DHCPCTL(3)

💖 DHCPCTL(3) — dhcpctl library initialization 💖

okay so we’re diving into DHCPCTL(3), bestie, and this is part 1 — more parts are literally on the way, so don’t panic. 🫶

🌸 NAME 🌸

dhcpctl_initialize — dhcpctl library initialization.

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✨ SYNOPSIS ✨

First you grab the header, then you meet the whole function lineup. Every prototype below is copied exactly — no renaming, no “fixing,” no vibes-based edits. 💅

       #include <dhcpctl/dhcpctl.h>

       dhcpctl_status
       dhcpctl_initialize(void);

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

       dhcpctl_status
       dhcpctl_timed_connect(dhcpctl_handle *cxn, const char *host, int port,
           dhcpctl_handle auth, dhcpctl_status *status);

       dhcpctl_status
       dhcpctl_disconnect(dhcpctl_handle *cxn, int force);

       dhcpctl_status
       dhcpctl_wait_for_completion(dhcpctl_handle object,
           dhcpctl_status *status);

       dhcpctl_status
       dhcpctl_timed_wait_for_completion(dhcpctl_handle object,
           dhcpctl_status *status, struct timeval *timeout);

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

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

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

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

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

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

       dhcpctl_status
       dhcpctl_object_update(dhcpctl_handle connection, dhcpctl_handle object);

       dhcpctl_status
       dhcpctl_object_refresh(dhcpctl_handle connection,
           dhcpctl_handle object);

       dhcpctl_status
       dhcpctl_object_remove(dhcpctl_handle connection, dhcpctl_handle object);

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

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

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

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

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

       isc_result_t
       dhcpctl_data_string_dereference(dhcpctl_data_string *, const char *,
           int);

✨ DESCRIPTION ✨

okay so bestie, 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 reflect 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 — no cap, it’s literally the first move. ✨

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, "127.0.0.1", 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_timed_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. How long the function waits for complete the connection is dictated by the value of the parameter, timeout. If the value is null, it will wait indefinitely. Otherwise it will wait for the amount of time specified by timeout (tv_sec:tv_usec). Values of zero for both fields are valid but not recommended. An example is shown below:

For example:

struct timeval timeout;
timeout.tv_sec = 5;   /* wait for 5 seconds */
timeout.tv_usec = 0;

s = dhcpctl_connect(&cxn, "127.0.0.1", 7911, NULL, &timeout);

connects to the DHCP server on the localhost via port 7911 (the standard OMAPI port). No authentication is used for the connection. It allows 5 seconds for the connect to complete. ⏳

dhcpctl_disconnect() closes the open connection specified by the first parameter, cxn. Note that this call will free the connection object and cxn will be set to nul. If the second parameter, force, is nonzero, the connection will be closed immediately. Otherwise the receiving end will be shut down but any unsent data will be sent before actually closing the socket. Note that disconnecting only destroys the connection object, any other objects previously created will still exist. 🙈

For example:

s = dhcpctl_disconnect(&cxn, 1);

will close the connection immediately. This function should be considered EXPERIMENTAL.

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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)
        local_failure(s);
else if (wv != ISC_R_SUCCESS)
        server_failure(wc);

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_timed_wait_for_completion() flushes a pending message to the server and waits for the response. How long the function waits for a response is dictated by the value of the third parameter, timeout. If the value is null, it will wait indefinitely or until the connection is lost. Otherwise it will wait for the amount of time specified by timeout (tv_sec:tv_usec). Values of zero for both fields are valid but not recommended. The result of the request as processed on the server is returned via the second parameter. An example is shown below:

struct timeval timeout;
timeout.tv_sec = 5;   /* wait for 5 seconds */
timeout.tv_usec = 0;

s = dhcpctl_wait_for_completion(cxn, &wv, &timeout);
if (s != ISC_R_SUCCESS) {
        local_failure(s);
} else if (wv != ISC_R_SUCCESS) {
        server_failure(wc);
}

heads up bestie: if the function times out, the status returned will be ISC_R_TIMEDOUT. Please note that even though the function is no longer waiting for a response, the server does not abandon the request and may still respond by writing the response to the socket. A subsequent call to either this function or dhcpctl_wait_for_completion() will see that data and read it. Depending on the application logic flow this may or may not be desired. Currently though only mechanism for “flushing” this data is to close the connection by calling disconnet(), and then reconnecting via connect(). Please note this function should be considered EXPERIMENTAL.

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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. simple as that. ✨

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. self-care for your handle, honestly. 🌸

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

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(). 🦋

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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 structures, 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 setting:

• DHCPCTL_CREATE — if the object does not exist then the remote will create it
• DHCPCTL_UPDATE — update the object on the remote side using the attributes already set in the handle
• DHCPCTL_EXCL — return and error if the object exists and DHCPCTL_CREATE was also specified

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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. 🧵

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. 🫶

🎀 EXAMPLES 🎀

Okay bestie, this is the part you actually came for, so let's go 💖. 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 10.0.0.101. It will then print out the time the lease is due to expire. Note that most error checking has been omitted for brevity — yes, queen, that's on purpose, it's a demo, not your final project 🫶.

#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 "omapip/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, "127.0.0.1",
                         7911, 0);

        dhcpctl_new_object (&lease, connection,
                            "lease");

        memset (&ipaddrstring, 0, sizeof
                ipaddrstring);

        inet_pton(AF_INET, "10.0.0.101",
                  &convaddr);

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

        dhcpctl_set_value (lease, ipaddrstring,
                           "ip-address");

        dhcpctl_open_object (lease, connection, 0);

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

        dhcpctl_data_string_dereference(&ipaddrstring,
                                        MDL);

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

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

        dhcpctl_data_string_dereference(&value, MDL);

        fprintf (stdout, "ending time is %s",
                 ctime(&thetime));
}

A lil' walkthrough so the vibe check passes 💅: it calls dhcpctl_initialize (), then dhcpctl_connect to "127.0.0.1" on port 7911 with 0 as the last arg, then dhcpctl_new_object to make a "lease" object on that connection. The IP gets converted with inet_pton(AF_INET, "10.0.0.101", &convaddr), a data string of size 4 is allocated with omapi_data_string_new (&ipaddrstring, 4, MDL) and the raw address bytes get memcpy'd in — iconic lowkey byte-level behavior, we love to see it ✨. That value gets set as "ip-address" via dhcpctl_set_value, the object is opened with dhcpctl_open_object (lease, connection, 0), and then dhcpctl_wait_for_completion blocks until the server replies. If waitstatus != ISC_R_SUCCESS it's giving "/* server not authoritative */" energy and the program just exit (0)s 😭. Then it frees the address data string, pulls "ends" with dhcpctl_get_value, memcpys the time into a time_t, frees that data string too (no leaks in this house, periodt 🧹), and finally prints "ending time is %s" with ctime(&thetime). Obsessed honestly 💜.

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👀 SEE ALSO 👀

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

Go read them all, they're the extended universe bestie 🌈📚.

💖 AUTHOR 💖

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

(Yeah, the sentence just ends there in the original — the way that it's giving cliffhanger, we simply must stan a mysterious icon 💀✨. Also important: if you're shipping this in your own project, the manual page carries a copyright and license line, so keep an eye out for that wherever this doc ships from — don't be out here republishing without the receipts, k? 🙈)

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And that's a wrap on the whole page, bestie! 🎀 This one's from DHCP 3, dated Nov 15, 2000, and it lives at section DHCPCTL(3) — go forth and query some leases, you absolute main character 👑🦋💅.