Category Archives: Linux

7 new features in RHEL7!

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Listing new features in RHEL7. These 7 new features making RHEL7 stand out from its predecessor. 

New features in RHEL7

Its been a while RHEL7 is launched and nicely accommodated in Linux world by now. What’s new in RHEL7? What is the difference between RHEL7 and its precedence versions? these kinds of questions are flowing through interviews these days. So thought of jotting them down. In this post I will quickly walk through some new key features launched in RHEL7 by Red Hat.

What’s new in RHEL7?

RHEL7 officially released in June 2014 with codename Maipo. It took time for the market to absorb this new release since there are many new features launched in this release. Red Hat launched many new ways, commands to do traditional stuff. We are going to see them now –

  1. The default file system is XFS. RHEL6 was launched with EXT4 as the default file system. XFS is a highly scalable, high-performance file system. XFS supports metadata journaling, which helps in quicker crash recovery. This means file system checks will take very little time. The XFS file system can be defragmented and extended while mounted. This makes it more admin friendly in a production environment since it avoids downtime of file systems for activities. Supports only 64 bit systems.
  2. Introduction of systemctl. A new way to manage services on RHEL7 is systemctl. Older service and chkconfig command is being replaced with systemctl.
  3. Run levels being called as targets. In RHEL7 run levels are called targets. Default target (run-level) is defined in /etc/systemd/system/default.target. 
  4. Fast boot. RHEL7 boots faster than its predecessors. This is achieved by simultaneously starting services that are not dependent on each other. In older versions, services used to start one after another. So if one service stuck or delays to start it subsequently delays the following process and boot time. This hurdle is removed in RHEL7 allowing it to boot much faster.
  5. New system service manager: systemd. The former init process is no more PID 1 or first process. Systemd is introduced which controls standard base init scripts.
  6. Bigger filesystem limits. RHEL7 now supports filesystem size up to 500TB. This limit is also the same for an individual file. This is due to the XFS file system on a 64-bit machine. RHEL6 supports the 16TB filesystem.
  7. New firewall identity. Former firewall iptables now replaces by firewalld (Firewall Dynamic). iptables still exist in the system and you can disable firewalld and use iptables.

These are key differences in the new RHEL7 release. apart from this there are many new things added in RHEL7. The whole list is compiled by Red Hat here.

Let us know your suggestions/feedback in comments section below!

Process states in Linux

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Learn different process states in Linux. Guide explaining what are they, how to identify them, and what do they do.

Different process states

In this article we will walk you through different process states in Linux. This will be helpful in analyzing processes during troubleshooting. Process states define what process is doing and what it is expected to do in the near time. The performance of the system depends on a major number of process states.

From birth (spawn) till death (kill/terminate or exit), the process has a life cycle going through several states. Some processes exist in process table even after they are killed/died, those processes are called zombie processes. We have seen much about the zombie process in this article. Let’s check different process states now. Broadly process states are :

  1. Running or Runnable
  2. Sleeping or waiting
  3. Stopped
  4. Zombie

How to check process state

top command lists total count of all these states in its output header.

top - 00:24:10 up 4 min,  1 user,  load average: 0.00, 0.01, 0.00
Tasks:  88 total,   1 running,  87 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1018308k total,   187992k used,   830316k free,    15064k buffers
Swap:        0k total,        0k used,        0k free,    67116k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
    1 root      20   0 19352 1552 1240 S  0.0  0.2   0:01.80 init
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthreadd
----- output clipped -----

See highlighted row named Tasks above. It shows the total number of processes and their state-wise split up.

Later in above top output observe column with heading S. This column shows process states. In the output we can see 2 processes in the sleeping state.

You can even use ps command to check process state. Use below syntax :

# ps -o pid,state,command
  PID S COMMAND
 1661 S sudo su -
 1662 S su -
 1663 S -bash
 1713 R ps -o pid,state,command

In the above output you can see column titled S shows state of the process. We have here 3 sleeping and one running process. Let’s dive into each state.

Process state: Running

The most healthy state of all. It indicates the process is active and serving its requests. The process is properly getting system resources (especially CPU) to perform its operations. Running process is a process which is being served by CPU currently. It can be identified by state flag R in ps or top output.

