Debian Administration

Building Debian CD-ROMS Part 1 - dfsbuild

dfsbuild is a relatively recent tool which allows you to build livecds based upon Debian. It's intended purpose is to allow you to install Debian from scratch, but because it is a simple means of building livecds it is useful for people who use Debian and don't wish to reinstall.

This is the first of two systems we're going to look at for building directly bootable, custom, Debian CD-ROMs the other being bootcd next time.

Installing the software is as simple as running:

root@mystery:~# apt-get install dfsbuild
Reading Package Lists... Done
Building Dependency Tree... Done
The following extra packages will be installed:
  apt-move cdebootstrap libdebian-installer-extra4 libdebian-installer4
The following NEW packages will be installed:
  apt-move cdebootstrap dfsbuild libdebian-installer-extra4
  libdebian-installer4
0 upgraded, 5 newly installed, 0 to remove and 0 not upgraded.
Need to get 1131kB of archives.
After unpacking 3316kB of additional disk space will be used.
Do you want to continue? [Y/n] 

Once the package is installed you'll find the default configuration created and placed in the file /etc/dfsbuild/dfs.cfg.

The configuration file contains details of the repositories you wish to use, for downloading software from along with details of the packages you wish to install upon your new system.

By default the image includes testing, stable, and unstable repositories and builds the image against packages from testing. I changed this by updating the configuration to read:

# Repositories to mirror.  Details about each one are configured below.
dlrepos = testing

# Repository to build the CD with.  Must be in above list.
suite = testing

This saves you from mirroring packages from systems you don't care about.

As I'm running on an x86 system I also removed all references to amd64 from the default configuration file, once that was done I had to tell the software where it should mirror the relevent packages from. This is as simple as adding a line next to the relevent repository:

[repo testing]
suite = testing
mirror = http://http.us.debian.org/debian 

The only other thing I had to do was update the following section which tells the build script which kernel to use. The kernel here comes from my local filesystem - so you'll either have to download it, or update the file:

# Debs from local fs to unpack on live FS (will not be configured)
unpackdebs = /var/cache/apt/archives/kernel-image-2.6.8-2-k7_2.6.8-13_i386.deb 

The other thing you might wish to adjust is the list of packages which are to be included in the image:

# Packages to install on live FS, on all archs, besides base system
allpackages = util-linux mkisofs cdrecord dvd+rw-tools ocaml hugs ghc6 perl
        vim nano joe libncurses5-dev ftp ssh telnet elinks less zip unzip tar 
        info man-db manpages-dev manpages cdebootstrap diff patch gawk 
        tcpdump bash devfsd module-init-tools modutils 
        rsh-client tftp traceroute iputils-tracepath strace iputils-ping iptraf
        iproute ipchains ipfwadm iptables ifupdown dhcp-client
        bind9-host whois dnsutils rsync rdiff-backup mutt netcat
        cpio buffer alien bzip2 dpkg-dev devscripts busybox-static
        dash sash pciutils hotplug discover buffer cramfsprogs minicom
        hdparm ntpdate disktype ddrescue umsdos recover dpkg-repack
        devscripts debhelper emacs21-nox mtr-tiny python-dev build-essential
        g++ wget lftp lynx cu debconf grep-dctrl lsof sysutils epic4 screen
        gnupg dfsbuild less sudo

These pacakgs are enough to test that the software works, and also have a useful system afterwards. By default the configuration file contained a few packages which weren't installable from testing, such as ext2-resize, this is another reason why my configuration file contains fewer packages than those in the default setup.

(My complete configuration file can be studied for reference).

Once the configuration file has been amended for your preferences you can then build the CD-ROM with the following command:

dfsbuild -c /etc/dfsbuild/dfs.cfg dir

The directory you give should be empty it is wiped with no prompting as the imaging creation process starts.

dfsbuild will download the named repositories and packages to place upon the CD-ROM, and then proceed to build the bootable image.

All being well the bootable image will be found in the working director you specified image.iso.

As well as just including raw packages from the Debian mirror of your choice you can also create files and directories upon the bootable image, append data to existing files, remove files, or create symbolic links.

In my configuration file you can see examples of each of these operations.

For example I create the /etc/resolve.conf file specific to my home network so that DNS lookups will work on the booted system.

Creating a file is as simple as placing the following in the configuration file:

[createfiles]
/etc/hostname = bootablesystemshostname

[createfiles]
/etc/resolve.conf = search my.flat
search 192.168.1.1

You can also append to existing files with the following:

[appendfiles]
/etc/network/interfaces = iface eth0 inet dhcp
  iface eth1 inet dhcp
  iface eth2 inet dhcp
  iface wlan0 inet dhcp
  iface wlan1 inet dhcp
  iface wlan2 inet dhcp
  iface ath0 inet dhcp

Finally symbolic links can be created as you can see in the sample configuratoin file:

[symlinks]
/etc/mtab = /proc/mounts

If you have Qemu installed you can test out your new image without booting a real machine:

qemu -boot c -cdrom image.iso

If you're wondering what you'd use a bootable Debian CD-ROM for then here are a couple of suggestions:

• Use as a rescue disk in case you render your machine unbootable.
• Use to install Debian on a new machine.
• Creating a booting disk which will serve as a webserver, mounting the documents its serving via NFS.
• Acting as a proxy server, or traffic sniffer in a simple manner.
• Testing a potentially compromised machine without disturbing it.

There are many other possible uses!

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