bsnmpclient(3)

NAME

snmp_client, snmp_send_cb_f, snmp_timeout_cb_f,
snmp_timeout_start_f,
snmp_timeout_stop_f, snmp_open, snmp_close, snmp_pdu_create, snmp_add_binding, snmp_pdu_check, snmp_pdu_send,
snmp_oid_append,
snmp_parse_server, snmp_receive, snmp_table_cb_f,
snmp_table_fetch,
snmp_table_fetch_async, snmp_dialog - SNMP client library

LIBRARY

Begemot SNMP library (libbsnmp, -lbsnmp)

SYNOPSIS

#include <asn1.h>
#include <snmp.h>
#include <snmpclient.h>
typedef void
(*snmp_send_cb_f)(struct  snmp_pdu  *req,  struct   snmp_pdu
*resp,
        void *uarg);
typedef void
(*snmp_timeout_cb_f)(void *uarg);
typedef void *
(*snmp_timeout_start_f)(struct timeval *timeout,
        snmp_timeout_cb_f callback, void *uarg);
typedef void
(*snmp_timeout_stop_f)(void *timeout_id);
extern struct snmp_client snmp_client;
void
snmp_client_init(struct snmp_client *client);
int
snmp_client_set_host(struct  snmp_client *client, const char
*host);
int
snmp_client_set_port(struct snmp_client *client, const  char
*port);
int
snmp_open(const  char  *host,  const  char *port, const char
*read_community,
        const char *write_community);
void
snmp_close(void);
void
snmp_pdu_create(struct snmp_pdu *pdu, u_int op);
int
snmp_add_binding(struct snmp_pdu *pdu, ...);
int
snmp_pdu_check(const  struct  snmp_pdu  *req,  const  struct
snmp_pdu *resp);
int32_t
snmp_pdu_send(struct  snmp_pdu  *pdu,  snmp_send_cb_f  func,
void *uarg);
int
snmp_oid_append(struct asn_oid *oid, const char *fmt, ...);
int
snmp_parse_server(struct snmp_client *sc, const char *str);
int
snmp_receive(int blocking);
typedef void
(*snmp_table_cb_f)(void *list, void *arg, int res);
int
snmp_table_fetch(const  struct   snmp_table   *descr,   void
*list);
int
snmp_table_fetch_async(const  struct snmp_table *descr, void
*list,
        snmp_table_cb_f callback, void *uarg);
int
snmp_dialog(struct snmp_pdu *req, struct snmp_pdu *resp);

DESCRIPTION

The SNMP library contains routines to easily build SNMP
client applications that use SNMP versions 1 or 2. Most of the routines
use a struct
snmp_client:

