snmp_table_fetch_async, snmp_dialog -- SNMP client library

LIBRARY

     Begemot SNMP library (libbsnmp, -lbsnmp)

SYNOPSIS

     #include <asn1.h>
     #include <snmp.h>
     #include <snmpclient.h>

     typedef void
     (*snmp_send_cb_f)(struct snmp_pdu *req, struct snmp_pdu *resp,
         void *uarg);

     typedef void
     (*snmp_timeout_cb_f)(void *uarg);

     typedef void *
     (*snmp_timeout_start_f)(struct timeval *timeout,
         snmp_timeout_cb_f callback, void *uarg);

     typedef void
     (*snmp_timeout_stop_f)(void *timeout_id);

     extern struct snmp_client snmp_client;

     void
     snmp_client_init(struct snmp_client *client);

     int
     snmp_client_set_host(struct snmp_client *client, const char *host);

     int
     snmp_client_set_port(struct snmp_client *client, const char *port);

     int
     snmp_open(const char *host, const char *port, const char *read_community,
         const char *write_community);

     void
     snmp_close(void);

     void
     snmp_pdu_create(struct snmp_pdu *pdu, u_int op);

     int
     snmp_add_binding(struct snmp_pdu *pdu, ...);

     int
     snmp_pdu_check(const struct snmp_pdu *req, const struct snmp_pdu *resp);

     int32_t
     int
     snmp_table_fetch(const struct snmp_table *descr, void *list);

     int
     snmp_table_fetch_async(const struct snmp_table *descr, void *list,
         snmp_table_cb_f callback, void *uarg);

     int
     snmp_dialog(struct snmp_pdu *req, struct snmp_pdu *resp);

DESCRIPTION

     The SNMP library contains routines to easily build SNMP client applica-
     tions that use SNMP versions 1 or 2.  Most of the routines use a struct
     snmp_client:

           struct snmp_client {
                   enum snmp_version version;
                   int     trans;  /* transport type to use */

                   /* these two are read-only for the application */
                   char    *cport; /* port number as string */
                   char    *chost; /* host name or IP address as string */

                   char    read_community[SNMP_COMMUNITY_MAXLEN + 1];
                   char    write_community[SNMP_COMMUNITY_MAXLEN + 1];

                   struct timeval  timeout;
                   u_int   retries;

                   int     dump_pdus;

                   size_t  txbuflen;
                   size_t  rxbuflen;

                   int     fd;

                   int32_t next_reqid;
                   int32_t max_reqid;
                   int32_t min_reqid;

                   char    error[SNMP_STRERROR_LEN];

                   snmp_timeout_start_f timeout_start;
                   snmp_timeout_stop_f timeout_stop;

                   /* private */
                   char    local_path[sizeof(SNMP_LOCAL_PATH)];
           };

     The fields of this structure are described below.

     version        This is the version of SNMP to use.  See bsnmplib(3) for
     chost          The SNMP agent's host name, IP address or UNIX domain
                    socket path name.  If this is NULL (the default) localhost
                    is assumed.  This field should not be changed directly but
                    rather through calling snmp_client_set_host().

     read_community
                    This is the community name to be used for all requests
                    except SET requests.  The default is `public'.

     write_community
                    The community name to be used for SET requests.  The
                    default is `private'.

     timeout        The maximum time to wait for responses to requests.  If
                    the time elapses, the request is resent up to retries
                    times.  The default is 3 seconds.

     retries        Number of times a request PDU is to be resent.  If set to
                    0, the request is sent only once.  The default is 3
                    retransmissions.

     dump_pdus      If set to a non-zero value all received and sent PDUs are
                    dumped via snmp_pdu_dump(3).  The default is not to dump
                    PDUs.

     txbuflen       The encoding buffer size to be allocated for transmitted
                    PDUs.  The default is 10000 octets.

     rxbuflen       The decoding buffer size to be allocated for received
                    PDUs.  This is the size of the maximum PDU that can be
                    received.  The default is 10000 octets.

     fd             After calling snmp_open() this is the file socket file
                    descriptor used for sending and receiving PDUs.

     next_reqid     The request id of the next PDU to send.  Used internal by
                    the library.

     max_reqid      The maximum request id to use for outgoing PDUs.  The
                    default is INT32_MAX.

     min_reqid      The minimum request id to use for outgoing PDUs.  Request
                    ids are allocated linearily starting at min_reqid up to
                    max_reqid.

     error          If an error happens, this field is set to a printable
                    string describing the error.

     timeout_start  This field must point to a function setting up a one shot
                    timeout.  After the timeout has elapsed, the given call-
                    back function must be called with the user argument.  The
                    timeout_start() function must return a void * identifying

     library must be a call to snmp_client_init() to initialize this global
     variable to the default values.  After this call and before calling
     snmp_open() the fields of the variable may be modified by the user.  The
     modification of the chost and cport fields should be done only via the
     functions snmp_client_set_host() and snmp_client_set_port().

