omapip/omapi.3

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  31 .TH omapi 3
  32 .SH NAME
  33 OMAPI - Object Management Application Programming Interface
  34 .SH DESCRIPTION
  35 .PP
  36 OMAPI is an programming layer designed for controlling remote
  37 applications, and for querying them for their state. It is currently
  38 used by the ISC DHCP server and this outline addresses the parts of
  39 OMAPI appropriate to the clients of DHCP server. It does this by also
  40 describing the use of a thin API layered on top of OMAPI called
  41 \'dhcpctl\'
  42 .PP
  43 OMAPI uses TCP/IP as the transport for server communication, and
  44 security can be imposed by having the client and server
  45 cryptographically sign messages using a shared secret.
  46 .PP
  47 dhcpctl works by presenting the client with handles to objects that
  48 act as surrogates for the real objects in the server. For example a
  49 client will create a handle for a lease object, and will request the
  50 server to fill the lease handle's state. The client application can
  51 then pull details such as the lease expiration time from the lease
  52 handle.
  53 .PP
  54 Modifications can be made to the server state by creating handles to
  55 new objects, or by modifying attributes of handles to existing
  56 objects, and then instructing the server to update itself according to
  57 the changes made.
  58 .SH USAGE
  59 .PP
  60 The client application must always call dhcpctl_initialize() before
  61 making calls to any other dhcpctl functions. This initializes
  62 various internal data structures.
  63 .PP
  64 To create the connection to the server the client must use
  65 dhcpctl_connect() function. As well as making the physical connection
  66 it will also set up the connection data structures to do
  67 authentication on each message, if that is required.
  68 .PP
  69 All the dhcpctl functions return an integer value of type
  70 isc_result_t. A successful call will yield a result of
  71 ISC_R_SUCCESS. If the call fails for a reason local to the client
  72 (e.g. insufficient local memory, or invalid arguments to the call)
  73 then the return value of the dhcpctl function will show that. If the
  74 call succeeds but the server couldn't process the request the error
  75 value from the server is returned through another way, shown below.
  76 .PP
  77 The easiest way to understand dhcpctl is to see it in action. The
  78 following program is fully functional, but almost all error checking
  79 has been removed to make is shorter and easier to understand. This
  80 program will query the server running on the localhost for the details
  81 of the lease for IP address 10.0.0.101. It will then print out the time
  82 the lease ends.
  83 .PP
  84 .nf
  85                 #include <stdarg.h>
  86                 #include <sys/time.h>
  87                 #include <sys/socket.h>
  88                 #include <stdio.h>
  89                 #include <netinet/in.h>
  90
  91                 #include <isc/result.h>
  92                 #include <dhcpctl/dhcpctl.h>
  93
  94                 int main (int argc, char **argv) {
  95                         dhcpctl_data_string ipaddrstring = NULL;
  96                         dhcpctl_data_string value = NULL;
  97 .fi
  98 .PP
  99 All modifications of handles and all accesses of handle data happen
 100 via dhcpctl_data_string objects.
 101 .PP
 102 .nf
 103                         dhcpctl_handle connection = NULL;
 104                         dhcpctl_handle lease = NULL;
 105                         isc_result_t waitstatus;
 106                         struct in_addr convaddr;
 107                         time_t thetime;
 108
 109                         dhcpctl_initialize ();
 110 .fi
 111 .PP
 112 Required first step.
 113 .PP
 114 .nf
 115                         dhcpctl_connect (&connection, "127.0.0.1",
 116                                          7911, 0);
 117 .fi
 118 .PP
 119 Sets up the connection to the server. The server normally listens on
 120 port 7911 unless configured to do otherwise.
 121 .PP
 122 .nf
 123                         dhcpctl_new_object (&lease, connection,
 124                                             "lease");
 125 .fi
 126 .PP
 127 Here we create a handle to a lease. This call just sets up local data
 128 structure. The server hasn't yet made any association between the
 129 client's data structure and any lease it has.
