Introduction

Did you know that you can assign more that one IP address to a single physical network interface? This technique is quite useful, for example when working with Apache and virtual hosts, as it allows you to access same Apache server by using two different IP addresses.

Temporary virtual network interface

The process of creating a virtual network interface in Linux is a quite simple matter. It involves a single execution of the ifconfig command.

ifconfig eth0:0 123.123.22.22

The above command will create a new virtual network interface based on original eth0 physical interface. The only most important condition for creating the virtual network interface is the physical network interface, as in our case eth0 must exists. The whole example is shown below:

Introduction

For those of you who haven't yet heard about John the Ripper (hereby called John for brevity), it is a free password cracking tool written mostly in C. Before going any further, we must tell you that although we trust our readers, we do not encourage or condone any malicious activities that may be performed using this tool or any other tools we talked about in the past. Security-related tools are often like a double-edged sword, in that they can be used for good but also for bad things. So although it might sound tempting, we recommend you to refrain from any damaging activities, if for nothing else, just because you have great chances to land in a jail cell. Password cracking with John the Ripper on LinuxThis article will deal with John from a system administrator's perspective, so we expect you to have intermediate knowledge about your Linux system, whatever distribution that may be, and that you are a security-conscious person with basic security knowledge. However, this article might appeal to you also if you are a home user wanting to learn about these kind of things, but be warned: some of the commands presented below will ask a great deal of your CPU time, so maybe it would be better if you had a test machine and/or lots of time and patience, because password cracking attempts may take days, even on a relatively new machine. As usual please refer to our new Linux Forum for additional help or information.

CD's and DVDs are using ISO9660 filesystem. The aim of ISO9660 is to provide a data exchange standard between various operating systems. As a result any Linux operating system is capable of handling the ISO9660 file system. This guide describes a way on how to mount / umount ISO9660 file-system in Linux and thus enabling user to read data from CD or DVD media.

If you still have some questions after reading this article please try our new Linux Forum.

Detecting CD/DVD-ROM drives

At first we need to find usable devices capable reading ISO9660 data. In other words we need to find CD/DVD drives available on our Linux system. In order to do that, we can use "wodim" command and its --devices option. wodim will scan and output symbolic device names found in /dev/* directory:

# wodim --devices 

If a wodim command is not available on your system make sure cdrecord package is installed on your system.

Debian and Ubuntu:

# apt-get install wodim
OR DEPENDING ON YOUR VERSION
# apt-get install cdrecord

RedHat, Fedora, CentOS:

# yum install cdrecord

Once you execute wodim command and you have some CD/DVD device hardware available in your system you should see an output similar to the one below:

$ wodim --devices
wodim: Overview of accessible drives (1 found) :
-------------------------------------------------------------------------
 0  dev='/dev/scd0'     rwrw-- : 'TSSTcorp' 'CD/DVDW SH-S183L'
-------------------------------------------------------------------------

Normal data deletion does not erase all data from SSD as same parts are reserved and omitted by removal process. The function secure erase function allows for a complete data removal from all cells. The secure erase function is offered by SSD manufactures and not all hard drives or Linux kernels support it. In the below examples we will refer to /dev/sda block device as our test drive. To find whether your SSD hard drive supports secure erase run a following linux command:
Warning:
ATA Security Feature Set These switches are DANGEROUS to experiment with, and might not work with some kernels. USE AT YOUR OWN RISK.
# hdparm -I /dev/sda | grep erase
		supported: enhanced erase

First let's explain what is write-back caching and how it works. Write-back caching is a feature available on most hard drive's to allow hard drive collect all data in hard drive's cache memory before are permanently written. Once certain amount of data is collected in hard drive's cache memory, the whole data chunk are transferred and stored with a single event.

As a result the reduction write events can improve hard drive's data transfer thus improve write speed. To check whether write-back caching is enable on your hard drive use:
# hdparm -W /dev/sda

/dev/sda:
 write-caching =  1 (on)

Change hard drive's sleep/standby mode timer to reduce power consumption Depends on your system's usage and environment the time your hard drive is in idle state may wary. Every time a hard drive has nothing to do it waits certain period of time and then it enters sleep mode. To enter the sleep/standby mode, had drive has to park it's head and stops plate spin. by reducing a timer before hard drive goes to sleep mode we can save some energy.
Use hdparm command to determine what is the current enter sleep mode timer value (APM LEVEL):
# hdparm -B /dev/sda

/dev/sda:
 APM_level	= 254

Most of the non-SSD hard drives allow for a noise reduction by decreasing head movement speed while accessing data. This ability is called Automatic Acoustic Management or AAM. This tutorial will show how to manipulate AAM values to reduce or increase head movement thus directly affect hard drive's noise level.
The best tool for this job is hdparm. hdparm is available for all major Linux distributions and is available for install via hdparm package. If hdparm command is not available on your system yet, you can install it using following linux commands:
UBUNTU/DEBIAN
# apt-get install hdparm
OR
FEDORA/RED HAT
# yum install hdparm
First find a correct block device for a hard drive you would like to work with. This can be done by:
]$ lsscsi -g
[2:0:0:0]    disk    ATA      HTS721060G9SA00  MC3I  /dev/sda   /dev/sg0

