bsnmp/lib/bsnmpagent.3

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.\" Copyright (c) 2004-2005
.\"	Hartmut Brandt.
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.\"	Fraunhofer Institute for Open Communication Systems (FhG Fokus).
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.\" Author: Harti Brandt <harti@FreeBSD.org>
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.\" $Begemot: bsnmp/lib/bsnmpagent.3,v 1.10 2005/10/04 08:46:49 brandt_h Exp $
.\"
.Dd October 4, 2005
.Dt BSNMPAGENT 3
.Os
.Sh NAME
.Nm bsnmpagent ,
.Nm snmp_depop_t ,
.Nm snmp_op_t ,
.Nm tree ,
.Nm tree_size ,
.Nm snmp_trace ,
.Nm snmp_debug ,
.Nm snmp_get ,
.Nm snmp_getnext ,
.Nm snmp_getbulk ,
.Nm snmp_set ,
.Nm snmp_make_errresp ,
.Nm snmp_dep_lookup ,
.Nm snmp_init_context ,
.Nm snmp_dep_commit ,
.Nm snmp_dep_rollback ,
.Nm snmp_dep_finish
.Nd "SNMP agent library"
.Sh LIBRARY
Begemot SNMP library
.Pq libbsnmp, -lbsnmp
.Sh SYNOPSIS
.In asn1.h
.In snmp.h
.In snmpagent.h
.Ft typedef int
.Fn (*snmp_depop_t) "struct snmp_context *ctx" "struct snmp_dependency *dep" "enum snmp_depop op"
.Ft typedef int
.Fn (*snmp_op_t) "struct snmp_context *ctx" "struct snmp_value *val" "u_int len" "u_int idx" "enum snmp_op op"
.Vt extern struct snmp_node *tree ;
.Vt extern u_int tree_size ;
.Vt extern u_int snmp_trace ;
.Vt extern void (*snmp_debug)(const char *fmt, ...) ;
.Ft enum snmp_ret
.Fn snmp_get "struct snmp_pdu *pdu" "struct asn_buf *resp_b" "struct snmp_pdu *resp" "void *data"
.Ft enum snmp_ret
.Fn snmp_getnext "struct snmp_pdu *pdu" "struct asn_buf *resp_b" "struct snmp_pdu *resp" "void *data"
.Ft enum snmp_ret
.Fn snmp_getbulk "struct snmp_pdu *pdu" "struct asn_buf *resp_b" "struct snmp_pdu *resp" "void *data"
.Ft enum snmp_ret
.Fn snmp_set "struct snmp_pdu *pdu" "struct asn_buf *resp_b" "struct snmp_pdu *resp" "void *data"
.Ft enum snmp_ret
.Fn snmp_make_errresp "const struct snmp_pdu *pdu" "struct asn_buf *req_b" "struct asn_buf *resp_b"
.Ft struct snmp_dependency *
.Fn snmp_dep_lookup "struct snmp_context *ctx" "const struct asn_oid *base" "const struct asn_oid *idx" "size_t alloc" "snmp_depop_t func"
.Ft struct snmp_context *
.Fn snmp_init_context "void"
.Ft int
.Fn snmp_dep_commit "struct snmp_context *ctx"
.Ft int
.Fn snmp_dep_rollback "struct snmp_context *ctx"
.Ft void
.Fn snmp_dep_finish "struct snmp_context *ctx"
.Sh 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
.Xr bsnmpd 1
loadable module, that handles the new MIB (see
.Xr snmpmod 3 ) .
.Pp
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:
.Bd -literal -offset indent
extern struct snmp_node *tree;
extern u_int  tree_size;
.Ed
.Pp
The elements of the array are of type
.Vt struct snmp_node :
.Bd -literal -offset indent
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 */
};
.Ed
.Pp
The fields of this structure are described below.
.Bl -tag -width "syntax"
.It Va oid
Base OID of the scalar or table column.
.It Va name
Name of this variable.
.It Va type
Type of this variable.
One of:
.Bd -literal -offset indent
enum snmp_node_type {
	SNMP_NODE_LEAF = 1,
	SNMP_NODE_COLUMN
};
.Ed
.It Va syntax
The SNMP syntax of this variable.
.It Va op
The user supplied handler for this variable.
The handler is called with the following arguments:
.Bl -tag -width "ctx"
.It Fa ctx
A pointer to the context (see below).
.Li NULL .
.It Fa 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
.Li SNMP_ERR_NOSUCHNAME .
For all other operations the oid in
.Fa val
is the oid to fetch or set.
.It Fa len
The length of the base oid without index.
