Saturday, 1 March 2014

Procedure to upgrade Linux 6.1 to 6.4 without CD from a local repository

We are doing an upgrade of RHEL 6.1 to 6.4. It is applicable to other versions and distributions like CentOS.
1) Create a local repository to have all the packages of the new version available.
Create a file called /etc/yum.repos.d/server64.repo with the following contents:
name=Red Hat Enterprise Linux 6.4 x64

2) Verify what is the current running version and kernel.
# uname -a
Linux SERVER1.domain.local 2.6.32-131.0.15.el6.x86_64 #1 SMP Tue May 10 15:42:40 EDT 2011 x86_64 x86_64 x86_64 GNU/Linux
# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.1 (Santiago)

3) List the available packages and specially the kernel version. Limit yum to use the new repository only.
yum --disablerepo="*" --enablerepo="server64x64" list available
yum --disablerepo="*" --enablerepo="server64x64" list kernel

4) Run the update command and wait it for completion.
For interactive mode, when questions have to be answered to proceed with the update, use:
yum --disablerepo="*" --enablerepo="server64x64" update
For automated mode, assuming all questions will be answered as “yes”, use:
yum –y --disablerepo="*" --enablerepo="server64x64" update

Example of what will be presented on the screen:
Transaction Summary
Install      21 Package(s)
Upgrade     278 Package(s)

Total download size: 224 M
Is this ok [y/N]:

  abrt-plugin-logger.x86_64 0:1.1.16-3.el6
  abrt-plugin-mailx.x86_64 0:1.1.16-3.el6
  abrt-plugin-rhtsupport.x86_64 0:1.1.16-3.el6
  abrt-plugin-sosreport.x86_64 0:1.1.16-3.el6
  libjpeg.x86_64 0:6b-46.el6


5) Verify the new version of the OS
# uname -a
Linux SERVER1.domain.local 2.6.32-131.0.15.el6.x86_64 #1 SMP Tue May 10 15:42:40 EDT 2011 x86_64 x86_64 x86_64 GNU/Linux
#cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.4 (Santiago)

6) Reboot to finish the process and to allow the new kernel and configuration be activated.

PS: RHEL 6.4 ships samba version 3.6. It has different configuration parameters in /etc/samba/smb.conf file when doing authentication thru AD.
Below are the required changes:


;    idmap uid = 10000-20000
;    idmap gid = 10000-20000
;    idmap backend = rid:DOMAIN=10000-20000


    idmap config * : backend = tdb
    idmap config * : range = 20001-30000
    idmap config DOMAIN : backend = rid
    idmap config DOMAIN : range = 10000-20000
    idmap config DOMAIN : base_rid = 0

(Replace DOMAIN with the short name of your domain name)

7) End

Tuesday, 25 February 2014

Fix hidden NIC devices after P2V conversion of Windows servers

Fix hidden NIC devices after P2V conversion of Windows servers

After a physical conversion a number of “stale devices” will exist in the new virtual machine. These are hardware devices which are no longer required.

Windows 2000 & 2003

To identify which devices are no longer physically installed on your virtual machine, use the Microsoft environment variable devmgr_show_nonpresent_devices. This is explained in the Microsoft article:

1. From the command prompt, type:

set devmgr_show_nonpresent_devices = 1

2. Start the search. Type:

start devmgmt.msc

3. From the Device Manager, list the devices.

Select: View > Show hidden devices.

4. Deinstall the listed devices:

Select the device and uninstall it from the Device Manager.


Tuesday, 4 February 2014

How to configure the right amount of swap space in Linux

Linux and other Unix-like operating systems use the term "swap" to describe both the act of moving memory pages between RAM and disk, and the region of a disk the pages are stored on. It is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions. Now, many admins (both Windows and Linux/UNIX) follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. Let us say I've 32GB RAM, should I set swap space to 64 GB? Is 64 GB of swap space really required? How big should your Linux / UNIX swap space be?

Old dumb memory managers

I think the '2x swap space' rule came from Old Solaris and Windows admins. Also, earlier memory mangers were very badly designed. There were not very smart. Today, we have very smart and intelligent memory manager for both Linux and UNIX.

Nonsense rule: Twice the size of your main system RAM for Servers

According to OpenBSD FAQ:

Many people follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. This rule is nonsense. On a modern system, that's a LOT of swap, most people prefer that their systems never swap. You don't want your system to ever run out of RAM+swap, but you usually would rather have enough RAM in the system so it doesn't need to swap.

