Technical Support

Internet Exchange Frequently Asked Questions

Internet Exchange Messaging Server
Dynamic DNS FAQ

Introduction
In order to receive non-tunneled SMTP mail messages across the Internet, the machine SMTPD is hosted on must be addressable and visible to other machines on the Internet. This is normally achieved through a DNS entry pointing to a well-defined (static) IP address. However, for some connection types (dialup, ADSL, cable, etc.) the IP address may be determined by the ISP, and periodically changed. The IP address is typically communicated to the connected host via PPP or DHCP, making static reference by a conventional DNS server impossible.

Dynamic DNS Solution
A solution to this problem is offered by Dynamic DNS Providers such as TZO. These organizations manage special DNS servers that can be updated either manually through a web form, or in an automated way by a small application running on the user's server. In the second case, the application monitors the IP address of the interface connecting the server to the Internet, and when it detects changes (or possibly at fixed intervals of time), it contacts the Dynamic DNS server asking to update the record for that machine. This change is then propogated to the non-authoritave DNS servers relatively quickly, as the TTL (time-to-live) value maintained for the machine is low (typically in the region of 5 minutes). The end result is that most of the time, the machine identified by such domain is addressable just as any conventional host with a static DNS record.

Comparison with BSMTP Tunneling
Dynamic DNS as well as BSMTP Tunneling can both be used to provide local messaging services for sites without dedicated IP addresses. Each approach has its own advantages and disadvantages.

Dynamic DNS is used to provide name service regardless of the protocols being used. This allows sites to run in addition to STMP service, other services such as HTTP (web), IMAP, POP3, FTP, and others. If the site is available online most of the time, this is an attractive option, as it makes the provision of other services the same as if it was using statically allocated IP addresses.

For sites that are infrequently connected to the Internet, and running their own SMTP servers, this can be problematic if the average connection time is not long enough. External mail servers which have mail to send to the usually disconnected site must queue the messages, usually employing a backoff algorithm on retries. This results in longer delays between attempts to reconnect as time goes on. If the retry time of remote hosts with mail to deliver is greater than the connect time of Internet Exchange host, this can result in mail never getting delivered.

To solve this problem, the connecting site must either increase the frequency of connection and/or the connection time, or employ another mechanism such as Batch SMTP Tunneling. Advantages of the BSMTP approach include no time limit on how often a host must connect to the Internet, making long disconnect times practical. Disadvantages however include having to find a BSMTP forwarder willing to tunnel BSMTP messages and store them, and the inability of the solution to provide services for protocols other than SMTP. If the site needs to provide support for other protocols, this has to be handled through other mechanisms. In the case of HTTP (web), this is typically done through remote web hosting services.

Where Can I Get More Information About Specific Dynamic DNS Providers?
The various Dynamic DNS providers each provide information pertaining to their individual service or services. Links to representative pages are found below: