Computing – Page 37 – More Zonkyness

DNS: How Does It Work?

Nov 022013

The DNS (the domain name system) is one of those Internet services that everybody uses; but most don’t even know it exists. That is partially a good thing – it is supposed to be invisible in the sense that it just works rather than causing problems. But everything – Internet Explorer, Firefox, Chrome, and anything that uses the network – uses the DNS.

But What Does It Do?

The DNS in a very simple sense is the way that applications such as Chrome (or any web browser) finds out what network address a name points to. When we visit a web page such as http://www.bbc.co.uk/, the web browser needs to know what network address to make a connection to. So the web browser asks the DNS “what network address does www.bbc.co.uk point to” and the DNS answers “212.58.246.92 and 212.58.246.93” (as of the time of writing). The DNS does quite a bit more than that – even ignoring the details of how the servers operate – as it can answer other kinds of questions than just what the network address of a name is. But the process works pretty much the same way whatever kind of question is asked, so we’ll concentrate on the name questions. Technically the name www.bbc.co.uk is a fully-qualified domain name, and the network address is either an IPv4 address or an IPv6 address which can be seen if we perform a lookup on www.google.co.uk instead of www.bbc.co.uk (as the BBC doesn’t have an IPv6 address as yet) :-

# host www.google.co.uk 
www.google.co.uk has address 74.125.132.94
www.google.co.uk has IPv6 address 2a00:1450:400c:c06::5e

That’s a command-line way of performing a DNS lookup, which is rather irrelevant to this discussion except that it shows just the DNS answer.

So How Does It Work?

When you perform a DNS lookup (or more usually an application performs a DNS lookup on your behalf), it makes use of a piece of software on your computer called the resolver. This is more complex than is described here, and can use mechanisms other than the DNS. But ignoring all of that, the resolver composes a question in terms that a DNS server would understand. It then sends the question to all of the DNS servers it knows about.

Hopefully one or more of those DNS servers will answer the question, and the application can get on with whatever it is doing.

If an answer is not returned, the question is sent again, and this carries on until the resolver decides that enough is enough and returns an error to the application. Which of course results in an unexpected error such as a web browser saying that Google doesn’t exist!

There’s a fair bit more to it than this of course – particularly how the DNS servers find out the answer to your question, but this is enough for now.

And Apologies For Any Disruption …

Computing 2 Responses »

Oct 252013

As you may have guessed from my previous post, my server is currently being upgraded so there may be episodes when it isn’t available.

Hopefully nothing dramatic.

If this distresses you bear in mind that I don’t get enough banner ad clicks to run a resilient service! At least not under the stairs.

FreeBSD: Updating the Source

Computing, Working Notes No Responses »

Oct 252013

This is about updating FreeBSD once you have broken free of binary updates – probably not the best thing to do, but I needed to do it for various reasons. This isn’t a recommended method; merely the method I use. It is more about gathering the instructions I need.

First update the source :-

cd /usr/src
make update SVN_UPDATE=yes

Next step is to build the world :-

make -j 4 buildworld

This step takes quite a while, so it’s very helpful to add the “-j 4” to build in parallel.

The next step is to build the locally configured kernel. This usually starts with configuring it, but comparing the GENERIC configuration with the local configuration shows that in my case at the time of writing I didn’t have to re-configure.

make buildkernel KERNCONF=${NAME-OF-YOUR-CONFIG}

Once built the kernel needs to be installed :-

make installkernel KERNCONF=${NAME-OF-YOUR-CONFIG}

Next is to boot to single user mode which is done by pressing a key other than Return during the 10 second countdown, and entering boot -s at the prompt. Once booted into single user mode, the following steps need to be taken :-

# mount -u /; zfs mount -a
(To mount all filesystems with zfs in read-write mode)
# mergemaster -p
(To start the update of the configuration files)
# cd /usr/src
# make installworld
(To install the new user land)
# mergemaster
(To complete the update of the configuration files)
# make delete-old

That’s it for single-user mode. Now reboot into full multi-user and carry on …

Next step is to update the ports :-

# portsnap fetch
# portsnap update
# pkgdb -F
# cd /usr/ports/ports-mgmt/portupgrade
# make deinstall
# make install clean
# portupgrade -af --batch

This of course takes a very long time to complete.

And that should be it … seems to have been successful for me.

