Friday, 24 March 2017

how to set display resolution permanently even after reboot

Use System -> Preferences -> Display menu option or gnome-display-properties tool to configure desired display resolution for user.

This tool will create '~/.config/monitors.xml' file which will be referred each time upon user login.

The same file can be copied to /var/lib/gdm/.config/ directory for gdm to have that display resolution upon reboot.

Friday, 20 January 2017

Increase Disk Size VirtualBox Host CentOS 6.5 and guest windows 7

Power off the guest virtual machine.

#VBoxManage modifyhd /root/VirtualBox VMs/windows/windows.vdi --resize 46080


That will resize a virtual disk image to 45GB.

After that check the disk management in windows 7 then you will find unallocated space. allocate that space to drive.

that's it.

Tuesday, 19 July 2016

How to Mount External Hard Drive as a partition on CentOS 6.5

First, list devices on your system using one of the following command :

#fdisk -l | grep '^Disk'

Now, assuming your new drive is “sda”, use the following command to invoke the fdisk partition utility :

#fdisk /dev/sda

To create a new partition (assuming this is a new unused drive), just type “n“. Then, save your work typing “w” and “q” to exit the utility.
Any doubt or need help about fdisk utility, just type “m“.

Let’s format the partition as traditional EXT4 Linux file system :

#mkfs.ext4 /dev/sda

Okay, the hard disk partitioning and formatting is done now.

Create a directory where you want to have your drive mounted (replace all “external_hd” value with the name you want to use ) :

#mkdir /data

#mount /dev/sda /data

You’re now ready to use it! However, this mount will not survive to a reboot. To make it permanent, you need to edit fstab :

#vi /etc/fstab

/dev/sda        /data          ext4        defaults     0 0

NOTE : If you manually mount the drive instead doing the fstab way, do not forget to manually unmount it before unplug it! Serious data loss problem may occur if you skip this step!

#umount /dev/sda

How to install Webmin on CentOS 6.5

Download rpm file 
# wget
Install from rpm file 
# rpm -Uvh webmin-1.660-1.noarch.rpm
Check the status of daemon 
# service webmin status
Start at boot 
# chkconfig webmin on

Saturday, 9 July 2016

Starting libvirtd daemon: libvirtd: relocation error: libvirtd: symbol dm_task_get_info_with_deferred_remove, version Base not defined in file with link time reference

[root@org Desktop]# /etc/init.d/libvirtd start
Starting libvirtd daemon: libvirtd: relocation error: libvirtd: symbol dm_task_get_info_with_deferred_remove, version Base not defined in file with link time reference

#yum upgrade device-mapper-libs

Tuesday, 21 June 2016

How to install OpenLDAP Server/Client and Configure on RHEL 6 Centos 6

Step 1. Install OpenLDAP packages via YUM 
#yum install openldap*

Step 2. Now generate a encrypted password for Administrator User That is "Manager"
New password: redhat
Re-enter new password: redhat

The above command will generate the password something like 

NOTE: You need to copy above generated password

Step 4. Now Configure OpenLDAP Server, so edit the following file:
#vim /etc/openldap/slapd.d/"cn=config"/"olcDatabase={2}bdb.ldif"

Inside this file do the following changes:
olcSuffix: dc=example,dc=com

olcRootDN: cn=Manager,dc=example,dc=com

Inside this file create the following lines:
olcTLSCertificateFile: /etc/pki/tls/certs/example.pem
olcTLSCertificateKeyFile: /etc/pki/tls/certs/examplekey.pem

:wq (save and exit) 

Step 5. Now specify the Monitoring privileges 
#vim /etc/openldap/slapd.d/"cn=config"/"olcDatabase={1}monitor.ldif"

Inside this file search the following "cn=manager,dc=my-domain,dc=com" 
and change this into "cn=Manager,dc=example,dc=com"

:wq (save and exit)

Step 6. Now copy the sample database file 
#cp /usr/share/openldap-servers/DB_CONFIG.example /var/lib/ldap/DB_CONFIG

You need to change owner and group ownership of this Database
#chown -R ldap:ldap /var/lib/ldap/

Now update the database

Step 7.  Configure OpenLDAP to listen on SSL/TLS 
#vim /etc/sysconfig/ldap 

SLAPD_LDAPS=yes #(default is no)

:wq (save and exit)

Step 8. Now you need to create a certificate for OpenLDAP Server. you can configure CA Server or something else, But in this example, I am creating a self sign certificate. 

# openssl req -new -x509 -nodes -out /etc/pki/tls/certs/example.pem -keyout /etc/pki/tls/certs/examplekey.pem -days 365

Country Name (2 letter code) [XX]:IN
State or Province Name (full name) []:Delhi
Locality Name (eg, city) [Default City]:New Delhi
Organization Name (eg, company) [Default Company Ltd]:Example, Inc.
Organizational Unit Name (eg, section) []:IT
Common Name (eg, your name or your server's hostname) []
Email Address []

Step 9. You need to change owner and group ownership of certificate and keyfile
#chown -Rf root:ldap /etc/pki/tls/certs/example.pem 
#chown -Rf root:ldap /etc/pki/tls/certs/examplekey.pem

You can also check, owner and group ownership changed or not
# ls -l /etc/pki/tls/certs/example*

Step 10. Start/Restart the service of OpenLDAP
# service slapd restart
#chkconfig slapd on

Step 11. Now you need to create base objects in OpenLDAP. 