Select right size for your setup

Here is my rule for normal server (Web / Mail etc):

Swap space == Equal RAM size (if RAM < 2GB)

Swap space == 2GB size (if RAM > 2GB)

Many Oracle experts recommend something as follows for heavy duty Oracle server with fast storage such as RAID 10:

Swap space == Equal RAM size (if RAM < 8GB)

Swap space == 0.50 times the size of RAM (if RAM > 8GB)

However, newer servers can have 

Red Hat Recommendation

Red hat recommends setting as follows for RHEL 5:

The reality is the amount of swap space a system needs is not really a function of the amount of RAM it has but rather the memory workload that is running on that system. A Red Hat Enterprise Linux 5 system will run just fine with no swap space at all as long as the sum of anonymous memory and system V shared memory is less than about 3/4 the amount of RAM. In this case the system will simply lock the anonymous and system V shared memory into RAM and use the remaining RAM for caching file system data so when memory is exhausted the kernel only reclaims pagecache memory.

Considering that 1) At installation time when configuring the swap space there is no easy way to predetermine the memory a workload will require, and 2) The more RAM a system has the less swap space it typically needs, a better swap space

Systems with 4GB of ram or less require a minimum of 2GB of swap space

Systems with 4GB to 16GB of ram require a minimum of 4GB of swap space

Systems with 16GB to 64GB of ram require a minimum of 8GB of swap space

Systems with 64GB to 256GB of ram require a minimum of 16GB of swap space

Swap will just keep running servers...

Swap space will just keep operation running for a while on heavy duty servers by swapping process. You can always find out swap space utilization using any one of the following command:

cat /proc/swaps

swapon -s

free -m


See how to find out disk I/O and related information under Linux. In the end, you need to add more RAM, adjust software (like controlling Apache workers or using lighttpd web server to save RAM) or use some sort of load balancing.

Also, refer Linux kernel documentation for /proc/sys/vm/swappiness. With this you can fine tune swap space.

A note about Desktop and Laptop

If you are going to suspend to disk, then you need swap space more than actual RAM. For example, my laptop has 1GB RAM and swap is setup to 2GB. This only applies to Laptop or desktop but not to servers.

Kernel hackers need more swap space

If you are a kernel hacker (debugging and fixing kernel issues) and generating core dumps, you need twice the RAM swap space.


If Linux kernel is going to use more than 2GB swap space at a time, all users will feel the heat. Either, you get more RAM (recommend) and move to faster storage to improve disk I/O. There are no rules, each setup and configuration is unique. Adjust values as per your requirements. Select amount of swap that is right for you.

Monday, 6 May 2013

Setting up tftp server on Redhat Linux and allow write access to tftpboot

A TFTP Server is often used by Cisco devices to manage their file transfers. Here is a quick guide on how to setup a TFTP server on Centos/Redhat.
First obtain the RPM. In Centos, use yum to search and install tftp-server.
yum search tftp
yum install tftp-server
After it's installed, the RPM creates a new folder named 'tftpboot' in the root partition. You must change the access rights for it be read and write accessible.
chmod 777 /tftpboot
The tftp-server is controlled by xinetd which is a Linux daemon that handles certain lightweight network applications. Edit the file name 'tftp' located in /etc/xinetd.d/ . The TFTP service is defaulted to off so change the disable option to 'no'
# default: off
# description: The tftp server serves files using the trivial file transfer \
#       protocol.  The tftp protocol is often used to boot diskless \
#       workstations, download configuration files to network-aware printers, \
#       and to start the installation process for some operating systems.
service tftp
        socket_type             = dgram
        protocol                = udp
        wait                    = yes
        user                    = root
        server                  = /usr/sbin/in.tftpd
        server_args             = -s -c /tftpboot
        disable                 = no
        per_source              = 11
        cps                     = 100 2
        flags                   = IPv4
Make sure the "disable = yes" option is set to "no" to enable the service.
TFTP server also comes default read only. In order to enable write access the -c argument needs to be added to the server_args.
server_args = -s -c /tftpboot
Ensure you restart the xinetd service when making changes to this file.
service xinetd restart
You may verify the server is started by verifying that it's listening on UDP port 69.
netstat -nap |grep :69
The output should look like this:
udp        0      0        *                               19141/xinetd
If you have a firewall, make sure that UDP port 69 is opened.
End of posting.
Reliable and unexpensive web hosting and domain names at

Thursday, 21 July 2011

How to kill the process running on specific port in Linux

I would like to stop the process which is running on port 8080 in a Linux server, but I don't know the process that owns it.

'lsof -w -n -i tcp:8080'


'fuser -n tcp 8080'


'netstat -anp | grep :8080[[:blank:]]'

should show you the process ID.

If there is no (init) script to use to shut down the offending service (since TCP/8080 means proxy) to kill you can by PID 'kill -9 pidnumber'.