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Man Pages

Manual Reference Pages  -  BSNMPLIB (3)


snmp_value_free, snmp_value_parse, snmp_value_copy, snmp_pdu_free, snmp_pdu_decode, snmp_pdu_encode, snmp_pdu_decode_header, snmp_pdu_decode_scoped, snmp_pdu_decode_secmode, snmp_pdu_init_secparams, snmp_pdu_dump, snmp_passwd_to_keys, snmp_get_local_keys, snmp_calc_keychange, TRUTH_MK, TRUTH_GET, TRUTH_OK - SNMP decoding and encoding library


See Also


Begemot SNMP library (libbsnmp, -lbsnmp)


.In bsnmp/asn1.h
.In bsnmp/snmp.h void snmp_value_free struct snmp_value *value int snmp_value_parse const char *buf enum snmp_syntax union snmp_values *value int snmp_value_copy struct snmp_value *to const struct snmp_value *from void snmp_pdu_free struct snmp_pdu *value enum snmp_code snmp_pdu_decode struct asn_buf *buf struct snmp_pdu *pdu int32_t *ip enum snmp_code snmp_pdu_encode struct snmp_pdu *pdu struct asn_buf *buf enum snmp_code snmp_pdu_decode_header struct snmp_pdu *pdu struct asn_buf *buf enum snmp_code snmp_pdu_decode_scoped struct asn_buf *buf struct snmp_pdu *pdu int32_t *ip enum snmp_code snmp_pdu_decode_secmode struct asn_buf *buf struct snmp_pdu *pdu void snmp_pdu_init_secparams struct snmp_pdu *pdu void snmp_pdu_dump const struct snmp_pdu *pdu enum snmp_code snmp_passwd_to_keys struct snmp_user *user char *passwd enum snmp_code snmp_get_local_keys struct snmp_user *user uint8_t *eid uint32_t elen enum snmp_code snmp_calc_keychange struct snmp_user *user uint8_t *keychange int TRUTH_MK F int TRUTH_GET T int TRUTH_OK T


The SNMP library contains routines to handle SNMP version 1, 2 and 3 PDUs. There are several basic structures used throughout the library:
struct snmp_value {
        struct asn_oid          var;
        enum snmp_syntax        syntax;
        union snmp_values {
          int32_t               integer;/* also integer32 */
          struct {
            u_int               len;
            u_char              *octets;
          }                     octetstring;
          struct asn_oid        oid;
          u_char                ipaddress[4];
          uint32_t              uint32; /* also gauge32, counter32,
                                           unsigned32, timeticks */
          uint64_t              counter64;
        }                       v;

This structure represents one variable binding from an SNMP PDU. The field var is the ASN.1 of the variable that is bound. syntax contains either the syntax code of the value or an exception code for SNMPv2 and may be one of:

enum snmp_syntax {
        SNMP_SYNTAX_NULL        = 0,
        SNMP_SYNTAX_INTEGER,    /* == INTEGER32 */
        SNMP_SYNTAX_GAUGE,      /* == UNSIGNED32 */

        /* v2 additions */         SNMP_SYNTAX_COUNTER64,         /* exceptions */         SNMP_SYNTAX_NOSUCHOBJECT,         SNMP_SYNTAX_NOSUCHINSTANCE,         SNMP_SYNTAX_ENDOFMIBVIEW, };

The field v holds the actual value depending on syntax. Note, that if syntax is SNMP_SYNTAX_OCTETSTRING and v.octetstring.len is not zero, v.octetstring.octets points to a string allocated by malloc(3).
#define SNMP_ENGINE_ID_SIZ              32

struct snmp_engine {         uint8_t                 engine_id[SNMP_ENGINE_ID_SIZ];         uint32_t                engine_len;         int32_t                 engine_boots;         int32_t                 engine_time;         int32_t                 max_msg_size; };

This structure represents an SNMP engine as specified by the SNMP Management Architecture described in RFC 3411.

