Windows XP Hacks

Hack 59 Make Servers Always Available by Mapping a Hostname to a Dynamic IP Address

Make sure that the web site or other kind of Internet server you run at home is always available to the world.

If you run your own web server, mail server, or other kind of server at home and are connected to the Internet via a cable modem or DSL modem, people frequently may not be able to connect to your server. That's because, typically, broadband ISPs assign you a dynamic IP address that changes regularly, even if you don't turn off your PC. Because your IP address constantly changes, there is no way for people to connect to you. One day its IP address might be 66.31.42.96, the next it might be 66.41.42.136, and if people don't know your server's current IP address they won't be able to find it. You won't be able to solve the problem by getting your own domain (such as www.gralla.com) and publishing that, because DNS servers won't be able to keep track of your changing IP address either. If people type in your domain name, the servers won't be able to report on your IP address—and again, your server won't be able to be reached.

There is a way to solve the problem, however: you can map your server's hostname to a dynamic IP address. When you do this, it doesn't matter that your IP address changes; when people type in your web site's URL, they will be forwarded to your new IP address automatically.

You can do this for free by signing up with a service that provides automatic mapping. A number of services will do it for free, such as No-IP.com (http://www.no-ip.com). When you sign up for the service, you choose a hostname for your server and give that hostname out to people who want to connect to the server. Whatever name you choose will end in .no-ip.com—for example, grallasite.no-ip.com.

After you get your hostname, you download client software that continually monitors your IP address. It reports on your server's current IP address to the No-IP.com site. Whenever the IP address changes, it reports that new IP address to the site. The client checks your IP address every three seconds.

Whenever a PC tries to connect to your server, it first goes to a No-IP.com server, which looks up your server's current address and then redirects the PC to your server, based on your current IP address. The person contacting your site will not have to do anything different than he normally does; he just types in your URL and is connected to your site.

If you own a domain and want to map that hostname to a dynamic IP address instead of using a No-IP.com address, you'll have to sign up for No-IP.com's No-IP Plus service for $24.95 a month.

If you're using a residential gateway at home to share Internet access among several PCs, you may run into problems using the service. Many gateways use Network Address Translation (NAT), in which all PCs on the network share a single external Internet address but are assigned internal network addresses. The No-IP.com client will track your external address, but because that single address is used by all PCs on the network, not just the server, incoming traffic won't be routed to your server. You can fix the problem by using the port forwarding feature of your route to send the incoming traffic to the server [Hack #49].

You may run into another problem as well: when you try to test your server by connecting to it from a PC inside your network, you might not be able to connect to it. That's because you might not be able to connect to the external IP address from inside the network. If this happens, the only solution is to connect to the site from a PC outside of your network or ask a friend to connect to it.

One more thing to watch out for: if you're behind a firewall, the No-IP.com client may have trouble connecting back to the No-IP.com site to report on your changing IP address. If you're using a firewall like ZoneAlarm [Hack #48] or a similar one that blocks outbound connections, tell it to allow the client to make outbound connections. Also, depending on the firewall you use, you may need to configure it to open TCP port 8245, because that's the port the client uses to contact No-IP.com with your new IP address.

5.19.1 See Also

• [Hack #40]