     The function snmp_open() creates a UDP or UNIX domain socket and connects
     it to the agent's IP address and port.  If any of the arguments of the
     call is not NULL the corresponding field in the global snmp_client is set
     from the argument.  Otherwise the values that are already in that vari-
     able are used.  The function snmp_close() closes the socket, stops all
     timeouts and frees all dynamically allocated resources.

     The next three functions are used to create request PDUs.  The function
     snmp_pdu_create() initializes a PDU of type op.  It does not allocate
     space for the PDU itself.  This is the responsibility of the caller.
     snmp_add_binding() adds bindings to the PDU and returns the (zero based)
     index of the first new binding.  The arguments are pairs of pointer to
     the OIDs and syntax constants, terminated by a NULL.  The call

           snmp_add_binding(&pdu,
               &oid1, SNMP_SYNTAX_INTEGER,
               &oid2, SNMP_SYNTAX_OCTETSTRING,
               NULL);

     adds two new bindings to the PDU and returns the index of the first one.
     It is the responsibility of the caller to set the value part of the bind-
     ing if necessary.  The functions returns -1 if the maximum number of
     bindings is exhausted.  The function snmp_oid_append() can be used to
     construct variable OIDs for requests.  It takes a pointer to an struct
     asn_oid that is to be constructed, a format string, and a number of argu-
     ments the type of which depends on the format string.  The format string
     is interpreted character by character in the following way:

     i    This format expects an argument of type asn_subid_t and appends this
          as a single integer to the OID.

     a    This format expects an argument of type struct in_addr and appends
          to four parts of the IP address to the OID.

     s    This format expects an argument of type const char * and appends the
          length of the string (as computed by strlen(3)) and each of the
          characters in the string to the OID.

     (N)  This format expects no argument.  N must be a decimal number and is
          stored into an internal variable size.

     b    This format expects an argument of type const char * and appends
          size characters from the string to the OID.  The string may contain
          NUL characters.

     c    This format expects two arguments: one of type size_t and one of
     response is received or the retransmission count is exceeded the callback
     func is called with the orignal request PDU, the response PDU and the
     user argument uarg.  If the retransmit count is exceeded, func is called
     with the original request PDU, the response pointer set to NULL and the
     user argument uarg.  The caller should not free the request PDU until the
     callback function is called.  The callback function must free the request
     PDU and the response PDU (if not NULL ).

     The function snmp_receive() tries to receive a PDU.  If the argument is
     zero, the function polls to see whether a packet is available, if the
     argument is non-zero, the function blocks until the next packet is
     received.  The packet is delivered via the usual callback mechanism (non-
     response packets are silently dropped).  The function returns 0, if a
     packet was received and successfully dispatched, -1 if an error occurred
     or no packet was available (in polling mode).

     The next two functions are used to retrieve tables from SNMP agents.
     They use the following input structure, that describes the table:

           struct snmp_table {
                   struct asn_oid          table;
                   struct asn_oid          last_change;
                   u_int                   max_iter;
                   size_t                  entry_size;
                   u_int                   index_size;
                   uint64_t                req_mask;

                   struct snmp_table_entry {
                       asn_subid_t         subid;
                       enum snmp_syntax    syntax;
                       off_t               offset;
                   }                       entries[];
           };

     The fields of this structure have the following meaning:

     table        This is the base OID of the table.

     last_change  Some tables have a scalar variable of type TIMETICKS
                  attached to them, that holds the time when the table was
                  last changed.  This OID should be the OID of this variable
                  (without the .0 index).  When the table is retrieved with
                  multiple GET requests, and the variable changes between two
                  request, the table fetch is restarted.

     max_iter     Maximum number of tries to fetch the table.

     entry_size   The table fetching routines return a list of structures one
                  for each table row.  This variable is the size of one struc-
                  ture and used to malloc(3) the structure.

     index_size   This is the number of index columns in the table.
                  all the index columns of the table, the rest are normal col-
                  umns.  If for the column at `entries[N]' the expression
                  `req_mask & (1 << N)' yields true, the column is considered
                  a required column.  The fields of this the array elements
                  have the following meaning:

                  subid   This is the OID subid of the column.  This is
                          ignored for index entries.  Index entries are
                          decoded according to the syntax field.

                  syntax  This is the syntax of the column or index.  A syntax
                          of SNMP_SYNTAX_NULL terminates the array.

                  offset  This is the starting offset of the value of the col-
                          umn in the return structures.  This field can be set
                          with the ISO-C offsetof() macro.

