NAME

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

LIBRARY

Begemot SNMP library (libbsnmp, -lbsnmp)

SYNOPSIS

#include
  <bsnmp/asn1.h>

#include
  <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);

DESCRIPTION

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_OCTETSTRING, 
	SNMP_SYNTAX_OID, 
	SNMP_SYNTAX_IPADDRESS, 
	SNMP_SYNTAX_COUNTER, 
	SNMP_SYNTAX_GAUGE,	/* == UNSIGNED32 */ 
	SNMP_SYNTAX_TIMETICKS, 
 
	/* 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, 
	SNMP_AUTH_HMAC_MD5, 
	SNMP_AUTH_HMAC_SHA 
};

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, 
	SNMP_PRIV_AES 
};

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, 
	SNMP_V2c, 
	SNMP_V3 
};

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, 
	SNMP_SECMODEL_UNKNOWN 
};

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 successful, 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.

DIAGNOSTICS

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).

ERRORS

snmp_pdu_decode() will return one of the following return codes:

[SNMP_CODE_OK]: Success.
[SNMP_CODE_FAILED]: The ASN.1 coding was wrong.
[SNMP_CODE_BADLEN]: A variable binding value had a wrong length field.
[SNMP_CODE_OORANGE]: A variable binding value was out of the allowed range.
[SNMP_CODE_BADVERS]: The PDU is of an unsupported version.
[SNMP_CODE_BADENQ]: There was an ASN.1 value with an unsupported tag.
[SNMP_CODE_BADSECLEVEL]: The requested securityLevel contained in the PDU is not supported.
[SNMP_CODE_BADDIGEST]: The PDU authentication parameter received in the PDU did not match the calculated message digest.
[SNMP_CODE_EDECRYPT]: Error occurred while trying to decrypt the PDU.

snmp_pdu_encode() will return one of the following return codes:

[SNMP_CODE_OK]: Success.
[SNMP_CODE_FAILED]: Encoding failed.

CAVEAT

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.

STANDARDS

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

AUTHORS

The Begemot SNMP library was originally written by Hartmut Brandt ⟨harti@FreeBSD.org⟩

Shteryana Shopova ⟨syrinx@FreeBSD.org⟩ added support for the SNMPv3 message proccessing and User-Based Security model message authentication and privacy.