tools:
ibmonitor for real-time traffic consumed: http://ibmonitor.sourceforge.net/

jnettop for real-time traffic visual analysis : http://download.fedora.redhat.com/pub/epel/4/i386/repoview/jnettop.html

tcpdump for real-time packet capture
iptables for block unwanted traffic

tcpdump howto:
1. dump traffic without port 22 and write it to file log.cap

tcpdump -i eth0 not port 22 -n -w log.cap

2. tcpdump for dummies: http://www.alexonlinux.com/tcpdump-for-dummies

iptables in centos howto:
1. block outgoing traffic to specific ip
-A OUTPUT -d 10.79.2.145 -j DROP

2. block incoming traffic from specific ip
-A RH-Firewall-1-INPUT -s 10.79.2.145 -j DROP