struct snmp_client {
enum snmp_version version;
int trans; /* transport type to use */
/* these two are read-only for the application
*/
char *cport; /* port number as string */
char *chost; /* host name or IP address as
string */
char read_community[SNMP_COMMUNITY_MAXLEN +
1];
char write_community[SNMP_COMMUNITY_MAXLEN
+ 1];
struct timeval timeout;
u_int retries;
int dump_pdus;
size_t txbuflen;
size_t rxbuflen;
int fd;
int32_t next_reqid;
int32_t max_reqid;
int32_t min_reqid;
char error[SNMP_STRERROR_LEN];
snmp_timeout_start_f timeout_start;
snmp_timeout_stop_f timeout_stop;
/* private */
char local_path[sizeof(SNMP_LOCAL_PATH)];
};
The fields of this structure are described below.
version This is the version of SNMP to use. See bsn
mplib(3) for
applicable values. The default version is
SNMP_V2c.
trans If this is SNMP_TRANS_LOC_DGRAM a local data
gram socket is
used. If it is SNMP_TRANS_LOC_STREAM a local
stream
socket is used. For SNMP_TRANS_UDP a UDP
socket is created. It uses the chost field as the path to
the server's
socket for local sockets.
cport The SNMP agent's UDP port number. This may
be a symbolic
port number (from /etc/services) or a numeric
port number.
If this field is NULL (the default) the stan
dard SNMP port
is used. This field should not be changed
directly but
rather by calling snmp_client_set_port().
chost The SNMP agent's host name, IP address or
UNIX domain
socket path name. If this is NULL (the de
fault) localhost
is assumed. This field should not be changed
directly but
rather through calling
snmp_client_set_host().
read_community
This is the community name to be used for all
requests
except SET requests. The default is `pub
lic'.
write_community
The community name to be used for SET re
quests. The
default is `private'.
timeout The maximum time to wait for responses to re
quests. If
the time elapses, the request is resent up to
retries
times. The default is 3 seconds.
retries Number of times a request PDU is to be re
sent. If set to
0, the request is sent only once. The de
fault is 3
retransmissions.
dump_pdus If set to a non-zero value all received and
sent PDUs are
dumped via snmp_pdu_dump(3). The default is
not to dump
PDUs.
txbuflen The encoding buffer size to be allocated for
transmitted
PDUs. The default is 10000 octets.
rxbuflen The decoding buffer size to be allocated for
received
PDUs. This is the size of the maximum PDU
that can be
received. The default is 10000 octets.
fd After calling snmp_open() this is the file
socket file
descriptor used for sending and receiving
PDUs.
next_reqid The request id of the next PDU to send. Used
internal by
the library.
max_reqid The maximum request id to use for outgoing
PDUs. The
default is INT32_MAX.
min_reqid The minimum request id to use for outgoing
PDUs. Request
ids are allocated linearily starting at
min_reqid up to
max_reqid.
error If an error happens, this field is set to a
printable
string describing the error.
timeout_start This field must point to a function setting
up a one shot
timeout. After the timeout has elapsed, the
given callback function must be called with the user
argument. The
timeout_start() function must return a void *
identifying
the timeout.
timeout_stop This field must be set to a function that
stops a running
timeout. The function will be called with
the return
value of the corresponding timeout_start()
function.
local_path If in local socket mode, the name of the
clients socket.
Not needed by the application.
In the current implementation there is a global variable

extern struct snmp_client snmp_client;
that is used by all the library functions. The first call
into the
library must be a call to snmp_client_init() to initialize
this global
variable to the default values. After this call and before
calling
snmp_open() the fields of the variable may be modified by
the user. The
modification of the chost and cport fields should be done
only via the
functions snmp_client_set_host() and snmp_client_set_port().
The function snmp_open() creates a UDP or UNIX domain socket
and connects
it to the agent's IP address and port. If any of the argu
ments of the
call is not NULL the corresponding field in the global
snmp_client is set
from the argument. Otherwise the values that are already in
that variable are used. The function snmp_close() closes the socket,
stops all
timeouts and frees all dynamically allocated resources.
The next three functions are used to create request PDUs.
The function
snmp_pdu_create() initializes a PDU of type op. It does not
allocate
space for the PDU itself. This is the responsibility of the
caller.
snmp_add_binding() adds bindings to the PDU and returns the
(zero based)
index of the first new binding. The arguments are pairs of
pointer to
the OIDs and syntax constants, terminated by a NULL. The
call

