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INSERT_OBJECT_OID_LINK_INDEX, INSERT_OBJECT_INT_LINK_INDEX, FIND_OBJECT_OID_LINK_INDEX, NEXT_OBJECT_OID_LINK_INDEX, FIND_OBJECT_INT_LINK_INDEX, NEXT_OBJECT_INT_LINK_INDEX, INSERT_OBJECT_OID_LINK, INSERT_OBJECT_INT_LINK, FIND_OBJECT_OID_LINK, NEXT_OBJECT_OID_LINK, FIND_OBJECT_INT_LINK, NEXT_OBJECT_INT_LINK, INSERT_OBJECT_OID, INSERT_OBJECT_INT, FIND_OBJECT_OID, FIND_OBJECT_INT, NEXT_OBJECT_OID, NEXT_OBJECT_INT, this_tick, start_tick, get_ticks, systemg, comm_define, community, oid_zeroDotZero, reqid_allocate, reqid_next, reqid_base, reqid_istype, reqid_type, timer_start, timer_stop, fd_select, fd_deselect, fd_suspend, fd_resume, or_register, or_unregister, buf_alloc, buf_size, snmp_input_start, snmp_input_finish, snmp_output, snmp_send_port, snmp_send_trap, string_save, string_commit, string_rollback, string_get, string_free, ip_save, ip_rollback, ip_commit, ip_get, oid_save, oid_rollback, oid_commit, oid_get, index_decode, index_compare, index_compare_off, index_append, index_append_off - "SNMP daemon loadable module interface"


Module Loading And Unloading
Implementing Tables
Daemon Timestamps
The System Group
Well Known Oids
Request Id Ranges
File Descriptor Support
Object Resources
Transmit And Receive Buffers
Processing Pdus
Simple Action Support
Table Index Handling
See Also


Begemot SNMP library (libbsnmp, -lbsnmp)


.Vt extern uint64_t this_tick ;
.Vt extern uint64_t start_tick ; uint64_t get_ticks "void"
.Vt extern struct systemg systemg ; u_int comm_define "u_int priv" "const char *descr" "struct lmodule *mod" "const char *str" const char * comm_string "u_int comm"
.Vt extern u_int community ;
.Vt extern const struct asn_oid oid_zeroDotZero ; u_int reqid_allocate "int size" "struct lmodule *mod" int32_t reqid_next "u_int type" int32_t reqid_base "u_int type" int reqid_istype "int32_t reqid" "u_int type" u_int reqid_type "int32_t reqid" void * timer_start "u_int ticks" "void (*func)(void *)" "void *uarg" "struct lmodule *mod" void timer_stop "void *timer_id" void * fd_select "int fd" "void (*func)(int, void *)" "void *uarg" "struct lmodule *mod" void fd_deselect "void *fd_id" void fd_suspend "void *fd_id" int fd_resume "void *fd_id" u_int or_register "const struct asn_oid *oid" "const char *descr" "struct lmodule *mod" void or_unregister "u_int or_id" void * buf_alloc "int tx" size_t buf_size "int tx" enum snmpd_input_err
.Fo snmp_input_start "const u_char *buf" "size_t len" "const char *source" "struct snmp_pdu *pdu" "int32_t *ip" "size_t *pdulen"
.Fc enum snmpd_input_err
.Fo snmp_input_finish "struct snmp_pdu *pdu" "const u_char *rcvbuf" "size_t rcvlen" "u_char *sndbuf" "size_t *sndlen" "const char *source" "enum snmpd_input_err ierr" "int32_t ip" "void *data"
.Fc void
.Fo snmp_output "struct snmp_pdu *pdu" "u_char *sndbuf" "size_t *sndlen" "const char *dest"
.Fc void
.Fo snmp_send_port "void *trans" "const struct asn_oid *port" "struct snmp_pdu *pdu" "const struct sockaddr *addr" "socklen_t addrlen"
.Fc void snmp_send_trap "const struct asn_oid *oid" "..." int string_save "struct snmp_value *val" "struct snmp_context *ctx" "ssize_t req_size" "u_char **strp" void string_commit "struct snmp_context *ctx" void string_rollback "struct snmp_context *ctx" "u_char **strp" int string_get "struct snmp_value *val" "const u_char *str" "ssize_t len" void string_free "struct snmp_context *ctx" int ip_save "struct snmp_value *val" "struct snmp_context *ctx" "u_char *ipa" void ip_rollback "struct snmp_context *ctx" "u_char *ipa" void ip_commit "struct snmp_context *ctx" int ip_get "struct snmp_value *val" "u_char *ipa" int oid_save "struct snmp_value *val" "struct snmp_context *ctx" "struct asn_oid *oid" void oid_rollback "struct snmp_context *ctx" "struct asn_oid *oid" void oid_commit "struct snmp_context *ctx" int oid_get "struct snmp_value *val" "const struct asn_oid *oid" int index_decode "const struct asn_oid *oid" "u_int sub" "u_int code" "..." int index_compare "const struct asn_oid *oid1" "u_int sub" "const struct asn_oid *oid2" int index_compare_off "const struct asn_oid *oid1" "u_int sub" "const struct asn_oid *oid2" "u_int off" void index_append "struct asn_oid *dst" "u_int sub" "const struct asn_oid *src" void index_append_off "struct asn_oid *dst" "u_int sub" "const struct asn_oid *src" "u_int off"


