How to set multiple IP addresses using Linux command line

Go to /etc/sysconfig/network-scripts
Make multiple copies of ifcfg-eth0 and name them as ifcfg-eth0:0, ifcfg-eth0:1 etc.
Open each of them and at least change the IPADDR to your chosen IP address and the NETMASK to the subnet mask for that IP address.
A sample entry would look like this:

DEVICE=eth0:0
BOOTPROTO=static
IPADDR=34.123.111.21
NETMASK=255.255.255.0
ONBOOT=yes

Restart the network:
/sbin/service network restart
/etc/init.d/ipaliases restart (for cpanel servers) 

If you have more than 100's of IP, it is difficult to choose above method. For that  you can try the following method.

[root@server ~]#d /etc/sysconfig/network-scripts/ – this is where the network settings are being searched for
[root@server network-scripts]# vi ifcfg-eth0-range0 – this will be the file where we define the new IP class(considering that our main interface is called eth0)
Add this content to the file:

IPADDR_START=192.168.0.1
IPADDR_END=192.168.0..28
CLONENUM_START=1
NETMASK=255.255.255.240

Explanation:
IPADDR_START – the first IP of the class you want to add
IPADDR_END – last IP of the class you want to add
CLONENUM_START – the number with which the virtual interface will start, this will create the virtual interfaces eth0:1 to eth0:28.
If you have more classes that you want to add on the next class CLONENUM will start at 29.
NETMASK – the mask for the IP class
After all this was added to the file and edited accordingly just save and restart the network.

.

Install and Configure Cacti Network Graphing Tool

Cacti is a complete frontend to RRDTool, it stores all of the necessary information to create graphs and populate them with data in a MySQL database. The frontend is completely PHP driven. Along with being able to maintain Graphs, Data Sources, and Round Robin Archives in a database, cacti handles the data gathering. There is also SNMP support for those used to creating traffic graphs with MRTG.

Required software(s)

You need to install the following software on RHEL / Fedora / CentOS Linux:

1. MySQL Server : Store cacti data.
2. NET-SNMP server - SNMP (Simple Network Management Protocol) is a protocol used for network management.
3. PHP with net-snmp module - Access SNMP data using PHP.
4. Apache / lighttpd / ngnix webserver : Web server to display graphs created with PHP and RRDTOOL

First, login as root user and type the following command to install mysql, apache and php
# yum install mysql-server mysql php-mysql php-pear php-common php-gd php-devel php php-mbstring php-cli php-snmp php-pear-Net-SMTP php-mysql httpd

Configure MySQL server

First, set root password:
# mysqladmin -u root password NEWPASSWORD

Create cacti MySQL database
Create a database called cacti, enter:
# mysql -u root -p -e 'create database cacti'

Create a user called cacti with a password called zYn95ph43zYtq, enter:
# mysql -u root -p

mysql> GRANT ALL ON cacti.* TO cacti@localhost IDENTIFIED BY 'zYn95ph43zYtq';
mysql> FLUSH privileges;
mysql> \q

Install snmpd

Type the following command to install net-snmpd
# yum install net-snmp-utils php-snmp net-snmp-libs
Configure snmpd, open /etc/snmp/snmpd.conf
# vi /etc/snmp/snmpd.conf

Append / modify it as follows (see snmpd.conf man page for details):

com2sec local     localhost           public
group MyRWGroup v1         local
group MyRWGroup v2c        local
group MyRWGroup usm        local
view all    included  .1                               80
access MyRWGroup ""      any       noauth    exact  all    all    none
syslocation Unknown (edit /etc/snmp/snmpd.conf)
syscontact Root  (configure /etc/snmp/snmp.local.conf)
pass .1.3.6.1.4.1.4413.4.1 /usr/bin/ucd5820stat

Save and close the file. Turn on snmpd service:
# /etc/init.d/snmpd start
# chkconfig snmpd on

Make sure you are getting information from snmpd:
# snmpwalk -v 1 -c public localhost IP-MIB::ipAdEntIfIndex
Sample ouptut:

IP-MIB::ipAdEntIfIndex.10.10.29.68 = INTEGER: 2
IP-MIB::ipAdEntIfIndex.67.yy.zz.eee = INTEGER: 3
IP-MIB::ipAdEntIfIndex.127.0.0.1 = INTEGER: 1

Install cacti

First, make sure EPEL repo is enabled. Type the following command to install cacti:
# yum install cacti

To install EPEL repo
http://www.cyberciti.biz/faq/rhel-fedora-centos-linux-enable-epel-repo/ 



Install cacti tables

Type the following command to find out cacti.sql path:
# rpm -ql cacti | grep cacti.sql
Sample output:

/usr/share/doc/cacti-0.8.7d/cacti.sql

Type the following command to install cacti tables (you need to type the cacti user password):
# mysql -u cacti -p cacti < /usr/share/doc/cacti-0.8.7d/cacti.sql

Configure cacti

Open /etc/cacti/db.php file, enter:
# vi /etc/cacti/db.php
Make changes as follows:

 
/* make sure these values refect your actual database/host/user/password */
$database_type = "mysql";
$database_default = "cacti";
$database_hostname = "localhost";
$database_username = "cacti";
$database_password = "zYn95ph43zYtq";
$database_port = "3306";
 

Save and close the file.

Configure httpd

Open /etc/httpd/conf.d/cacti.conf file, enter:
# vi /etc/httpd/conf.d/cacti.conf


You need to update allow from line. Either set to ALL or your LAN subnet to allow access to cacti:

 
#
# Cacti: An rrd based graphing tool
#
Alias /cacti    /usr/share/cacti
 

        Order Deny,Allow
        Deny from all
        Allow from 10.0.0.0/8
 

Another option is create /usr/share/cacti/.htaccess file and password protect the directory. Finally, restart httpd:
# service httpd restart

Setup cacti cronjob

Open /etc/cron.d/cacti file, enter:
# vi /etc/cron.d/cacti
Uncomment the line:

*/5 * * * *     cacti   /usr/bin/php /usr/share/cacti/poller.php > /dev/null 2>&1

Save and close the file.

Run cacti installer

Now cacti is ready to install. Fire a webbrowser and type the url:
http://your.example.com/cacti/
OR
http://your.server.ip.address/cacti/

The default username and password for cacti is admin / admin. Upon first login, you will be force to change the default password.


.

Semaphore issue - Unable to start apache.

Semaphores are used for communicating between the active processes of a certain application. In the case of Apache, they're used to communicate between the parent and child processes. If Apache can't write these things down, then it can't communicate properly with all of the processes it starts.
I'd assume if you're reading this article, Apache has stopped running. Run this command as root
# ipcs -s


If you see a list of semaphores, Apache has not cleaned up after itself, and some semaphores are stuck. Clear them out with this command:
# for i in `ipcs -s | awk '/httpd/ {print $2}'`; do (ipcrm -s $i); done

Now, in almost all cases, Apache should start properly.  If it doesn't, 
you may just be completely out of available semaphores.  You may want to
 increase your available semaphores, and you'll need to tickle your 
kernel to do so.  Add this to /etc/sysctl.conf:

kernel.msgmni = 1024
kernel.sem = 250 256000 32 1024
And then run sysctl -p to pick up the new changes.

how to know the OS version in linux

uname -a
cat /proc/version
cat /etc/issue

/etc/redhat-release
/etc/debian_version
/etc/SuSE-release
/etc/slackware-version
/etc/gentoo-release