This Blog is to share our knowledge and expertise on Linux System Administration and VMware Administration

Tuesday, June 14, 2016

Stages of Linux Boot Process

Tuesday, June 14, 2016 0

The following are the 6 high level stages of a typical Linux boot process.

1. BIOS

    BIOS stands for Basic Input/Output System
    Performs some system integrity checks
    Searches, loads, and executes the boot loader program.
    It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
    Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
    So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

    MBR stands for Master Boot Record.
    It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda
    MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.
    It contains information about GRUB (or LILO in old systems).
    So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

    GRUB stands for Grand Unified Bootloader.
    If you have multiple kernel images installed on your system, you can choose which one to be executed.
    GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.
    GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
    Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

    #boot=/dev/sda
    default=0
    timeout=5
    splashimage=(hd0,0)/boot/grub/splash.xpm.gz
    hiddenmenu
    title CentOS (2.6.18-194.el5PAE)
              root (hd0,0)
              kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/
              initrd /boot/initrd-2.6.18-194.el5PAE.img

    As you notice from the above info, it contains kernel and initrd image.
    So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

    Mounts the root file system as specified in the “root=” in grub.conf
    Kernel executes the /sbin/init program
    Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.
    initrd stands for Initial RAM Disk.
    initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

    Looks at the /etc/inittab file to decide the Linux run level.
    Following are the available run levels
        0 – halt
        1 – Single user mode
        2 – Multiuser, without NFS
        3 – Full multiuser mode
        4 – unused
        5 – X11
        6 – reboot
    Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.
    Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level
    If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.
    Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

    When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.
    Depending on your default init level setting, the system will execute the programs from one of the following directories.
        Run level 0 – /etc/rc.d/rc0.d/
        Run level 1 – /etc/rc.d/rc1.d/
        Run level 2 – /etc/rc.d/rc2.d/
        Run level 3 – /etc/rc.d/rc3.d/
        Run level 4 – /etc/rc.d/rc4.d/
        Run level 5 – /etc/rc.d/rc5.d/
        Run level 6 – /etc/rc.d/rc6.d/
    Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
    Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.
    Programs starts with S are used during startup. S for startup.
    Programs starts with K are used during shutdown. K for kill.
    There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.
    For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

That is what happens during the Linux boot process. Thanks for  Reading.

How Traceroute Works?

Tuesday, June 14, 2016 0
Traceroute utility uses the TTL field in the IP header to achieve its operation. For users who are new to TTL field, this field describes how much hops a particular packet will take while traveling on network.

So, this effectively outlines the lifetime of the packet on network. This field is usually set to 32 or 64. Each time the packet is held on an intermediate router, it decreases the TTL value by 1. When a router finds the TTL value of 1 in a received packet then that packet is not forwarded but instead discarded.

After discarding the packet, router sends an ICMP error message of “Time exceeded” back to the source from where packet generated. The ICMP packet that is sent back contains the IP address of the router.

So now it can be easily understood that traceroute operates by sending packets with TTL value starting from 1 and then incrementing by one each time. Each time a router receives the packet, it checks the TTL field, if TTL field is 1 then it discards the packet and sends the ICMP error packet containing its IP address and this is what traceroute requires. So traceroute incrementally fetches the IP of all the routers  between the source and the destination.

Tuesday, June 7, 2016

When up2date/yum fail with "Error Class Code 31" - Solved.

Tuesday, June 07, 2016 0
Whenever Running up2date or yum update fails with below error

Error Message    : Service not enabled for system profile: "system1.example.com"

Error Class Code: 31
Error Class Info   :This system does not have a valid entitlement for Red Hat Network.

    Please visit https://rhn-server/rhn/systems/SystemEntitlements. or 

    login at https://rhn-server, and from the "Overview" tab,
    select "Subscription Management" to enable Redhat Network service for this system.
Situation

  • System registration fails with above error.
  • Redhat Network entitlements missing after Redhat contract renewal.
  • After executing rhn_register, the system appears in host list, but as unentitled.
  • Cannot entitle system.
  • System does not have a valid entitlement for Red Hat Network.
  • When trying to install a package, an error was received that said the system does not have a valid entitlement.
  • No longer able to update system.
  • Satellite certificate activation is failing with "Error Class Code 31"?