#define SNMP_ADM_STR32_SIZ              (32 + 1)
#define SNMP_AUTH_KEY_SIZ               40
#define SNMP_PRIV_KEY_SIZ               32

enum snmp_usm_level {         SNMP_noAuthNoPriv = 1,         SNMP_authNoPriv = 2,         SNMP_authPriv = 3 };

struct snmp_user {         char                            sec_name[SNMP_ADM_STR32_SIZ];         enum snmp_authentication        auth_proto;         enum snmp_privacy               priv_proto;         uint8_t                         auth_key[SNMP_AUTH_KEY_SIZ];         uint8_t                         priv_key[SNMP_PRIV_KEY_SIZ]; };

This structure represents an SNMPv3 user as specified by the User-based Security Model (USM) described in RFC 3414. The field sec_name is a human readable string containing the security user name. auth_proto contains the id of the authentication protocol in use by the user and may be one of:

enum snmp_authentication {
        SNMP_AUTH_NOAUTH = 0,

priv_proto contains the id of the privacy protocol in use by the user and may be one of:
enum snmp_privacy {
        SNMP_PRIV_NOPRIV = 0,
        SNMP_PRIV_DES = 1,

auth_key and priv_key contain the authentication and privacy keys for the user.
#define SNMP_COMMUNITY_MAXLEN           128
#define SNMP_MAX_BINDINGS               100
#define SNMP_CONTEXT_NAME_SIZ           (32 + 1)
#define SNMP_TIME_WINDOW                150

#define SNMP_USM_AUTH_SIZE              12 #define SNMP_USM_PRIV_SIZE              8

#define SNMP_MSG_AUTH_FLAG              0x1 #define SNMP_MSG_PRIV_FLAG              0x2 #define SNMP_MSG_REPORT_FLAG            0x4

#define SNMP_MPM_SNMP_V1                0 #define SNMP_MPM_SNMP_V2c               1 #define SNMP_MPM_SNMP_V3                3

struct snmp_pdu {         char                    community[SNMP_COMMUNITY_MAXLEN + 1];         enum snmp_version       version;         u_int                   type;

        /* SNMPv3 PDU header fields */         int32_t                 identifier;         uint8_t                 flags;         int32_t                 security_model;         struct snmp_engine      engine;

        /* Associated USM user parameters */         struct snmp_user        user;         uint8_t                 msg_digest[SNMP_USM_AUTH_SIZE];         uint8_t                 msg_salt[SNMP_USM_PRIV_SIZE];

        /* View-based Access Model */         uint32_t                context_engine_len;         uint8_t                 context_engine[SNMP_ENGINE_ID_SIZ];         char                    context_name[SNMP_CONTEXT_NAME_SIZ];

        /* trap only */         struct asn_oid          enterprise;         u_char                  agent_addr[4];         int32_t                 generic_trap;         int32_t                 specific_trap;         uint32_t                time_stamp;

        /* others */         int32_t                 request_id;         int32_t                 error_status;         int32_t                 error_index;

        /* fixes for encoding */         size_t                  outer_len;         size_t                  scoped_len;         u_char                  *outer_ptr;         u_char                  *digest_ptr;         u_char                  *encrypted_ptr;         u_char                  *scoped_ptr;         u_char                  *pdu_ptr;         u_char                  *vars_ptr;

        struct snmp_value       bindings[SNMP_MAX_BINDINGS];         u_int                   nbindings; };

This structure contains a decoded SNMP PDU. version is one of
enum snmp_version {
        SNMP_Verr = 0,
        SNMP_V1 = 1,

and type is the type of the PDU. security_model is the security model used for SNMPv3 PDUs. The only supported value currently is 3 (User-based Security Model). Additional values for any, unknown, SNMPv1 and SNMPv2c security models are also enumerated
enum snmp_secmodel {
        SNMP_SECMODEL_ANY = 0,
        SNMP_SECMODEL_SNMPv1 = 1,
        SNMP_SECMODEL_SNMPv2c = 2,
        SNMP_SECMODEL_USM = 3,

The function snmp_value_free is used to free all the dynamic allocated contents of an SNMP value. It does not free the structure pointed to by value itself.

