Administering virtual machines with MLN

Watching the Flock

© Diana Kosaric, Fotolia

© Diana Kosaric, Fotolia

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This simple command-line tool helps you keep ahead of your virtual infrastructure.

The simplest virtualization scenarios are very easy to manage. In the real world, though, virtualization does not always simplify system administration. In fact, many of the most powerful uses of virtualization result in additional administrative tasks and challenges. For example, large data centers use virtualization to consolidate several servers onto the same physical hardware, thereby minimizing space, cost, and power requirements.

These virtual machines (VMs) can migrate between servers to achieve higher uptimes and flexible load balancing. However, managing such a complex virtual infrastructure is at least as challenging as administering the equivalent number of separate server systems, and it requires very different skills.

Many high-end commercial virtualization products come with sophisticated VM management features. If you are wondering whether the free software community offers an equivalent application, the open source MLN VM administration tool might be just what you need. MLN, which stands for "Manage Large Networks," lets you build sophisticated, dynamic, virtual infrastructures with the use of freely available virtualization platforms. The versatile MLN currently supports the widely used VMware Server [1] and Xen [2] packages, as well as User-Mode Linux [3].

For Xen, MLN also provides live migration of entire network topologies, which is otherwise available in applications only at the highest end. MLN's simple command-line and config file interfaces serve the needs of complex virtualization scenarios, but it is easy enough for administrators who are new to virtualization.

About MLN

The MLN environment helps you manage large numbers of virtual machines through a simple command-line interface. MLN has three main design goals:

  • The user should be able to design and implement complex VM deployments.
  • The process from design to actual deployment should be as efficient as possible.

The configuration language should be extensible, so users can customize MLN easily for their unique needs.

MLN is designed to work in concert with other VM management tools. For example, if you have a VMware Server, you can use MLN for efficient building and redeploying of complex virtual networks and also use VMware Server Console for starting and stopping individual virtual machines.

Download MLN from SourceForge [4]. The installation process is simple and similar to the procedure for other Linux programs:

# wget
# tar xzfv mln-latest.tar.gz
# cd mln-latest
# ./mln setup

Defining Projects

MLN organizes virtual machines into units called projects. Projects are groups of virtual machines and their associated virtual networks. The simplest project consists of a single virtual machine. A more powerful approach is to group virtual machines that belong together: all virtual machines that are web servers, an entire replica of a small business network, all the test systems for a software test suite.

Projects are defined in MLN configuration files. The configuration file shown in Listing 1 (ldaptest.mln) defines two virtual machines connected by a virtual LAN. Annotations in the file highlight the most important configuration settings.

Listing 1



Listing 1 starts with the global stanza, wherein the name of the project is defined. Next follows a declaration of a virtual machine called ldapclient – a Xen host with 128MB of memory built from the template ubuntu-client.ext3. The host has one network interface configured to use DHCP for its IP address. The third stanza defines another Xen host, ldapserver, with similar settings, although it is given more memory. Note that the second host uses a different template as its initial operating system image and has a static IP address. The final stanza defines a virtual switch to which both virtual machines are connected; this switch allows them to communicate with one another.

Most of the settings are straightforward, but the template specifications deserve special mention. Templates are pre-fabricated VM filesystems. Each time you build a new virtual machine, you specify a template to use as its base. The virtual machine will be individualized by MLN during the build process. Some people like to use very generic templates for all their virtual machines and customize them later, whereas others create very specialized templates, like a fully functional LAMP server or a copy of a typical employee desktop.

The definition for a VMware Server virtual machine differs only slightly from the host definition in Listing 1. The vmware keyword is used in the VM host definition stanzas instead of xen, and a VMware format template is used instead of a filesystem image. The switch definition stanza also requires the vmware keyword.

Valid host and switch names are case sensitive and can contain lowercase and uppercase letters, numbers, and hyphens.