     Both table fetching functions return TAILQ (see queue(3)) of struc-
     tures--one for each table row.  These structures must start with a
     TAILQ_ENTRY() and a uint64_t and are allocated via malloc(3).  The list
     argument of the table functions must point to a TAILQ_HEAD().  The
     uint64_t fields, usually called found is used to indicate which of the
     columns have been found for the given row.  It is encoded like the
     req_mask field.

     The function snmp_table_fetch() synchronously fetches the given table.
     If everything is ok 0 is returned.  Otherwise the function returns -1 and
     sets an appropriate error string.  The function snmp_table_fetch_async()
     fetches the tables asynchronously.  If either the entire table is fetch,
     or an error occurs the callback function callback is called with the
     callers arguments list and uarg and a parameter that is either 0 if the
     table was fetched, or -1 if there was an error.  The function itself
     returns -1 if it could not initialize fetching of the table.

     The following table description is used to fetch the ATM interface table:

           /*
            * ATM interface table
            */
           struct atmif {
                   TAILQ_ENTRY(atmif) link;
                   uint64_t        found;
                   int32_t         index;
                   u_char          *ifname;
                   size_t          ifnamelen;
                   uint32_t        node_id;
                   uint32_t        pcr;
                   int32_t         media;
                   uint32_t        vpi_bits;
                   uint32_t        vci_bits;
                   uint32_t        max_vpcs;
                   uint32_t        max_vccs;
                   {
                     { 0, SNMP_SYNTAX_INTEGER,
                           offsetof(struct atmif, index) },
                     { 1, SNMP_SYNTAX_OCTETSTRING,
                           offsetof(struct atmif, ifname) },
                     { 2, SNMP_SYNTAX_GAUGE,
                           offsetof(struct atmif, node_id) },
                     { 3, SNMP_SYNTAX_GAUGE,
                           offsetof(struct atmif, pcr) },
                     { 4, SNMP_SYNTAX_INTEGER,
                           offsetof(struct atmif, media) },
                     { 5, SNMP_SYNTAX_GAUGE,
                           offsetof(struct atmif, vpi_bits) },
                     { 6, SNMP_SYNTAX_GAUGE,
                           offsetof(struct atmif, vci_bits) },
                     { 7, SNMP_SYNTAX_GAUGE,
                           offsetof(struct atmif, max_vpcs) },
                     { 8, SNMP_SYNTAX_GAUGE,
                           offsetof(struct atmif, max_vccs) },
                     { 9, SNMP_SYNTAX_OCTETSTRING,
                           offsetof(struct atmif, esi) },
                     { 10, SNMP_SYNTAX_INTEGER,
                           offsetof(struct atmif, carrier) },
                     { 0, SNMP_SYNTAX_NULL, 0 }
                   }
           };

           ...
                   if (snmp_table_fetch(&atmif_table, &atmif_list) != 0)
                           errx(1, "AtmIf table: %s", snmp_client.error);
           ...

     The function snmp_dialog() is used to execute a synchonuous dialog with
     the agent.  The request PDU req is sent and the function blocks until the
     response PDU is received.  Note, that asynchonuous receives are handled
     (i.e. callback functions of other send calls or table fetches may be
     called while in the function).  The response PDU is returned in resp.  If
     no response could be received after all timeouts and retries, the func-
     tion returns -1.  If a response was received 0 is returned.

     The function snmp_parse_server() is used to parse an SNMP server specifi-
     cation string and fill in the fields of a struct snmp_client.  The syntax
     of a server specification is

           [trans::][community@][server][:port]

     where trans is the transport name (one of udp, stream or dgram),
     community is the string to be used for both the read and the write commu-
     nity, server is the server's host name in case of UDP and the path name
     in case of a local socket, and port is the port in case of UDP transport.
     The function returns 0 in the case of success and return -1 and sets the
     error string in case of an error.
     Hartmut Brandt <harti@freebsd.org>
     Kendy Kutzner <kutzner@fokus.gmd.de>

BSD                             October 4, 2005                            BSD