Simple Virtual Appliances with Linux and Xen

Everyone is familiar with hardware appliances in one form or another. It could be a wireless access point at home or a DNS server appliance in the data center. Appliances offer a prebuilt software solution (with hardware) that can be deployed rapidly with minimal hassle. When you couple the “appliance” concept with virtualization, you get virtual appliances—a prebuilt software solution, ready to run on your own hardware with minimal work.

In this article, I provide a hands-on introduction to constructing a simple virtual appliance by assembling readily available components. The framework can be used to build a wide range of appliances.

Virtual appliances share many attributes in common with their hardware cousins. In general, both types of appliances have a small footprint, use an embedded or “thin” OS, are single-purpose, provide easy backup and restore, and are Web-managed. Most important, they come ready to rock and roll with minimal configuration. Virtual appliances have the additional benefit of being hosted on your own hardware, so you can host multiple virtual appliances on a single physical host.

Many Linux-based virtual appliances are constructed with an extremely thin OS. This can make installing common software complicated due to dependencies, especially for a beginner. For this example, I decided to use an off-the-shelf free distribution, specifically CentOS, because it uses tools most people are used to. However, we'll cut it to the bone as much as possible.

We are going to build our virtual appliances using the Xen hypervisor, because it's free and comes with most Linux distributions these days. In my examples, I am using CentOS 5.3 for both the host and appliance. The host needs the Virtualization option selected during install, or you can retro-fit an existing Linux system by installing the xen and kernel-xen packages. I chose Xen because it's easy; alternatively, you could use VMware, KVM or any other hypervisor.

You can install CentOS directly from the Internet if you have a good connection, or download it to a local Web or NFS server. In this example, I point to mirror.centos.org for the install sources and to a local NFS server for the kickstart config.

We will use the Webmin package to provide Web-based management of our appliance. Webmin has been around for a long time and will provide our appliance with a lot of functionality, like complete Web-based management and simple backup/restore. I downloaded the webmin-1.480-1 RPM from www.webmin.com for our appliance. Everything else will be provided by standard CentOS packages.

To create a minimal CentOS install for our appliance, we will use a custom kickstart with the --nobase option set. One of the most important concepts of good system management is repeatability—a fully automated kickstart install is repeatable and self-documenting. Our entire OS installation will fit quite comfortably in a 2GB virtual disk and 256MB of memory. We are creating our appliance under /xen, which is a standard location for Xen virtual machines (also known as guests). If you choose another location, make sure either to disable SELinux or adjust your settings. Wherever you put Xen, the disk images need the system_u:object_r:xen_image_t context set.

First, let's create an “appliance-base” guest, which will be used like a template. All the files for this guest will be stored in /xen/appliance-base/. Start by logging in to the Xen host as root and create the virtual disk. Then, grab the Xen vmlinuz and initrd files from the install media:

xenhost$ mkdir -p /xen/appliance-base
xenhost$ cd /xen/appliance-base
xenhost$ dd if=/dev/zero of=appliance-base.img \
            oflag=direct bs=1M seek=2048 count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB) copied, 0.071271 seconds, 14.7 MB/s
xenhost$ cd /xen
xenhost$ wget \
  http://mirror.centos.org/centos/5.3/os/i386/images/xen/initrd.img
xenhost$ wget \
  http://mirror.centos.org/centos/5.3/os/i386/images/xen/vmlinuz

You have just created a 2GB virtual disk for your appliance. Now, create an appliance-base.install.cfg file and a ks.cfg file, as shown in Listings 1 and 2. Be sure to substitute your CentOS URL or a mirror on the Internet. The last three bytes of the MAC address in the .cfg file are made up; just make sure all your Xen guests are unique.