Managing VMs with the mln Command

Once you have defined a project in a configuration file, you can use the mln command to build the virtual machines and virtual network inside it. For example, the following commands create the virtual hosts and network defined in Listing 1 and then start the hosts and virtual network:

# mln build -f ldaptest.mln
# mln start -p ldaptest

The build subcommand instantiates the entities in the specified project file, and the start subcommand boots the virtual machines and activates the virtual network.

Typically, the build subcommand is used only once – initially to create the project – whereas the start subcommand and its sibling stop are used to activate and deactivate the project hosts and network.

By including the -h option, you can apply the command to a single host within a project:

# mln stop -p ldaptest -h ldapserver

Table 1 summarizes the most important subcommands related to project management.

Clearly, writing host stanzas for large numbers of virtual machines would get tedious. The MLN configuration language allows you to define superclasses: named groups of settings that can be applied to multiple hosts.


Listing 2 illustrates the use of a superclass. The superclass, which is called basic in Listing 2, provides a bundle of settings that are then available for host definitions. Listing 2 defines three hosts based on the superclass. The third host illustrates the technique of overriding a setting from the superclass. Of course, many more settings could be placed into the superclass or individual host stanzas as required. One project file can include multiple superclasses, and the superclasses can even be nested.

Substanzas, such as network interface definitions, which appear both within superclass and host definitions, are merged. Directives that appear in the host definition have precedence over directives within the superclass.

Listing 2

Working with a Superclass


Distributed Projects and Live Migration

So far, all the examples have been located on a single server. However, MLN supports distributed projects in which virtual machines are located on one or more remote hosts. The host and switch definitions in a distributed project require an additional service_host directive, which specifies the server location:

host rem-vm {
   template rhel5.vmdk
   service_host bigvmbox

Superclasses can include the service_host directive as well (as shown in Listing 2).

For servers that will host virtual machines, the MLN daemon must be running. In addition, the MLN daemon's configuration file, /etc/mln/mln.conf, must define the server to be referenced in project files and must grant access to remote systems from which projects will be managed. For example, the MLN daemon configuration file on the target computer for the rem-vm virtual machine would contain directives like the following:

service_host bigvmbox
daemon_allow 192.168.10.*

In this case, the daemon_allow setting permits access only from systems on the local subnet.

The argument to service_host can be either a resolvable hostname or an IP address.

With MLN, it is a very small step from distributed projects to live VM migration in Xen environments.

To move virtual machines from one server to another, all you need do is modify the service_host settings within the project file to reflect the desired new destination and then rebuild the project with the mln upgrade command. For example, the following commands modify and rebuild the italy project shown in Listing 2, resulting in the virtual machines migrating from the server denoted by bigvmbox to the one denoted newvmbox:

# vi italy.mln
Change bigvmbox to newvmbox
# mln upgrade -f italy.mnl

Note that this command can run even when the virtual machines within the project are currently running.

Live migration also requires a few preparatory steps:

  • All of the servers must run the MLN daemon. Access control for the daemon must grant access to the other servers involved in the migration.
  • The virtual machines must use shared storage to hold images that will be accessible to all relevant servers. The best solution is usually a storage area network (SAN) controlled by the logical volume manager (LVM), but you can also use a shared NFS directory for testing, especially when performance is not a consideration. The location is specified in the host definitions in the project (see Listing 3).

In addition, with a SAN, the /etc/mln/mln.conf file on all relevant servers must contain the san_path directive. The argument for san_path specifies the location of the SAN, which is either the volume group name (like lvm_vg within the host definition) or the local mount point for the SAN.

Xen must be configured to allow migration on all relevant servers via the following entries in the Xen daemon's configuration file /etc/xen/xend-config.sxp:

(xend-address '')
(xend-relocation-hosts-allow '^localhost$ ^.*\\.ahania\\.com$')

The argument in the second directive is a single-quoted list of regular expressions specifying allowed hosts. In this case, we specify the local host and hosts anywhere within the domain.

