Change Linux Login banner

Last Updated on Friday, 29 April 2011 09:41 Written by A.Jesin Friday, 29 April 2011 09:41

Banner screens provide a lot of important and warm welcome messages, but they also expose the system to a number of threats by displaying the exact version number. In Linux by default after your system starts when you come to the terminal you may see a screen like this

The default banner in Linux displays the OS name and version number

You can customize this banner according to your liking to add a personalized message or to make it more informative. Read More…

Linux iptables LOG everything

Last Updated on Thursday, 28 April 2011 08:55 Written by A.Jesin Thursday, 28 April 2011 08:55

Using iptable’s LOG action certain things in the network tracffic can be logged. In this article I’ll explain how to log each and every minute network traffic using iptables. You can choose which Chain rules and tables should be logged. Information on network traffic is stored in /var/log/messages. This information can be very useful for debugging firewall rules or when you are configuring NAT. Read More…

RHCE exam experience

Last Updated on Saturday, 16 July 2011 11:24 Written by A.Jesin Wednesday, 27 April 2011 10:06

In this article I’ll be sharing my experience of going through RHCE exam and a few tips on how to crack the one-of -the-kind performance based exams. Before I proceed let me tell you that I will NOT be sharing any exam questions because I’ve agreed to the NDA (Non Disclosure Agreement). For those who haven’t heard RHCE (Red Hat Certified Engineer) is a certification offered by Red Hat Inc. for which you have to pass two “performance” based exams, RHCSA (Red Hat Certified System Administrator) and RHCE. Performance based means you have to practically configure everything on the system they provide no objective type questions or theoretical ones where you can practice a couple of “dumps” and breeze through it. Read More…

How to unregister RHN (Red Hat Network)

Last Updated on Wednesday, 20 April 2011 09:54 Written by A.Jesin Wednesday, 20 April 2011 09:54

The RHN (Red Hat Network) is a service provided by Red Hat for receiving updates and support for your Red Hat Enterprise Linux installation. A RHEL copy can be added to RHN by executing the rhn_register command. If for some reason you want to unregister a machine from your RHN network login to RHN from http://rhn.redhat.com/ select you system and click “Delete System” located on the top right.

It isn’t over yet because your computer still “thinks” it is a part of RHN network. To unregister the computer a file has to be removed. Login as root user and execute the following command

rm /etc/sysconfig/rhn/systemid

if you aren’t comfortable with the command line open a file browser with super user permissions (root) navigate to /etc/sysconfig/rhn and delete the file systemid. You can execute rhn_register to reregister that system in future.

SSH verify host public key

Last Updated on Saturday, 9 April 2011 10:51 Written by A.Jesin Saturday, 9 April 2011 10:51

Usually when you connect to a computer via SSH for the first time you might see a message stating “The authenticity of the host can’t be established Are you sure you want to continue connecting ?” even if you blindly give yes it has a lot of meaning in it. If you’re accessing your workplace computer through SSH just by entering ssh server.workplace.com you can’t be sure the connection is between you and your workplace server. A malicious user tampering with your internet connection can also create a man-in-the-middle attack and find out your Linux user account password. By importing the public key of the host to which you’re connecting you can be sure you’re “talking” to the right person. Read More…

SSH configure key based authentication

Last Updated on Monday, 4 April 2011 06:10 Written by A.Jesin Monday, 4 April 2011 06:10

Configure key based authentication in SSH to allow users to login without entering their password. Key based authentication uses public key cryptography to make sure only the owner of the correct private pair logs in through SSH. If the user logs in using password based authentication anyone knowing that user’s password can login. But with key based authentication only the person possessing the correct private key pair for the public key located on the SSH server can login. To add more security you can even add a passphrase to the SSH key. Read More…