The runnable state is when the process has got all the system resources to perform its operation except CPU. This means the process is ready to go once the CPU is free. Runnable processes are also flagged with state flag R

Process state: Sleeping

The sleeping process is the one who waits for resources to run. Since its on the waiting stand, it gives up CPU and goes to sleep mode. Once its required resource is free, it gets placed in the scheduler queue for CPU to execute. There are two types of sleep modes: Interruptible and Uninterruptible

Interruptible sleep mode

This mode process waits for a particular time slot or a specific event to occur. If those conditions occur, the process will come out of sleep mode. These processes are shown with state S in ps or top output.

Uninterruptible sleep mode

The process in this sleep mode gets its timeout value before going to sleep. Once the timeout sets off, it awakes. Or it awakes when waited-upon resources become available for it. It can be identified by the state D in outputs.

Process state : Stopped

The process ends or terminates when they receive the kill signal or they enter exit status. At this moment, the process gives up all the occupied resources but does not release entry in the process table. Instead it sends signals about termination to its parent process. This helps the parent process to decide if a child is exited successfully or not. Once SIGCHLD received by the parent process, it takes action and releases child process entry in the process table.

Process state: Zombie

As explained above, while the exiting process sends SIGCHLD to parents. During the time between sending a signal to parent and then parent clearing out process slot in the process table, the process enters zombie mode. The process can stay in zombie mode if its parent died before it releases the child process’s slot in the process table. It can be identified with Z in outputs.

So complete life cycle of process can be circle as –

  • Spawn
  • Waiting
  • Runnable
  • Running
  • Stopped
  • Zombie
  • Removed from process table

Difference between tmpfs and swap

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Learn what is tmpfs, what is the use of tmpfs, what is swap, what is the use of swap and differences between tmpfs, and swap.

tmpfs and swap

On the social share of our last post about RAM disk in Linux we got a comment “what is the difference between RAM disk and SWAP?” So I decided to explain it a bit in an article on our blog. In this post I will try to explain how swap and RAM disk i.e. tmpfs/ramfs is different and how they work.

What is tmpfs?

Tmpfs also mounted as shared memory /dev/shm. tmpfs is a portion of a virtual memory mounted as a file system that helps to speed up applications. It normally is used to transfer data between programs. It appears as a file system but it does not use persistent devices such as a hard disk. Instead it uses virtual memory (a portion of a RAM).

That’s why if you create any file in tmpfs it’s not created on your system disks but in your memory. Whenever you un-mount tmpfs, everything within is lost. Its volatile storage. Even if you add an entry of tmpfs to re-mount at boot, it will be mounted blank. Data does not persist over reboots or shutdowns in tmpfs.

What is SWAP?

swap is a portion of your hard disks used to extend RAM. Its roughly extended RAM by use of persistent storage device. swap only comes in action once your RAM (physical memory) is full. The normal thumb rule is the size of the swap should be double of your physical ram size. But these changes depend on the conditions and system you have. Read how to create extra swap here & check swap on the server.

Even if it uses persistent devices, it still is a volatile memory. It does not hold data over reboot or shutdowns. Since it plays the role of RAM, its characteristics are still of ram even if it uses hard disks.

Difference between tmpfs and swap

  • tmpfs uses memory while as swap uses persistent storage devices.
  • tmpfs can be viewed as a file system in df output whereas swap doesn’t
  • swap has general size recommendations, tmpsfs not. tmpfs size varies on system purpose.
  • tmpfs makes applications fasters on loaded systems. swap helps the system breathe in-memory full situations.
  • swap full indicates system heavily loaded, degraded performance, and may crash. tmpfs being full not necessarily means heavy load or prone to crash.
  • tmpfs is enhancement whereas swap is a must-have feature!

How to create RAM disk in Linux

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Short tutorial explaining what is RAM disk and how to create a RAM disk in Linux. It also includes differences between ramfs and tmpfs.

RAM disk in Linux

Recently one of our readers asked “how to create a RAM disk in Linux?”. So I thought of writing this small tutorial which will help you to understand what is a RAM disk, what is the use of it and how to create a RAM disk in Linux.

What is RAM disk?

Roughly RAM disk can be termed as a portion of your RAM mounted as a directory. It uses tmpfs or ramfs. Refer below table for the difference between ramfs and tmpfs.

ramfs
tmpfs
Old type New type and replacing ramfs now
Can not be limited in size Size limit can be defined
Since can not be limited may lead to system crash Once limit reached, disk full error written. No system crash issue.
Entry is not visible in ‘df’ output. Need to calculate by using ‘Cached’ number in ‘free’ output. Can be seen in ‘df’ command output
Work mechanism as file system cache Work mechanism is as partition of physical disk

RAM disk is a very high speed, high performance and almost zero latency area to store application files. Due to its performance-oriented nature, its mostly used for temporary data like caching application files.

