Example:

SSHD: 1.1.1.1 #Client 1

The hosts.allow file will have to be updated in this format every time you decide to add a new IP.

Once you have whitelisted all of the allowed IP’s for SSH you must then edit the hosts.deny file. Open the file /etc/hosts.deny and add the following line:

sshd: ALL


cat /etc/issue

Update the hosts file – Look for the primary IP of the server. Follow the same format and replace the old hostname with the new hostname on the same line as the primary IP.
vi /etc/hosts

Update your network file with the new hostname:
vi /etc/sysconfig/network

Instead, you can use the find command which will find each file in a specific folder and then remove it. This is extremely helpful when you have to remove thousands of files from a mail queue after a spam bomb.

BE CAREFUL – To remove all of the files in a folder use the following:
find /$fullpathtofolder -type f -exec rm -rf {} \;

i.e.
find /var/spool/mqueue -type f -exec rm -rf {} \;

By placing setting the -type to f you will ensure you only remove files and not folders within that folder.

This will show you the number of processors/cores and the speed/memory capabilities of them.

This will ensure nothing can change the file unless it removes the chattr bit prior to writing to the file. If you need to update the file run:
#remove protection
chattr -i filename

This will enable editing so you can make changes. However, be sure to reset the file as read only once you’re finished to ensure the file doesn’t change. This is extremely useful with cpanel and other systems that change system files and reset them to defaults each time the service is restarted. It will allow you to implement customizations without them being overwritten every time a service restarts.

echo 0 > /selinux/enforce

perl -pi -e ‘s/replaceme/reoplacewith/g’ filename.txt

This is a VERY basic replacement without and special characters. Be VERY careful if you have special characters that you’re trying to replace. If you have characters like:
/ you will have to proceed them with a \ to ensure you don’t wipe out the file.

1 = The minute after the hour that you want it done
2 = The hour you want it done (Military Time)
3 = Day of the Month
4 = Month of the Year
5 = Day of the week

6 = ‘command’ that you want it to run

Example:

5 0 * * * /usr/local/bin/email

means -
5 – 5th minute after the hour
0 – In the 0 hour
* – Everyday of the month
* – Every month of the year
* – Everyday of the week

minute This controls what minute of the hour the command will run on,
and is between ’0′ and ’59′
hour This controls what hour the command will run on, and is specified in
the 24 hour clock, values must be between 0 and 23 (0 is midnight)
dom This is the Day of Month, that you want the command run on, e.g. to
run a command on the 19th of each month, the dom would be 19.
month This is the month a specified command will run on, it may be specified
numerically (0-12), or as the name of the month (e.g. May)
dow This is the Day of Week that you want a command to be run on, it can
also be numeric (0-7) or as the name of the day (e.g. sun).
user This is the user who runs the command.
cmd This is the command that you want run. This field may contain
multiple words or spaces.

The command above will check for badblocks on the partition /dev/sda3 and save the results to the log file badblocks.log

checks for badblocks and it writes the results to: badblocks.log

If you want to check a specific drive or partition you should run:
fdisk -l|awk ‘{print $1}’|grep “/”
This command will provide you with a list of partitions on your system that you can check.