So Apparently I’m An Exceptionally Dull Weirdo …

Computing, General, Media, Politics No Responses »

Oct 252013

Apparently this idiot thinks that all coders are exceptionally dull weirdos. I’ll quite happily admit to being a weirdo, but as to exceptionally dull … I only seem that way to the exceptionally thick, or prejudiced.

Concentrating for the moment about this journalist’s (in the loosest possible sense of the word) insult to a good number of people, we can probably deduce the following :-

He doesn’t know many developers socially. If he accidentally encounters one in a social setting one of the following occurs :-
1. The developer denies all knowledge of computers because of the prejudice of idiots like him. This is a bit of self defence we geeks learned in the past and used in the past. So we can also assume he’s a bit of a dinosaur (like me).
2. Once he learns someone is a developer, he will climb the walls to get out of the way.
3. He converses in his own specialised area, and anyone who runs away is classified as an exceptionally dull weirdo.
If he thinks that coding is mechanistic, he is totally clueless about programming, and thinks that anyone can simply start writing code immediately. As a little hint, the majority of the time spent programming is thinking; simply grinding out code is a relatively small part of the job.
Lumping coding in with car mechanics, or plumbing in a derogatory manner indicates this guy is one of those fools who think that anything technical is a low-status activity. Hope his plumber tells him to fix it himself when his next water leak occurs, or charges him double!

Now onto the main point of his ill educated rant on the subject of teaching “coding” to school children. Hopefully the government plans to teach “programming” rather than “coding”, but does he have a point? Perhaps, although it’s a bit difficult to take an idiot like this seriously.

It really depends on exactly what and how it is being taught.

If the plan is to turn out vast swarms of fully fledged developers, everyone is going to be disappointed; apart from anything else, if we were going to turn out fully fledged developers it would have a catastrophic effect on every other subject being taught. After all, it would take so much teaching time away from other subjects, there wouldn’t be enough time for Maths, English, History, etc.

But if the intention is to teach programming in a fun way (say with Logo and robots), with the intention of giving students a better idea of how computers work and how they are instructed, it could well be a good thing.

Not everyone needs to code he claims. Actually most people may find themselves coding in ways that may not be thought of as programming – setting up formula in a spreadsheet, setting up a database, automating a task in a word processor. Although none of these are “true” programming, they do share some elements with it – not least to think about the task in hand, dividing it up into sub-tasks, and setting about telling the computer how to do those tasks in a way that the computer will understand.

Does everyone need this? Perhaps not, but they will find using computers much more effective if they have a better idea of what is going on.

What Happens When You Have 100 Partitions?

Computing, Working Notes No Responses »

Oct 232013

Crazy experiment time. What happens when you have a disk with 100 partitions? The replacement for the old MBR standard for partitions on PC hardware is slowly being replaced with GUID partitions. The later increased the maximum number of partitions to 128 which is probably far more than anyone needs, but what happens when you have a disk with 100 partitions?

As it happens, I had a spare external drive to play with, so set something up :-

for x in {1..99}     
do
  parted /dev/sdc mkpart FAT $(($x * 100)) $((x * 100 + 99))
  mkfs -t vfat /dev/sdc${x} 
done

This took a surprising amount of time to run with two interesting effects :-

The mkfs tool refused to make a filesystem on /dev/sdc16 and /dev/sdc80 as it claimed it would be creating a filesystem on a full disk device. I suspect that this is a bug due to simplistic assumption of what constitutes a full disk device based on minor device numbers (/dev/sdc16 happened to be 0 and /dev/sdc80 happened to be 64). This could probably be solved by using device nodes within /dev/disk/by-${something}/${whatever}.
The Unity Launcher appeared to attempt to populate itself with the new filesystems as they were being created, but very rapidly decided not to bother. This happened several times.

Once the creation process was complete, I reconnected the external drive to my Ubuntu machine, and yes the launcher does contain a ton of hard disk icons. The launcher is still full functional, but having a hundred (or so) devices below the normal icons does make using it a little clumsy.

