Author Archive

VirtualBox and Networking – How you do it.

April 25, 2011 by flibs. 4 comments

I have often had to answer questions on setting up advanced networking with VirtualBox.  The most common ones are along the lines of:

Connecting to a VirtualBox machine from the host, using an ip address

and others of a similar ilk.

Well, let’s delve into the mysteries of VirtualBox’s networking (and networking in general) to unravel the secrets behind setting it all up right.

VirtualBox has 4 basic types of network available:

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Linux permissions demystified

April 22, 2011 by flibs. 8 comments

I have seen a large number of questions on Super User recently all around the same topic of Linux and Unix file permissions. For example:

unix file permissions

File permissions in UNIX are frequently specified as an octal number. Why is octal the preferred base for this purpose?

… and …

Linux file permissions

I own a particular file on a Linux system. I would like to give 2 groups (accounting, shipping) read access and only read access, and 3 users(Mike, Raj and Wally) write access and only write access. How can I accomplish this?

In a world of Windows where file permissions can be granted on a per-user basis, Linux and Unix permissions seem to be very hokey and restricted.

Well, let me tell you, they’re not.  For such a seemingly basic arrangement they are an incredibly powerful tool.

Most people forget that to do anything even remotely fancy with permissions in Linux you really have to couple them with groups.  And you really can do some fancy things with them!

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Appeal for laptops

April 18, 2011 by flibs. 0 comments

An acquaintance of mine, Chris John, will be traveling out to Nicaragua on the 5th of June for 3 weeks.  He is taking a stack of laptops with him, and his aim is to teach children the basics of using office tools – word processing, spreadsheets, presentations etc.

He has some laptops already, but he has room to take 5 more.

If you have an old laptop you no longer want it would be great to see it put to good use rather than it being consigned to the rubbish tip or the bottom of the bedroom cupboard. Chris is resident in the UK, so obviously this is only really applicable to fellow UK residents.

The trip is being arranged in association with the Peace and Hope Trust.

If you would like to send your laptop to Nicaragua with Chris please:

  • Make sure it’s in good working order
  • It has a clean installation of an operating system which is capable of running OpenOffice (even better if you can install OpenOffice as well)
  • Contact me directly to check there is still room for it in the luggage and that your laptop is suitable.

Thanks one and all.

Stress Testing your Network

April 17, 2011 by flibs. 0 comments

Welcome to part 2 of my little network masterclass.

In this episode I shall show you how to stress test your network to the max. We’re going to try to get those wires to burst, spilling data all over the carpet.

Why would we want to do such a destructive thing? Well, we want our networks to run as fast and as reliably as they can, and we can’t know what the limits of speed and reliability are unless we really push our networks to the max. From this we will be able to work out ways to make the network better, faster, stronger…

So what sort of things are we going to be looking at? Well, here’s a brief summary:

Network Throughput

  • Just how much data can we push through those wires at once without it breaking?

Packet Sizes

  • What happens if we use lots of small packets, or some really really huge ones?

Advanced Routing

  • Trip over a wire and pull it out of the wall – could cripple your network. But, could it go unnoticed..?

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Confused by the output of ‘du’ in Linux?

April 16, 2011 by flibs. 0 comments

One of the questions on Super User that really hasn’t had the exposure it deserves is this one asked by Jason:

I am confused with the output of command ‘du’ in Linux.

Actually, it’s not so much a question and trying to understand some confusion, so let’s take a look at what he’s confused over and pull it all apart shall we?

I create a file named file.o, i want to check the size of the file.o file.
du -h file.o ====> 4.0K
du -b file.o ====> 1120
according to ‘du -b file.o’, i get to know file.o is 1120bytes large. But why ‘du -h file.o’ outputs 4.0K(means 4*1024 bytes)?

So when he creates a file that is 1120 bytes in size and looks at it with ‘du‘, if he gets the ‘Human Readable’ form of the result, -h, it claims the file is about 4 times the size that it actually is. But, if he requests the number of bytes, -b, it shows the real size. What is going on here?

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How do I preserve an unused computer?

April 11, 2011 by flibs. 4 comments

Yes, this is a question that gets asked. Apparently some people like to hold on to their old computers rather than recycle them or donate them to charity. Agnel Kurian asked on Super User:

How do I preserve an unused computer?

I have a netbook that I am not using but would like to preserve for posterity. What precautions do I need to take to ensure that the machine does not undergo any deterioration? The machine will remain switched off most of the time.

This question some good practical responses, so we at the Super User Blog thought we’d combine them into one definitive answer for your edification.

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Diagnosing Your Network

April 10, 2011 by flibs. 4 comments

Many of you will have had cause to ask (shortly after screaming and swearing)

My network isn’t working – what can I do to find out why?

Well, at Super User we receive many questions each day very much like that. They are all different, and they are all hard to diagnose when you’re a third party just going by what the user is telling you. So what can you do to make your life easier and get as much information about your network issue as possible to help get it all sorted as quickly as possible?

I am going to introduce you to some of the tools that are at your finger tips for finding out where the fault lies in the network and how to fix it when you find it.

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Compression and Encryption: The ZIP Years.

April 2, 2011 by flibs. 1 comments

In a comment to my last post Compression and Encryption, nhinkle asked:

Do you know then how encrypted ZIP files work? Encryption seems to be built into many encryption formats like zip, rar, 7z, etc. Do these usually compress and then encrypt, or somehow do both at once?

Well, ZIP handles this in its own special way. First let’s look at how a ZIP file is made up. A ZIP file consists of one or more ‘file entries’ – blocks of data that make up the actual content of the zip file, followed by a final ‘central directory’:


As you can see each file in the ZIP file has its own local header which contains the information about how the file is compressed. This allows each file in the ZIP file to be compressed in a different way – from “Store” (no compression – ideal for adding pre-compressed files) right up to the maximum and slowest compression available.

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April Fools!

April 1, 2011 by flibs. 0 comments

Every Friday here at the SuperUser Blog is WTF Friday and what better way to start this one out on April Fool’s.

Super users love a good practical joke. Ever since BOFH came on the scene there have been bids to out-do each other with practical jokes. A while back we were asked what a few of our favorites are, and here some of them are. Since it’s April Fool’s day we hope you find them useful! Just remember that some of these may cause significant problems for people, so use discretion when tricking your friends and colleagues. We don’t want something like this:


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Compression and Encryption

March 21, 2011 by flibs. 4 comments

I recently posed the question:

Should I compress and then encrypt, or vice-versa?

to the Super User community. Well, this seemingly simple question sparked quite a discussion, so I have collated all the comments and answers and come up with what I hope is a definitive answer, along with the why and wherefore of it all.

In order to not only answer this question, but also to understand the answer, we first need to know how both compression and encryption work.

Let’s start with compression.

We’ve all come across compression in some form or other, but actually compression and how does it work?

Well, the actual meaning of compression in the English language is:

the result of the subjection of a material to compressive stress, which results in reduction of volume as compared to an uncompressed but otherwise identical state.

In other words, squashing something to make it smaller. And this can be applied to data as well as physical objects. No, data compression doesn’t mean physically pushing the magnetic particles in your hard drive closer together (though that would be a neat trick), but to change the data so it takes up less space but can still be interpreted in the same way.

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