Posts Tagged ‘Linux’

Download: (History of) Free Software Foundation In A Nutshell

Tuesday, October 9th, 2012

Here there are the PDF, MP3, M4B (audiobook) and iBooks versions of ‘Free Software Foundation In A Nutshell’

mp3: Free Software Foundation In A Nutshell

m4b: Free Software Foundation In A Nutshell

pdf: Free Software Foundation In A Nutshell

ibooks: Free Software Foundation In A Nutshell

Download and Share, please.

 

How-To Configure Wireless LAN on RaspberryPi With Raspbian Kernel 3.2.27+ And Solwise RTL8188CUS WiFi dongle

Friday, September 28th, 2012


RaspberryPi and Solwise WiFi dongle RTL8188CUS chipsetThe configuration of WLAN with a RTL8188CUS dongle on RaspberryPi is quite trivial now. It doesn’t’ rely on esoteric scripts, of manual installation of third-party kernel modules anymore.

I’ve been digging the solution for days before, it came alone at the beginning of September with a release of new ‘firmware’ for the RPi (see ‘A little of Story’ at the end of the post).

The procedure to install and configure a wireless network interface with Raspbian requires as little as a system upgrade and minimal understanding of the ‘wpa_supplicant’ utility.

Prerequisites

  1. The RaspberryPi must be powered with a 2A output USB charger because the WiFi dongle is very energy-thirsty, especially when it’s scanning the network for available SSIDs or when it’s creating the connection with the assigned SSID.
  2. The RaspberryPi must be installed with Raspbian version 2012-08-16-wheezy-raspbian or greater.
  3. The RaspberryPi must be connected to the internet via the ethernet card.
  4. It’s advisable to have the ‘avahi-daemon’ package installed (and running)
  5. The WiFi Dongle must not be plugged to the RaspberryPi until specified in the following procedure.
  6. It’s better to use a USB extension lead to connect the WiFi dongle to avoid the RaspberryPi to self-restart if the dongle is hot-plugged (or hot-unplugged).

Notes

I did notice that when the WiFi dongle is installed and active, sometimes it interferes with the usb keyboard (with both normal and wireless keyboards).

System Preparation

If your RPi is running a Raspbian version greater than 2012-08-16-wheezy-raspbian you can skip the System Preparation.

  1. launch a repository update:
    $ sudo apt-get update
  2. run a system upgrade:
    $ sudo apt-get upgrade
  3. make sure that the latest RaspberryPi firmware version is installed
    $ sudo apt-get install raspberrypi-bootloader
    The recent RPi firmwares include the Linux kernel version 3.2.27+ or greater.
    On Raspbian the RPi firmware is packaged as ‘raspberrypi-bootloader
     
  4. Install the wpa_supplicant utility:
    $ apt-get install wpasupplicant

WLAN configuration and wpa_supplicant set-up

We suppose that the WiFi dongle will be recognised as the wlan0 device.

Under some circumstances it may be recognised as wlan1 (..or wlan2 on so on), in such case modify the configuration accordingly.

Otherwise if you want your system to forcibly recognise the dongle as wlan0 you will have to play with the /etc/udev/ configuration files.

  1. Generate a PSK version of your WLAN password with wpa_passphrase utility
    $  wpa_passphrase My_WiFi_SSID mypassword

    the output will be similar to

    network={
        ssid="My_WiFi_SSID"
        #psk="mypassword"
        psk=b2abb0fcd2f4527e11817de0823a57bb19ba4622f4595062c94ec4dd1370b5fe
    }
    This output is meat to be an entry for a network configuration blocks of a wpa_supplicant.conf file.
    By the way we e will use it differently.

  2. Copy the ‘psk’ value of the wpa_passphrase output
    i.e. b2abb0fcd2f4527e11817de0823a57bb19ba4622f4595062c94ec4dd1370b5fe
  3. edit the /etc/network/interfacesand add the wlan0 configurations as follow:
    ...
    
    auto wlan0
    allow-hotplug wlan0
    iface wlan0 inet dhcp
      wpa-ssid "My_WiFi_SSID"
      wpa-psk b2abb0fcd2f4527e11817de0823a57bb19ba4622f4595062c94ec4dd1370b5fe
    alternatively you can use the clear-text version of the password
      wpa-psk "mypassword"
  4. Shutdown the RPi.
  5. Unplug the ethernet cable.
  6. Plug the WiFi dongle in the RPi’s USB port.
  7. Restart the RPi and wait that it connects to the Wireless LAN.

If the dongle will lighten up and you can ping or ssh into the Raspbian, congratulations, you’ve done it!

A little of story:

The Linux kernel 3.x comes with the module rtl8192cu.ko that is not able to properly recognised the WiFi dongle with the RTL8188CUS chipset, and when plugging the device, the RPi will hang on device detection of may even freeze.

The most recent versions on the RPi firmware (Sep 2012) have removed the buggy kernel module, and substituted it with a ‘manually’ compiled module called 8192cu.ko probably sources from the source code available at Realtek home page.

 

How the UNIX system Load Avarage is calculated on Mac OS X and Linux

Wednesday, August 24th, 2011

The Load Avarage values that are shown using the ‘top’ commad or the ‘uptime’ command represent the number of blocking processes in the run queue averaged over a certain time period.

An example fo HIGH Load Avarage:

 load average: 12.54, 12.71, 12.19

these values represent CPU, Disk I/O and Network I/O.

if you run the ‘top’ utility you can seek for the CPU usage and for the CPU I/O waiting time that are represented by the ‘us‘ and ‘wa‘ abbriaviations.

If these uage is permanently around 100% then chances are the problem is related to your CPU.

If the I/O waiting time is mostly above the 80% it means that there could be some problem with the I/O devices suche network cards or failing hard drives.

In my spefic case I notices that in ReadyNAS with 6 disks in RAIDX-2 the load avarage is constatly over 10.00,10.00,10.00. I believe this is normal considering that the ‘md’ process has to keep the RAID chain continuosly checksummed and alligned.

This information has been sources from Andy Millar’s really nice blog I have found digging around: http://www.andymillar.co.uk/blog/2006/12/24/linux-load-average-explained/

 

How-To flush the DHCP server lease cache

Monday, January 17th, 2011

On a GNU/Linux server locate the files  dhcpd.leases and dhcpd.leases~
Mine is a Debian so the location is /var/lib/dhcpd3/ .

  1. delete the temporary file dhcpd.leases~:
    $ sudo rm dhcpd.leases~
  2. flush the lease cache dhcpd.leases:

    $ sudo echo "" > dhcpd.leases

the next time the clients will request a lease they will probably obtain a different IP respect the one they had before.