Appendix B. Dynamic DNS Services

If your IP address is assigned dynamically, you may find it helpful to use a dynamic DNS service. A dynamic DNS service provides you with an automated way to notify them whenever your IP address changes so that they can immediately publish new DNS records for your domain. Without dynamic DNS, you would have to contact your ISP to have them change your DNS records, and your web site and other services would be unavailable for several days until the change was processed. You can easily enable the usage of a dynamic DNS service by selecting it on your server console.

A dynamic DNS service can be a great solution when used with a dedicated connection. With a typical dialup connection, your IP address changes much more frequently (possibly every time your server connects) and, because the server only connects intermittently, there is a delay in informing the dynamic DNS service of the change. This means that the risk of misdirected information is much greater with a dialup connection. For this reason, we recommend and support the use of dynamic DNS services only for dedicated connections. has tested four dynamic DNS services, of which two are free services and two are commercial services: (free), (commercial), (commercial) and (free). For simplicity, we have preprogrammed the server to work with these services (including pre-installing their client software). If you have arranged dynamic IP address assignment from your ISP and you wish to use one of these services, all you need to do is visit the appropriate web site to sign up for service, and enable that particular function in the server console. In pre-configuring the server for this particular service, we in no way interfere with nor prevent you from using another dynamic DNS service if you wish. To do so, however, would require some customized configuration on your part. has tested the functionality of these services with our software. However, we accept no liability for any breach of service on their part. A failure on the part of your dynamic DNS service can result in your network becoming temporarily unreachable from the Internet. During these times, your e-mail may be undeliverable.

If your IP address is assigned dynamically and you intend to receive all your e-mail directly (rather than having it stored at an ISP and retrieving it via POP or IMAP), but you decide not to use a dynamic DNS service, you should implement multidrop e-mail as your e-mail solution as this will ensure that no e-mail is misdirected to another IP address (See Some important notes on Service list D (multidrop mail) in Chapter 3.)

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Dynamic DNS services are not perfect. They merely point hostnames to IP addresses. If your system receives an IP address via DHCP or PPPoE, it will automatically update the dynamic DNS service each time it comes online. However, when your server disconnects from the Internet, with most dynamic DNS services your server does not indicate that it is offline in any way to the dynamic DNS service. If your system is offline for a period of time, it is possible that someone else will be assigned your IP address by your ISP. If this occurs, with most dynamic DNS services this other system will now start receiving your e-mail and web page requests until your server comes back online and updates the service with your new IP address. There is not much you can do about this, but you should be aware of this fact if there is any chance your system will be offline for a long period of time.