DHIS Principle and Architecture:
================================

	DHIS is a client-server architecture meant to update databases
	for systems which are assigned a dynamic IP[v4] address.

	By the means of a DHIS client a host which is assigned a dynamic
	IP address (either from its ISP or from DHCP) is able to 
	communicate with a DHIS server in order to advertise its newly
	acquired IP address.

	DHIS comprises a UDP based protocol to achieve this purpose. 

	A DHIS client has a unique identification number and a set of
	authentication keys, runs in background, and attempts to reach
	its server. 
	
	The DHIS server (permanently online) listens to UDP messages
	from its clients and authenticates these against its knowledge
	of keys. When authentication is successful the DHIS server 
	updates one or more databases with the newly received IP
	address for the given client. 

	The client then keeps sending, every period of time, refresh
	messages to the server. If the server doesn't hear from a client
	for an amount of time (3x refresh period) it considers that the
	client has gone offline and marks it accordingly with 192.168.255.0
	(a private unrechable IP address).

	Alternativelly the server may receive an OFFLINE_REQ
	packet from the client, in which case the DNS record
	is updated at once and the online state droped.

	In essence the DHIS server:

		Listens to a UDP port for messages from clients
		Authenticates clients
		Checks that clients are still connected
		If not, marks them offline

Compatibility:
==============

	The DHIS release 5 server dhisd is compatible with release 3 and
	4 clients.

	Release 3 and 4 servers are not, however, compatible with 
	release 5 clients.
	
Secure QRC Authentication:
==========================

	On a secure level DHIS 5 has built in public key authentication based
	on QRC (Quadratic Residue Cypher). The server(dhisd) supports
	two methods of authentication, password and qrc. Furthermore,
	dhisd 5 is compatible with R3 authentication scheme.

	In R5 authentication (and protocol) the client sends an
	ECHO_REQ packet to the server from which it expects an
	ECHO_REP. If received the connection is established and
	the client proceeds to authentication. If mode is password,
	a simple password is sent raw over the network upon which
	the server confirms or denies the online state. If scheme
	is QRC the server sends the client an AUTH_SY packet to
	which the client must reply with an AUTH_SX before authentication
	can be confirmed.

	The QRC algorithm, as implemented in DHIS 5, is as follows:

	The client has two 100 digit keys P and Q. The server
	has (for each client) the public key N obtained by
	P*Q. P and Q are both prime and congruent to 3 mod 4.
	When authentication is requested the server generates
	a random number (prime relative to N) called X and 
	squares it mod N. It then sends its square to the client (Y)
	which by its turn has to calculte its square root
	mod N (X') using the chinese remainder theorem. X' is sent 
	back and compared with X. X' may only be calculated 
	knowing the two private keys factors P and Q. 

	In order to make use of QRC dhisd uses the GNU Multi 
	Precision Library gmp. 

Signals:
========

	dhisd accepts HUP and TERM signals. A kill -HUP will
	make it reload the hosts database and kill -TERM
	will terminate it. Its pid number is recorded a the
	text file (default: /var/run/dhis/dhisd.pid)

	Before terminating with SIGTERM dhisd will attempt to bring all 
	online clients offline.

	The pid file location can be changed with either the -P option 
	of the PidFile statement under dhisd.conf

	In addition, a kill -USR1 signal will increate the debug level of 
	dhisd by 1 and a -USR2 will reset its debug level to 0.

Logging:
========

	dhisd logs online and offline transitions on a text file
	(default: /var/log/dhis/dhisd.log)

	The log file location can be changed with either the -l option 
	of the LogFile statement under dhisd.conf

Command Line Options:
=====================

	dhisd supports the following command line options:

	-b <ipv4addr> binds the server to the interface with the specified
		      IPv4 address instead of the default 0.0.0.0 ANY

	-p <udp port> allows the server to listen to an 
	              alternative port. Default is 58800.
	
	-P <pid_file> allows the specification of an alternative port

                      e.g. dhisd -P /var/run/dhisd.pid

	-l <log_file> allows specifying a path for the log file

		      e.g. dhisd -l /var/log/dhisd.log
	
	-d <dbase_file> allows specifying a path for the database file

		      e.g. dhisd -d /usr/local/etc/dhis.db

		      If the special keyword "mysql" is given as a file, 
		      and dhisd has been compiled with MySQL support,
		      dhisd uses the authentication information present 
		      in dhisd.conf to connect to a MySQL database and 
		      use it as a database instead.

	-D	      Increases the verbosity log level by 1
	
	All options may be used in conjunction with each other.

Confirguration File
===================

	If specified and if it exists, parameters can be put on a 
	configuration file (default is /usr/local/etc/dhisd.conf)
	
	The following parameters are supported:

	LogFile		a-log-file-location
	PidFile		a-pid-file-location
	BindAddress	The address to bind the server to, default is ANY
	BindPort	The UDP port to use, default is 58000
	DBFile		The location of the database file

	See dhisd.conf.sample for an example.

MySQL Support
=============

	dhisd 5.4 adds MySQL support in that, instead of using a dhis.db file
	for keeping the hosts, these can be kept on a MySQL database table.

	In order to add MySQL suport the source code must be compiled with
	WITH_MYSQL enabled (in the Makefile) and must be linked against
	the mysqlclient library. 

	If enabled, dhisd will try to use an SQL database instead of the 
	traditional text file if and only if the DBFile is set to the special
	keyword "mysql".

