BSNMPCLIENT(3) FreeBSD Library Functions Manual BSNMPCLIENT(3)
NAME
snmp_client, snmp_client_init, snmp_client_set_host,
snmp_client_set_port, 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_discover_engine - 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);)(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);)(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);)(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);
int
snmp_discover_engine(void);
DESCRIPTION
The SNMP library contains routines to easily build SNMP client
applications that use SNMP versions 1, 2 or 3. Most of the routines use
a struct snmp_client:
struct snmp_client {
enum snmp_version version;
int trans; /* which transport 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];
/* SNMPv3 specific fields */
int32_t identifier;
int32_t security_model;
struct snmp_engine engine;
struct snmp_user user;
/* SNMPv3 Access control - VACM*/
uint32_t clen;
uint8_t cengine[SNMP_ENGINE_ID_SIZ];
char cname[SNMP_CONTEXT_NAME_SIZ];
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;
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 bsnmplib(3) for
applicable values. The default version is SNMP_V2c.
trans If this is SNMP_TRANS_LOC_DGRAM a local datagram socket is
used. If it is SNMP_TRANS_LOC_STREAM a local stream
socket is used. For SNMP_TRANS_UDP a UDPv4 socket and for
SNMP_TRANS_UDP6 a UDPv6 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 standard 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 default) 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 `public'.
write_community
The community name to be used for SET requests. The
default is `private'.
identifier The message identifier value to be used with SNMPv3 PDUs.
Incremented with each transmitted PDU.
security_model
The security model to be used with SNMPv3 PDUs. Currently
only User-Based Security model specified by RFC 3414
(value 3) is supported.
engine The authoritive SNMP engine parameters to be used with
SNMPv3 PDUs.
user The USM SNMP user credentials to be used with SNMPv3 PDUs.
clen The length of the context engine id to be used with SNMPv3
PDUs.
cengine The context engine id to be used with SNMPv3 PDUs. Default
is empty.
cname The context name to be used with SNMPv3 PDUs. Default is
`'.
timeout The maximum time to wait for responses to requests. 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 resent. If set to
0, the request is sent only once. The default 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 arguments 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
decimal number and is stored into an internal
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 response 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 original 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 available, if the
argument is non-zero, the function blocks until the next packet is
received. The packet is delivered via the usual callback mechanism (non-
response packets are silently dropped). The function returns 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 retrieved 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 column that is
required. Bit 0 corresponds to the first element (index 0)
in the array entries, bit 1 to the second (index 1) and so
on. SNMP tables may be sparse. For sparse columns the bit
should not be set. If the bit for a given column is set and
the column value cannot be retrieved for a given 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 descriptors. This
array is terminated by an element with syntax
SNMP_SYNTAX_NULL. The first index_size elements 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 array elements
have the following meaning:
subid This is the OID subid of the column. This is
ignored for index entries. Index entries are
decoded according to the syntax field.
syntax This is the syntax of the column or index. A syntax
of SNMP_SYNTAX_NULL terminates the array.
offset This is the starting offset of the value of the
column 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 malloc(3). The list
argument of the table functions must point to a TAILQ_HEAD(). The
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 either 0 if the
table was fetched, or -1 if there was an error. The function itself
returns -1 if it could not initialize fetching of the table.
The following table description is used to fetch the ATM interface 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, &atmif_list) != 0)
errx(1, "AtmIf table: %s", snmp_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 fetches may be
called while in the function). The response PDU is returned in resp. If
no response could be received after all timeouts and retries, the
function returns -1. If a response was received 0 is returned.
The function snmp_discover_engine() is used to discover the authoritative
snmpEngineId of a remote SNMPv3 agent. A request PDU with empty USM user
name is sent and the client's engine parameters are set according to the
snmpEngine parameters received in the response PDU. If the client is
configured to use authentication and/or privacy and the snmpEngineBoots
and/or snmpEngineTime in the response had zero values, an additional
request (possibly encrypted) with the appropriate user credentials is
sent to fetch the missing values. Note, that the function blocks until
the discovery process is completed. If no response could be received
after all timeouts and retries, or the response contained errors the
function returns -1. If the discovery process was completed 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", "udp6", "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.
The function snmp_parse_serverr() fills the transport, the port number
and the community strings with reasonable default values when they are
not specified. The default transport is SNMP_TRANS_UDP. If the host
name contains a slash the default is modified to SNMP_TRANS_LOC_DGRAM.
If the host name looks like a numeric IPv6 address the default is
SNMP_TRANS_UDP6. For numeric IPv6 addresses the transport name udp is
automatically translated as SNMP_TRANS_UDP6. The default port number
(for udp or udp6) is "snmp". The default read community is "public" and
the default write community "private".
snmp_parse_server() recognizes path names, host names and numerical IPv4
and IPv6 addresses. A string consisting of digits and periods is assumed
to be an IPv4 address and must be parseable by inet_aton(3). An IPv6
address is any string enclosed in square brackets. It must be parseable
with gethostinfo(3).
The port number for snmp_parse_server() can be specified numerically or
symbolically. It is ignored for local sockets.
DIAGNOSTICS
If an error occurs in any of the functions an error indication as
described above is returned. Additionally the function sets a printable
error string in the error field 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>
FreeBSD 13.1-RELEASE-p6 March 31, 2020 FreeBSD 13.1-RELEASE-p6
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