omapip/omapi.3

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