The function snmp_value_parse parses the ASCII representation of an SNMP value into its binary form. This function is mainly used by the configuration file reader of bsnmpd(1).

The function snmp_value_copy makes a deep copy of the value pointed to by from to the structure pointed to by to. It assumes that to is uninitialized and will overwrite its previous contents. It does not itself allocate the structure pointed to by to.

The function snmp_pdu_free frees all the dynamically allocated components of the PDU. It does not itself free the structure pointed to by pdu.

The function snmp_pdu_decode decodes the PDU pointed to by buf and stores the result into pdu. If an error occurs in a variable binding the (1 based) index of this binding is stored in the variable pointed to by ip.

The function snmp_pdu_encode encodes the PDU pdu into the an octetstring in buffer, and if authentication and privacy are used, calculates a message digest and encrypts the PDU data in the buffer buf.

The function snmp_pdu_decode_header decodes the header of the PDU pointed to by buf. The uncoded PDU contents remain in the buffer.

The function snmp_pdu_decode_scoped decodes the scoped PDU pointed to by buf.

The function snmp_pdu_decode_secmode verifies the authentication parameter contained in the PDU (if present) and if the PDU is encrypted, decrypts the PDU contents pointed to by buf. If successfull, a plain text scoped PDU is stored in the buffer.

The function snmp_pdu_init_secparams calculates the initialization vector for the privacy protocol in use before the PDU pointed to by pdu may be encrypted or decrypted.

The function snmp_pdu_dump dumps the PDU in a human readable form by calling snmp_printf.

The function snmp_passwd_to_keys calculates a binary private authentication key corresponding to a plain text human readable password string. The calculated key is placed in the auth_key field of the user.

The function snmp_get_local_keys calculates a localazied authentication and privacy keys for a specified SNMPv3 engine. The calculateds keys are placed in the auth_key and priv_key fields of the user.

The function snmp_calc_keychange calculates a binary key change octet string based on the contents of an old and a new binary localized key. The rezult is placed in the buffer pointer to by keychange and may be used by an SNMPv3 user who wishes to change his/her password or localized key.

The function TRUTH_MK takes a C truth value (zero or non-zero) and makes an SNMP truth value (2 or 1). The function TRUTH_GET takes an SNMP truth value and makes a C truth value (0 or 1). The function TRUTH_OK checks, whether its argument is a legal SNMP truth value.


When an error occurs in any of the function the function pointed to by the global pointer
extern void (*snmp_error)(const char *, ...);

with a printf(3) style format string. There is a default error handler in the library that prints a message starting with 'SNMP:' followed by the error message to standard error.

The function pointed to by

extern void (*snmp_printf)(const char *, ...);

is called by the snmp_pdu_dump function. The default handler is printf(3).


snmp_pdu_decode will return one of the following return codes:
  The ASN.1 coding was wrong.
  A variable binding value had a wrong length field.
  A variable binding value was out of the allowed range.
  The PDU is of an unsupported version.
  There was an ASN.1 value with an unsupported tag.
  The requested securityLevel contained in the PDU is not supported.
  The PDU authentication parameter received in the PDU did not match the calculated message digest.
  Error occured while trying to decrypt the PDU.

snmp_pdu_encode will return one of the following return codes:
  Encoding failed.


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


The SNMPv3 message digests, encryption and decryption, and key routines use the cryptographic functions from crypto(3). The library may optionally be built without references to the crypto(3) library. In such case only plain text SNMPv3 PDUs without message digests may be proccessed correctly.


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


The Begemot SNMP library was originally written by
.An Hartmut Brandt Aq

.An Shteryana Shopova Aq added support for the SNMPv3 message proccessing and User-Based Security model message authentication and privacy.

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