The following post explains how to set ch attribute on a file. 
root@me [~]# chattr +i /etc/fstab
root@me [~]# lsattr /etc/fstab
----i--A----- /etc/fstab
root@me [~]# chattr -i /etc/fstab

root@me [~]# lsattr /etc/fstab
-------A----- /etc/fstab
root@me [~]#
The letters "acdijsuADST" select the new attributes for the files: append only (a), compressed (c), no dump (d), immutable (i), data journalling (j), secure deletion (s), no tail-merging (t), undeletable (u),no ...

This post explains how to block all ips except one accessing your system through ssh. Here we are using iptables firewall for blocking and allowing  ips. These rules are tested on Centos linux and will work with other linux distros like redhat, fedora, etc.

Suppose you have your main server with ip 192.168.1.10
And you want to allow access from 192.168.1.4 only

Here is the rules :
Initial state [all accept]

root@test [~]# iptables -L
Chain IN...

What is the use of  screen command in linux? How to use it? The following post explains it.
Screen is a powerful tool useful for working in a remote environment. You can start a screen and execute the commands inside the screen. So even if the connection goes down between your system and remote system, you can later attach to the screen. The command will be working inside the screen. 

Installation:
#yum install screen

#screen
Then execute t...

To know your public ip. It shows location and other details too.
http://whatismyipaddress.com/

For checking nameservers, mx record, A record, PTR record and other DNS details.
http://www.intodns.com/

Example:
http://www.intodns.com/randeeppr.me

For checking the accessibility of your ip address or url. It gives ping results from many parts of the world.
http://www.just-ping.com/

For checking the domain details such as owner of the domain and contact details.
http:/...