	If so, dhisd needs to know how to reach the database so the following 
	dhisd.conf configuration parameters become mandatory:

	MySQLServer	127.0.0.0	; Or the IP address of the server
	MySQLUser	auser		
	MySQLPassword	userpassword
	MySQLDBase	mysql-database-to-use

	auser with userpassword must exist as a user in the SQL server and 
	must have read/write access to the DHISHost table of the database in
	use. 

	And the parameters must be correct in order to dhisd to connect 
	to the database and load it. If any of them are missed or incorrect
	dhisd won't run. 

	The server queries/updates a table named DHISHost under the specified
	database. The table should have the following schema at least, although
	more parameters can be added for other purposes without impact on the
	server.

	+-------------------+--------------+------+-----+---------+-------+
	| Field             | Type         | Null | Key | Default | Extra |
	+-------------------+--------------+------+-----+---------+-------+
	| DHISHostID        | int(11)      | NO   | PRI | 0       |       | 
	| DHISHostName      | varchar(127) | NO   | UNI |         |       | 
	| DHISHostPassword  | varchar(127) | YES  |     | NULL    |       | 
	| DHISAuthN0        | varchar(127) | YES  |     | NULL    |       | 
	| DHISAuthN1        | varchar(127) | YES  |     | NULL    |       | 
	| DHISAuthN2        | varchar(127) | YES  |     | NULL    |       | 
	| DHISAuthN3        | varchar(127) | YES  |     | NULL    |       | 
	| DHISStatus        | int(11)      | NO   |     | 0       |       | 
	| DHISOnlineCmd     | varchar(255) | YES  |     | NULL    |       | 
	| DHISOfflineCmd    | varchar(255) | YES  |     | NULL    |       | 
	| DHISLastLogin     | bigint(20)   | NO   |     | 0       |       | 
	+-------------------+--------------+------+-----+---------+-------+

Text Database
=============

	For details on using a text file as a database instead of MySQL
	(the traditional method) please see dhis.db.sample

Running dhisd as dhis user
==========================

	The server can and should be run by a user created for that purpose,
	namely dhis. Running it as root is no longer required.

Multi-server mode:
==================

	Since release 5 (due to the R5 client) DHIS may be used
	in more than one server at the same time for redundancy or
	load sharing purposes.

	DHIS R5 clients has the possibility of specifying multiple
	redundant servers. When connected the clients try to reach
	one of the available servers and use the one that provides
	a faster reply (or a reply at all if others are down).

	The only restriction to running DHIS in redundant multi-servers
	is to keep exactly the same database files on all redundant systems
	at all times.

	An example for a redundant or load-balanced dynamic DNS service:

	
		Primary DNS server A               Secondary DNS server B
                        |         +-----------------+          |
                        |                                      |
              --------------------------------------------------
              |                   |
        DHIS Server C        DHIS Server D
               \                  /
                ----- client -----

	Client attempts to communicate with C and D. Client uses whichever
	replies (first) and authenticates with it. Say, if C picks up the
	request, C issues an nsupdate to A or B. An Update is then sent to B
	or A if DNS NOTIFY is configured.

	If C is down D will keep the system running.

	If is also possible to run multiple DHIS servers on the
	same machine (using different UDP port and file locations)
	in either independent or redundant mode.


Firewalls:
==========

	In order to configure a firewall for a DHIS server ensure that:

	The server can receive UDP packets on its listening port
	(default: 58800)

	The server can send UDP packets to anywhere/anyport
	(Since the client has the possibility of changing its UDP port)


Refreshing and keeping the connection alive:
============================================

	The new R5 method of keep-alive for DHIS is as follows:

	The client authenticates with the server. Optionally the
	client (in the authentication request) sends a refresh 
	rate proposal. 

	The server updates the client with its subscribed services
	and initialises the refresh period to a default of 60 seconds.
	
	If the refresh period was proposed by the client in the 
	AUTH request the server compares this value against minimum
	and maximum limits and, if valid, uses this refresh delay
	instead.

	When N seconds have elapsed the client should send an ECHO_REQ
	to the server. If the server doesn't receive ECHO_REQs for three
	periods of time it considers the host offline and marks it 
	accordingly.


Online and Offline Commands:
============================

	The server may run individual commands for a given client at online 
	and offline transitions.

	For each client in dhis.db:

	If the client record has a line in the form:

		OnCmd /path/to/on/cmd

	/path/to/on/cmd is executed when that client becomes online.

	If the client record has a line in the form:

		OffCmd /path/to/off/cmd

	/path/to/off/cmd is executed when that client becomes offline.

	Both OffCmd and OnCmd executions occur with 2 parameters,
	the HostID of the client and the IP address acquired via DHIS.

	In addition both oncmd and offcmd commands may be invoqued
	with additional parameters (3, 4, 5, ...) as they appear
	in the dhis.db file. The first "word" after the command
	is passed to the external program as argument 3 (1 and 2
	are id and address from above), the second as argument 4
	and so on...

	Example of a simple logging test:

	1000 {
		hostpass something
		oncmd /etc/oncmd
	}


	# /etc/oncmd
	#!/bin/sh	
	#
	echo I am $1 online now at $2

	OnCmd = oncmd and OffCmd = offcmd as keywords are not case
	sensitive.


Support:
========

	Please address any questions or bugs regarding dhisd to 
        support (at) dhis.org