LIBRARY

     Begemot SNMP library (libbsnmp, -lbsnmp)

SYNOPSIS

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

     typedef int
     (*snmp_depop_t)(struct snmp_context *ctx, struct snmp_dependency *dep,
         enum snmp_depop op);

     typedef int
     (*snmp_op_t)(struct snmp_context *ctx, struct snmp_value *val, u_int len,
         u_int idx, enum snmp_op op);

     extern struct snmp_node *tree;
     extern u_int tree_size;
     extern u_int snmp_trace;
     extern void (*snmp_debug)(const char *fmt, ...);

     enum snmp_ret
     snmp_get(struct snmp_pdu *pdu, struct asn_buf *resp_b,
         struct snmp_pdu *resp, void *data);

     enum snmp_ret
     snmp_getnext(struct snmp_pdu *pdu, struct asn_buf *resp_b,
         struct snmp_pdu *resp, void *data);

     enum snmp_ret
     snmp_getbulk(struct snmp_pdu *pdu, struct asn_buf *resp_b,
         struct snmp_pdu *resp, void *data);

     enum snmp_ret
     snmp_set(struct snmp_pdu *pdu, struct asn_buf *resp_b,
         struct snmp_pdu *resp, void *data);

     enum snmp_ret
     snmp_make_errresp(const struct snmp_pdu *pdu, struct asn_buf *req_b,
         struct asn_buf *resp_b);

     struct snmp_dependency *
     snmp_dep_lookup(struct snmp_context *ctx, const struct asn_oid *base,
         const struct asn_oid *idx, size_t alloc, snmp_depop_t func);

     struct snmp_context *
     snmp_init_context(void);

     int
     snmp_dep_commit(struct snmp_context *ctx);

     variables:

           extern struct snmp_node *tree;
           extern u_int  tree_size;

     The elements of the array are of type struct snmp_node:

           typedef int (*snmp_op_t)(struct snmp_context *, struct snmp_value *,
               u_int, u_int, enum snmp_op);

           struct snmp_node {
                   struct asn_oid oid;
                   const char      *name;          /* name of the leaf */
                   enum snmp_node_type type;       /* type of this node */
                   enum snmp_syntax syntax;
                   snmp_op_t       op;
                   u_int           flags;
                   u_int32_t       index;          /* index data */
                   void            *data;          /* application data */
                   void            *tree_data;     /* application data */
           };

     The fields of this structure are described below.

     oid     Base OID of the scalar or table column.

     name    Name of this variable.

     type    Type of this variable.  One of:

                   enum snmp_node_type {
                           SNMP_NODE_LEAF = 1,
                           SNMP_NODE_COLUMN
                   };

     syntax  The SNMP syntax of this variable.

     op      The user supplied handler for this variable.  The handler is
             called with the following arguments:

             ctx  A pointer to the context (see below).  NULL.

             val  The value to be set or retrieved.  For GETNEXT and GETBULK
                  operations the oid in this value is the current OID.  The
                  function (called in this case only for table rows) must find
                  the lexically next existing OID within the same column and
                  set the oid and value subfields accordingly.  If the table
                  column is exhausted the function must return
                  SNMP_ERR_NOSUCHNAME.  For all other operations the oid in
                  val is the oid to fetch or set.

             len  The length of the base oid without index.
             The user handler must return an appropriate SNMP v2 error code.
             If the original PDU was a version 1 PDU, the error code is mapped
             automatically.

     flags   Currently only the flag SNMP_NODE_CANSET is defined and set for
             nodes, that can be written or created.

     index   This word describes the index for table columns.  Each part of
             the index takes 4 bits starting at bit 4.  Bits 0 to 3 hold the
             number of index parts.  This arrangement allows for tables with
             up to seven indexes.  Each bit group contains the syntax for the
             index part.  There are a number of macros to help in parsing this
             field:

                   #define SNMP_INDEXES_MAX        7
                   #define SNMP_INDEX_SHIFT        4
                   #define SNMP_INDEX_MASK 0xf
                   #define SNMP_INDEX_COUNT(V)     ((V) & SNMP_INDEX_MASK)
                   #define SNMP_INDEX(V,I) \
                           (((V) >> (((I) + 1) * SNMP_INDEX_SHIFT)) & \
                           SNMP_INDEX_MASK)

     data    This field may contain arbitrary data and is not used by the
             library.

     The easiest way to construct the node table is gensnmptree(1).  Note,
     that one must be careful when changing the tree while executing a SET
     operation.  Consult the sources for bsnmpd(1).

     The global variable snmp_trace together with the function pointed to by
     snmp_debug help in debugging the library and the agent.  snmp_trace is a
     bit mask with the following bits:

           enum {
                   SNMP_TRACE_GET,
                   SNMP_TRACE_GETNEXT,
                   SNMP_TRACE_SET,
                   SNMP_TRACE_DEPEND,
                   SNMP_TRACE_FIND,
           };

     Setting a bit to true causes the library to call snmp_debug() in strate-
     gic places with a debug string.  The library contains a default implemen-
     tation for the debug function that prints a message to standard error.