snmp_add_binding(&pdu,
&oid1, SNMP_SYNTAX_INTEGER,
&oid2, SNMP_SYNTAX_OCTETSTRING,
NULL);
adds two new bindings to the PDU and returns the index of
the first one.
It is the responsibility of the caller to set the value part
of the binding if necessary. The functions returns -1 if the maximum
number of
bindings is exhausted. The function snmp_oid_append() can
be used to
construct variable OIDs for requests. It takes a pointer to
an struct
asn_oid that is to be constructed, a format string, and a
number of arguments the type of which depends on the format string. The
format string
is interpreted character by character in the following way:
i This format expects an argument of
type
asn_subid_t and appends this as a
single integer
to the OID.
a This format expects an argument of
type struct
in_addr and appends to four parts of
the IP
address to the OID.
s This format expects an argument of
type const char
* and appends the length of the
string (as computed by strlen(3)) and each of the
characters in
the string to the OID.
(N) This format expects no argument. N
must be a dec
imal number and is stored into an in
ternal variable size.
b This format expects an argument of
type const char
* and appends size characters from
the string to
the OID. The string may contain NUL
characters.
c This format expects two arguments:
one of type
size_t and one of type const u_char
*. The first
argument gives the number of bytes to
append to
the OID from the string pointed to by
the second
argument.
The function snmp_pdu_check() may be used to check a re
sponse PDU. A
number of checks are performed (error code, equal number of
bindings,
syntaxes and values for SET PDUs). The function returns +1
if everything
is ok, 0 if a NOSUCHNAME or similar error was detected, -1
if the
response PDU had fatal errors and -2 if resp is NULL (a
timeout
occurred).
The function snmp_pdu_send() encodes and sends the given
PDU. It records
the PDU together with the callback and user pointers in an
internal list
and arranges for retransmission if no response is received.
When a
response is received or the retransmission count is exceeded
the callback
func is called with the orignal request PDU, the response
PDU and the
user argument uarg. If the retransmit count is exceeded,
func is called
with the original request PDU, the response pointer set to
NULL and the
user argument uarg. The caller should not free the request
PDU until the
callback function is called. The callback function must
free the request
PDU and the response PDU (if not NULL ).
The function snmp_receive() tries to receive a PDU. If the
argument is
zero, the function polls to see whether a packet is avail
able, if the
argument is non-zero, the function blocks until the next
packet is
received. The packet is delivered via the usual callback
mechanism (nonresponse packets are silently dropped). The function re
turns 0, if a
packet was received and successfully dispatched, -1 if an
error occurred
or no packet was available (in polling mode).
The next two functions are used to retrieve tables from SNMP
agents.
They use the following input structure, that describes the
table:

struct snmp_table {
struct asn_oid table;
struct asn_oid last_change;
u_int max_iter;
size_t entry_size;
u_int index_size;
uint64_t req_mask;
struct snmp_table_entry {
asn_subid_t subid;
enum snmp_syntax syntax;
off_t offset;
} entries[];
};
The fields of this structure have the following meaning:
table This is the base OID of the table.
last_change Some tables have a scalar variable of type
TIMETICKS
attached to them, that holds the time when the
table was
last changed. This OID should be the OID of
this variable
(without the .0 index). When the table is re
trieved with
multiple GET requests, and the variable changes
between two
request, the table fetch is restarted.
max_iter Maximum number of tries to fetch the table.
entry_size The table fetching routines return a list of
structures one
for each table row. This variable is the size
of one structure and used to malloc(3) the structure.
index_size This is the number of index columns in the
table.
req_mask This is a bit mask with a 1 for each table col
umn that is
required. Bit 0 corresponds to the first ele
ment (index 0)
in the array entries, bit 1 to the second (in
dex 1) and so
on. SNMP tables may be sparse. For sparse
columns the bit
should not be set. If the bit for a given col
umn is set and
the column value cannot be retrieved for a giv
en row, the
table fetch is restarted assuming that the
table is currently being modified by the agent. The bits
for the index
columns are ignored.
entries This is a variable sized array of column de
scriptors. This
array is terminated by an element with syntax
SNMP_SYNTAX_NULL. The first index_size ele
ments describe
all the index columns of the table, the rest
are normal
columns. If for the column at `entries[N]' the
expression
`req_mask & (1 << N)' yields true, the column
is considered
a required column. The fields of this the ar
ray elements
have the following meaning:
subid This is the OID subid of the column.
This is
ignored for index entries. Index en
tries are
decoded according to the syntax field.
syntax This is the syntax of the column or in
dex. A syntax
of SNMP_SYNTAX_NULL terminates the ar
ray.
offset This is the starting offset of the val
ue of the col
umn in the return structures. This
field can be set
with the ISO-C offsetof() macro.
Both table fetching functions return TAILQ (see queue(3)) of
structures--one for each table row. These structures must start
with a
TAILQ_ENTRY() and a uint64_t and are allocated via mal
loc(3). The list
argument of the table functions must point to a