Listing 3

MLN with a SAN


One Is Easy, and So Are 60

Consider the case of an operating system class taught to, say, 120 students. The students are organized in groups of two for lab work, with each group using a network of one Linux and one Windows virtual machine to solve the course exercises. The Linux virtual machine will have two network cards and share its connection to the LAN with the Windows virtual machine.

The challenge for the instructor is to create 60 of those mini-networks quickly, each with an individual public IP address and password.

The main philosophy for this task is "design once, deploy often." All 60 mini-networks must be as consistent as possible, and the system also must be easy to reconfigure, so if you later decide that the Windows virtual machines need more memory, you can modify all of them easily. Putting all of the virtual machines into a single project might seem like an easy way to accomplish this, but a single project would make it more difficult to manage one group's virtual machines separately.

A better solution is to use one project per student group. However, it will not be necessary to write 60 complete project definitions. The #include statement lets you compress as much information as possible, so each project file only contains the information unique to that project. For example, you can create 60 small project files and let them all point to the same main configuration file (see Listing 4).

Listing 4

Use of the #include Statement


The items beginning with a dollar sign are variable definitions, which will be used within the main configuration file.

Listing 5 shows the main file, oscourse.mln. (Note that this file has no global stanza.)

Listing 5



The hvm directive in the definition of the virtual machine running Windows XP indicates that it uses Xen full (hardware) virtualization. The Linux virtual machine uses Xen paravirtualization. As such, networking configuration is internal to this virtual machine (as IP address, so it is not configured by MLN and there is no network substanza. This configuration file also introduces the root_passwd keyword, as well as two keywords that set up the VNC display.

With a shell or Perl script, you can loop over the various project files with the mln create, the mln update, or both commands if necessary, as in when a change occurs to a customized template file used by the Linux virtual machine. Also, you can run mln commands manually.

Defining Virtual Clusters

Virtualization is also an effective way of using a cluster of physical machines. A cluster can essentially act as a server farm for virtualization; in fact, the student networks described previously could run on the various nodes in a cluster. The physical cluster can serve as physical hardware for one or more virtual clusters. Listing 6 shows a project file that automatically creates a virtual cluster of 36 nodes.

Listing 6

Creating a Virtual Cluster


This project uses the autoenum plugin (included with MLN) to automatically generate virtual machines with successive IP addresses and service hosts taken from the list specified in the plugin's service_hosts substanza.

Listing 6 creates 36 hosts on the basis of the settings in the cluster-node superclass, and the list of service hosts comes from an external file referenced with the #include directive.

The superclass definition also introduces two more configuration file entries that specify the required amount of free space within the filesystem in which the VM image will be stored (another way of specifying its size) and the default gateway (router) IP for a network interface.

Plugins for Special-Purpose Configuration Options

The preceding example illustrated the use of a plugin designed for a specific configuration task: in this case, assigning sequential IP addresses and service hosts.

The MLN configuration language has an open architecture that simplifies the task of writing plugins. MLN plugins are Perl routines that are invoked within the global stanza of a project file.

Listing 7 shows two more examples of plugins. One (apache) sets various configuration parameters for the Apache web server within a virtual machine. Another (sshkey) installs the specified SSH key file(s) into the designated user account. This technique is very practical because it allows you to build virtual machines that can be automatically accessed from other systems, such as by a monitoring tool like Nagios.

Listing 7



The apache plugin will cause MLN to set the Apache document root location and the maximum number of simultaneous connections (once again, the directives in the global stanza and the host stanza are merged).

The sshkey plugin will copy the public key from the local nagios user account to the .ssh/authorized_keys file in the home directory of the user nagios in the virtual machine web14 (creating the file and directory as needed).

Configuring MLN Defaults

Within the main configuration file, /etc/mln/mln.conf, you can set many MLN defaults. The installed version of this file contains extensive comments documenting the available entries. Some of the most useful and important settings are shown in Table 2.

Note that the -P, -T, and -F options to the mln command, which override default directory locations, appear before the desired subcommand (for example, mln -P /mln/projects start ldaptest).

