Your DNS cache stores the locations (IP addresses) of pages you have recently viewed. DNS cache help to speeds up the lookups within the same host.  But sometimes if the website page change before your DNS cache is updated, you will failed to access the page. For example, you received a web page with HTML 404 error codes. So you need to flush your DNS cache. Once you clear your DNS cache, your computer will query nameservers for the new DNS information you require.

Clear DNS cache on Windows XP, Vista, Win7

1. Open the Start menu.
2. Go to Run.
   • If you do not see the Run command in Vista, search for “run” in the Search bar.
3. In the Run text box, type: ipconfig /flushdns
4. Press Enter or Return, and your cache will be flushed.

To clear DNS cache in Mac OS X 10.5 and above, use the following command:

1. Go to Applications.
2. Go to Utilities.
3. Open the Terminal application.
4. Type: dscacheutil -flushcache
5. Press Enter or Return, and your cache will be flushed.

Clear DNS cache on Linux (Red Hat / Fedora / CentOS)

Launch your terminal and enter following command to flush DNS:-

$ sudo /etc/init.d/nscd restart

OR

# service nscd restart

I just installed Apache web server on my Fedora 15 system. After starting HTTP (Apache web server) in Linux & pointing your web browser to http://localhost, you will see below test page. This means your server has configures & ready to use.

Question: Is it anyway to remove this Apache Test Page?

Answer: Yes, you can disable this page if you don’t want it.

To remove apache test page in CentOS/REHL/Fedora, follow the steps below:-

• Start your Terminal and login to your web server thru SSH
• Once logged in, enter following command to remove the test page (Pls make sure you login as root)

[root@www ~]# rm -f /etc/httpd/conf.d/welcome.conf

[root@www ~]# rm -f /var/www/error/noindex.html

• now reload your apache web server

[root@www ~]# service httpd reload

Now  try to open your browser and enter http://localhost.  You shouldn’t see any Apache Test Page again.

I wanted to install phpmyadmin on Centos 5.6 Machine. I check from phpmyadmin official web site, phpmyadmin requires at least PHP 5.2 and MySQL 5. I have to check my php version 1st before start install.

Here is a useful command to display or show what version of php you have installed in your Linux Machine.

Just enter following command to check your PHP Version

root@raptor(~)
[ 2]# php -v
PHP 5.1.6 (cli) (built: Nov 29 2010 16:47:46)
Copyright (c) 1997-2006 The PHP Group
Zend Engine v2.1.0, Copyright (c) 1998-2006 Zend Technologies

Once you enter the php -v command, you should able to see the result like what i show you above, if you don’t get something like the above, then it means that you don’t have php installed in your server.

I noticed that my Centos Server logs is not on the correct time. I tried to check the time from the command line and found that the timezone is pointing to the wrong timezone

root@thundercat ~]# date
Mon May 23 00:21:56 EDT 2011

How to change it? There are two ways to change the timezone on CentOS.

1. Set the timezone by running system-config-date command

Type the system-config-date command at the command line to start the time and date properties tool. After that, just follow on screen instructions to change timezone.

2. Set timezone using /etc/localtime configuration file

It is very easy to change the Timezone from /etc/localtime. Just create a new soft link for /etc/localtime to /usr/share/zoneinfo (directory contains the time zone files)

Below are the steps to do it:-

• Just make a backup of the existing localtime file

mv /etc/localtime /etc/localtime.bak

• Create a new soft link for /etc/localtime

ln -s /usr/share/zoneinfo/Asia/Kuala_Lumpur /etc/localtime

• To test your change. Run the “date” command to ensure that the appropriate time, date, and timezone are reported.

[root@thundercat ~]# date
Mon May 23 12:23:27 MYT 2011

Below are the few steps :-

1) Open /etc/hosts.allow

Use the following command to open the hosts.allow file:

Once this file is open, add the following line:

2) Open /etc/hosts.deny

Use the following command to open the hosts.deny file:

Once it opens, add the following line to the file:

Save and exit, and you have successfully limited SSH access by IP address.

Dropbox is a free service that lets you bring all your photos, docs, and videos anywhere. Dropbox provides users with 2 GB of space for free, and they can pay for more. Dropbox  support Windows, Mac OS X, Fedora, Ubuntu, iPad, iPhone, Android and Blackberry.

After you install Dropbox on your computer, any file you save to your Dropbox will automatically save to all your computers, your iPhone and iPad and even the Dropbox website.

To know more about Dropbox, click HERE to visit Dropbox web site.

To download Dropbox, just click HERE to download.

You will see the installation guide on the download page. Just follow the instruction to install it.

Recently i downloaded a eBook in EPUB format. I failed to open it using Adobe Reader (PDF reader) and Chmox (.chm reader). How to open it?? What is EPUB?? After i google it, i know that ePub format is an open standard for ebooks by the International Digital Publishing Forum. All the ePub files have the extension .epub.

There are several software programs support the ePub format. Below are the few software allow you to read .ePub file on your computer. You can just click the following link to download it.

• Adobe Digital Editions (Support Windows and OS X)
• EPUBReader (Firefox Add-on)
• Lucidor (Supports Windows, OS X, and Linux)
• Openberg (plug-in for Firefox,)
• FBReader (Windows, Linux, OS X)

Among all the software that i mention above, i tried to read .epub ebook using Adobe Digital Edition on my Mac OS X machines. It work fine for me.

