Make a Custom LiveCD with vmbetter

Introduction

vmbetter is a versatile tool for making customized Debian-based environments. It can generate kernel+initramfs pairs, QCOW hard disk images, and ISO CDROM images. This document focuses on the latter, specifically on how you can use vmbetter to easily make a customized LiveCD.

In this article, we'll create a LiveCD which boots you straight into a GUI with Wireshark running--it's just an example, but one that could be useful for someone trying to make a ready-to-deploy security toolkit ala Kali Linux.

Set up vmbetter

To get vmbetter, you'll want to download and compile the minimega repository. First, make sure you have a Go compiler installed. Then, install some libraries needed to build minimega and the associated tools:

# fetch pre-requisites
$ sudo apt-get install libpcap-dev libreadline-dev syslinux 
# On ubuntu 16.04lts you need these as well
$ sudo apt-get debootstrap genisoimage extlinux

With those installed, you can pull down the minimega repo and build:

# get the repository
$ git clone git@github.com:sandia-minimega/minimega.git
$ cd minimega
$ ./all.bash

There should now be a binary called vmbetter in the bin subdirectory.

Making a vmbetter config

vmbetter builds images based on rules specified in a configuration file. It also inserts any additional files you may want into the filesystem of the image it creates.

Write the config file

We'll start by creating our configuration file. From the minimega repository root, open a new file called misc/vmbetter_configs/wireshark.conf. This directory contains several other configurations which could be instructive, but for now just paste the following text into the file and save it:

parents = "default_amd64.conf"
packages = "traceroute dnsutils wget tcpdump telnet wireshark xserver-xorg-core xserver-xorg-input-all xserver-xorg-video-fbdev xserver-xorg-video-cirrus xterm xinit blackbox"
overlay = "misc/vmbetter_configs/wireshark_overlay"
postbuild = `
    sed -i 's/nullok_secure/nullok/' /etc/pam.d/common-auth
    sed -i 's/PermitRootLogin without-password/PermitRootLogin yes/' /etc/ssh/sshd_config
    sed -i 's/PermitEmptyPasswords no/PermitEmptyPasswords yes/' /etc/ssh/sshd_config
    passwd -d root
`

The configuration file specifies several important options: one or more parent configs, a list of packages to install, the "overlay" directory, and the "postbuild" commands. The overlay directory contains any files you want to add to the disc's filesystem. The postbuild commands are a list of shell commands to be run as the final step of the building process; we use it here to set up passwordless root login for convenience.

Note that wireshark.conf has a parent configuration, default_amd64.conf. That file contains some very basic packages which you'll want in almost every case, so most configurations will want to either include default_amd64.conf as a parent, or set their parent to another configuration which uses default_amd64.conf as its parent.

You'll see that we install Wireshark, the basic X.org packages, and some bare minimum X utilities: xterm, xinit, blackbox. If you want additional programs, you can add their package names to the list. vmbetter will automatically figure out any dependencies they may have and install them for you.

Populate the overlay filesystem

Our configuration file says we're going to put any additional files we want into misc/vmbetter_configs/wireshark_overlay. This directory contains a hierarchy of files and directories, based at /, that we want copied into the image we build. Let's start by copying over an existing overlay to get a baseline:

$ cp -r misc/vmbetter_configs/default_overlay misc/vmbetter_configs/wireshark_overlay

If you look in that directory, you'll see a few basic files in a hierarchy:

$ du -a misc/vmbetter_configs/wireshark_overlay/
4       misc/vmbetter_configs/wireshark_overlay/init
8       misc/vmbetter_configs/wireshark_overlay/

In order to make the LiveCD automatically launch X and Wireshark as soon as we log in as root, we can set up a few configuration files:

$ cd misc/vmbetter_configs/wireshark_overlay
$ mkdir -p root
$ cat > root/.profile << EOF
#!/bin/sh
xinit -- /usr/bin/Xorg &
EOF
$ cat > root/.xinitrc << EOF
#!/bin/sh
wireshark &
exec blackbox
EOF
$ cd ../../..    # go back to the root of the minimega repo
# On ubuntu init may not have proper permissions
$ chmod 777 misc/vmbetter_conf/wireshark_overlay/init

Our .profile launches X automatically as soon as root logs in, and then .xinitrc starts Wireshark and the Blackbox window manager once X starts.

Build the image

Now that we've specified how to build the image, we can fire up vmbetter and actually build it. Here's the command I used to build:

$ sudo ./bin/vmbetter -branch stable -iso -level debug  -mbr /usr/lib/syslinux/mbr/mbr.bin misc/vmbetter_configs/wireshark.conf

It's rather complex, so I'll break down each part:

(Depending on your system, you may not need to specify the -mbr flag, or you may find that mbr.bin is in yet a different location.)

When you run vmbetter, it will print lots of debugging information as it calculates dependencies, fetches packages (this will take a long time), installs the packages in a temporary environment, and finally creates an ISO image. If all goes well, you should end up with a file called wireshark.iso in the current directory

Test the image

If everything worked right, you should have a file called wireshark.iso in the current directory. We can test it using minimega:

$ sudo ./bin/minimega
minimega$ vm config cdrom wireshark.iso
minimega$ vm launch kvm wireshark
minimega$ vm start wireshark
minimega$ web 9001

Now, if you point your web browser at http://localhost:9001, you should be able to see minimega's web front-end. There should be one VM listed on the main page; click the screenshot to open a VNC session to it.

Once you're in the VNC session, you will probably see a login prompt. Log in as root, no password necessary. As soon as you log in, it should start up X, run the window manager, and start up Wireshark.

If you want to run the image on real computers, simply burn wireshark.iso to a CD and use it to boot.

Authors

The minimega authors

22 Mar 2016