Resolution

If the system is not registered with rhn-server, follow the below steps to have an entitlement.


  • Log in to Satellite Customer Portal
  • Click on My Subscriptions
  • Under Redhat Network Classic select All Registered Systems
  • Click on system name
  • Click on Edit These Properties beside System Properties
  • Ensure either Update or Management is selected for Base Entitlement.
  • Click the Update Properties button located in the bottom-right corner.

Root Cause

  • Error Class Code: 31 means that a valid entitlement is not assigned to your system profile.
  • When you register a system, the base entitlement gets assigned to either Update / Management (as per the free entitlement in account) along with the base channel. But if the base entitlement is removed for the system profile then while updating the system it fails with Error Class Code: 31

Saturday, April 23, 2016

Find command with more example

Saturday, April 23, 2016 0
1. Find Files Using Name in Current Directory

[root@nsk nskselvan]# find . -name kal.txt
./kal.txt

2. Find Files Under Home Directory

[root@nsk nskselvan]# find /home -name kal.txt
/home/nskselvan/kal.txt

3. How to run the last executed find command?

[root@nsk nskselvan]# !find
find /home -name kal.txt
/home/nskselvan/kal.txt


4. Find Files Using Name and Ignoring Case sensitive

[root@nsk nskselvan]# find /home -iname Kal.txt
/home/nskselvan/kal.txt


5. Find Directories under / directory by using particular name.

[root@nsk nskselvan]# find / -type d -name kal
/home/nskselvan/kal

6. Find all txt Files in Directory

[root@nsk nskselvan]# find / -type f -name "*.txt"
/etc/pki/nssdb/pkcs11.txt
/var/cache/yum/x86_64/7/base/mirrorlist.txt
/var/cache/yum/x86_64/7/extras/mirrorlist.txt
..
.
/usr/share/hwdata/oui.txt
/home/nskselvan/kal.txt


7. Find Files With 777 Permissions

[root@nsk nskselvan]# find . -type f -perm 0777 -print
./tes1.txt

8. Find Files Without 777 Permissions

[root@nsk nskselvan]# find / -type f ! -perm 777 
/boot/grub2/device.map
/boot/grub2/i386-pc/gcry_rmd160.mod
/boot/grub2/i386-pc/acpi.mod
/boot/grub2/i386-pc/gcry_rsa.mod
/boot/grub2/i386-pc/adler32.mod

9. Find Sticky Bit Files with 551 Permissions

[root@nsk nskselvan]#  find / -perm 1551
/home/nskselvan/tes.txt

10. Find SUID Files

[root@nsk nskselvan]# find / -perm /u=s
/usr/bin/umount
/usr/bin/chage
/usr/bin/gpasswd
/usr/bin/staprun

11. Find SGID Files

[root@nsk nskselvan]# find / -perm /g+s
/run/log/journal
/run/log/journal/57b26cb527b64b9f88803ad24fd81384
/usr/bin/wall
/usr/bin/write

/usr/bin/ssh-agent


12. Find Read Only Files

[root@nsk nskselvan]# find / -perm /u=r 
/
/boot
/boot/efi
/boot/efi/EFI
/boot/efi/EFI/centos
/boot/grub2
/boot/grub2/device.map
/boot/grub2/i386-pc


13. Find Executable Files

[root@nsk nskselvan]# find / -perm /a=x | more
/
/boot
/boot/efi
/boot/efi/EFI
/boot/efi/EFI/centos
/boot/grub2