Fortunately it did not mount all the filesystems automatically – closing that many windows would be very tedious. Mounting them all via a file manager window was pretty tedious, but it worked :-

/dev/sdc56             95M     0   95M   0% /media/mike/8663-39C5
/dev/sdc65             95M     0   95M   0% /media/mike/8673-0919
/dev/sdc71             95M     0   95M   0% /media/mike/873E-FEE7
/dev/sdc72             95M     0   95M   0% /media/mike/8741-47B3
/dev/sdc79             95M     0   95M   0% /media/mike/874D-4B53
/dev/sdc81             95M     0   95M   0% /media/mike/874E-D280
/dev/sdc82             95M     0   95M   0% /media/mike/8752-1ACE
/dev/sdc83             95M     0   95M   0% /media/mike/8754-2562
/dev/sdc84             95M     0   95M   0% /media/mike/8755-D262
/dev/sdc86             95M     0   95M   0% /media/mike/8759-0D82
/dev/sdc87             95M     0   95M   0% /media/mike/875A-E5C5
/dev/sdc89             95M     0   95M   0% /media/mike/875E-035B
/dev/sdc92             95M     0   95M   0% /media/mike/8763-8FB5
/dev/sdc93             95M     0   95M   0% /media/mike/8765-7A2F
/dev/sdc94             95M     0   95M   0% /media/mike/8767-1DBC
/dev/sdc95             95M     0   95M   0% /media/mike/8768-D314
/dev/sdc96             95M     0   95M   0% /media/mike/876A-A46E
/dev/sdc97             95M     0   95M   0% /media/mike/876B-F064
/dev/sdc98             95M     0   95M   0% /media/mike/876D-9D90
/dev/sdc58             95M     0   95M   0% /media/mike/8666-B9AA
/dev/sdc61             94M     0   94M   0% /media/mike/866B-8EFA
/dev/sdc62             95M     0   95M   0% /media/mike/866D-1726
/dev/sdc64             95M     0   95M   0% /media/mike/8671-5EE1
/dev/sdc66             95M     0   95M   0% /media/mike/8736-C2F5
/dev/sdc67             95M     0   95M   0% /media/mike/8737-EE95
/dev/sdc68             95M     0   95M   0% /media/mike/8739-7213
/dev/sdc69             94M     0   94M   0% /media/mike/873B-181F
/dev/sdc70             95M     0   95M   0% /media/mike/873C-E80C
/dev/sdc73             95M     0   95M   0% /media/mike/8743-11E7
/dev/sdc74             95M     0   95M   0% /media/mike/8745-28A8
/dev/sdc75             95M     0   95M   0% /media/mike/8746-CA94
/dev/sdc77             95M     0   95M   0% /media/mike/874A-1D30
/dev/sdc78             95M     0   95M   0% /media/mike/874B-C1C7
/dev/sdc85             95M     0   95M   0% /media/mike/8757-77A0
/dev/sdc88             94M     0   94M   0% /media/mike/875C-6DF9
/dev/sdc90             95M     0   95M   0% /media/mike/8760-8FD5
/dev/sdc91             94M     0   94M   0% /media/mike/8762-01DA
/dev/sdc99             94M     0   94M   0% /media/mike/8770-0F74
/dev/sdc1              93M     0   93M   0% /media/mike/8609-229A
/dev/sdc17             95M     0   95M   0% /media/mike/8621-921D
/dev/sdc21             95M     0   95M   0% /media/mike/8628-8CDB
/dev/sdc22             95M     0   95M   0% /media/mike/862A-2217
/dev/sdc23             94M     0   94M   0% /media/mike/862B-8EF9
/dev/sdc25             95M     0   95M   0% /media/mike/862F-0BE5
/dev/sdc27             95M     0   95M   0% /media/mike/8633-1F9D
/dev/sdc28             95M     0   95M   0% /media/mike/8634-A26F
/dev/sdc34             95M     0   95M   0% /media/mike/863E-14EB
/dev/sdc37             95M     0   95M   0% /media/mike/8643-1F63
/dev/sdc4              95M     0   95M   0% /media/mike/860D-2753
/dev/sdc40             95M     0   95M   0% /media/mike/8647-8E49
/dev/sdc41             95M     0   95M   0% /media/mike/8649-033D
/dev/sdc42             94M     0   94M   0% /media/mike/864A-A12A
/dev/sdc43             95M     0   95M   0% /media/mike/864C-6EEF
/dev/sdc44             95M     0   95M   0% /media/mike/864E-3469
/dev/sdc45             95M     0   95M   0% /media/mike/8650-8796
/dev/sdc46             95M     0   95M   0% /media/mike/8652-64DF
/dev/sdc47             95M     0   95M   0% /media/mike/8653-F743
/dev/sdc48             95M     0   95M   0% /media/mike/8655-B14B
/dev/sdc49             95M     0   95M   0% /media/mike/8657-34FF
/dev/sdc5              95M     0   95M   0% /media/mike/860E-EBD7
/dev/sdc50             94M     0   94M   0% /media/mike/8658-A04A
/dev/sdc51             95M     0   95M   0% /media/mike/865A-D4D3
/dev/sdc52             95M     0   95M   0% /media/mike/865C-33D1
/dev/sdc53             95M     0   95M   0% /media/mike/865D-FA56
/dev/sdc54             95M     0   95M   0% /media/mike/8660-6C95
/dev/sdc55             95M     0   95M   0% /media/mike/8661-D456
/dev/sdc57             95M     0   95M   0% /media/mike/8665-0AFD
/dev/sdc59             95M     0   95M   0% /media/mike/8668-3D53
/dev/sdc6              95M     0   95M   0% /media/mike/8610-F9B0
/dev/sdc60             95M     0   95M   0% /media/mike/866A-0A0E
/dev/sdc63             95M     0   95M   0% /media/mike/866E-F6E7
/dev/sdc76             95M     0   95M   0% /media/mike/8748-8D02
/dev/sdc10             95M     0   95M   0% /media/mike/8616-B29F
/dev/sdc11             95M     0   95M   0% /media/mike/8618-6462
/dev/sdc12             94M     0   94M   0% /media/mike/861A-5208
/dev/sdc13             95M     0   95M   0% /media/mike/861B-BA6E
/dev/sdc14             95M     0   95M   0% /media/mike/861D-5133
/dev/sdc15             95M     0   95M   0% /media/mike/861E-C384
/dev/sdc18             95M     0   95M   0% /media/mike/8623-BFCF
/dev/sdc19             95M     0   95M   0% /media/mike/8625-9D85
/dev/sdc2              95M     0   95M   0% /media/mike/860A-504E
/dev/sdc20             94M     0   94M   0% /media/mike/8627-1391
/dev/sdc24             95M     0   95M   0% /media/mike/862D-457F
/dev/sdc26             95M     0   95M   0% /media/mike/8631-5F8A
/dev/sdc29             95M     0   95M   0% /media/mike/8636-2F58
/dev/sdc3              95M     0   95M   0% /media/mike/860B-8C77
/dev/sdc30             95M     0   95M   0% /media/mike/8637-F726
/dev/sdc31             94M     0   94M   0% /media/mike/8639-6B19
/dev/sdc32             95M     0   95M   0% /media/mike/863A-FBBC
/dev/sdc33             95M     0   95M   0% /media/mike/863C-AE68
/dev/sdc35             95M     0   95M   0% /media/mike/8640-3A10
/dev/sdc36             95M     0   95M   0% /media/mike/8641-93A6
/dev/sdc38             95M     0   95M   0% /media/mike/8644-AFCF
/dev/sdc39             94M     0   94M   0% /media/mike/8646-1BAE
/dev/sdc7              95M     0   95M   0% /media/mike/8612-54E8
/dev/sdc8              95M     0   95M   0% /media/mike/8613-C38C
/dev/sdc9              95M     0   95M   0% /media/mike/8615-3522

