ISC DHCP 4.1 Manual Pages - ctl
  • 23 Dec 2021
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ISC DHCP 4.1 Manual Pages - ctl

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dhcpctl_initialize - dhcpctl library initialization.


 #include <dhcpctl/dhcpctl.h>


 dhcpctl_connect(dhcpctl_handle *cxn, const char *host, int port,
     dhcpctl_handle auth);

 dhcpctl_wait_for_completion(dhcpctl_handle object,
     dhcpctl_status *status);

 dhcpctl_get_value(dhcpctl_data_string *value, dhcpctl_handle object,
     const char *name);

 dhcpctl_get_boolean(int *value, dhcpctl_handle object, const char *name);

 dhcpctl_set_value(dhcpctl_handle object, dhcpctl_data_string value,
     const char *name);

 dhcpctl_set_string_value(dhcpctl_handle object, const char *value,
     const char *name);

 dhcpctl_set_boolean_value(dhcpctl_handle object, int value,
     const char *name);

 dhcpctl_set_int_value(dhcpctl_handle object, int value,
     const char *name);

 dhcpctl_object_update(dhcpctl_handle connection, dhcpctl_handle object);

 dhcpctl_object_refresh(dhcpctl_handle connection, dhcpctl_handle object);

 dhcpctl_object_remove(dhcpctl_handle connection, dhcpctl_handle object);

 dhcpctl_set_callback(dhcpctl_handle object, void *data,
     void (*function) (dhcpctl_handle, dhcpctl_status, void *));

 dhcpctl_new_authenticator(dhcpctl_handle *object, const char *name,
     const char *algorithm, const char *secret, unsigned secret_len);

 dhcpctl_new_object(dhcpctl_handle *object, dhcpctl_handle connection,
     const char *object_type);

 dhcpctl_open_object(dhcpctl_handle object, dhcpctl_handle connection,
     int flags);

 omapi_data_string_new(dhcpctl_data_string, *data, unsigned, int, length,
     const, char, *filename,, int, lineno);

 dhcpctl_data_string_dereference(dhcpctl_data_string *, const char *,


The dhcpctl set of functions provide an API that can be used to communicate with and manipulate a running ISC DHCP server. All functions return a value of isc_result_t. The return values reflects the result of operations to local data structures. If an operation fails on the server for any reason, then the error result will be returned through the second parameter of the dhcpctl_wait_for_completion() call.

dhcpctl_initialize() sets up the data structures the library needs to do its work. This function must be called once before any other.

dhcpctl_connect() opens a connection to the DHCP server at the given host and port. If an authenticator has been created for the connection, then it is given as the 4th argument. On a successful return the address pointed at by the first argument will have a new connection object assigned to it.

For example:

       s = dhcpctl_connect(&cxn, "", 7911, NULL);

connects to the DHCP server on the localhost via port 7911 (the standard OMAPI port). No authentication is used for the connection.

dhcpctl_wait_for_completion() flushes a pending message to the server and waits for the response. The result of the request as processed on the server is returned via the second parameter.

       s = dhcpctl_wait_for_completion(cxn, &wv);
       if (s != ISC_R_SUCCESS)
       else if (wv != ISC_R_SUCCESS)

The call to dhcpctl_wait_for_completion() won't return until the remote message processing completes or the connection to the server is lost.

dhcpctl_get_value() extracts a value of an attribute from the handle. The value can be of any length and is treated as a sequence of bytes. The handle must have been created first with dhcpctl_new_object() and opened with dhcpctl_open_object(). The value is returned via the parameter named “value”. The last parameter is the name of attribute to retrieve.

       dhcpctl_data_string value = NULL;
       dhcpctl_handle lease;
       time_t thetime;

       s = dhcpctl_get_value (&value, lease, "ends");
       assert(s == ISC_R_SUCCESS && value->len == sizeof(thetime));
       memcpy(&thetime, value->value, value->len);

dhcpctl_get_boolean() extracts a boolean valued attribute from the object handle.

The dhcpctl_set_value(), dhcpctl_set_string_value(), dhcpctl_set_boolean_value(), and dhcpctl_set_int_value() functions all set a value on the object handle.

dhcpctl_object_update() function queues a request for all the changes made to the object handle be sent to the remote for processing. The changes made to the attributes on the handle will be applied to remote object if permitted.

dhcpctl_object_refresh() queues up a request for a fresh copy of all the attribute values to be sent from the remote to refresh the values in the local object handle.

dhcpctl_object_remove() queues a request for the removal on the server of the object referenced by the handle.

The dhcpctl_set_callback() function sets up a user-defined function to be called when an event completes on the given object handle. This is needed for asynchronous handling of events, versus the synchronous handling given by dhcpctl_wait_for_completion(). When the function is called the first parameter is the object the event arrived for, the second is the status of the message that was processed, the third is the same value as the second parameter given to dhcpctl_set_callback().

The dhcpctl_new_authenticator() creates a new authenticator object to be used for signing the messages that cross over the network. The “name”, “algorithm”, and “secret” values must all match what the server uses and are defined in its configuration file. The created object is returned through the first parameter and must be used as the 4th parameter to dhcpctl_connect(). Note that the 'secret' value must not be base64 encoded, which is different from how the value appears in the dhcpd.conf file.

dhcpctl_new_object() creates a local handle for an object on the server. The “object_type” parameter is the ascii name of the type of object being accessed. e.g. "lease". This function only sets up local data struc‐ tures, it does not queue any messages to be sent to the remote side, dhcpctl_open_object() does that.

dhcpctl_open_object() builds and queues the request to the remote side. This function is used with handle created via dhcpctl_new_object(). The flags argument is a bit mask with the following values available for set‐ ting:

           if the object does not exist then the remote will create it

           update the object on the remote side using the attributes
           already set in the handle.

           return and error if the object exists and DHCPCTL_CREATE was
           also specified

The omapi_data_string_new() function allocates a new dhcpctl_data_string object. The data string will be large enough to hold “length” bytes of data. The “file” and “lineno” arguments are the source file location the call is made from, typically by using the __FILE__ and __LINE__ macros or the MDL macro defined in the ISC DHCP source file omapip/omapip.h.

dhcpctl_data_string_dereference() deallocates a data string created by omapi_data_string_new(). The memory for the object won't be freed until the last reference is released.


The following program will connect to the DHCP server running on the local host and will get the details of the existing lease for IP address It will then print out the time the lease is due to expire. Note that most error checking has been omitted for brevity.

       #include <sys/time.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <stdarg.h>

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <arpa/inet.h>

       #include "isc-dhcp/result.h"
       #include "dhcpctl.h"

       int main (int argc, char **argv) {
               dhcpctl_data_string ipaddrstring = NULL;
               dhcpctl_data_string value = NULL;
               dhcpctl_handle connection = NULL;
               dhcpctl_handle lease = NULL;
               isc_result_t waitstatus;
               struct in_addr convaddr;
               time_t thetime;

               dhcpctl_initialize ();

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

               dhcpctl_new_object (&lease, connection,

               memset (&ipaddrstring, 0, sizeof

               inet_pton(AF_INET, "",

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

               dhcpctl_set_value (lease, ipaddrstring,

               dhcpctl_open_object (lease, connection, 0);

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


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

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

               dhcpctl_data_string_dereference(&value, MDL);

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


omapi(3), omshell(1), dhcpd(8), dhclient(8), dhcpd.conf(5), dhclient.conf(5).


dhcpctl is maintained by ISC. To learn more about Internet Systems Consortium, see