Manual Pages  — BSNMPCLIENT

struct snmp_client {
        enum snmp_version       version;
        int                     trans;  /* which transport 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];

        /* SNMPv3 specific fields */
        int32_t                 identifier;
        int32_t                 security_model;
        struct snmp_engine      engine;
        struct snmp_user        user;

        /* SNMPv3 Access control - VACM*/
        uint32_t                clen;
        uint8_t                 cengine[SNMP_ENGINE_ID_SIZ];
        char                    cname[SNMP_CONTEXT_NAME_SIZ];

        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;

        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 applicable values. The default version is SNMP_V2c.
trans
	If this is SNMP_TRANS_LOC_DGRAM a local datagram socket is used. If it is SNMP_TRANS_LOC_STREAM a local stream socket is used. For SNMP_TRANS_UDP a UDP socket is created. It uses the chost field as the path to the server's socket for local sockets.
cport
	The SNMP agent's UDP port number. This may be a symbolic port number (from /etc/services) or a numeric port number. If this field is NULL (the default) the standard SNMP port is used. This field should not be changed directly but rather by calling snmp_client_set_port().
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'.
identifier
	The message identifier value to be used with SNMPv3 PDUs. Incremented with each transmitted PDU.
security_model
	The security model to be used with SNMPv3 PDUs. Currently only User-Based Security model specified by RFC 3414 (value 3) is supported.
engine
	The authoritive SNMP engine parameters to be used with SNMPv3 PDUs.
user
	The USM SNMP user credentials to be used with SNMPv3 PDUs.
clen
	The length of the context engine id to be used with SNMPv3 PDUs.
cengine
	The context engine id to be used with SNMPv3 PDUs. Default is empty.
cname
	The context name to be used with SNMPv3 PDUs. Default is ''.
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 callback function must be called with the user argument. The timeout_start() function must return a void * identifying the timeout.
timeout_stop
	This field must be set to a function that stops a running timeout. The function will be called with the return value of the corresponding timeout_start() function.
local_path
	If in local socket mode, the name of the clients socket. Not needed by the application.

In the current implementation there is a global variable

that is used by all the library functions. The first call into the 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 variable 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 binding 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 arguments 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 type const u_char *. The first argument gives the number of bytes to append to the OID from the string pointed to by the second argument.

The function snmp_pdu_check() may be used to check a response PDU. A number of checks are performed (error code, equal number of bindings, syntaxes and values for SET PDUs). The function returns +1 if everything is ok, 0 if a NOSUCHNAME or similar error was detected, -1 if the response PDU had fatal errors and -2 if resp is NULL (a timeout occurred).

The function snmp_pdu_send() encodes and sends the given PDU. It records the PDU together with the callback and user pointers in an internal list and arranges for retransmission if no response is received. When a response is received or the retransmission count is exceeded the callback func is called with the original 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 amp;.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 structure and used to malloc(3) the structure.
index_size
	This is the number of index columns in the table.
req_mask
	This is a bit mask with a 1 for each table column that is required. Bit 0 corresponds to the first element (index 0) in the array entries, bit 1 to the second (index 1) and so on. SNMP tables may be sparse. For sparse columns the bit should not be set. If the bit for a given column is set and the column value cannot be retrieved for a given row, the table fetch is restarted assuming that the table is currently being modified by the agent. The bits for the index columns are ignored.
entries
	This is a variable sized array of column descriptors. This array is terminated by an element with syntax SNMP_SYNTAX_NULL. The first index_size elements describe all the index columns of the table, the rest are normal columns. 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 column in the return structures. This field can be set with the ISO-C offsetof() macro.