 130 .PP
 131 .nf
 132                         memset (&ipaddrstring, 0, sizeof
 133                                 ipaddrstring);
 134
 135                         inet_pton(AF_INET, "10.0.0.101",
 136                                   &convaddr);
 137
 138                         omapi_data_string_new (&ipaddrstring,
 139                                                4, MDL);
 140 .fi
 141 .PP
 142 Create a new data string to storing in the handle.
 143 .PP
 144 .nf
 145                         memcpy(ipaddrstring->value, &convaddr.s_addr, 4);
 146
 147                         dhcpctl_set_value (lease, ipaddrstring,
 148                                            "ip-address");
 149 .fi
 150 .PP
 151 We're setting the ip-address attribute of the lease handle to the
 152 given address. We've not set any other attributes so when the server
 153 makes the association the ip address will be all it uses to look up
 154 the lease in its tables.
 155 .PP
 156 .nf
 157                         dhcpctl_open_object (lease, connection, 0);
 158 .fi
 159 .PP
 160 Here we prime the connection with the request to look up the lease in
 161 the server and fill up the local handle with the attributes the server
 162 will send over in its answer.
 163 .PP
 164 .nf
 165                         dhcpctl_wait_for_completion (lease,
 166                                                      &waitstatus);
 167 .fi
 168 .PP
 169 This call causes the message to get sent to the server (the message to
 170 look up the lease and send back the attribute values in the
 171 answer). The value in the variable waitstatus when the function
 172 returns will be the result from the server. If the message could
 173 not be processed properly by the server then the error will be
 174 reflected here.
 175 .PP
 176 .nf
 177                         if (waitstatus != ISC_R_SUCCESS) {
 178                                 /* server not authoritative */
 179                                 exit (0);
 180                         }
 181
 182                         dhcpctl_data_string_dereference(&ipaddrstring,
 183                                                         MDL);
 184 .fi
 185 .PP
 186 Clean-up memory we no longer need.
 187 .PP
 188 .nf
 189                         dhcpctl_get_value (&value, lease, "ends");
 190 .fi
 191 .PP
 192 Get the attribute named ``ends'' from the lease handle. This is a
 193 4-byte integer of the time (in unix epoch seconds) that the lease
 194 will expire.
 195 .PP
 196 .nf
 197
 198                         memcpy(&thetime, value->value, value->len);
 199                         dhcpctl_data_string_dereference(&value, MDL);
 200
 201                         fprintf (stdout, "ending time is %s",
 202                                  ctime(&thetime));
 203                 }
 204
 205 .fi
 206 .SH AUTHENTICATION
 207 If the server demands authenticated connections then before opening
 208 the connection the user must call dhcpctl_new_authenticator.
 209 .PP
 210 .nf
 211                 dhcpctl_handle authenticator = NULL;
 212                 const char *keyname = "a-key-name";
 213                 const char *algorithm = "hmac-md5";
 214                 const char *secret = "a-shared-secret";
 215
 216                 dhcpctl_new_authenticator (&authenticator,
 217                                            keyname,
 218                                            algorithm,
 219                                            secret,
 220                                            strlen(secret) + 1);
 221 .fi
 222 .PP
 223 The keyname, algorithm and must all match what is specified in the server's
 224 dhcpd.conf file, excepting that the secret should appear in \'raw\' form, not
 225 in base64 as it would in dhcpd.conf:
 226 .PP
 227 .nf
 228                 key "a-key-name" {
 229                         algorithm hmac-md5;
 230                         secret "a-shared-secret";
 231                 };
 232
 233                 # Set the omapi-key value to use
 234                 # authenticated connections
 235                 omapi-key a-key-name;
 236 .fi
 237 .PP
 238 The authenticator handle that is created by the call to
 239 dhcpctl_new_authenticator must be given as the last (the 4th) argument
 240 to the call to dhcpctl_connect(). All messages will then be signed
 241 with the given secret string using the specified algorithm.
 242 .SH SEE ALSO
 243 dhcpctl(3), omshell(1), dhcpd(8), dhclient(8), dhcpd.conf(5), dhclient.conf(5).
 244 .SH AUTHOR
 245 .B omapi
 246 is maintained by ISC.  To learn more about Internet Systems Consortium,
 247 see
 248 .B https://www.isc.org
 249