brctl: Creating a network bridge

April 28th, 2009

Creating a layer 2 ethernet bridge under Debian (or Linux in general really) is incredibly easy, as are most things. What a L2 bridge actually is, is outside the scope of this guide, however Google has plenty of information. Essentially, ethernet frames are mirrored between the two interfaces, therefore what Layer 3 protocol data (TCP, UDP, ICMP, etc) is within the ethernet frame is irrelevant. Anyway, back to the point..

In case you don’t have the package, apt-get install bridge-utils and you’ll also need bridge support in your kernel, however unless you specifically removed it, you should have it.
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cPanel rebuild all DNS zones

April 20th, 2009

I recently had my datacenter set up a new cPanel/Fantastico server for our hosting platform. Unfortunately they spelled the site name incorrectly which caused a load of problems. You can change the hostname with relative ease from within the basic setup section, however once the wrong nameservers are specified and domains are generated you’re stuck.

After fixing the nameserver addresses, you’d think that ‘/scripts/rebuildnsdzones’ or ‘/scripts/rebuildnamedconf’ would be obvious candidates however no such luck.

The answer came in the form of a 3rd party script. Back up your /var/named directory, and then download/run this perl script: http://mirrors.ndchost.com/pub/ndchost/scripts/cpanel_rebuildbindzones

Did the job perfectly for me 🙂

at, batch scheduling

April 13th, 2009

‘batch’ is a very useful tool for scheduling jobs to run when the server is sufficiently idle to process them.

The default is to run jobs when the server load is less than 1.5 however this can be overwritten via the atd tool with -l.

batch takes input via STDIN, i.e.:

mysqldump -u backup –all-databases |batch

‘at’ executes a command at a specific time, ‘atq’ lists the current jobs, ‘atrm’ deletes a job by ID, and ‘batch’ executes jobs when server load drops below the defined level.

The timing of batch is obviously not predictable. If predictable scheduling is required, use ‘at’ for one shots or ‘cron’ for all others. The purpose of batch is to run non time sensitive tasks when the server is sufficiently idle. Examples include mailings, bulk updates such as ‘updatedb’ or ‘apt-get update’, and log rotating or log analyzing such as awstats.

Simple text sorting

April 5th, 2009

On the command line we have a number of powerful tools available to us. I’m going to cover some text sorting methods here.

I have a file called ‘testfile’ within this file is the following:

test:~# cat testfile
line1
line3
abcdefg

test
line9

this is a test
test file

test
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Creating an MD5 on Linux with md5sum

April 1st, 2009

An MD5 is a type of Hash, also, a Checksum.

An MD5 hash is a one way verification sum which can be used to verify a string or contents of a file. Once you have a file and an MD5 checksum, the recipient of the file can also perform an MD5 calculation to ensure that the file’s contents are unchanged. They may have been changed maliciously such as in the case of a binary file, or simply by data corruption. An MD5 is NOT a type of encryption. It can not be reversed.

In the case that you know the length of the data, say between 5 and 8 characters for a password, you can attempt to brute force (try every combination automatically until something hits) the password. For that reason passwords are often salted before being MD5’d however salts and their purpose are outside the scope of this article.

We can use PHP to do the following:

<?php
$string = “teststring”;
$checksum = md5($string);
echo “The checksum is: ” . $checksum . “n”;
?>

The output is:

The checksum is: d67c5cbf5b01c9f91932e3b8def5e5f8

We can also use the ‘md5sum’ linux command and pipe input to it via STDIN.

echo “teststring”|md5sum

test:~# echo “teststring”|md5sum
50be80a7a199c13e2bb09e2e745ba233  –

Why is the output of this md5sum different to that above? Well, ‘echo’ automatically adds a newline to the string to make it “teststringn”. We can surpress this with -n:

test:~# echo -n “teststring”|md5sum
d67c5cbf5b01c9f91932e3b8def5e5f8  –

We can also run the md5sum command against a file:

test:~# md5sum /bin/bash
c8770eb0a3f2b6088914b4bc29301113  /bin/bash