Controlling the MLN Daemon

The MLN daemon, mlnd, is started at boot time via the file /etc/init.d/mlnd (linked to the appropriate rcn.d directory).

Also, you can run this script manually with the usual start, stop, and restart arguments.

To start the daemon manually, use the following command:

# mln daemon -D /var/run/

The following command will display the status of the MLN daemon on all hosts specified in the daemon_status_query lines in /etc/mln/mln.conf:

# mln daemon_status

When you set up MLN to manage virtual machines and networks, it is a good idea to use LVM for flexible VM storage, including expansion capabilities.

Anticipate resource use before deploying virtual machines, and monitor it on an ongoing basis with software like Munin or Cacti.

To limit remote VM management and live migration, use access control and don't forget security. Virtual machines are not inherently more secure than physical systems, contrary to many vendor claims. In fact, in the absence of precautions, they can even be less secure because they offer new forms of attack. Apply the usual system hardening techniques to virtual machines and, especially, to the physical servers that host them.


Also, think about backups. Either you can choose to back up virtual machines in the usual manner, within your enterprise backup scheme, or back up virtual machines at the virtual level.


Virtualization products are everywhere. What makes MLN so different is its ability to work in a very wide range of deployments. MLN works well for virtualization beginners because it removes the gritty details of VM configuration files, and, at the same time, you can use MLN to deploy far more complex scenarios than most vendors offer.

Creating Templates for Virtual Machines

Under both Xen and various free and commercial flavors of VMware, creating a virtual machine starts with making an empty virtual machine. On the first boot, an operating system is installed just as it would be on physical hardware, often from the same installation media, or, more recently, from the corresponding CD/DVD image files. Once you have a virtual machine with an installed virtual system, its image could be copied to create new virtual machines, although the copies might require customization.

MLN is designed for complex virtualization tasks. As such, it does not install operating systems from standard media or ISO images; rather, it relies on installed operating system image files – what it calls templates – as the basis for instantiating virtual machines (relying on the ability of VMware and Xen to create fully installed virtual machines as well as empty ones).

A few of the options for creating MLN templates are as follows:

  • Copy existing virtual machines and use them as templates. Before copying, however, it is a good idea to boot the virtual machine and make it as generic as possible to allow for different deployment contexts and scenarios. This includes removing local users and groups (because MLN can configure these as required for each virtual machine created from the template); eliminating specific network configurations; and removing device names, /etc/fstab entries, and so on.
  • Download Xen templates from Internet sites (e.g., Note that such images typically correspond to paravirtualized virtual machines, meaning that the included operating system knows that it is running in a Xen virtual environment and contains special features for efficient execution. This also means these systems do not contain bootable kernels but rather rely on the kernel and initial ramdisk on the virtualization server for booting.
  • Download VM images for VMware Server from Internet sites (e.g.,, These images are typically special-purpose virtual machines ready to run a specific application or fulfill a specific purpose. They are normal VMware VM image files (usually -flat.vmdk preallocated disk image files). Note that you can convert VMware images for use with Xen with the qemu-img convert command.
  • The Xen Tools package provides an easy way to create templates for Xen paravirtualized virtual machines from Debian, Ubuntu, Fedora, and other Linux distributions. See the xen-create-image command for easy creation and customization.
  • With the dd command, you can harvest an image from an installed operating system, copying the entire partition into an image file.

Once you have a template, you can modify it easily by mounting it in loopback mode, as in the following examples.

For Xen images:

# mount -o loop guest.img /somewhere

and VMware images:

# mount -o loop,offset=32256 guest-flat.vmdk /somewhere

If the image is a Linux operating system, you can chroot /somewhere to access the image. This allows you to use the VM operating system's own tools to make modifications, something that is especially helpful for ensuring proper functioning when you add software. If the image is a Windows operating system, you will have to use external tools to modify items within it.

Once prepared, templates must be registered with the MLN daemon before you can use them to build virtual machines:

# mln register_template -t file-system-image-file

Also, you can use rt as an abbreviation for the register_template subcommand.