1omapi(3) Library Functions Manual omapi(3)
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6 OMAPI - Object Management Application Programming Interface
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9 OMAPI is an programming layer designed for controlling remote applica‐
10 tions, and for querying them for their state. It is currently used by
11 the ISC DHCP server and this outline addresses the parts of OMAPI
12 appropriate to the clients of DHCP server. It does this by also
13 describing the use of a thin API layered on top of OMAPI called
14 ´dhcpctl´
15 OMAPI uses TCP/IP as the transport for server communication, and secu‐
17 rity can be imposed by having the client and server cryptographically
18 sign messages using a shared secret.
19 dhcpctl works by presenting the client with handles to objects that act
21 as surrogates for the real objects in the server. For example a client
22 will create a handle for a lease object, and will request the server to
23 fill the lease handle's state. The client application can then pull
24 details such as the lease expiration time from the lease handle.
25 Modifications can be made to the server state by creating handles to
27 new objects, or by modifying attributes of handles to existing objects,
28 and then instructing the server to update itself according to the
29 changes made.
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32 The client application must always call dhcpctl_initialize() before
33 making calls to any other dhcpctl functions. This initializes various
34 internal data structures.
35 To create the connection to the server the client must use dhcpctl_con‐
37 nect() function. As well as making the physical connection it will also
38 set up the connection data structures to do authentication on each mes‐
39 sage, if that is required.
40 All the dhcpctl functions return an integer value of type isc_result_t.
42 A successful call will yield a result of ISC_R_SUCCESS. If the call
43 fails for a reason local to the client (e.g. insufficient local memory,
44 or invalid arguments to the call) then the return value of the dhcpctl
45 function will show that. If the call succeeds but the server couldn't
46 process the request the error value from the server is returned through
47 another way, shown below.
48 The easiest way to understand dhcpctl is to see it in action. The fol‐
50 lowing program is fully functional, but almost all error checking has
51 been removed to make is shorter and easier to understand. This program
52 will query the server running on the localhost for the details of the
53 lease for IP address 10.0.0.101. It will then print out the time the
54 lease ends.
55 #include <stdarg.h>
57 #include <sys/time.h>
58 #include <sys/socket.h>
59 #include <stdio.h>
60 #include <netinet/in.h>
61 #include <isc/result.h>
63 #include <dhcpctl/dhcpctl.h>
64 int main (int argc, char **argv) {
66 dhcpctl_data_string ipaddrstring = NULL;
67 dhcpctl_data_string value = NULL;
68 All modifications of handles and all accesses of handle data happen via
70 dhcpctl_data_string objects.
71 dhcpctl_handle connection = NULL;
73 dhcpctl_handle lease = NULL;
74 isc_result_t waitstatus;
75 struct in_addr convaddr;
76 time_t thetime;
77 dhcpctl_initialize ();
79 Required first step.
81 dhcpctl_connect (&connection, "127.0.0.1",
83 7911, 0);
84 Sets up the connection to the server. The server normally listens on
86 port 7911 unless configured to do otherwise.
87 dhcpctl_new_object (&lease, connection,
89 "lease");
90 Here we create a handle to a lease. This call just sets up local data
92 structure. The server hasn't yet made any association between the
93 client's data structure and any lease it has.
94 memset (&ipaddrstring, 0, sizeof
96 ipaddrstring);
97 inet_pton(AF_INET, "10.0.0.101",
99 &convaddr);
100 omapi_data_string_new (&ipaddrstring,
102 4, MDL);
103 Create a new data string to storing in the handle.
105 memcpy(ipaddrstring->value, &convaddr.s_addr, 4);
107 dhcpctl_set_value (lease, ipaddrstring,
109 "ip-address");
110 We're setting the ip-address attribute of the lease handle to the given
112 address. We've not set any other attributes so when the server makes
113 the association the ip address will be all it uses to look up the lease
114 in its tables.
115 dhcpctl_open_object (lease, connection, 0);
117 Here we prime the connection with the request to look up the lease in
119 the server and fill up the local handle with the attributes the server
120 will send over in its answer.
121 dhcpctl_wait_for_completion (lease,
123 &waitstatus);
124 This call causes the message to get sent to the server (the message to
126 look up the lease and send back the attribute values in the answer).
127 The value in the variable waitstatus when the function returns will be
128 the result from the server. If the message could not be processed prop‐
129 erly by the server then the error will be reflected here.
130 if (waitstatus != ISC_R_SUCCESS) {
132 /* server not authoritative */
133 exit (0);
134 }
135 dhcpctl_data_string_dereference(&ipaddrstring,
137 MDL);
138 Clean-up memory we no longer need.
140 dhcpctl_get_value (&value, lease, "ends");
142 Get the attribute named ``ends'' from the lease handle. This is a
144 4-byte integer of the time (in unix epoch seconds) that the lease will
145 expire.
146 memcpy(&thetime, value->value, value->len);
149 dhcpctl_data_string_dereference(&value, MDL);
150 fprintf (stdout, "ending time is %s",
152 ctime(&thetime));
153 }
154
157 If the server demands authenticated connections then before opening the
158 connection the user must call dhcpctl_new_authenticator.
159 dhcpctl_handle authenticator = NULL;
161 const char *keyname = "a-key-name";
162 const char *algorithm = "hmac-md5";
163 const char *secret = "a-shared-secret";
164 dhcpctl_new_authenticator (&authenticator,
166 keyname,
167 algorithm,
168 secret,
169 strlen(secret) + 1);
170 The keyname, algorithm and must all match what is specified in the
172 server's dhcpd.conf file, excepting that the secret should appear in
173 ´raw´ form, not in base64 as it would in dhcpd.conf:
174 key "a-key-name" {
176 algorithm hmac-md5;
177 secret "a-shared-secret";
178 };
179 # Set the omapi-key value to use
181 # authenticated connections
182 omapi-key a-key-name;
183 The authenticator handle that is created by the call to
185 dhcpctl_new_authenticator must be given as the last (the 4th) argument
186 to the call to dhcpctl_connect(). All messages will then be signed with
187 the given secret string using the specified algorithm.
188
190 dhcpctl(3), omshell(1), dhcpd(8), dhclient(8), dhcpd.conf(5),
191 dhclient.conf(5).
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194 omapi is maintained by ISC. To learn more about Internet Systems Con‐
195 sortium, see https://www.isc.org
196 omapi(3)