How to create RAM disk?

RAM disk can be created in simple two steps. One is to create a directory on which it should be mounted and the second step is to mount it on that directory using specific FS type. Make sure you have enough free RAM on the system so that portion of it can be used in RAM disk. You can check it using free command.

Lets create directory /mnt/ram_disk and mount RAM disk on it.

# mkdir /mnt/ram_disk
# mount -t tmpfs -o size=1024m new_ram_disk /mnt/ram_disk

In above mount command, -t should be followed by tmpfs or ramfs type. For ramfs, size is the starting size of RAM disk since ramfs have limitless size. Size followed by the name of the disk (of your choice ex. new_ram_disk). You can verify if it mounted properly using df command.

# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1      5.8G  2.9G  2.7G  53% /
tmpfs           498M     0  498M   0% /dev/shm
new_ram_disk          1.0G     0  1.0G   0% /mnt/ram_disk

You can see newly created tmpfs of 1GB size is mounted on /mnt/ram_disk (highlighted above).

You can add below entry in /etc/fstab as well to persist it over reboots as well. But keep in mind that data within RAM disk flushes for each reboot since its backed memory is volatile.

new_ram_disk    /mnt/ram_disk   tmpfs    nodev,nosuid,noexec,nodiratime,size=1024M   0 







					

		
		
	


	

				

			
						

				All you need to know about hostname in Linux
			

										

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Learn what is hostname, how to set hostname and how to change hostname in Debian and RedHat based Linux systems.





Learn about hostname in Linux





The hostname is a prime identity of Linux servers in the human world! Obviously, the IP address is the main component to identify the system in the environment. In this article, we are going to see anything and everything about the hostname. We will walk through what is the hostname, how to set hostname, how to change hostname etc. Let’s start with the basics of the hostname.





What is hostname





The hostname is the humanly readable identity of the server. Any server is identified by IP address in the network but to identify easily hostname is also given. Normally FQDN (Fully Qualified Domain Name) is expected for the system but even Domain name (the name before the dot) is also fine for systems under private networks. The hostname can be alpha-numeric





Generally hostname standards to the maximum of 255 bytes long. But normally people prefer to keep it 10-12 characters long so that it’s easy to remember. Kernel variables _POSIX_HOST_NAME_MAX or HOST_NAME_MAX defines your current max limit of hostname. You can get their values using getconf a command like below :




# getconf HOST_NAME_MAX
64





How to set hostname in Linux





A quick command in all-new Linux distros is hostnamectl. Use set-hostname switch and your new hostname as an argument.




# hostnamectl set-hostname kerneltalks





For more details read on …





Hostname is defined in files





  • /etc/hosts for networking
  • /etc/hostname : This will be read by boot scripts on boot time and set its value.
  • /proc/sys/kernel/hostname : Current hostname.
  • /etc/sysconfig/network : Networking (HOSTNAME=”server1″ parameter)





In above files, you can only view current hostname (being used by the live kernel) under proc file only. Rest all files are used to lookup or set hostname at boot time. So if you change hostname using hostname command then it won’t reflect in rest files. It will only reflect in the proc file.





You can set the hostname of your choice in /etc/hostname or /etc/sysconfig/network and restart network service to notify kernel about it.





How to change hostname in Linux





The current hostname can be checked by typing hostname command without any argument. The hostname can be changed by simply using hostname command followed by the name of your choice.





Cautions : Do not change hostname on live production systems!




# hostname
server5
# hostname kerneltalks.com
# hostname
kerneltalks.com





Please make a note that change is dynamic and not permanent. After the system reboot, the hostname will be returned to what it was earlier.





Change hostname permanently in Linux





On RedHat systems : You can edit file /etc/sysconfig/network (define in HOSTNAME=”xyz”)  & reboot system




# cat /etc/sysconfig/network
HOSTNAME=kerneltalks.com





On Debian systems : You can edit file /etc/hostname & call /etc/init.d/hostname.sh script (/etc/init.d/hostname.sh start)





You can even change the hostname using the system control command. Use parameter kernel.hostname and define its value like below :




# sysctl kernel.hostname=kerneltalks
kernel.hostname = kerneltalks





On Suse systems: Edit file /etc/HOSTNAME and add hostname in it. There will be no parameter and value format. Only you have to enter hostname like below :




# cat /etc/HOSTNAME
kerneltalks.com










Change hostname permanently in clone, template VM & cloud clones





If you have a system which is prepared using clone, template from VMware or cloud clone deploy then you should do the following :





