omapi(3)                   Library Functions Manual                   omapi(3)

       OMAPI - Object Management Application Programming Interface

       OMAPI is an programming layer designed for controlling remote
       applications, 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
       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 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_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 below.

       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 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
       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

                      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), omapi(3), dhcpd(8), dhclient(8), dhcpd.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.