snmp_depop_t, snmp_set_finish_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_set_atfinish, snmp_init_context,
snmp_dep_commit, snmp_dep_rollback -- SNMP agent library
Begemot SNMP library (libbsnmp, -lbsnmp)
#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 void
(*snmp_set_finish_t)(struct snmp_context *ctx, int fail, void *uarg);
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);
int
snmp_set_atfinish(struct snmp_context *ctx, snmp_set_finish_t func,
void *uarg);
struct snmp_context *
snmp_init_context(void);
int
snmp_dep_commit(struct snmp_context *ctx);
int
snmp_dep_rollback(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 snmpd(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 */
};
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.
idx For table columns this is the index expression from the node
(see below).
op This is the operation to execute, one of:
enum snmp_op {
SNMP_OP_GET = 1,
SNMP_OP_GETNEXT,
SNMP_OP_SET,
SNMP_OP_COMMIT,
SNMP_OP_ROLLBACK,
};
The user handler must return an appropiate 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 arrangment 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 snmpd(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;
void *data; /* user data */
};
struct snmp_scratch {
void *ptr1;
void *ptr2;
u_int32_t int1;
u_int32_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 occures. 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 occures 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 interdependend
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 opposite
order. This allows all variables to undo the effect of
the SET operation. After this all the dependencies are freed
and the finalizers are executed with a fail flag of 1. Then
the function returns to the caller with an appropriate error
indication.
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. 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.
If a SNMP_OP_SET operation fails, the dependency callbacks are never
called. The nodes SNMP_OP_ROLLBACK operations have to ensure, that any
dynamically allocated data is freed.
Finalizers are a `last change' to do processing. They are called after
everything has been done, just before returning to the user. They get a
flag, that tells them, whether the return to the user is a good one or
not. The typical use is to finally remove deleted table elements. Finalizers
are created with snmp_set_atfinish() which takes the callback function
and a user data pointer as argument.
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 three functions allow some parts of the SET operation to be executed.
This is only used in snmpd(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
SNMP_DEPOP_COMMIT for all dependencies in the context stopping at the
first error. The function snmp_dep_rollback() executes SNMP_DEPOP_ROLLBACK
starting at the previous of the current dependency in the context.
If an error occures in any of the function an error indication as
described above is returned. Additionally the functions may call
snmp_error on unexected errors.
snmpd(1), gensnmptree(1), bsnmplib(3) bsnmpclient(3), snmpmod(3)
This implementation conforms to the applicable IETF RFCs and ITU-T recommendations.
Hartmut Brandt <[email protected]>
FreeBSD 5.2.1 August 16, 2002 FreeBSD 5.2.1 [ Back ] |
