It's been a month since I last climbed a mountain. First, I had to scan all the great photos I took during my Dachstein vacation, and then winter paid its first visit to the Alps. It came with such force that one unlucky mountaineer had to spend three nights in a snow storm before he was rescued. Otherwise, deep snow at the end of September is pretty normal.

Today, the snow was gone at 2400m, the sun was shining, and the trees looked like they do in autumn. I didn't feel like exploring a new mountain, so I paid a visit to the Sonnjoch. Even my place at the summit was the same as in 2001.

After our server had a near-death experience last week (its hard disk disappeared after a reboot), a couple of friends and I have been busy setting up a new server during the last few days. Not only a good opportunity to switch from SuSE 8 to RHEL 4, but also for installing the latest and greatest software, and for setting things up in the right way.

ipac -> MRTG

To monitor network traffic, I have used ipac-ng so far (screenshot). It is good for monitoring traffic on a single host, but what I really wanted was the Multi Router Traffic Grapher. Last time, I just couldn't figure out how to use it for anything but monitoring external devices with an SNMP interface.

This time, I searched for "mrtg snmp howto" on Google, and came up with a PDF that described how to set it up without SNMP. You just need a script that returns one or two values, and configure MRTG to call it. Couldn't be easier. Here's part of the configuration and the script, and a screenshot.

mod_throttle -> mod_bwshare

Putting your photo collection into a Web server has the disadvantage that from time time a braindead mirroring tool comes along and hammers the server with a never-ending stream of requests. With Apache 1.3, I have used mod_throttle to block ill-mannered visitors. It keeps track of how many requests have been received from an IP address, and blocks or slows down any further requests once a limit has been reached. One problem with this module is that to reset his count to 0, a visitor must not make any request for a certain time.

Because mod_throttle has not been ported to Apache 2 yet, I had to look for a replacement. What I found is mod_bwshare, which uses statistical shaping to detect unusual request patterns. Instead of a timeout after which the counter is reset, it keeps track of the average number of requests per second, and allows clients to exceed the limit for some time (screenshot). Once they accumulate too much debt, further requests are blocked.

I first followed the installation instructions in the README. It worked and I even was able to make a shared module for the RPM-installed Apache, but as stated in the documentation, the shared module could not be configured from httpd.conf. Then I discovered apach2-mod_bwshare.rpm, which works quite well on RHEL 4.

Update: After switching the DNS records to the new server yesterday and getting some real traffic, Apache crashed two times. Probably mod_bwshare cannot handle graceful restarts - after I started doing full restarts, it seems to work fine.

When I did my first training with a Linux-powered IBM ThinkPad, I naively entered the training room, connected the beamer, and expected things to work. For the rest of the day, a quarter of my desktop was projected on the screen, while the rest was only visible on the laptop's LCD. Not too bad, because I could do a few things in the private area of the desktop. As long as I did not move the mouse.

The next day, I figured out the right order in which to switch on and connect the beamer and the laptop, and was able to use the full desktop, albeit with a scaled-down resolution. And I no longer had a private workspace.

Today, I found the solution on Alexander Wagner's ThinkPad page: A dual head setup for X, and VNC for controlling what gets sent to the beamer. The dual head setup (configuration file for X.org 6.7 and ThinkPad A31p) gives you two separate displays, one for external displays, and one for the LCD. Each with its own resolution, and set up so the mouse can move from one to the other.

Next, we need to install a rearview mirror on the laptop, to check the beamer display without turning our back on the audience. VNC gives us exactly that, with its ability to put a complete X display in a window which is then displayed on another X display, and also allowing us to have multiple windows showing the same VNC display. The present script (tested with Fedora Core 2 and the above X.org configuration) starts a VNC X server, displays it in fullscreen mode on the beamer display, and also opens it in a window on the LCD.

Update 27 Nov 2004: Preparing for another training next week, I noticed that there is a little problem with the dual-head setup, which may be caused by the upgrade to Fedora Core 3. When I boot with an external display connected, Fedora Core 3 hangs while starting the HAL daemon. And when I connect the external display after X has started, the external display gets the same screen resolution as the internal LCD. Solution:

1. Keep the external display disconnected while booting.
2. After the graphical login appears, change to a text console and log in as root.
3. Run "init 3", wait a bit, run "init 5".
4. Use the graphical login with your user account, and everything should be fine.