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omapi(3)										 omapi(3)

       OMAPI - Object Management Application Programming Interface

       OMAPI is an programming layer designed for controlling remote applications, and for query-
       ing them for their state. It is currently used by the ISC DHCP  server  and  this  outline
       addresses  the  parts  of OMAPI appropriate to the clients of DHCP server. It does this by
       also describing the use of a thin API layered on top of OMAPI called

       OMAPI uses TCP/IP as the transport for server communication, and security can  be  imposed
       by having the client and server cryptographically sign messages using a shared secret.

       dhcpctl	works by presenting the client with handles to objects that act as surrogates for
       the real objects in the server. For example a client will create  a  handle  for  a  lease
       object,	and will request the server to fill the lease handle's state. The client applica-
       tion can then pull details such as the lease expiration time from the lease handle.

       Modifications can be made to the server state by creating handles to new  objects,  or  by
       modifying  attributes  of  handles to existing objects, and then instructing the server to
       update itself according to the changes made.

       The client application must always call dhcpctl_initialize() before making  calls  to  any
       other dhcpctl functions. This initializes various internal data structures.

       To  create the connection to the server the client must use dhcpctl_connect() function. As
       well as making the physical connection it will also set up the connection data  structures
       to do authentication on each message, if that is required.

       All  the dhcpctl functions return an integer value of type isc_result_t. A successful call
       will yield a result of ISC_R_SUCCESS. If the call fails for a reason local to  the  client
       (e.g.  insufficient  local memory, or invalid arguments to the call) then the return value
       of the dhcpctl function will show that. If the  call  succeeds  but  the  server  couldn't
       process the request the error value from the server is returned through another way, shown

       The easiest way to understand dhcpctl is to see it in action.  The  following  program  is
       fully  functional,  but	almost all error checking has been removed to make is shorter and
       easier to understand. This program will query the server running on the localhost for  the
       details	of the lease for IP address It will then print out the time the lease

		 #include <stdarg.h>
		 #include <sys/time.h>
		 #include <sys/socket.h>
		 #include <stdio.h>
		 #include <netinet/in.h>

		 #include <isc/result.h>
		 #include <dhcpctl/dhcpctl.h>

		 int main (int argc, char **argv) {
		      dhcpctl_data_string ipaddrstring = NULL;
		      dhcpctl_data_string value = NULL;

       All  modifications  of  handles	 and   all   accesses	of   handle   data   happen   via
       dhcpctl_data_string objects.

		      dhcpctl_handle connection = NULL;
		      dhcpctl_handle lease = NULL;
		      isc_result_t waitstatus;
		      struct in_addr convaddr;
		      time_t thetime;

		      dhcpctl_initialize ();

       Required first step.

		      dhcpctl_connect (&connection, "",
				 7911, 0);

       Sets up the connection to the server. The server normally listens on port 7911 unless con-
       figured to do otherwise.

		      dhcpctl_new_object (&lease, connection,

       Here we create a handle to a lease. This call just  sets  up  local  data  structure.  The
       server  hasn't  yet made any association between the client's data structure and any lease
       it has.

		      memset (&ipaddrstring, 0, sizeof

		      inet_pton(AF_INET, "",

		      omapi_data_string_new (&ipaddrstring,
				       4, MDL);

       Create a new data string to storing in the handle.

		      memcpy(ipaddrstring->value, &convaddr.s_addr, 4);

		      dhcpctl_set_value (lease, ipaddrstring,

       We're setting the ip-address attribute of the lease handle to the given address. We've not
       set  any  other attributes so when the server makes the association the ip address will be
       all it uses to look up the lease in its tables.

		      dhcpctl_open_object (lease, connection, 0);

       Here we prime the connection with the request to look up the lease in the server and  fill
       up the local handle with the attributes the server will send over in its answer.

		      dhcpctl_wait_for_completion (lease,

       This  call  causes the message to get sent to the server (the message to look up the lease
       and send back the attribute values in the answer). The value in	the  variable  waitstatus
       when  the function returns will be the result from the server. If the message could not be
       processed properly by the server then the error will be reflected here.

		      if (waitstatus != ISC_R_SUCCESS) {
			   /* server not authoritative */
			   exit (0);


       Clean-up memory we no longer need.

		      dhcpctl_get_value (&value, lease, "ends");

       Get the attribute named ``ends'' from the lease handle. This is a 4-byte  integer  of  the
       time (in unix epoch seconds) that the lease will expire.

		      memcpy(&thetime, value->value, value->len);
		      dhcpctl_data_string_dereference(&value, MDL);

		      fprintf (stdout, "ending time is %s",

       If  the	server	demands  authenticated connections then before opening the connection the
       user must call dhcpctl_new_authenticator.

		 dhcpctl_handle authenticator = NULL;
		 const char *keyname = "a-key-name";
		 const char *algorithm = "hmac-md5";
		 const char *secret = "a-shared-secret";

		 dhcpctl_new_authenticator (&authenticator,
				   strlen(secret) + 1);

       The keyname, algorithm and secret must  all  match  what  is  specified	in  the  server's
       dhcpd.conf file:

		 key "a-key-name" {
		      algorithm hmac-md5;
		      secret "a-shared-secret";

		 # Set the omapi-key value to use
		 # authenticated connections
		 omapi-key "a-key-name";

       The  authenticator handle that is created by the call to dhcpctl_new_authenticator must be
       given as the last (the 4th) argument to the call to dhcpctl_connect(). All  messages  will
       then be signed with the given secret string using the specified algorithm.

       dhcpctl(3), omapi(3), dhcpd(8), dhclient(8), dhcpd.conf(5), dhclient.conf(5).

       omapi  was  created  by	Ted Lemon of Nominum, Inc.  Information about Nominum and support
       contracts for DHCP and BIND can be found at This documentation was
       written by James Brister of Nominum, Inc.

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