 Quick
and Dirty Debian Preseeds
Joe Blaylock
For a long time, my co-workers and I have been taking part in
an evolutionary movement that will sound familiar to many readers.
We've been migrating from huge, expensive, proprietary systems,
to small, inexpensive, open ones. Like the fast-breeding shrew-like
rodents that replaced the dinosaurs, our systems have been proliferating
wildly. Clusters of cheap, relatively disposable systems are replacing
powerful singleton hosts. Like the prolific mammals, the survival
of our service no longer depends too much on any single host. If
a given node or set of nodes dies, there are ample replacements.
At the heart of our appliance-computing evolution, we've placed
the Debian GNU/Linux operating system. Debian is famous (some would
say infamous) as a Linux distribution committed to free software,
portability, and stability. Being appropriately conservative with
releases in their quest for stability, the Debian project has gotten
a reputation for cruftiness in some circles. Releases have been
known to take years to freeze. Still, it is the extreme amount of
care taken that makes Debian so appealing to so many. When you run
a Debian system, you know what to expect, from package documentation
to upgrade procedures.
All is not puppies and chocolate-chip cookies in the land of Debian
systems administration, however. In particular, when you find yourself
handling large numbers of systems, host automation and easy installation
become critical. This is an area in which Debian has historically
lagged. There has been at least one attempt over the years to make
things better; Progeny packaged Kickstart for its Debian-based distribution.
However, this didn't really catch on in the larger Debian community,
and systems administrators were left to fend for themselves. Until
Sarge.
Since the release of Sarge, Debian has supported partially or
fully automated installation using pre-configuration files ("preseeds").
Preseed files are simple sets of key/value pairs that tell most
Debian software (including the Debian Installer) how you would like
various debconf package configuration questions answered. This frees
you, the extremely valuable human being, from having to answer them
yourself. Preseed files can be stored on modified installation media,
on removable media, such as a USB stick, or even on the network
(provided you can help your host onto the network, one way or another).
Thus, they can provide significant help to the working systems administrator,
whether you have one system or one hundred.
Getting Started
Starting a Debian installation with a preseed file is easy. At
the boot prompt, simply provide the kernel with an extra parameter,
"preseed/url" or "preseed/file". If you wanted to install Sarge
using the default startup options and a preseed file on your trusty
floppy disk, you would just use:
linux preseed/file=/floppy/preseed.cnf
This would boot Linux, passing the preseed/file parameter to the Debian
Installer, which would then look in that location to load a set of
defaults. Similarly, a preseed file can be passed in from the network
using:
linux preseed/url=http://example.com/preseed.cnf
But take care! If no local resource provides the Debian Installer
with a meaningful set of parameters to help it set up the networking,
it will default to trying to get a DHCP lease. If it can't get one,
then it won't be able to reach your preseed file. This might be okay
-- you might be perfectly happy to sit at the keyboard long enough
to set up networking on the machine, and then go for coffee. Try not
to be too surprised, though, if you seed your installation from the
network and go to lunch, only to find later that the machine hasn't
done much of anything because it's not plugged into the Ethernet or
you forgot to register your MAC address.
Simple Sample, Explained
Listing 1 (first.cnf) contains a simple pre-configuration
for the Debian Installer, which sets a few installation parameters
and then loads additional data from the file second.cnf (Listing
2). As you can see, the format has the (possibly abbreviated) name
of the package to be preconfigured, the package configuration query
to be configured, and the setting to assign, separated by whitespace.
In Listing 1, the time zone is preconfigured, and in Listing 2,
the system's default locale setting. This shows how preseeding might
be useful for often-repeated and rarely altered settings that would
be made to all of your nodes -- even if a fully automated installation
would be overdoing it.
FAI and Preseed
To prepare a set of pre-configuration files for a fully automated
system installation, it is best to begin by installing the system
manually and to keep careful notes. In particular, a record of the
extra packages installed from outside of base would be useful. Any
services configured outside of debconf should be recorded, too.
If your system has the "debconf-utils" package, then a small script
like the one in Listing 3 might come in handy. The debconf-dump-settings.sh
script will create a file, /root/selections-tidy, which will
contain a listing of all of the debconf selections (both explicit
and implicit) made during your system installation, sorted for easy
reference. Using this, you can add entries to your preseed file,
and then try installing the system again -- this time, specifying
your new preseed file to try things out. A certain amount of trial
and error may be necessary to get things just right, particularly
if you're going for a fully automatic system installation.
The effort to develop a smooth system installation is well worth
it, however. Preseeding can make the Debian installation itself
painless. Even better, an annotated preseed file set serves as comprehensive
documentation of the system's initial state. Keeping the preseed
in CVS ensures a record of systems architecture changes over time.
What working systems administrator isn't excited by the thought
of systems documentation that comes almost for free?
References
1. The Debian GNU/Linux Installation Guide, found at: http://www.debian.org/releases/stable/i386/
has a section on automated installations, with a more complete sample
configuration than provided here: http://www.debian.org/releases/stable/i386/ch04s07.html.en
2. John Borwick wrote an article about installation automation
with a lot of great ideas, which was published in last October's
Sys Admin, "System Automation with PXE, Kickstart, and Cfengine"
(http://www.samag.com/documents/s=9881/sam0510b).
3. The DebianInstallerPreseed node on the Debian Wiki, with links
to other valuable resources, tips, and gotchas: http://wiki.debian.org/?DebianInstallerPreseed
Since 2002, Joe Blaylock has been administering Linux and Unix
systems for the Indiana University Knowledge Base team. He has great
fun preaching about open systems, pursuing his Bachelor's degree,
and attending meetings. Actually, that bit about meetings is a lie.
Questions, comments? Send to: blaylock@indiana.edu. |