Dealing with dynamic IPs

With all this discussion about MX and DNS records, it might appear that running a simple Postfix SMTP server from ones home broadband connection is out of reach, since the IP address of these services are almost always dynamic. A static IP address is desired, but for many, a static IP address is either priced out of reach by their provider, or not available in residential networks at all.

Your service provider can change your IP address at any time they want. And it doesn't matter to most folks since there broadband connection is used only for web browsing and email fetching. But anyone wanting to host a web site or run a mail server from the home office has a bit of a problem, since DNS records bind a name to an IP address - if that IP address is subject to whimsical change, things don't work so good.

But! all is not lost. Remember what we said about SMTP being a store and forward protocol? This is a good thing. There are numerous facilities available that serve to bridge the gap between dynamic and static IP addresses - programs that will constantly monitor what IP address is assigned and take action if it changes.

These programs will check at some periodic interval - say every 30 minutes or so, it see what IP address the service provider has assigned your broadband connection. In the event of a change in IP address, these programs are configured to be able to log on to your DNS hosting service and make the necessary change on your behalf.

In use by this author is a program called ddclient - an open source package available on all Linux platforms and derivations. Similar ports and programs are probably available for those running MDaemon on a Windows box too.

A simple http GET command to a any URL that reports the IP address of the visitor fetches the current IP address. These URLs are readily available, and often provided by to author of the monitoring program. By configuring ddclient with information sufficient to login to DNS hosts such as zoneedit.com, easydns.com, or dyndns.org, ddclient can login and make the needed changes to your DNS records when needed.

Part of the configuration specifies which domains are subject to change. So if for example you have your mail server running on a dynamic IP and your web page from a static, you specify in the configuration, exactly which records are subject to change. That way, you don't wind up changing all records at any DNS hosting service, only the ones subject to change.

Check it out.