     Many of the functions use a so called context:

           struct snmp_context {
                   u_int   var_index;
                   struct snmp_scratch *scratch;
                   struct snmp_dependency *dep;
                   void    *data;          /* user data */

                by the library.  In all other functions this is undefined.

     scratch    For the node operation callback this is a pointer to a per
                variable binding scratch area that can be used to implement
                the commit and rollback.  Set by the library.

     dep        In the dependency callback function (see below) this is a
                pointer to the current dependency.  Set by the library.

     data       This is the data argument from the call to the library and is
                not used by the library.

     The next three functions execute different kinds of GET requests.  The
     function snmp_get() executes an SNMP GET operation, the function
     snmp_getnext() executes an SNMP GETNEXT operation and the function
     snmp_getbulk() executes an SNMP GETBULK operation.  For all three func-
     tions the response PDU is constructed and encoded on the fly.  If every-
     thing is ok, the response PDU is returned in resp and resp_b.  The caller
     must call snmp_pdu_free() to free the response PDU in this case.  One of
     the following values may be returned:

     SNMP_RET_OK   Operation successful, response PDU may be sent.

     SNMP_RET_IGN  Operation failed, no response PDU constructed.  Request is
                   ignored.

     SNMP_RET_ERR  Error in operation.  The error code and index have been set
                   in pdu.  No response PDU has been constructed.  The caller
                   may construct an error response PDU via
                   snmp_make_errresp().

     The function snmp_set() executes an SNMP SET operation.  The arguments
     are the same as for the previous three functions.  The operation of this
     functions is, however, much more complex.

     The SET operation occurs in several stages:

           1.   For each binding search the corresponding nodes, check that
                the variable is writeable and the syntax is ok.  The writeable
                check can be done only for scalars.  For columns it must be
                done in the node's operation callback function.

           2.   For each binding call the node's operation callback with func-
                tion SNMP_OP_SET.  The callback may create dependencies or
                finalizers (see below).  For simple scalars the scratch area
                may be enough to handle commit and rollback, for interdepen-
                dent table columns dependencies may be necessary.

           3.   If the previous step fails at any point, the node's operation
                callback functions are called for all bindings for which
                SNMP_OP_SET was executed with SNMP_OP_ROLLBACK, in the oppo-
                site order.  This allows all variables to undo the effect of
                with a fail flag of 1 and the function returns to the caller
                with an appropriate error indication.

           5.   If the dependency commits were successful, for each binding
                the node's operation callback is called with SNMP_OP_COMMIT.
                Any error returned from the callbacks is ignored (an error
                message is generated via snmp_error().)

           6.   Now the dependencies are freed and the finalizers are called
                with a fail flag of 0.  For each dependency just before free-
                ing it its callback is called with SNMP_DEPOP_FINISH. Then the
                function returns SNMP_ERR_OK.

     There are to mechanisms to help in complex SET operations: dependencies
     and finalizers.  A dependency is used if several bindings depend on each
     other.  A typical example is the creation of a conceptual row, which
     requires the setting of several columns to succeed.  A dependency is
     identified by two OIDs.  In the table case, the first oid is typically
     the table's base OID and the second one the index.  Both of these can
     easily be generated from the variables OID with asn_slice_oid().  The
     function snmp_dep_lookup() tries to find a dependency based on these two
     OIDs and, if it cannot find one creates a new one.  This means for the
     table example, that the function returns the same dependency for each of
     the columns of the same table row.  This allows during the SNMP_OP_SET
     processing to collect all information about the row into the dependency.
     The arguments to snmp_dep_lookup() are: the two OIDs to identify the
     dependency (they are copied into newly created dependencies), the size of
     the structure to allocate and the dependency callback.

     When all SNMP_OP_SET operations have succeeded the dependencies are exe-
     cuted.  At this stage the dependency callback has all information about
     the given table row that was available in this SET PDU and can operate
     accordingly.

     It is guaranteed that each dependency callback is executed at minimum
     once - with an operation of SNMP_OP_ROLLBACK.  This ensures that all
     dynamically allocated resources in a callback can be freed correctly.

     The function snmp_make_errresp() makes an error response if an operation
     has failed.  It takes the original request PDU (it will look only on the
     error code and index fields), the buffer containing the original PDU and
     a buffer for the error PDU.  It copies the bindings field from the origi-
     nal PDUs buffer directly to the response PDU and thus does not depend on
     the decodability of this field.  It may return the same values as the
     operation functions.

     The next four functions allow some parts of the SET operation to be exe-
     cuted.  This is only used in bsnmpd(1) to implement the configuration as
     a single transaction.  The function snmp_init_context() creates and ini-
     tializes a context.  The function snmp_dep_commit() executes
     SNMP_DEPOP_COMMIT for all dependencies in the context stopping at the
     first error.  The function snmp_dep_rollback() executes SNMP_DEPOP_ROLL-
     mendations.

AUTHORS

     Hartmut Brandt <harti@freebsd.org>

BSD                             October 4, 2005                            BSD