SNMP_API(3)

NAME
snmp_open, snmp_close, snmp_send, snmp_read, snmp_free_pdu, snmp_select_info, snmp_timeout - send and receive SNMP messages

SYNOPSIS
#include <sys/types>
#include <netinet/in.h>
#include <sys/time.h>
#include "snmp.h"
#include "snmp_impl.h"
#include "asn1.h"
#include "snmp_api.c"

extern int snmp_errno;

struct snmp_session *snmp_open(session)
struct snmp_session *session;

int snmp_close(session)
struct snmp_session *session;

int snmp_send(session, pdu)
struct snmp_session *session;
struct snmp_pdu *pdu;

void snmp_read(fdset)
fd_set *fdset;

int snmp_select_info(numfds, fdset, timeout, block)
int *numfds;
fd_set *fdset;
struct timeval *timeout;
int *block;

void snmp_timeout()

void snmp_free_pdu(pdu)
struct snmp_pdu *pdu;

DESCRIPTION
Snmp_open, snmp_close, and snmp_send each take as input a pointer to an object with the following structure. In addition, snmp_open also returns a pointer to an object with this structure. This structure contains information for a set of transactions that will share similar transport characteristics.

    typedef struct sockaddr_in  ipaddr;
    struct snmp_session {
        u_char  *community;	/* community for outgoing requests. */
        int	    community_len;  /* Length of community name. */
        int	    retries;	/* Number of retries before timeout. */
        long    timeout;    /* Number of uS until first timeout, then exponential backoff */
        char    *peername;	/* Domain name or dotted IP address of default peer */
        u_short remote_port;/* UDP port number of peer. */
        u_short local_port; /* My UDP port number, 0 for default, picked randomly */
        /* Authentication function or NULL if null authentication is used */
        u_char  *(*authenticator)();
        int	    (*callback)();  /* Function to interpret incoming data */
        /* Pointer to data that the callback function may consider important */
        void    *callback_magic;
    };
    /*
     * Set fields in session and pdu to the following to get a default or unconfigured value.
     */
    #define SNMP_DEFAULT_COMMUNITY_LEN  0/* for default community name */
    #define SNMP_DEFAULT_RETRIES	    -1
    #define SNMP_DEFAULT_TIMEOUT	    -1
    #define SNMP_DEFAULT_REMPORT	    0
    #define SNMP_DEFAULT_REQID	    0
    #define SNMP_DEFAULT_ERRSTAT	    -1
    #define SNMP_DEFAULT_ERRINDEX	    -1
    #define SNMP_DEFAULT_ADDRESS	    0
    #define SNMP_DEFAULT_PEERNAME	    NULL
    #define SNMP_DEFAULT_ENTERPRISE_LENGTH	0
    #define SNMP_DEFAULT_TIME	    0
    /*
     * This routine must be supplied by the application:
     *
     * u_char *authenticator(pdu, length, community, community_len)
     * u_char *pdu;		The rest of the PDU to be authenticated
     * int *length;		The length of the PDU (updated by the authenticator)
     * u_char *community;	The community name to authenticate under.
     * int	community_len	The length of the community name.
     *
     * Returns the authenticated pdu, or NULL if authentication failed.
     * If null authentication is used, the authenticator in snmp_session can be
     * set to NULL(0).
     */
    /*
     * This routine must be supplied by the application:
     *
     * int callback(operation, session, reqid, pdu, magic)
     * int operation;
     * struct snmp_session *session;    The session authenticated under.
     * int reqid;			    The request id of this pdu (0 for TRAP)
     * struct snmp_pdu *pdu;	    The pdu information.
     * void *magic			    A link to the data for this routine.
     *
     * Returns 1 if request was successful, 0 if it should be kept pending.
     * Any data in the pdu must be copied because it will be freed elsewhere.
     * Operations are defined below:
     */
    #define RECEIVED_MESSAGE   1
    #define TIMED_OUT	   2

Snmp_send and snmp_free_pdu each take as input a pointer to an object with the following structure. This structure contains information that describes a transaction that will be performed over an open session.

    struct snmp_pdu {
        ipaddr  address;	/* Address of peer */
        int	    command;	/* Type of this PDU */
        u_long  reqid;	/* Request id */
        u_long  errstat;	/* Error status */
        u_long  errindex;	/* Error index */
        /* Trap information */
        oid	    *enterprise;/* System OID */
        int	    enterprise_length;
        ipaddr  agent_addr;	/* address of object generating trap */
        int	    trap_type;	/* trap type */
        int	    specific_type;  /* specific type */
        u_long  time;	/* Uptime */
        struct variable_list *variables;
    };
    struct variable_list {
        struct variable_list *next_variable;    /* NULL for last variable */
        oid	    *name;  /* Object identifier of variable */
        int	    name_length;    /* number of subid's in name */
        u_char  type;   /* ASN type of variable */
        union { /* value of variable */
    	long	*integer;
    	u_char	*string;
    	oid	*objid;
        } val;
        int	    val_len;
    };

Snmp_read, snmp_select_info, and snmp_timeout provide an interface for the use of the select(2) system call so that SNMP transactions can occur asynchronously.

Snmp_select_info is given the information that would have been passed to select in the absence of SNMP. For example, this might include window update information. This information is modified so that SNMP will get the service it requires from the call to select. In this case, numfds, fdset, and timeout correspond to the nfds, readfds, and timeout arguments to select, respectively. The only exception is that timeout must always point to an allocated (but perhaps uninitialized) struct timeval. If timeout would have been passed as NULL, block is set to true, and timeout is treated as undefined. This same rule applies upon return from snmp_select_info.

After calling snmp_select_info, select is called with the returned data. When select returns, snmp_read is called with the fd_set returned from select. This will read all SNMP sockets with input. If select times out, snmp_timeout should be called to see if the timeout was intended for SNMP.

DIAGNOSTICS

Error return status from snmp_open is indicated by return of a null pointer. Error return status from snmp_close and snmp_send is indicated by return of 0. A successful status will return a 1. The external integer snmp_errno may then be checked to see what type of error has occurred

snmp_errno can have the following values:

    SNMPERR_GENERR
        A generic error occurred.

    SNMPERR_BAD_LOCPORT
        The local port was bad because it had already been allocated or permission was denied.

    SNMPERR_BAD_ADDRESS
        The host name or address given was not useable.

    SNMPERR_BAD_SESSION
        The specified session was not open.

SEE ALSO
select(2), snmp_api.h