Edit file /etc/cloud/cloud.cfg and change parameter 'preserve_hostname' to true. You can do it using one-line sed script as below :




root@kerneltalks # sed --in-place 's/preserve_hostname: false/preserve_hostname: true/' /etc/cloud/cloud.cfg





Also, change DHCP related parameter DHCLIENT_SET_HOSTNAME in file /etc/sysconfig/network/dhcp to no. So that hostname wont be changed by DHCP in the next reboot. Again, you can use one line sed to do that as below :




root@kerneltalks # sed --in-place 's/DHCLIENT_SET_HOSTNAME="yes"/DHCLIENT_SET_HOSTNAME="no"/' /etc/sysconfig/network/dhcp





That’s it. These are two extra steps you need to take on cloud or VM servers.





How to configure FQDN in Linux





Another thing around the hostname is to set FQDN for Linux server i.e. Fully Qualified Domain Name. Generally you should be doing in via DNS in your environment but /etc/hosts always get checked first. So its good practise to define FQDN at /etc/hosts file





Use <IP> <FQDN> <Hostname> format to add/edit entry in /etc/hosts and you are good to go. Sample entry below – 




root@testsrv1 # echo "10.1.1.5 testsrv1.kerneltalks.com testsrv1">>/etc/hosts





You can verify Linux server’s FQDN by using command hostname -f




root@testsrv1 # hostname -f
testsrv1.kerneltalks.com

					

		
		
	


	

				

			
						

				How to tune kernel parameters in Linux
			

										

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An article explaining how to tune kernel parameters in the Linux system using command or using a configuration file.





Tune kernel parameters in Linux





In this article we will be discussing how to set or tune the kernel parameter in any Linux system. There are many ways you can do it like setting them in their configuration files or using a system control command sysctl.





sysctl command is used to configure kernel parameters at runtime. Your current kernel parameters values can be viewed with -a switch.




# sysctl -a
abi.vsyscall32 = 1
crypto.fips_enabled = 0
debug.exception-trace = 1
debug.kprobes-optimization = 1
dev.hpet.max-user-freq = 64
dev.mac_hid.mouse_button2_keycode = 97
dev.mac_hid.mouse_button3_keycode = 100
dev.mac_hid.mouse_button_emulation = 0
dev.parport.default.spintime = 500

----- output clipped -----





In the above output you can see parameters on left and their current value on the right. Parameters are sorted with alphabetical order and both columns in output are delimited with = sign so that you can sort this output easily using this delimiter.





There are a few parameters you can even view using the proc file system. You can cat their respective files and view values.




# cat /proc/sys/kernel/shmmni
2048





In above example we can see shmmni value is set to 2048.





How to tune kernel parameter





To change the kernel parameter you can define it under configuration file /etc/sysctl.conf and it will be applied at the next reboot. You need to define parameter=value format in this file (ex. kernel.shmmni=4096).





Each new line represents a new parameter and value pair. Values in this file will be loaded at the next reboot. If you want to load this file immediately then you can can do it by using sysctl -p command. It will load /etc/sysctl.conf file in kernel. You can even define values with -w switch explained below.





To change the kernel parameter using sysctl, you should use a write switch -w along with parameter and value. In the below example we are changing kernel.shmmni value to 2048.




# sysctl kernel.shmmni
kernel.shmmni = 4096
# sysctl -w kernel.shmmni=2048
kernel.shmmni = 2048





You can observe previously kernel.shmmni value was 4096, using -w we changed it to 2048. This change is immediate and does not need a reboot to comes in effect.

					

		
		
	


	

				

			
						

				lolcat: a tool to rainbow color Linux terminal
			

										

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Paint your command outputs with rainbow color! Use lolcat (Ruby gem) tool and add some spice to the black putty terminal!





Rainbow color outputs with lolcat





Another article to have some fun on your Linux terminal. In the past, we have seen few articles about fun in Linux terminal –









In this article, we will cover lolcat command which colors your terminal texts in rainbow fashion! See below GIF to start with –





lolcat command sample output





See how lolcat command colors output in rainbow color scheme!





lolcat is available at its Git Repository for download. Lets setup lolcat on your server.





How to install lolcat tool





lolcat is Ruby gem hence you need to install Ruby first. Install packages rubygems ruby-devel & ruby on your system using yum or apt-get. Once successfully install, download the latest version of lolcat  from its Git repository using wget and any Linux downloader.





