.com/ubuntu-7.10-server-install-xen-from-ubuntu-repositories-p2
 * http://deshantm.livejournal.com/29959.html?thread=24327
 * http://www.howtoforge.com/installing-xen-3.3-with-kernel-2.6.27-on-ubuntu-8.10-x86_64-p2
 * http://howtoforge.com/ubuntu-8.04-server-install-xen-from-ubuntu-repositories-p2
 * http://www.howtoforge.org/installing-xen-3.3-with-kernel-2.6.27-on-ubuntu-8.10-x86_64
 * https://help.ubuntu.com/community/Xen
* CentOS DomU on Ubuntu Hardy
 * http://www.howtoforge.com/create-centos5.2-domu-on-ubuntu-hardy-dom0
 * http://www.xen.org/support/
 * http://wiki.debian.org/Xen
 * http://wiki.xensource.com/xenwiki/XenDom0Kernels/
 * http://ganto.no-ip.org/?cat=20
* Installing Xen 3.0 upon Debian Unstable, with a custom Kernel
 * http://www.debian-administration.org/articles/320
* Installing a Kernel and Generating a Menu List
 * http://library.linode.com/advanced/pv-grub-howto
* Fedora Core 8 as DomU
 * http://www.ilande.co.uk/index.php?/categories/2-Blog
* An interesting article related to Xen
 * http://www.debian-administration.org/articles/505
 * http://pdg86.wordpress.com/2009/09/16/howto-compile-2-6-31-kernel-in-slackware-13-0/
* Compile a Dom0 kernel the Debian way
 * http://lists-archives.org/debian-user/3481939-how-to-compile-a-xen-dom0-kernel-the-     debian-way.html
 * http://mercurial.selenic.com/wiki/FAQ#General_Questions
* Relevant tutorials from IBM
 * http://www.ibm.com/developerworks/linux/
 * http://www.ibm.com/developerworks/linux/library/l-pci-passthrough/index.html?   S_TACT=105AGX03&S_CMP=HP
 * http://www.ibm.com/developerworks/linux/library/l-top-10.html?  S_TACT=105AGX03&S_CMP=HP
* 7 Features to consider before hypervisor installation
 * http://searchservervirtualization.techtarget.com/tip/0,289483,sid94_        gci1375660_mem1,00.ht   ml?asrc=SS_CLA_304683&psrc=CLT_94
* Building a Xen Virtual Guest Filesystem on a Disk Image (Cloning Host   System)
 * http://www.virtuatopia.com/index.php/Building_a_Xen_Virtual_Guest_Fi        lesystem_on_a_Di        sk_Image_(Cloning_Host_System)
* Red Hat System Administration Guide
 * http://web.mit.edu/rhel-doc/4/RH-DOCS/index-en.html
 * http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/security-guide/s1-firewall-         state.html
* Building a Xen Virtual Guest Filesystem on a Disk Image (Cloning Host   System)
 * http://www.virtuatopia.com/index.php/Building_a_Xen_Virtual_Guest_Filesystem_on_a_Disk_Image_(Cloning_Host_System)
* Virtualization Saves Microsoft Customers Nearly a Half-Million Dollars Per      Year
 * http://www.microsoft.com/presspass/press/2009/feb09/02- 09virtualizationsavingspr.mspx
* Overview of Virtualization
 * http://www.virtuatopia.com/index.php/An_Overview_of_Virtualization_Techniques
  . Happy Xenning!!!