[Beowulf] Which distro for the cluster?

Robert G. Brown rgb at phy.duke.edu
Wed Jan 3 06:51:44 PST 2007

On Wed, 3 Jan 2007, Leif Nixon wrote:

> "Robert G. Brown" <rgb at phy.duke.edu> writes:
>> Also, plenty of folks on this list have done just fine running "frozen"
>> linux distros "as is" for years on cluster nodes.  If they aren't broke,
>> and live behind a firewall so security fixes aren't terribly important,
>> why fix them?
> Because your users will get their passwords stolen.
> If your cluster is accessible remotely, that firewall doesn't really
> help you very much. The attacker can simply login as a legitimate user
> and proceed to walk through your wide-open local security holes.


   a) Our cluster wasn't remotely accessible.  In fact, it was on a
192.168 network and in order to even touch it, one had to login to an up
to date, carefully defended desktop workstation login server in the

   b) If an attacker has compromised a user account on one of these
workstations, IMO the security battle is already largely lost.  They
have a choice of things to attack or further evil they can try to wreak.
Attacking the cluster is one of them, and as discussed if the cluster is
doing real parallel code it is likely to be quite vulnerable regardless
of whether or not its software is up to date because network security is
more or less orthogonal to fine-grained code network performance.

Still, a cluster is paradoxically one of the best monitored parts of a
network.  Although it would make a gangbusters DoS platform, network
traffic on the cluster, cpu consumption on the cluster, user access to
the cluster are all relatively carefully monitored.  The cluster
installation is likely to be different enough and "odd" enough to make
standard rootkit encapsulations fail for anyone but the legendary
Ubercracker (who can always do whatever they want anyway, right?;-) In
an organization that tightly monitors everything all the time on general
security principles (first line of defense, really, as one can NEVER be
sure all exploitable holes are closed even with a yum-updated, stable,
currently supported distro and human eyes are better at picking up
anomalies in system operation than any automated tool) I think it is
pretty likely that any attempt to take over a cluster and use it for
diabolical ends would be almost instantly detected.

BTW, the cluster's servers were not (and I would not advise that servers
ever be) running the old distro -- we use a castle keep security model
where servers have extremely limited access, are the most tightly
monitored, and are kept aggressively up to date on a fully supported
distro like Centos.  The idea is to give humans more time to detect
intruders that have successfully compromised an account at the
workstation LAN level and squash them like the nasty little dung beetles
that they are.

FWIW, our department is entirely linux at the server level, and almost
entirely linux at the workstation level.  A very few experimental groups
and individuals run either Windows boxes (usually to be able to use some
particular software package) or Macs (because they are, umm, "that kind
of user":-).  I'm guessing that the ratio is something like 4:1 linux to
Win at the workstation level (Macs down there in the noise) and maybe
10:1 linux to win if you include cluster nodes, whatever OS they might
be running.

Since Seth introduced yup on top of RH (maybe 7-8 years ago?  How time
flies...), and then proceeded to write yum to replace yup for RPM
distros in general, we haven't had a single successful promotion to root
in the department.  Nothing done locally can prevent some grad student's
password from being trapped as they login from some compromised
win-based system in their hometown over fall break, but the very few of
these that have occurred have been quickly detected and quickly squashed
without further compromise.

In that same interval, we had a WinXX system compromised and turned into
a pile of festering warez rot something like twice a year.  Pretty
amazing given that they are kept up to date as best as possible and they
make up only 10-20% of our total system count.

> But you know this already.

Oh yeah;-)

And we didn't do this "willingly" and aren't that likely to repeat it
ourselves.  We had some pretty specific reasons to freeze the node
distro -- the cluster nodes in question were the damnable Tyan dual
Athlon systems that were an incredible PITA to stabilize in the first
place (they had multiple firmware bugs and load-based stability issues
under the best of circumstances).  Once we FINALLY got them set up with
a functional kernel and library set so that they wouldn't crash, we were
extremely loathe to mess with it.  So we basically froze it and locked
down the nodes so they weren't easily accessible except from inside the
department, and then monitored them with xmlsysd and wulfstat in
addition to the usual syslog-ng and friends admin tools.

Odd usage patterns (that is, almost any sort of running binary that
wasn't a well-known numerical task associated with one of the groups,
logins by anyone who wasn't a known user) would have been noticed by any
of a half-dozen people, one of whom was me, almost immediately.  The
kernel was "barely stable" as it was and couldn't easily have been
replaced with a hacker kernel (to e.g. erase /proc trace) without a VERY
high probability that the hacker kernel would crash the system and
reveal the hacker on the first try. xmlsysd reads all sorts of stuff
from all over /proc and was custom code that I was working on and
periodically updating, even while Seth was working on yum and updating
THAT.  Somebody would have had to literally custom craft some very
advanced C code to stay hidden on the cluster and even then would have
been revealed by e.g. an update of xmlsysd unless they were a bit beyond
even Ubercracker status.

In general, though, it is very good advice to stay with an updated OS.
My real point was that WITH yum and a bit of prototyping once every
12-24 months, it is really pretty easy to ride the FC wave on MANY
clusters, where the tradeoff is better support for new hardware and more
advanced/newer libraries against any library issues that one may or may
not encounter depending on just what the cluster is doing.  Freezing FC
(or anything else) long past its support boundary is obviously less
desireable.  However, it is also often unnecessary.

On clusters that add new hardware, usually bleeding edge, every four to
six months as research groups hit grant year boundaries and buy their
next bolus of nodes, FC really does make sense as Centos probably won't
"work" on those nodes in some important way and you'll be stuck
backporting kernels or worse on top of your key libraries e.g. the GSL.
Just upgrade FC regularly across the cluster, probably on an "every
other release" schedule like the one we use.

On clusters (or sub-clusters) with a 3 year replacement cycle, Centos or
other stable equivalent is a no-brainer -- as long as it installs on
your nodes in the first place (recall my previous comment about the
"stars needing to be right" to install RHEL/Centos -- the latest release
has to support the hardware you're buying) you're good to go
indefinitely, with the warm fuzzy knowledge that your nodes will update
from a "supported" repo most of their 3+ year lifetime, although for the
bulk of that time the distro will de-facto be frozen except for whatever
YOU choose to backport and maintain.

And really, there isn't much stopping folks from adopting a range of
"mixed" strategies -- running FC-whatever on new nodes for a year or
whatever as needed in order to support their hardware or use new
libraries, then reinstalling them with Centos/RHEL (which is basically
FC-even-current-at-release-time frozen and supported or so it seems
recently anyway) as Centos support catches up with the hardware by
syncing with an FC-current on a new release.

Nowadays, with PXE/Kickstart/Yum (or Debian equivalents, or the OS of
your choice with warewulf, or...) reinstalling OR upgrading a cluster
node is such a non-event in terms of sysadmin time and effort that it
can pretty much be done at will.  Except for pathological cases (like
the Tyans) we're talking at most a few days of sysadmin time to set up a
prototyping node or four, flash over to the new distro via a discrete
node reboot (unattended automated reinstall or a new node diskless
image), and let selected users whack on it for a week or two.  If it
proves invisibly stable and satisfactory -- the rule rather than the
exception -- crank it on up across the cluster.  Even if it "fails" on
some untested pathway after you do this, it costs you at most a reboot
(again to a reinstall/replacement of a node image) to put things back as
they were while you fix things.

The worst thing that such a strategy might require is a rebuild of user
applications for both distros, but with shared libraries to my own
admittedly anecdotal experience this "usually" isn't needed going from
older to newer (that is, an older Centos built binary will "probably"
still work on a current FC node, although this obviously depends on the
precise libraries it uses and how rapidly they are changing).  It's a
bit harder to take binaries from newer to older, especially in packaged
form.  There you almost certainly need an rpmbuild --rebuild and a bit
of luck.

Truthfully, cluster installation and administration has never been


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

More information about the Beowulf mailing list