If you think your Linux box is under attack, print out a list of open connections on your box and sorts them by according to IP address, enter:
# netstat -atun | awk ‘{print $5}’ | cut -d: -f1 | sed -e ‘/^$/d’ |sort | uniq -c | sort -n
Output:
1 10.0.77.52
2 10.1.11.3
4 […]

To print list of all unique IP address connected to server, enter:
# netstat -nat | awk ‘{ print $5}’ | cut -d: -f1 | sed -e ‘/^$/d’ | uniq
To print total of all unique IP address, enter:
# netstat -nat | awk ‘{ print $5}’ | cut -d: -f1 | sed -e ‘/^$/d’ | uniq | […]

netstat command and shell pipe feature can be used to dig out more information about particular IP address connection. You can find out total established connections, closing connection, SYN and FIN bits and much more. You can also display summary statistics for each protocol using netstat.
This is useful to find out if your server is […]