NOTE: base objects means you have to create dn: for domain name, for OUs, so to creating dn:, you have to defining objectclass. 

there are two ways, (1). you can create it manually (2). you can use migration tools. In this example I am using migration tools. 

#yum install migrationtools 

# cd /usr/share/migrationtools/
# ls

You will see lot of files and scripts here. So you need to change some predefined values according to your domain name, for that do the following:

# vim

on the Line Number 61, change "ou=Groups" 
  $NAMINGCONTEXT{'group'}             = "ou=Groups";

 on the Line Number 71, change your domain name 

on the line number 74, change your base name 
$DEFAULT_BASE = "dc=example,dc=com";

on the line number 90, change schema value

:wq (save and exit)

Now generate a base.ldif file for your Domain, use the following:
#./ > /root/base.ldif

If you want to migrate your local users and groups on LDAP do the following:
first I am creating 5 local users and groups and then I will migrate to LDAP. 

#mkdir /home/guests
#useradd -d /home/guests/ldapuser1 ldapuser1
#useradd -d /home/guests/ldapuser2 ldapuser2
#useradd -d /home/guests/ldapuser3 ldapuser3
#useradd -d /home/guests/ldapuser4 ldapuser4
#useradd -d /home/guests/ldapuser5 ldapuser5

Now assign the password 
#passwd ldapuser1
#passwd ldapuser2
#passwd ldapuser3
#passwd ldapuser4
#passwd ldapuser5

Now you need to filter out these users from /etc/passwd to another file:
#getent passwd | tail -n 5 > /root/users

Now you need to filter out password information from /etc/shadow to another file:
# getent shadow | tail -n 5 > /root/passwords

Now you need to filter out user groups from /etc/group to another file:
# getent group | tail -n 5 > /root/groups

Now you have to generate ldif file of these filtered out files of users, passwords, and groups

So Open the following file to change the location of password file
# vim 

Inside this file search /etc/shadow and change it to /root/passwords and then save and exit

NOTE: "/etc/shadow" will be available approx the line number of 188. 

Now generate a ldif file for users 
# ./ /root/users > /root/users.ldif

Now Generate a ldif file for groups 
# ./ /root/groups > /root/groups.ldif

Step 12. Now it' time to upload these ldif file to LDAP Server 

#ldapadd -x -W -D "cn=Manager,dc=example,dc=com" -f /root/base.ldif 

# ldapadd -x -W -D "cn=Manager,dc=example,dc=com" -f /root/users.ldif

# ldapadd -x -W -D "cn=Manager,dc=example,dc=com" -f /root/groups.ldif 

NOTE: It will as a password of "Manager", you have to type the password which you generated in encrypted format. 

Now you can use "ldapsearch" command 

# ldapsearch -x -b "dc=example,dc=com"

Step 13. Now you need to share LDAP Users Home Directories via NFS they can mount the home directory on client machine. 

#vim /etc/exports 


:wq (save and exit)

# service nfs restart 
# chkconfig nfs on
# service iptables stop 
# chkconfig iptables off

Step 14. Now you need to copy your LDAP Server certificate in to /var/ftp/pub/. 
# cp -rvf /etc/pki/tls/certs/example.pem /var/ftp/pub/
# ln -s /var/ftp/pub/ /var/www/html/
# service vsftpd restart
# chkconfig vsftpd on
# service httpd restart 
# chkconfig httpd on

Now go to the Client Machine and configure it to use LDAP Server. 

# authconfig-gtk 

Click on "Identity & Authentication" Tab 
Click on drop down menu in "User Account Database" and Select "LDAP"
in LDAP Search Base DN: dc=example,dc=com
in LDAP Server: ldap://
Select the check Box of "Use TLS to encrypt connections
Click "Download CA Certificate
In Certificate URL: type
Click "OK"

# getent passwd ldapuser1

Now Configure your client machine to access the home directory as well 
# vim /etc/auto.master

create the following New Line 
/home/guests    /etc/auto.guests

:wq (save and exit)

# vim /etc/auto.guests
*       -rw

# service autofs reload

#su - ldapuser1

Friday, 17 June 2016

can't boot windows 8 after installing centos 7

The "/etc/grub2.cfg" is not updated even if it's detecting the windows partition at the terminal.

run the below command


check the section "### BEGIN /etc/grub.d/30_os-prober ###", and copied the menu entry from there in the terminal

add the menu entry to the grub2 config file "/etc/grub2.cfg" in the same section "### BEGIN /etc/grub.d/30_os-prober ###"

finally, boot to the windows os