The easiest way to obtain hard drive's model information is to use smartmontools and its smartctl command. If you do not have smartmontools package installed on your system yet you can do so by:
UBUNTU:
# apt-get install smartmontools
FEDORA:
# yum install smartmontools

If you are a strong command line user, performing package search and installations using command line package manager yum you may have noticed a frequent metadata updates when using this tool:
Loaded plugins: langpacks, refresh-packagekit
google-chrome   
rpmfusion-free-updates     
rpmfusion-nonfree-updates     
updates/20/x86_64/metalink
updates
virtualbox 
updates/20/x86_64/primary_db 
Although, this behavior is intentional and provides many benefits in order to keep your software updated, it can also however be quite frustrating if you see and have to wait for this update to finish couple times a day. The default expiry time of system's metadata is 90 minutes, and therefor every time 1,5 hour elapses your system will download new repository updates. For a production server this poses not obstacle or problem. However, for desktop systems this feature can be quite annoying.

Master PDF editor is a powerful tool to create or edit existing PDF documents. In case Master PDF editor is not available for your Linux distribution via standard repository then keep reading to find out how to install this software on any Linux System. Just a word of warning though, This is a proprietary software which means that you have absolutely no control of it and therefore, you run it on your own risk.
The only requirement for this software to work is a functional Graphical User Interface.
Master PDF Editor comes as a pre-compiled tarball and in the form of installable DEB and RPM package.

There may be a time that you wish to clear a Memory on your server. Normally you do not want to fiddle with it so do it only you are sure that it is necessary and that it would not affect your server's performance. For this we can use sysctl command which allows linux admins to configure kernel parameters at runtime.

Introduction

In this configuration tutorial we will guide you through the process of configuring sendmail to be an email relay for your gmail or google apps account. This allows you to send email from your bash scripts, hosted website or from command line using mail command. Other examples where you can utilize this setting is for a notification purposes such or failed backups etc. Sendmail is just one of many utilities which can be configured to rely on gmail account where the others include postfix, exim , ssmpt etc. In this tutorial we will use Debian and sendmail for this task.

Install prerequisites

# apt-get install sendmail mailutils sendmail-bin 

Create Gmail Authentication file

# mkdir -m 700 /etc/mail/authinfo/
# cd /etc/mail/authinfo/

next we need to create an auth file with a following content. File can have any name, in this example the name is gmail-auth:

Once you downloaded a Raspberry PI *.img file you may have a reason to look inside the image. Here is a shot config on how you do it:

First get your image file:

# ls -lh
total 1.9G
-rw-r--r-- 1 root root 1.9G Apr 24 14:35 2013-02-09-wheezy-raspbian.img

Hostgator VPS generates apache's httpd.conf file automatically. Therefore, any new lines you add to apache's configuration file will be overwritten. In order to add new configuration settings we need to include a configuration. Open up your main config file /etc/httpd/conf/httpd.conf and search for a virtual host with the domain name you wish to edit. Last two lines of your virtual host should look similar to this:

# To customize this VirtualHost use an include file at the following location
#    Include "/usr/local/apache/conf/userdata/std/2/linuxcon/linuxconfig.org/*.conf"

uncomment the Include line by remove leading "#".

Next, create a directory to reflect the Include location:

# mkdir -p /usr/local/apache/conf/userdata/std/2/linuxcon/linuxconfig.org/

What is DHCP?

Anyone with a basic knowledge of computer networking knows that for two hosts to communicate on the same network using TCP/IP model, both hosts need to have a unique IP address. There are two ways on how any given host on your network can obtain an IP address.

One way is to manually configure the network interface and assign an IP address by hand. The manual network configuration is called static configuration, which means that host's IP address does not change its IP address unless changed manually by the user or system administrator. If your company network includes more than 1000 hosts, the job of configuring each host with a static IP address becomes tiresome and more importantly extremely inefficient.

Another way to assign your network hosts with a proper IP address regardless of the actual network size is to assign an IP address to each host automatically. To perform an automatic host's IP configuration is where DHCP ( Dynamic Host Configuration Protocol ) comes handy.

The DHCP protocol lets a DHCP client, that is your network host to lease network configuration parameters such as an IP address. In fact, lease parameters are not limited to IP address only as they may also include the following configuration settings:

  • IP addresses and network masks
  • Domain Names servers ( DNS )
  • Default Gateways
  • WINS servers
  • Syslog hosts
  • Proxy servers
  • NTP servers
  • X Font servers
  • Syslog hosts

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