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omapi(3)                   Library Functions Manual                   omapi(3)

       OMAPI - Object Management Application Programming Interface

       OMAPI  is an programming layer designed for controlling remote applica‐
       tions, and for querying 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  secu‐
       rity  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 application 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_con‐
       nect() function. As well as making the physical connection it will also
       set up the connection data structures to do authentication on each mes‐
       sage, 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 below.

       The easiest way to understand dhcpctl is to see it in action. The  fol‐
       lowing  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 ends.

                 #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 configured 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 prop‐
       erly 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

                      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  must  all match what is specified in the
       server's dhcpd.conf file, excepting that the secret  should  appear  in
       ´raw´ form, not in base64 as it would in dhcpd.conf:

                 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),   omshell(1),   dhcpd(8),    dhclient(8),    dhcpd.conf(5),

       omapi  is maintained by ISC.  To learn more about Internet Systems Con‐
       sortium, see

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