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);
int
snmp_dep_rollback(struct snmp_context *ctx);
void
snmp_dep_finish(struct snmp_context *ctx);

DESCRIPTION

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.

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 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) & SN

MP_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 gensn

mptree(1). Note,
that one must be careful when changing the tree while exe

cuting 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 de

fault implementation for the debug function that prints a message to stan

dard 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 */
enum snmp_ret code; /* return code */

};

struct snmp_scratch {

void *ptr1;
void *ptr2;
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 re

turned 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 re

quests. 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 con

structed.

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 oper

ation 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 call

back with func
tion SNMP_OP_SET. The callback may create depen

dencies 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
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 dependen

cies are freed
and the finalizers are executed with a fail flag

of 1. Then
the function returns to the caller with an appro

priate error
indication.

4. If the SET step was successful for all bindings,

the depen
dency callbacks are executed in the order in

which the dependencies were created with an operation of SN

MP_DEPOP_COMMIT.
If any of the dependencies fails, all the commit

ted 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 SN

MP_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 SN

MP_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_DE

POP_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 de

pendency 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 dependen

cies), the size of
the structure to allocate and the dependency callback.

When all SNMP_OP_SET operations have succeeded the dependen

cies are executed. At this stage the dependency callback has all infor

mation 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 val

ues as the
operation functions.

The next four functions allow some parts of the SET opera

tion to be executed. This is only used in bsnmpd(1) to implement the con

figuration as
a single transaction. The function snmp_init_context() cre

ates and initializes a context. The function snmp_dep_commit() executes SNMP_DEPOP_COMMIT for all dependencies in the context stop

ping at the
first error. The function snmp_dep_rollback() executes SN

MP_DEPOP_ROLLBACK starting at the previous of the current dependency in

the context.
The function snmp_dep_finish() executes SNMP_DEPOP_FINISH

for all dependencies.

DIAGNOSTICS

If an error occurs in any of the function an error indica

tion 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), sn

mpmod(3)

STANDARDS

This implementation conforms to the applicable IETF RFCs and

ITU-T recommendations.

AUTHORS

Hartmut Brandt <harti@freebsd.org>

BSD October 4, 2005

BSD