Once downloaded, unzip it




# unzip master.zip
Archive:  master.zip
dfc68649f6bdac255d5be052d2123f3fbe3f555c
   creating: lolcat-master/
 extracting: lolcat-master/.gitignore
  inflating: lolcat-master/Gemfile
  inflating: lolcat-master/LICENSE
  inflating: lolcat-master/README.md
 extracting: lolcat-master/Rakefile
   creating: lolcat-master/ass/
  inflating: lolcat-master/ass/screenshot.png
   creating: lolcat-master/bin/
  inflating: lolcat-master/bin/lolcat
   creating: lolcat-master/lib/
  inflating: lolcat-master/lib/lolcat.rb
   creating: lolcat-master/lib/lolcat/
  inflating: lolcat-master/lib/lolcat/cat.rb
  inflating: lolcat-master/lib/lolcat/lol.rb
 extracting: lolcat-master/lib/lolcat/version.rb
  inflating: lolcat-master/lolcat.gemspec





and install it using Ruby gems.




# cd lolcat-master/bin
# gem install lolcat
Successfully installed lolcat-42.24.0
Parsing documentation for lolcat-42.24.0
1 gem installed





This confirms your successful installation of lolcat!





lolcat command to rainbow color output!





Its time to see lolcat in action. You can pipe it with any output of your choice and it will color your command output in rainbow color (a few examples below)!




# ps -ef |lolcat
# date | lolcat





Want some more fun?





lolcat comes with few options which will make it more fun on the terminal. Run command with -d and duration and it will color your output in running mode.





Running colors in terminal using lolcat





You can even combine it with text banners like figlet or toilet and have fun!

					

		
		
	


	

				

			
						

				RHEL6 boot process
			

										

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Understand step by step how RHEL6 system boots. Walkthrough of  RHEL6 boot process which lists all the tasks, activities happen during boot.










Anyone starting to learn Linux must know the boot process of Linux. Here is this post I will be explaining the boot process of Red Hat Enterprise Linux 6 i.e. RHEL6. In brief RHEL6 boot process can be summarized as below :





  1.  Powered on system loads boot loader once it completes POST. The boot loader in turn loads GRUB.
  2. GRUB loads kernel into memory which further loads necessary modules and mount root partition as read-only.
  3. Kernel invokes /sbin/init program and hands it over the boot process.
  4. Init program loads all services as per run level and mounts mount points
  5. The user is presented with a login screen.





Lets see each point in detail to understand RHEL6 boot process properly.





1. Power on and boot loaders:





Whenever the system turned on, it runs POST (Power on self-test) to check all hardware and its operating state. Once POST is cleared, the system runs BIOS. BIOS is a basic input-output system which is the lowest level interface for hardware. BIOS gets loaded in memory and checks system, connected peripherals, boot device path. Lastly, BIOS will load the first sector of the bootable disk in memory which is the MBR master boot record. Once MBR loaded, BIOS hand over boot control to it.





MBR is a small machine code that has a first stage boot loader. The first stage or stage-1 boot loader exists to locate the second stage boot loader and load it in memory only. The second stage or stage-2 boot loader is GRUB. Now boot control is with GRUB.





In UEFI based systems, BIOS is replaced by UEFI. It’s much powerful than BIOS. It has its own architecture, CPU, device drivers. It can mount and read file systems. Such systems have EFI partitions that have EFI’s own boot loaders which can load operating systems or stage-2 boot loaders.





2. GRUB:





GRUB displays a list of available kernels to the user in the graphical interface (like below). Its configuration file is /boot/grub/grub.conf (for BIOS) or /boot/efi/EFI/redhat/grub.conf (for UEFI). Here user can select its kernel to boot using arrow keys and press enter. If not then it will boot default selected kernel when selection time passes out. We can even reset the forgotten root password on this screen.










Once GRUB destined to load the kernel, it searches the kernel binary of it under /boot partition. The boot loader then places one or more appropriate initramfs (Initial RAM file system) images into memory (as seen in the above screenshot). The initramfs is used by the kernel to load drivers and modules necessary to boot the system. Once kernel and initramfs are loaded into memory, boot control is taken by the kernel.





3. Kernel:





Once kernel gets boot control, it quickly run though below tasks:





  • Initialize and configure memory, hardware, and attached peripherals.
  • Decompress initramfs into /sysroot and loads necessary drivers from it
  • Loads virtual devices related to file systems like LVM etc.
  • Free up memory by removing initramfs image
  • Create a root device, mount root partition (read-only)





Now, the kernel is fully loaded and operational. But no services loaded in the system yet so the system is not usable for humans. To load rest of the services kernel calls /sbin/init program and hand it over boot process to him.





