Ask Klaus!

MTU Settings

Question:

First of all – I just want to say that I really enjoy reading your column in Linux Magazine, and that the Knoppix CD is never far away for my work! Cheers!

I just read your response to the "Getting Online" letter in the May 2009 issue and I had a thought that could also be a factor. Because the reader can access Google and carry out a search, it seems as though the Internet access and DNS resolution is working. That probably means that the proxy is unlikely.

A common theme I've found lately when fixing other folks' computers is the MTU settings on the operating system and/or the router. Sometimes, a person's network or ISP warrants a lower MTU value, and as a consequence only certain websites and/or files are accessible.

Answer:

Should the ISP not provide an automatic setup for PPP/PPPoE/WLAN/LAN on the server side if a different MTU is required? Using a lower MTU setting (ifconfig devicename mtu number) may indeed help in some environments if the receiving side gets confused by the default MTU. The default MTU (maximum transfer unit) is usually 1500, but for ISDN I have also seen values like 1450, which may or may not give a better data transfer rate because of protocol header encapsulations that fit into the "remaining" space of a unit. This is of course just an educated guess. But in any case, you are right, under certain circumstances, changing the MTU setting repairs otherwise inexplicably slow network performance.

Contributing

Question:

I am by no means a pro at Linux but I do use it daily and enjoy it. I would like to get more in-depth with it and help to contribute to the open source community. Any words of wisdom or tips on small projects that would be interesting?

Answer:

The Internet contains enough information about GNU/Linux and other free software that any level of in-depth understanding is accessible to any user who is willing to explore. Some places I would recommend for a start, where you can also share your own experience, are wikis and forums. But even more exciting than networking on the Internet is getting involved in the various projects consisting of real people, not all of them programmers, most of whom are just casual computer users with interests far beyond geek stuff.

You might want to look around for Linux user groups or projects that you would like to know more about. Even your favorite distribution might have its own user group, with the possibility to meet the people behind it and maybe become part of the community on a non-technical level. Just helping someone install GNU/Linux or helping someone to find the right place to look for help is a contribution most welcome. And each application has its own community; LXDE, KDE, GIMP – you name it – have communities that share and contribute knowledge and experience to improve and promote their favorite hobby, working tool, or passion.

Cheat Codes

Question:

Hi Klaus. I am a Detective and Computer Forensics Examiner in California. I have been using Linux for four years, including SUSE, Knoppix, and Helix, both at home and for work. I use an LCD TV for my monitor at home, and it only supports a few resolutions, including 1024x768 60Hz. With some Live distros, I have to pass boot commands to set the correct resolution.

I have the most current release of the Knoppix CD, 6.0.1, and the cheat codes that I am familiar with for previous releases do not seem to work. Are there boot commands for screen resolution and refresh rate in this release? If not, can I remaster the CD so the default resolution is what I need?

Answer:

The Xorg server in Debian Lenny now uses the RandR extension by default, in which modelines and predefined resolutions in /etc/X11/xorg.conf are mostly ignored, and the "preferred" resolution from the graphics cards firmware is used.

To change this behavior, the old cheat codes, which only set the xorg.conf values for static resolutions, are insufficient. A new cheat code for falling back to the old behavior will be added in later versions, but for now, you can manually change /etc/X11/xorg.conf by disabling the "RandR" option in the ServerLayout section:

Section "ServerLayout"
...
Option "RandR" "False"
EndSection

For Knoppix, another option is to boot in framebuffer mode, either with the Knoppix-specific boot option "fb1024x768" or the more general "vga=791", which sets 1024x768 as the framebuffer resolution for text mode. Together with

Driver "fbdev"

in the "Device" section of /etc/X11/xorg.conf, you get a fixed resolution, which is slower than the native driver for your graphics cards chipset and does not support direct rendering or 3D but usually works well with TFT monitors.