====== Obtain external IP address ======

    http://ifconfig.me
    http://checkmyip.com
    http://checkip.dyndns.org|sed -e 's/.*: //;s/<.*//'
    http://api.ipify.org

Simple server which listens on a port and prints out received data

    nc -l -p portnumber

Show apps that use internet connection at the moment.

    lsof -P -i -n | cut -f 1 -d " "| uniq | tail -n +2

Block known dirty hosts from reaching your machine

    wget -qO - http://infiltrated.net/blacklisted|awk '!/#|[a-z]/&&/./{print "iptables -A INPUT -s "$1" -j DROP"}'

smtp server

    python -m smtpd -n -c DebuggingServer localhost:1025