We use a DNS because our external IP changes every time. So a normal reverse_tcp connects like this.
Victim’s internal IP—>Victim’s external IP—> Attacker’s external IP—> Attacker’s Internal IP
But most of the time the Attacker’s External IP is Dynamic and changes. For that we can use a free domain name server such as No-IP. First of all create an account on No-IP and configure your DUC client correctly. Next make sure you port forward you router to a new internal IP. That is your Back Track machine. Use a bridged connection in Network settings because we are going to add our BT machine to our internal IP Range. So here we use the payload as windows/meterpreter/reverse_tcp_dns in making our backdoor. As the DUC client sends our External IP and updates our no-ip domain DNS is acting static. This is how it works
read more........http://worldhack3r.wordpress.com/2013/04/17/using-dns-in-metasploit-backdoors/