14. Find Files with 777 Permissions and Chmod to 644 (dont use untill not checked)

# find / -type f -perm 0777 -print -exec chmod 644 {} \;

15. Find Directories with 777 Permissions and Chmod to 755 (dont use untill not checked)

# find / -type d -perm 777 -print -exec chmod 755 {} \;

16. Find and remove single File  

# find . -type f -name "kal.txt" -exec rm -f {} \;

17. Find and remove Multiple File ( list before using rm command)

# find . -type f -name "*.txt" -exec rm -f {} \;

18. Find all Empty Files under /tmp Directory

[root@nsk nskselvan]# find /tmp -type f -empty
/tmp/yum.log

19.  Find all Empty Directories under /tmp Directory

[root@nsk nskselvan]#  find /tmp -type d -empty
/tmp/.font-unix
/tmp/.Test-unix
/tmp/.ICE-unix
/tmp/.XIM-unix
/tmp/.X11-unix

20. File all Hidden Files 

[root@nsk nskselvan]# find / -type f -name ".*" | more
/boot/.vmlinuz-3.10.0-693.el7.x86_64.hmac
/boot/.vmlinuz-3.10.0-693.5.2.el7.x86_64.hmac
/run/initramfs/.need_shutdown
/sys/module/sg/notes/.note.gnu.build-id


21. Find Single File Based on User

[root@nsk nskselvan]# find / -user nskselvan -name kal.txt
/home/nskselvan/kal.txt

22. Find all Files Based on User

[root@nsk nskselvan]# find /  -user nskselvan
/var/spool/mail/nskselvan
/home/nskselvan
/home/nskselvan/.bash_logout
/home/nskselvan/.bash_profile
/home/nskselvan/.bashrc
/home/nskselvan/kal.txt
/home/nskselvan/kal
/home/nskselvan/tes.txt
/home/nskselvan/tes1.txt

/home/nskselvan/test2.txt


23. Find all Files Based on Group

[root@nsk nskselvan]# find / -group ntp
/etc/ntp/crypto
/var/lib/ntp

/var/log/ntpstats

24. Find Particular Files of User

[root@nsk nskselvan]# find / -user nskselvan -iname "*.txt"
/home/nskselvan/kal.txt
/home/nskselvan/tes.txt
/home/nskselvan/tes1.txt
/home/nskselvan/test2.txt


25. Find Last 5 Days Modified Files

[root@nsk nskselvan]# find / -mtime 5 
/
/boot/efi
/boot/efi/EFI
/boot/grub2/device.map
/boot/grub2/i386-pc
/boot/grub2/i386-pc/gcry_rmd160.mod
/boot/grub2/i386-pc/acpi.mod


26. Find Last 5 Days Accessed Files

[root@nsk nskselvan]#  find / -atime 5 | more
/boot/grub2/device.map
/boot/grub2/i386-pc/gcry_rmd160.mod
/boot/grub2/i386-pc/acpi.mod
/boot/grub2/i386-pc/gcry_rsa.mod
/boot/grub2/i386-pc/adler32.mod


27. Find Last 1-5 Days Modified Files

[root@nsk nskselvan]#  find / -mtime +1 -mtime -5
/var/cache/yum/x86_64/7/extras/gen/primary_db.sqlite
/var/cache/yum/x86_64/7/extras/repomd.xml


28. Find Changed Files in Last 1 Hour

[root@nsk nskselvan]# find / -cmin -60
/var/lib/rsyslog
/var/lib/rsyslog/imjournal.state
/var/log/messages
/var/log/audit/audit.log
/var/log/cron
/home/nskselvan
/home/nskselvan/kal.txt

/home/nskselvan/kal


29. Find Modified Files in Last 1 Hour

[root@nsk nskselvan]# find / -mmin -60
/dev/pts/0
/dev/ptmx
/proc/fb
/proc/fs

/proc/fs/xfs


30. Find Accessed Files in Last 1 Hour

[root@nsk nskselvan]# find / -amin -60 | more
/boot/efi
/boot/efi/EFI
/boot/efi/EFI/centos
/boot/grub2
/boot/grub2/i386-pc
/boot/grub2/locale