Below are the few step to show how to install Adobe Digital Edition :-

• Download Adobe Digital Edition

• Just follow the on screen instruction to download the installer files.
• After that, just follow the on screen instruction to install the application. Below are the screen shoot: -

• Try to open .epub format e-book

Besides Adobe Digital Editions, i tried to install Firefox add-on called EPUBReader to read ePub-files in the browser. With this Firefox add-on, you can read ePub files on multiple Operating System (Windows, Linux, Mac OS X, FreeBSD) without install additional software.

To install this Firefox Add-on, it is very easy to do it. Below are the few simple steps to show how to install it:-

• Click HERE to download the EPUBReader add-on

• Click Add to Firefox button to begin the installation

• After download, click Install Now

• Restart the Firefox application

How to use EPUBReader? Just view the following video:-

If you others way to read .epub files, please share with me by leave a comment.

It is easy to update your SSH via yum. To do so, simply SSH into your linux server. Once logged in, type the the following;

#yum update openssh

This will update the openssh process, openssh-clients and openssh-server.

To verify what version of openssh you are running, you can type the following commend;

#ssh -V

Your server will then reply back with the version;

[root@XXXXXXX ~]# ssh -V
OpenSSH_4.3p2, OpenSSL 0.9.8e-fips-rhel5 01 Jul 2008

The default port for the SSH user is port 22. For a hacker to determine ssh is running on your machine, he’ll most likely scan port 22 to determine this. So, we better change our default port number to some other port. For instance, you want to run the ssh service on port 99 instead of 22

To make the change, use any of you text editor to open /etc/ssh/sshd_config file:

vi /etc/sshd/sshd_config

Search for the port setting. By default, the port setting is 22. Change port 22 to port 99, save and exit your text editor.

To test it, just open your terminal and type following command to login:

ssh -p 99 root@techkaki.com

Just try it

By default, most Linux distributions do not enable password aging. This means passwords do not expire and is it possible for a user to have the same password indefinitely. This situation is not secure. For security reasons, it is advisable to require users to change their passwords periodically.

Password aging is a mechanism that allows the system to enforce a certain lifetime for passwords. The chage command is used to modify password aging.

# chage [options] username

Here are the common options used with the chage command:

Below are few example on how to use chage command:-

a) Displays the help information

Just type chage command

[root@myopensos ~]# chage
Usage: chage [options] user

Options:
-d, –lastday LAST_DAY    set last password change to LAST_DAY
-E, –expiredate EXPIRE_DATE    set account expiration date to EXPIRE_DATE
-h, –help            display this help message and exit
-I, –inactive INACTIVE    set password inactive after expiration
to INACTIVE
-l, –list            show account aging information
-m, –mindays MIN_DAYS    set minimum number of days before password
change to MIN_DAYS
-M, –maxdays MAX_DAYS    set maximim number of days before password
change to MAX_DAYS
-W, –warndays WARN_DAYS    set expiration warning days to WARN_DAYS

List the password and its related details for an user

b) Show account aging information

Just type chage -l username or chage –list username

[root@myopensos ~]# chage -l techkaki
Last password change                    : Aug 19, 2010
Password expires                    : never
Password inactive                    : never
Account expires                        : never
Minimum number of days between password change        : 0
Maximum number of days between password change        : 99999
Number of days of warning before password expires    : 7

c) Set Password Expiry Date

Example, user techkaki password is set to expire 3 days from the last password change.

Note: both “Password expires” and “Maximum number of days between password change” entries as shown below.

[root@myopensos ~]# chage -M 3 techkaki
[root@myopensos ~]# chage -l techkaki
Last password change                    : Aug 19, 2010
Password expires                    : Aug 22, 2010
Password inactive                    : never
Account expires                        : never
Minimum number of days between password change        : 0
Maximum number of days between password change        : 3
Number of days of warning before password expires    : 7

d) To disable password aging for an user account

To turn off the password expiration for an user account, set the following:

• -m 0 will set the minimum number of days between password change to 0
• -M 99999 will set the maximum number of days between password change to 99999
• -I -1 (number minus one) will set the “Password inactive” to never
• -E -1 (number minus one) will set “Account expires” to never.

[root@myopensos ~]# chage -m 0 -M 99999 -I -1 techkaki
[root@myopensos ~]# chage –list techkaki
Last password change                    : Aug 19, 2010
Password expires                    : never
Password inactive                    : never
Account expires                        : never
Minimum number of days between password change        : 0
Maximum number of days between password change        : 99999
Number of days of warning before password expires    : 7

e) To set Account Expiry Date for an user

With option -E, You can set the account expiry date

[root@myopensos ~]# chage -E “2010-08-20″ techkaki
[root@myopensos ~]# chage -l techkaki
Last password change                    : Aug 19, 2010
Password expires                    : never
Password inactive                    : never
Account expires                        : Aug 20, 2010
Minimum number of days between password change        : 0
Maximum number of days between password change        : 99999
Number of days of warning before password expires    : 7

If you want to know more on how to use chage command, please type man chage to view the manual.