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BSNMPAGENT(3) FreeBSD Library Functions Manual BSNMPAGENT(3)
NAME
bsnmpagent, snmp_depop_t, snmp_op_t, tree, tree_size, snmp_trace,
snmp_debug, snmp_get, snmp_getnext, snmp_getbulk, snmp_set,
snmp_make_errresp, snmp_dep_lookup, snmp_init_context, snmp_dep_commit,
snmp_dep_rollback, snmp_dep_finish - SNMP agent library
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);
The SNMP library contains routines to easily build SNMP agent
applications that use SNMP versions 1 or 2. Note, however, that it may
be even easier to build an bsnmpd(1) loadable module, that handles the
new MIB (see snmpmod(3)).
Most of the agent routines operate on a global array that the describes
the complete MIB served by the agent. This array is held in the two
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.
SNMP_OP_GET = 1,
SNMP_OP_GETNEXT,
SNMP_OP_SET,
SNMP_OP_COMMIT,
SNMP_OP_ROLLBACK,
};
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
strategic places with a debug string. The library contains a default
implementation 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;
uint32_t int1;
uint32_t int2;
};
The fields are used as follows:
va_index For the node operation callback this is the index of the
variable binding that should be returned if an error
occurs. Set 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
functions the response PDU is constructed and encoded on the fly. If
everything 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
function 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
interdependent 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
4. If the SET step was successful for all bindings, the
dependency callbacks are executed in the order in which the
dependencies were created with an operation of
SNMP_DEPOP_COMMIT. If any of the dependencies fails, all the
committed dependencies are called again in the opposite order
with SNMP_DEPOP_ROLLBACK. Than for all bindings from the last
to the first the node's operation callback is called with
SNMP_OP_ROLLBACK to undo the effect of SNMP_OP_SET. At the
end the dependencies are freed and the finalizers are called
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
freeing 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
executed. 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
original 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
executed. This is only used in bsnmpd(1) to implement the configuration
as a single transaction. The function snmp_init_context() creates and
initializes a context. The function snmp_dep_commit() executes
If an error occurs in any of the function an error indication as
described above is returned. Additionally the functions may call
snmp_error on unexpected errors.
SEE ALSO
gensnmptree(1), bsnmpd(1), bsnmpclient(3), bsnmplib(3), snmpmod(3)
STANDARDS
This implementation conforms to the applicable IETF RFCs and ITU-T
recommendations.
AUTHORS
Hartmut Brandt <harti@FreeBSD.org>
FreeBSD 14.0-RELEASE-p11 October 4, 2005 FreeBSD 14.0-RELEASE-p11