Fedora

Starting with Fedora 16, you can again use Fedora as a Dom0 environment for running virtual machines under xen.

Install xen

It's fairly simple to get Fedora 16 set up as the Dom0. Just install Fedora 16 as usual. After the installation is complete and you have rebooted, install xen:

# yum -y install xen

This will install a large number of package dependencies.

After installing xen, I disable selinux. While it's probably possible to get selinux running in the Dom0, for my testing purposes I've not gone through the hassle.

The next step is to configure Fedora 16 to automatically boot into xen. To do that, you must tell grub2 to use xen as the default:

# grub2-set-default "Xen 4.1"

Configure network bridging

Next, you'll want to enable network bridging. This will allow all of your DomU's to act as if they're on the same network as your Dom0. To do so, edit /etc/sysconfig/network-scripts/ifcfg-virbr0 to contain:

DEVICE=virbr0
TYPE=bridge
ONBOOT=yes
USERCTL=no
BOOTPROTO=dhcp
NM_CONTROLLED=no

And set your primary ethernet interface to use the bridge. For me, the interface is p49p1, so in /etc/sysconfig/network-scripts/ifcfg-p49p1 I have:

DEVICE=p49p1
HWADDR="01:23:45:67:89:ab"  # actual MAC address here
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
BOOTPROTO=none
BRIDGE=virbr0
NM_CONTROLLED=no

After rebooting, I see this output from ifconfig (with various values hidden):

lo      link encap:Local Loopback
        inet addr:127.0.0.1 Mask:255.0.0.0
        inet6 addr: ::1/128 Scope:Host
        UP LOOPBACK RUNNING  MTU:16436  Metric:1
        RX packets:0 errors:0 dropped:0 overruns:0 frame:0
        TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:0
        RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

p49p1   Link encap:Ethernet HWaddr 01:23:45:67:89:ab
        inet6 addr: xxxx::yyy:zzzz:aaaa:bbb/64 Scope:Link
        UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
        RX packets:201 errors:0 dropped:0 overruns:0 frame:0
        TX packets:140 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:1000
        RX bytes:25946 (25.3 KiB)  TX bytes:15704 (15.3 KiB)
        Interrupt:16 Memory:fbce0000-fbd00000

virbr0  Link encap:Ethernet  HWaddr 01:23:45:67:89:ab
        inet addr:10.10.10.2  Bcast:10.10.10.255 Mask:255.255.255.0
        inet6 addr: xxxx::yyy:zzzz:aaaa:bbb/64 Scope:Link
        UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
        RX packets:197 errors:0 dropped:0 overruns:0 frame:0
        TX packets:148 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:0
        RX bytes:21884 (21.3 KiB)  TX bytes:16488 (16.1 KiB)

Build the Fedora 14 appliance

The next step is to build and install a Fedora 14 appliance. For this, I use boxgrinder:

# yum -y install rubygem-boxgrinder-build

Then, create a Fedora 14 appliance configuration file named f14.appl:

name: f14-jeos
summary: Just Enough Operating System based on Fedora 14
os:
  name: fedora
  version: 14
hardware:
  partitions:
    "/":
      size: 2
packages:
  - @core

Run boxgrinder to create the appliance:

# boxgrinder-build f14.appl

boxgrinder will download all of the Fedora packages from the installation mirrors and create a Fedora 14 appliance, ready to install and run.

Install virt-install, which will be used to launch the appliance:

# yum -y install python-virtinst

Launch the appliance

Then run virt-install to launch the appliance:

# virt-install --graphics=none --name=f14 --ram=768 --import \
--disk=build/appliances/i686/fedora/14/f14-jeos/1.0/fedora-plugin/f14-jeos-sda.raw \
--network=bridge:virbr0

I'm installing with --graphics=none, meaning that only a text mode console is supported (no VNC). You should now be connected to the DomU's console. From within the Fedora 14 DomU you will be able to find your appliance's IP address and be able to connect to it via ssh.

Adding additional storage

For my Fedora 14 instance, I wanted to add some additional storage. The easiest way is to create a file in the Dom0 storage, then mount that in the DomU. In the Dom0, create the file to be used as storage. In my case, it's 65GB of space:

# dd if=/dev/zero of=build-f14-repository bs=1G count=65

Still in Dom0, attach the block device to the DomU:

# xm block-attach f14 file:$PWD/build-f14-repository xvdb w

Then switch to the DomU and format the device:

# mkfs.ext4 /dev/xvdb

Finally, mount the device in the Fedora 14 DomU:

# mount /dev/xvdb /mnt
# df -h
Filesystem            Size  Used Avail Use% Mounted on
rootfs                1.9G  604M  1.3G  32% /
udev                  362M  116K  362M   1% /dev
tmpfs                 374M     0  374M   0% /dev/shm
/dev/xvda1            1.9G  604M  1.3G  32% /
udev                  362M  116K  362M   1% /dev
/dev/xvdb              64G  180M   61G   1% /mnt

We've added 61GB of usable space on /mnt.

In order to reliably build our servers, we use kickstart and PXE to give us a simple, repeatable process. During this build process, we need user input to decide exactly which configuration to apply to the system being built.

In order to do that, we run a Python script in the ks.cfg %pre section that uses the snack library to get information from the user. Snack is the (poorly documented) UI toolkit that comes with kickstart/anaconda. Here's how you use it in %pre.

First, you need a %pre section that runs the python interpreter. To do that, start the section with:

%pre --interpreter /usr/bin/python

Next, you need to realize that the kickstart screen you usually see runs on tty3. But the snack UI will show up on tty1. So we use a little routine to switch tty's:

def set_tty(n):
    f = open('/dev/tty%d' % n, 'a')
    os.dup2(f.fileno(), sys.stdin.fileno())
    os.dup2(f.fileno(), sys.stdout.fileno())
    os.dup2(f.fileno(), sys.stderr.fileno())

Next comes the function that actually calls snack to get the user input. Don't worry about the host_config parameter, instead focus on how snack is used. This code build a dialog box with a listbox and an OK button:

def get_user_input(host_config, default=None):
    # get the hostname, from it the other params are computed
    # return the hostname and everything that's derived from it
    # which for now is just the disk layout scheme

    from snack import SnackScreen, Listbox, Grid, Label, Entry, Button, GridForm

    def host_list():
        def hosts():
            return sorted(host_config.keys())

        lb = Listbox(height=len(host_config), returnExit=True)
        for host in hosts():
            lb.append(host, host)
        if default in host_config.keys():
            lb.setCurrent(default)
        return lb

    screen = SnackScreen()
    form = GridForm(screen, 'Select host to configure', 1, 2)
    form.add(host_list(), 0, 0, (0, 0, 0, 1))
    form.add(Button('OK'), 0, 1)

    # now run the form
    result = form.runOnce()
    screen.finish()

    hostname = form.childList[0].current()
    return {'layout': host_config[hostname],
            'hostname': hostname,
            }

Finally, put it all together:

set_tty(1)  # change to tty1, we're called by kickstart with stdout as tty3
user_args = get_user_input(host_config, args.args.get('tb-host'))
set_tty(3)  # restore

If you ever need to find information on snack, it helps to know that it's a wrapper for newt. So you can Google on "snack newt python" to get some useful answers. But be warned that there's not much there and you might have to look through the source code to snack.