In order to add additional repositories on your Centos you have to run:

rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm

This command for example will add the EPEL repository. You can find additional repositories on http://wiki.centos.org/AdditionalResources/Repositories

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Searching for packages with yum:

yum list samba

Display list of updated software:

yum list updates

Apply all updates:

yum update

Update specified package

yum update bind

Check if there is an update for a specified package:

yum list updates bind

Install network setup tool

yum install system-config-network-tui

Find which package contains a specific command

yum provides OR whatprovides ifconfig

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Using tcpdump to capture packets

tcpdump -n -i eth0 port 25

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List The Open Ports And The Process That Owns Them

netstat -tulpn