DHIS Architecture and Technology

DHIS was the developed with the intention of keeping track of changing dynamic IP addresses and, more specifically, to update DNS accordingly. In the old days users typically connected to the Internet using dialup PPP connections over analog telephone lines. The systems were not always online but, when they were, it was desirable to have a unique means to quickly reach them without having to memorise or know the assigned IP address.

At present more and more broadband Internet connections are being deployed. A few of these are, or have the possibility of being always on. Typically Internet Service Providers tend to offer these services with dynamic IP addresses, meaning that if a user disconnects the address is returned to a pool and subsequently assigned to the next user that needs it.

DHIS can be useful regardless of the way that is used to connect. In fact, DHIS is technology independent and can work on dialup lines, ISDN, WLAN, xDSL, Cable, or any means used to gain Internet connectivity. The operating principle is simple; a central server is registered as the DNS authority for a given domain (in our case dhis.org). The user registers a dynamic fully qualified domain name in the central server (e.g. myhome.dhis.org). The client computer runs the client software which then talks to the server.

Upon successful negotiation and authentication the server marks the associated user domain name online with the IP address it is currently standing on. The record is created with a TTL (time-to-live) of 60 seconds, meaning that other systems on the Internet are not allowed to cache it for more than this period.

From then on the client and server keep exchanging messages to ensure that the IP address is still valid. If the client computer disconnects, the server detects this and changes its IP address to 192.168.255.0. This address is unroutable on the Internet (RFC1918) and hence, the computer becomes unreachable.

If on the other hand, instead of disconnecting the client computer changes its IP address (reconnects for example), the server also detects this and sets its IP address to the new one.

What does this all mean? Well, for as long as you are connected to the Internet, anywhere in the world, using any connection type, your computer is reachable by others via name.dhis.org. Your IP address may change, but the name always points to it. If you disconnect, the name is kept and made to point to an unreachable address.

The DHIS server is always online listening to client requests. Clients connect to it and tell about their current IP addresses. Upon changes, the DHIS server updates DNS with the new IP address.

The DHIS client is installed once on the client machine and runs in background. No interaction is needed. The only tasks required are the initial installation and configuration (setting the valid authentication parameters). You can run a DHIS client and not touch it for years!