A Computer Lab with No Windows, Part II

Finishing the lab setup, the benefits of OpenOffice.org and looking to the future.

In Part I of this article, I outlined my reasons for building a Linux terminal server network for my classroom lab. I also began the explanation of how I set up the lab. Here's Part II.

Why Two Hard Drives

With an Ultra320 SCSI port, we can connect as many as 15 hard drives to the bus. However, SCSI hard drives still are expensive, and it was beyond our budget to buy more than two hard drives. In my design, the second hard drive is for backup purposes only. I did not choose RAID 0 (data stripping for better performance) nor RAID 1 (mirroring), as I wanted to do the data backup manually.

Following a Linux mini how-to I found on hard-disk upgrades, I set up my second drive exactly the same as my first. I did not choose RAID 1 basically due to performance concerns. If RAID 1 is set up, every write (save to hard drive) triggers another write on the mirror disk and more time is needed. As for teaching purposes, my users' (student) data is important but not as critical as business data. Instead of having a second write each time a user saves his or her work on the server, I wrote a simple backup script and placed it under /etc/cron.daily. With it, all users' data is backed up to my second drive at 1:00am.

Two More Linux Servers Are Needed

In order not to overload my Linux terminal server, I set up two more Linux servers, one for Apache and the other for a router and Squid. With the support of the Manitoba Chapter of Computers for Schools, I got two not-very-old servers for $75 each. One is an old Dell dual Pentium Pro server and the other is an IBM Netfinity server. I put 512MB of RAM in the router/Squid server, as I needed more RAM for Squid. With a few commands in iptables, I was able to re-route all Web requests to the Squid server without any setup required on students' workstation:

[root@router root]# cat /sbin/transquid.sh

# written by C T Leung
# November 15, 2002
# for basic NAT function + transparent proxy using squid

# add this line to block all ip packets to/from chaos

/sbin/route add -host chaos.wsd1.org reject

IPTABLES="/sbin/iptables"       # iptables binary
INTIF="eth0"                    # internal interface
EXTIF="eth1"                    # external interface

# initialization of chains and rules
$IPTABLES -F -t nat
$IPTABLES -X    # delete any chains existing

# setting default rules for each flow (in this case, accept everything)

# adding masquerading function into "nat" chain
# with this, all the locals can go out to Internet
# through external interface (from internal interface)
# at the same time, every connection goes to port 80
# will be redirected to 8080, squid proxy server

#$IPTABLES -I INPUT -s chaos.wsd1.org -j DROP

echo 1 > /proc/sys/net/ipv4/ip_forward  # to enable ip_forward by
assigning 1
$IPTABLES -t nat -A PREROUTING -i $INTIF -p tcp --dport 80 -j REDIRECT
--to-port 8080
# listing the chains and rules set by above lines
$IPTABLES -t nat -L

Computer Programming with Shell Scripts, C/C++, Java and CGI

Most of the common programming languages, such as shell scripts, C and C++, are included in the LTSP download. If you want to have the latest Java development environment installed, however, you can download your choice of Java SDK from Sun and install it. Sun offers Java SDKs in both source as well as binary code. After installation, you might want to add a path to /opt/ltsp/i386/etc/lts.conf so any user can have access to the language. Here is mine configuration file as an example:

# Config file for the Linux Terminal Server Project (www.ltsp.org)
# See lts.conf.readme for a description of each configuration item
        SERVER             =
        XSERVER            = auto
        X_MOUSE_PROTOCOL   = "PS/2"
        X_MOUSE_DEVICE     = "/dev/psaux"
        X_MOUSE_RESOLUTION = 400
        X_MOUSE_BUTTONS    = 2
        X_USBMOUSE_DEVICE  = "/dev/input/mice"

# Keyboards
        XkbSymboles        = "us(pc101)"
        XkbModel           = "pc101"
        XkbLayout          = "us"

        USE_XFS            = N
        LOCAL_APPS         = N
        RUNLEVEL           = 5

In the final line of the config file, I added a current path (./) as well as the path for all the binary, such as JavaC and Java for every user.



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Re: A Computer Lab with No Windows, Part II

Anonymous's picture

Adding . to PATH:


is not secure. E.g., I can create an executable 'ls' in my directory, and trick you into cd'ing to my directory and typing 'ls'. Then you get my version of 'ls' which can do whatever I want, with your permissions.

Re: A Computer Lab with No Windows, Part II

Anonymous's picture

First off, This system seems pretty effective, great use of resources.

I recognize that this could pick up a command in a local directory, but does the shell look for commands from left to right in the PATH variable? So if "ls" is at /bin and ., and "PATH=/bin:." then it will grab the one from /bin first right?

After closer inspection, I noticed the example has it the other way.

Re: A Computer Lab with No Windows, Part II

Anonymous's picture

Just tell me, how often you mistype "ls" and write
sl -la
ls- la
ls-l a

each of these could be a "trojan" planted in a search path

Re: A Computer Lab with No Windows, Part II

Anonymous's picture

I recognize that this could pick up a command in a local directory, but does the shell look for commands from left to right in the PATH variable? So if "ls" is at /bin and ., and "PATH=/bin:." then it will grab the one from /bin first right?

PATH is searched until it finds a match. So, yes, order matters. In the example you've provided, any matches in /bin will go before ./. If you put ./ first, you're asking for trouble.

If you must have ./ in your path, it should come at the end. But it might be better to just specify the /full/path/to/executable, or drop your executables into /usr/local/bin (or other suitable place).

Re: A Computer Lab with No Windows, Part II

Anonymous's picture

It is not safe to have ./ last in the path. It only takes a little more to compromise. What you do is instead of creating an executable file ls you creat all files wich is typical misspelling of ls and is not a command in the path. You could creat an executable calles la for example and if a user types in la instead of ls in that directory he will run your script instead of what he thought he would. How many times have you not misspelld a command.

One other downside with having ./ in path is that you might belive you are running a command in the current directroy but you are not. A god example is test. If you just going to test something what better name to call the executable then test. So you creat the test in your home dir and the you run the command "test". Just to se that it does nothing. No matter what you do with that executable it still do nothing. Just because you are not running that executable you are runing /usr/bin/test wich with no arument does nothing. I know bean there done that. This /usr/bin/test is problebly installed an all Linux machines. So I think manny people fall in this trap.

If you do not have ./ in youre path the you just simple have to put ./ before youre command when you want to run something in the current dir. So to run my test executable I would have typed "./test" and then my test and not the one in the /usr/bin/test would have been executed and I would have been a much happier human with better thinks to do that writing long misspelled comment abouth why ./ in path is not so great idea.


Re: A Computer Lab with No Windows, Part II

Anonymous's picture

"In fact, with my old terminal server, I have to set Icewin (a lightweight X manager) as my default desktop manager and put GNOME and KDE off-line."

I'm sure you wanted to say IceWM instead of Icewin.
And, yes, IceWM (http://www.icewm.org) it's a great window manager. Combined with Rox (http://rox.sourceforge.net) it realy rulz.

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