31. Find 5MB Files

[root@nsk nskselvan]# find / -size 5M
/usr/bin/ld.gold

/usr/lib/x86_64-redhat-linux6E/lib64/libc.a


32. Find Size between 5MB – 7MB

[root@nsk nskselvan]# find / -size +5M -size -7M
/boot/vmlinuz-3.10.0-693.el7.x86_64
/boot/vmlinuz-0-rescue-57b26cb527b64b9f88803ad24fd81384
/boot/vmlinuz-3.10.0-693.5.2.el7.x86_64


33. Find and Delete 100MB Files (list before deleting the file)

# find / -size +100M -exec rm -rf {} \;

34. Find Specific Files and Delete (list before deleting the file)

# find / -type f -name *.mp3 -size +10M -exec rm {} \;

Saturday, March 5, 2016

vSphere 6 – new virtual hardware version 11

Saturday, March 05, 2016 0
This new hardware version enables new maximum configurations and features listed below. Below you can find a list with enhancements and a compatiblity list showing vSphere Version to supported virtual hardware versions.

New configuration maximums for virtual machines with vHW11:

    128 vCPUs
    4 TB vRAM
    32 serial ports

New features with vHW11:

    vNUMA aware hot-add RAM
    WDDM 1.1 GDI acceleration
    USB 3.0 xHCI controller
    extended support for virtual graphics incl. Nvidia vGPU

Expanded Guest OS Support:

    Solaris 11.2
    Oracle Unbreakable Enterprise Kernel Release 3 Quaterly Update 3
    Asanux 4 SP4
    Ubuntu 12.04.5 and 14.04.1
    Oracle Linux 7
    FreeBSD 9.3
    Mac OSX 10.10





Importing media into VMware Update Manager fails with the error: Cannot upload file, because it is an invalid package

Saturday, March 05, 2016 0
Whenever
Attempting to import an ESXi update image fail with below erros

    Cannot upload file, because it is an invalid package.
    Failed to import data. The uploaded upgrade package cannot be used with VMware vSphere Update Manager



The resolution will be

 “ You must use the same version of vSphere Update Manager to upgrade to the corresponding version of ESXi”

vSphere 6.0 – What’s New in VMware Fault Tolerance (FT)

Saturday, March 05, 2016 0
Finally, With vSphere 6.0, VMware broken the limitation lock of Fault Tolerance. FT VM now Supports upto 4 vCPUs and 64 GB of RAM (Which was 1 vCPu and 64 GB RAM in vSphere 5.5). With this vSMP support, Now FT can be used to protect your Mission Critical applications. Along with the vSMP FT support, There are lot more features has been added in FT with vSphere 6.0, Let’s take a look at what’s new in vSphere 6.0 Fault Tolerance(FT)

 
Benefits of Fault Tolerance
  •     Continuous Availablity with Zero downtime and Zero data loss
  •     NO TCP connections loss during failover
  •     Fault Tolerance is completely transparent to Guest OS.
  •     FT doesn’t depend on Guest OS and application
  •     Instantaneous Failover from Primary VM to Secondary VM in case of ESXi host failure
What’s New in vSphere 6.0 Fault Tolerance
  •     FT support upto 4 vCPUs and 64 GB RAM
  •     Fast Check-Pointing, a new Scalable technology is introduced to keep primary and secondary in Sync by replacing “Record-Replay”
  •     vSphere 6.0, Supports vMotion of both Primary and Secondary Virtual Machine
  •     With vSphere 6.0, You will be able to backup your virtual machines. FT supports for vStorage APIs for Data Protection (VADP) and it also supports all leading VADP solutions in Market like symantec, EMC, HP ,etc.
  •     With vSphere 6.0, FT Supports all Virtual Disk Type like EZT, Thick or Thin Provisioned disks. It supports only Eager Zeroed Thick with vSphere 5.5 and earlier versions
  •     Snapshot of FT configured Virtual Machines are supported with vSphere 6.0
  •     New version of FT keeps the Separate copies of VM files like .VMX, .VMDk files to protect primary VM from both Host and Storage failures. You are allowed to keep both Primary and Secondary VM files on different datastore.
  •