TAILQ_HEAD

uint64_t fields, usually called found is used to indicate
which of the
columns have been found for the given row. It is encoded
like the
req_mask field.
The function snmp_table_fetch() synchronously fetches the
given table.
If everything is ok 0 is returned. Otherwise the function
returns -1 and
sets an appropriate error string. The function
snmp_table_fetch_async()
fetches the tables asynchronously. If either the entire
table is fetch,
or an error occurs the callback function callback is called
with the
callers arguments list and uarg and a parameter that is ei
ther 0 if the
table was fetched, or -1 if there was an error. The func
tion itself
returns -1 if it could not initialize fetching of the table.
The following table description is used to fetch the ATM in
terface table:

/*
* ATM interface table
*/
struct atmif {
TAILQ_ENTRY(atmif) link;
uint64_t found;
int32_t index;
u_char *ifname;
size_t ifnamelen;
uint32_t node_id;
uint32_t pcr;
int32_t media;
uint32_t vpi_bits;
uint32_t vci_bits;
uint32_t max_vpcs;
uint32_t max_vccs;
u_char *esi;
size_t esilen;
int32_t carrier;
};
TAILQ_HEAD(atmif_list, atmif);
/* list of all ATM interfaces */
struct atmif_list atmif_list;
static const struct snmp_table atmif_table = {
OIDX_begemotAtmIfTable,
OIDX_begemotAtmIfTableLastChange, 2,
sizeof(struct atmif),
1, 0x7ffULL,
{
{ 0, SNMP_SYNTAX_INTEGER,
offsetof(struct atmif, index) },
{ 1, SNMP_SYNTAX_OCTETSTRING,
offsetof(struct atmif, ifname) },
{ 2, SNMP_SYNTAX_GAUGE,
offsetof(struct atmif, node_id) },
{ 3, SNMP_SYNTAX_GAUGE,
offsetof(struct atmif, pcr) },
{ 4, SNMP_SYNTAX_INTEGER,
offsetof(struct atmif, media) },
{ 5, SNMP_SYNTAX_GAUGE,
offsetof(struct atmif, vpi_bits) },
{ 6, SNMP_SYNTAX_GAUGE,
offsetof(struct atmif, vci_bits) },
{ 7, SNMP_SYNTAX_GAUGE,
offsetof(struct atmif, max_vpcs) },
{ 8, SNMP_SYNTAX_GAUGE,
offsetof(struct atmif, max_vccs) },
{ 9, SNMP_SYNTAX_OCTETSTRING,
offsetof(struct atmif, esi) },
{ 10, SNMP_SYNTAX_INTEGER,
offsetof(struct atmif, carrier) },
{ 0, SNMP_SYNTAX_NULL, 0 }
}
};
...
if (snmp_table_fetch(&atmif_table, &at
mif_list) != 0)
errx(1, "AtmIf table: %s", sn
mp_client.error);
...
The function snmp_dialog() is used to execute a synchonuous
dialog with
the agent. The request PDU req is sent and the function
blocks until the
response PDU is received. Note, that asynchonuous receives
are handled
(i.e. callback functions of other send calls or table fetch
es may be
called while in the function). The response PDU is returned
in resp. If
no response could be received after all timeouts and re
tries, the function returns -1. If a response was received 0 is returned.
The function snmp_parse_server() is used to parse an SNMP
server specification string and fill in the fields of a struct
snmp_client. The syntax
of a server specification is

[trans::][community@][server][:port]
where trans is the transport name (one of udp, stream or
dgram),
community is the string to be used for both the read and the
write community, server is the server's host name in case of UDP and
the path name
in case of a local socket, and port is the port in case of
UDP transport.
The function returns 0 in the case of success and return -1
and sets the
error string in case of an error.

DIAGNOSTICS

If an error occurs in any of the function an error indica
tion as
described above is returned. Additionally the function sets
a printable
error string in the error filed of snmp_client.

SEE ALSO

gensnmptree(1), bsnmpd(1), bsnmpagent(3), bsnmplib(3)

STANDARDS

This implementation conforms to the applicable IETF RFCs and
ITU-T recommendations.

AUTHORS

Hartmut Brandt <harti@freebsd.org>
Kendy Kutzner <kutzner@fokus.gmd.de>
BSD October 4, 2005

BSD

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