Yes I have cut the “interesting” filesystems out of that output.

Windows (7) does deal quite so well with the situation. After rebooting into Windows with the disk plugged in, the login process seemed to take longer than usual (although I don’t boot Windows enough to be sure).

Once logged in, everything seemed fine including the little popup window by the status bar saying it was configuring the plugged in disk drive. However that took longer than expected – after clicking on it for details, it took around 5 minutes to complete. At which point it stuck a red cross by the “Disk Drive” whilst it popped up an Autoplay window for drives E: through Z:. With an offer to format drive T: – so it could at least use the drive that Linux refused (by default) to format.

However except for that little red cross, there was no clear warning that it failed to do anything with nearly 80 partitions. And closing all those popup Autoplay windows was pretty tedious.

OSX (10.9) dealt a little better with the disk; it at least recognised all of the disks, and stuck up little icons for each one. And mounted them all. However Finder didn’t seem to respond to attempts to unmount the disks … I had to resort to the command-line. Perhaps I wasn’t patient enough.

And the moral of this little crazy experiment? Whilst we can perhaps throw a little mud at Microsoft, the main lesson learnt is that you too can annoy someone using Windows by handing them an external hard disk with 100 partitions. Especially if the information they want is not in the first 20-odd partitions <Evil Grin>

Oh! And just stating the obvious – it’s a good idea to remove the partitions before putting the spare disk away, or you may encounter a nasty surprise later!

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