Archives for October 2016

The output of the runlevel command will indicate your previous and current runlevel separated by a single space. If there was no previous runlevel ‘N‘ will be used as a placeholder. For example: # runlevel N 5 The output above indicates that the current runlevel is 5 and there was no previous runlevel. Changing current […]

Booting into single user mode using GRUB is accomplished by editing the kernel line. single user mode can be accessed by appending an “S“, “s“, or “single” to the kernel command line in GRUB. This assumes that either the GRUB boot menu is not password protected or that you have access to the password if […]

Device Mapper Multipathing (or DM-multipathing) is a Linux native multipath tool, which allows you to configure multiple I/O paths between server nodes and storage arrays into a single device. The post describes the steps to un-configure and remove device mapper multipath completely from the system. Disabling the service 1. Make sure device mapper multipath is […]

By default root is not allowed to login through telnet for security reasons. Passwords are transmitted in plain text when you use telnet. For this reason, the root user is not allowed to connect using telnet by default. To verify root is disabled we can check the config file /etc/xinetd.d/telnet. When the parameter disable is […]

Question : How do I secure SSH to disable direct root/non-root user login? Answer : The procedure described here disallows direct root login, so when you connect using SSH you need to first login as a normal user, then su to obtain root access. Disabling root login 1. Edit the /etc/ssh/sshd_config file with a text […]

Why to disable root login? – Having the root password defined is not necessary on a linux system. Root password can be disabled but it is not a good practice since the system prompts for the root password absolutely if in case it goes into the maintenance mode. It will not be possible to proceed […]

Locking down single-user mode in RHEL5 requires editing /boot/grub/grub.conf and /etc/inittab files. 1. Define the single user login shell in /etc/inittab by adding a the below line : # vi /etc/inittab … su:S:wait:/sbin/sulogin Or you can also use output redirection to have the entry placed in the file /etc/inittab : # echo “su:S:wait:/sbin/sulogin” >>/etc/inittab 2. […]

Locking down single-user mode in RHEL6 requires editing /boot/grub/grub.conf and /etc/sysconfig/init. 1. Change the definition of the single user login shell in /etc/sysconfig/init from sushell to sulogin # vi /etc/sysconfig/init … # Set to ‘/sbin/sulogin’ to prompt for password on single-user mode # Set to ‘/sbin/sushell’ otherwise SINGLE=/sbin/sulogin

Cron is a time-based job scheduler, it is configured it to run commands at given times or intervals. Each User has a cron table which defines what to execute and at what interval. crontab command is used to create, modify and view cron jobs. Configuration files and directories – Cron is controlled by a set […]

ISO files are whole disk images. They are a single image file that are used for burning to CD-ROM. In order to access the files inside the iso file, you need to mount it as a ‘Loop Device’. A loop device, vnd (vnode disk), or lofi (loopback file interface) is a pseudo-device that makes a […]