.It Fa idx
For table columns this is the index expression from the node (see below).
.It Fa op
This is the operation to execute, one of:
.Bd -literal -offset indent
enum snmp_op {
	SNMP_OP_GET 	= 1,
	SNMP_OP_GETNEXT,
	SNMP_OP_SET,
	SNMP_OP_COMMIT,
	SNMP_OP_ROLLBACK,
};
.Ed
.El
.Pp
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.
.It Va flags
Currently only the flag
.Li SNMP_NODE_CANSET is defined and set for nodes, that can be written or
created.
.It Va 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:
.Bd -literal -offset indent
#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) \e
	(((V) >> (((I) + 1) * SNMP_INDEX_SHIFT)) & \e
	SNMP_INDEX_MASK)
.Ed
.It Va data
This field may contain arbitrary data and is not used by the library.
.El
.Pp
The easiest way to construct the node table is
.Xr gensnmptree 1 .
Note, that one must be careful when changing the tree while executing a SET
operation.
Consult the sources for
.Xr bsnmpd 1 .
.Pp
The global variable
.Va snmp_trace
together with the function pointed to by
.Va snmp_debug
help in debugging the library and the agent.
.Va snmp_trace is a bit mask with the following bits:
.Bd -literal -offset indent
enum {
	SNMP_TRACE_GET,
	SNMP_TRACE_GETNEXT,
	SNMP_TRACE_SET,
	SNMP_TRACE_DEPEND,
	SNMP_TRACE_FIND,
};
.Ed
.Pp
Setting a bit to true causes the library to call
.Fn 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.
.Pp
Many of the functions use a so called context:
.Bd -literal -offset indent
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;
};
.Ed
.Pp
The fields are used as follows:
.Bl -tag -width ".It Va var_index"
.It Va 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.
.It Va 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.
.It Va dep
In the dependency callback function (see below) this is a pointer to the
current dependency.
Set by the library.
.It Va data
This is the
.Fa data
argument from the call to the library and is not used by the library.
.El
.Pp
The next three functions execute different kinds of GET requests.
The function
.Fn snmp_get
executes an SNMP GET operation, the function
.Fn snmp_getnext
executes an SNMP GETNEXT operation and the function
.Fn 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
.Fa resp
and
.Fa resp_b .
The caller must call
.Fn snmp_pdu_free
to free the response PDU in this case.
One of the following values may be returned:
.Bl -tag -width ".It Li SNMP_RET_ERR"
.It Li SNMP_RET_OK
Operation successful, response PDU may be sent.
.It Li SNMP_RET_IGN
Operation failed, no response PDU constructed.
Request is ignored.
.It Li SNMP_RET_ERR
Error in operation.
The error code and index have been set in
.Fa pdu .
No response PDU has been constructed.
The caller may construct an error response PDU via
.Fn snmp_make_errresp .
.El
.Pp
The function
.Fn 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.
.Pp
The SET operation occurs in several stages:
.Bl -enum -offset indent
.It
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.
.It
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.
.It
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.
.It
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.
.It
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
.Fn snmp_error ).
.It
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
.Li SNMP_DEPOP_FINISH.
Then the function returns
.Li SNMP_ERR_OK .
.El
.Pp
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
.Fn asn_slice_oid .
The function
.Fn 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
.Fn 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.
.Pp
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.
.Pp
It is guaranteed that each dependency callback is executed at minimum once
- with an operation of
.Li SNMP_OP_ROLLBACK .
This ensures that all dynamically allocated resources in a callback can be
freed correctly.
.Pp
The function
.Fn 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.
.Pp
The next four functions allow some parts of the SET operation to be executed.
This is only used in
.Xr bsnmpd 1
to implement the configuration as a single transaction.
The function
.Fn snmp_init_context
creates and initializes a context.
The function
.Fn snmp_dep_commit
executes SNMP_DEPOP_COMMIT for all dependencies in the context stopping at
the first error.
The function
.Fn snmp_dep_rollback
executes SNMP_DEPOP_ROLLBACK starting at the previous of the current
dependency in the context.
The function
.Fn snmp_dep_finish
executes SNMP_DEPOP_FINISH for all dependencies.
.Sh DIAGNOSTICS
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.
.Sh SEE ALSO
.Xr gensnmptree 1 ,
.Xr bsnmpd 1 ,
.Xr bsnmpclient 3 ,
.Xr bsnmplib 3 ,
.Xr snmpmod 3
.Sh STANDARDS
This implementation conforms to the applicable IETF RFCs and ITU-T
recommendations.
.Sh AUTHORS
.An Hartmut Brandt Aq harti@FreeBSD.org