4. Init program :





/sbin/init i.e. init process spawns very first in a system with PID 1 and it will be parent process for many system processes or zombie/defunct processes all the time. Init executes and calls various scripts as below :





  • Runs /etc/rc.d/rc.sysinit to start swap, set environment, FS checks, and some system initialization steps.
  • Process jobs in /etc/event.d directory which has run level specific settings
  • Set function library /etc/rc.d/init.d/functions
  • Runs background processes from their respective rc directories. Default specified in /etc/inittab. e.g. for run level 3, it will execute /etc/rc.d/rc3.d/ . Mostly rc directories are having symbolic links of start/stop services.
  • Once all processes started in the specified run level, init finishes, and spawns login screen.





5. Login screen:





Once init completes loading RC directories, it forks Upstart which in turns call /sbin/mingetty. mingetty will be forked for each virtual console. Run level 1 i.e. single user mode has 1 while run level 2 to 5 has 6 virtual consoles. /sbin/mingetty starts communication with tty devices, sets terminal modes, prints login screen (with messages if any), and prompt username to the user!





This completes RHEL6 boot process from power up to login prompt!

					

		
		
	


	

				

			
						

				Why and how to disable SELinux in your Linux distro
			

										

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Learn how to disable SELinux in most of the Linux distro. Understand what is SELinux; why and when you require to disable it. 










You may have come across many app/tools or utility configurations on Linux like FTP when its stated to disable SELinux. You do it and your config runs smoothly. Have you ever wondered what is this SELinux thing? We are going to discuss this in this post!





SELinux is a short form of Security-Enhanced Linux. It’s a set of modules that implement access control policies in the kernel. To disable all this policy enforcement, we disable SELinux. These access control policies might interfere and won’t let your app/utility etc work properly. This is why we see disabling SELinux steps in configurations. Vendors like Red Hat strongly recommends using ‘Permissive mode’ rather than completely disabling SELinux.





SELinux exists to secure your server! Do not disable it unless you have a good reason or you know what you are doing.





SELinux modes:





There are three modes defined in SELinux config files.





  1. Enforcing: Always enforce policies
  2. Permissive: Prints warning messages rather than actually enforcing policies
  3. Disabled: All SELinux policies disabled.





Its always to go with permissive mode if you are troubleshooting some system/app/utility behavior. So that it will give you a proper picture if SELinux really coming in your way and if you should permanently disable it or not.





Current SELinux mode can be checked using below command :




# getenforce
Permissive





Here SELinux is running in permissive mode.





How to disable SELinux temporarily :





Disabling SELinux permanently using the config file method (explained below) needs a reboot. If you want to avoid downtime then you can turn it into permissive mode temporarily using below method :




# echo 0 > /selinux/enforce
OR
# setenforce 0





Setting enforce file with value 0 makes it permissive. Turning it to 1 makes SELinux enforcing.





How to disable SELinux permanently:





SELinux has its own configuration file located at /etc/selinux/config. Here you can state SELINUX=disabled. But this file change needs a reboot to take effect. Here you can even choose permissive mode we talked about earlier.




# cat /etc/selinux/config

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
#     targeted - Targeted processes are protected,
#     mls - Multi Level Security protection.
SELINUXTYPE=targeted





Save file changes and reboot the system. Once rebooted you can see SELinux status using getenforce command.





This method works on RHEL, CentOS, Fedora, Ubuntu.





SELinux mode selection from GRUB:





You can even decide which mode SELinux starts with system boot. Edit /boot/grub/grub.conf file and appending  entry selinux=0 or selinux=1 in kernel line like below:




# cat /boot/grub/grub.conf
---- output clipped -----
splashimage=(hd0)/boot/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux (2.6.32-431.29.2.el6.x86_64)
        root (hd0)
        kernel /boot/vmlinuz-2.6.32-431.29.2.el6.x86_64 ro root=LABEL=/ rhgb quite selinux=0  
        initrd /boot/initramfs-2.6.32-431.29.2.el6.x86_64.img










Disabling SELinux for specific service:





Disabling SELinux for specific services also possible. So you can keep it in enforcing mode and only disable for your required services. You need to use setsebool command and feed it with service name nad required value like below.




# setsebool httpd_disable_trans 0





This sets its permissive mode for httpd services. Make a note that, you need to restart related service after disabling/enabling SELinux for it.