Tom Salmon

VMWare 7.1 – no /dev/vmnet0

admin — Wed, 11 Aug 2010 09:35:09 +0000

A frustrating error with VMWare:
“Ethernet0: can’t open vmnet device /dev/vmnet0″

Solution is to run ‘vmware-netcfg’ and save, then the problem is fixed until next reboot.

WiFi (2.4Ghz) Link Calculation

admin — Sat, 07 Aug 2010 19:09:49 +0000

While searching around on how to calculate the maximum distance of a WiFi connection, I came across this very useful link budget calculator.

So far we’re still searching for a way to calculate the area of a fresnel zone which falls below ground. Based on a 1km link the zone starts at the antenna (8m high), extends to the user (1m high), and the fresnel zone has a radius at its centre point of 5.6m. This zone will intersect the ground at 371.181m and 990.905m from the antenna – thanks to Rory McCann for calculating this.

We’re still trying to work out the area of the Freshnel which travels underground. After doing this, we only then need to calculate the loss in dB. Any ideas?

View ‘dd’ progress

admin — Fri, 16 Jul 2010 13:53:21 +0000

When reading/writing a disk image using Linux tool ‘dd’ launch as follows:
dd if=/dev/sdb of=filename.img & pid=$!
this will run ‘dd’ in the background.

To see the progress:
kill -USR1 $pid

Notes:

If you are doing it as a non-root user you’ll need to prefix those commands with sudo
If you do that do ’sudo ls’ first, otherwise the ’sudo dd’ will fail waiting on the password which it’ll never get as its backgrounded.

Thanks to Cian.

Exporting physical Linux hard disk to a Virtual Machine

admin — Fri, 16 Jul 2010 13:37:12 +0000

Shutdown and remove the hard disk, then connect to a separate PC with VirtualBox (virtualbox-ose) installed.

Take a ‘dd’ copy of the hard disk, eg: dd if=/dev/sdb of=filename.img
(warning this may take a while and requires enough free disk space to hold a copy of the entire hard disk being cloned)
After the image has been successfully created, convert it to an image suitable for VMWare or VirtualBox using the command:
vboxmanage convertfromraw filename.img -format VMDK filename.vmdk
This can be imported into VirtualBox by creating a new (Linux/Other Linux) Virtual Machine and “Use existing hard drive”. Click the browse button to go to the Virtual Media Manager, click ‘Add’ and select the VMDK file created in the previous step.

The virtual machine should now be ready to run in VirtualBox.
To import into VMWare, create a new virtual machine and virtual hard disk. Overwrite the new Virtual Hard disk by copying the VMDK file created earlier, into its place.

Clifford Chambers – new site containing photographs from the past

admin — Fri, 28 May 2010 17:00:09 +0000

Clifford Chambers the village where I grew up, now has a site cataloging pictures from the past century.

mogrify – convert the existing image with imagemagick

admin — Sat, 08 May 2010 13:13:53 +0000

Thanks to Rory McCann for pointing this one out.

It is possible to convert (resize, rotate etc) an image without creating a new file. mogrify takes the same options as convert but operates on the existing image. Both are tools included in imagemagick.

Asterisk-oh323, GnuGK with VPoint-HD

admin — Tue, 04 May 2010 10:39:52 +0000

A warning to all: audio does not work full-duplex when calling from a VPoint (VCON Emblaze) H323 client -> GNUGK -> Asterisk-oh323 -> SIP client. Tested with versions 8 and 10.

However audio works perfectly in the other direction.

The solution would appear to be to use OOH323. However the audio is again half duplex in the scenario where SIP clients are calling H323 clients.
Therefore calls originating from Asterisk to H323 clients use the ‘OH323′ module, and calls from GNUGK into Asterisk use the ‘OOH323′ module.

Linux CPU Scaling Governor – Performance and Overheating

admin — Sat, 01 May 2010 12:51:54 +0000

On my previous laptop I suffered with repeated over-heating problems whenever CPU load went high for a prolonged period. After several emergency shutdowns (performed by ACPI) I discovered the problem:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

returned the result: “performance”

Inserting the following line before the ‘exit’ line in /etc/rc.local:

echo "conservative" > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

resolved the problem.

For all available scaling governors:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors

Bash Scripts – exit on immediate fail

admin — Tue, 27 Apr 2010 21:43:50 +0000

With some bash scripts it can be useful to have them exit as soon as a command fails. To accomplish this have the following just after the “#!/bin/bash” line:

set -o errexit

This will cause the script to exit as soon as any command or program returns an exit status not equal to zero.
Thanks to Rory.

Skype, Linux and NFS

admin — Tue, 27 Apr 2010 19:11:58 +0000

A problem I found when using the Linux client for Skype is that it will try to perform file locking, which is a problem if your home partition is an NFS drive.

To overcome this limitation, create a directory on the local hard disk:

sudo mkdir /usr/local/skype
sudo chown tom.tom /usr/local/skype

Delete the configuration that Skype attempted to install, and symlink to the directory on the local harddisk:

rm -rf /home/tom/.Skype
ln -s /usr/local/skype /home/tom/.Skype

After this Skype worked as normal.