omapi.3

DHCP服务器源码

.\"	omapi.3
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.\" This software has been written for Internet Systems Consortium
.\" by Ted Lemon in cooperation with Vixie Enterprises and Nominum, Inc.
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.TH omapi 3
.SH NAME
OMAPI - Object Management Application Programming Interface
.SH DESCRIPTION
.PP
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
'dhcpctl'
.PP
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.
.PP
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. 
.PP
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.
.SH USAGE
.PP
The client application must always call dhcpctl_initialize() before
making calls to any other dhcpctl functions. This initializes 
various internal data structures. 
.PP
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.
.PP
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.
.PP
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 10.0.0.101. It will then print out the time
the lease ends.
.PP
.nf
		#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;
.fi
.PP
All modifications of handles and all accesses of handle data happen
via dhcpctl_data_string objects.
.PP
.nf
			dhcpctl_handle connection = NULL;
			dhcpctl_handle lease = NULL;
			isc_result_t waitstatus;
			struct in_addr convaddr;
			time_t thetime;

			dhcpctl_initialize ();
.fi
.PP
Required first step.
.PP
.nf
			dhcpctl_connect (&connection, "127.0.0.1",
					 7911, 0);
.fi
.PP
Sets up the connection to the server. The server normally listens on
port 7911 unless configured to do otherwise.
.PP
.nf
			dhcpctl_new_object (&lease, connection,
					    "lease");
.fi
.PP
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.
.PP
.nf
			memset (&ipaddrstring, 0, sizeof
				ipaddrstring);

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

			omapi_data_string_new (&ipaddrstring,
					       4, MDL);
.fi
.PP
Create a new data string to storing in the handle.
.PP
.nf
			memcpy(ipaddrstring->value, &convaddr.s_addr, 4);

			dhcpctl_set_value (lease, ipaddrstring,
					   "ip-address");
.fi
.PP
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.
.PP
.nf
			dhcpctl_open_object (lease, connection, 0);
.fi
.PP
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.
.PP
.nf
			dhcpctl_wait_for_completion (lease,
						     &waitstatus);
.fi
.PP
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.
.PP
.nf
			if (waitstatus != ISC_R_SUCCESS) {
				/* server not authoritative */
				exit (0);
			}

			dhcpctl_data_string_dereference(&ipaddrstring,
							MDL);
.fi
.PP
Clean-up memory we no longer need.
.PP
.nf
			dhcpctl_get_value (&value, lease, "ends");
.fi
.PP
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.
.PP
.nf
	
			memcpy(&thetime, value->value, value->len);
			dhcpctl_data_string_dereference(&value, MDL);

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

.fi
.SH AUTHENTICATION
If the server demands authenticated connections then before opening
the connection the user must call dhcpctl_new_authenticator.
.PP
.nf
		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, 
                                           keyname,
                                           algorithm,
                                           secret,
					   strlen(secret) + 1);
.fi
.PP
The keyname, algorithm and secret must all match what is specified in
the server's dhcpd.conf file:
.PP
.nf
		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";
.fi
.PP
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.
.SH SEE ALSO
dhcpctl(3), omapi(3), dhcpd(8), dhclient(8), dhcpd.conf(5), dhclient.conf(5).
.SH AUTHOR
.B omapi
was created by Ted Lemon of Nominum, Inc.  Information about Nominum
and support contracts for DHCP and BIND can be found at
.B http://www.nominum.com.   This documentation was written by James
Brister of Nominum, Inc.