The snmpd(1) SNMP daemon implements a minimal MIB which consists of the system group, part of the SNMP MIB, a private configuration MIB, a trap destination table, a UDP port table, a community table, a module table, a statistics group and a debugging group. All other MIBs are support through loadable modules. This allows snmpd(1) to use for task, that are not the classical SNMP task.


Modules are loaded by writing to the module table. This table is indexed by a string, that identfies the module to the daemon. This identifier is used to select the correct configuration section from the configuration files and to identify resources allocated to this module. A row in the module table is created by writing a string of non-zero length to the begemotSnmpdModulePath column. This string must be the complete path to the file containing the module. A module can be unloaded by writing a zero length string to the path column of an existing row.

Modules may depend on each other an hence must be loaded in the correct order. The dependencies are listed in the corresponding manual pages.

Upon loading a module the SNMP daemon expects the module file to a export a global symbol config. This symbol should be a variable of type
.Vt struct snmp_module :
typedef enum snmpd_proxy_err (*proxy_err_f)(struct snmp_pdu *, void *,
const struct asn_oid *, const struct sockaddr *, socklen_t,
enum snmpd_input_err, int32_t);

struct snmp_module {
const char *comment;
int (*init)(struct lmodule *, int argc, char *argv[]);
int (*fini)(void);
void (*idle)(void);
void (*dump)(void);
void (*config)(void);
void (*start)(void);
proxy_err_f proxy;
const struct snmp_node *tree;
u_int tree_size;
void (*loading)(const struct lmodule *, int);

This structure must be statically initialized and its fields have the following functions:

comment This is a string that will be visible in the module table. It should give some hint about the function of this module.
init This function is called upon loading the module. The module pointer should be stored by the module because it is needed in other calls and the argument vector will contain the arguments to this module from the daemons command line. This function should return 0 if everything is ok or an UNIX error code (see errno 3 ). Once the function returns 0, the fini function is called when the module is unloaded.
fini The module is unloaded. This gives the module a chance to free resources that are not automatically freed. Be sure to free all memory, because daemons tend to run very long. This function pointer may be NULL if it is not needed.
idle If this function pointer is not NULL, the function pointed to by it is called whenever the daemon is going to wait for an event. Try to avoid using this feature.
dump Whenever the daemon receives a SIGUSR1 it dumps it internal state via syslog(3). If the dump field is not NULL it is called by the daemon to dump the state of the module.
config Whenever the daemon receives a SIGHUP signal it re-reads its configuration file. If the config field is not NULL it is called after reading the configuration file to give the module a chance to adapt to the new configuration.
start If not NULL this function is called after successful loading and initializing the module to start its actual operation.
proxy If the daemon receives a PDU and that PDU has a community string who’s community was registered by this module and proxy is not NULL than this function is called to handle the PDU.
tree This is a pointer to the node array for the MIB tree implemented by this module.
tree_size This is the number of nodes in tree.
loading If this pointer is not NULL it is called whenever another module was loaded or unloaded. It gets a pointer to that module and a flag that is 0 for unloading and 1 for loading.

When everything is ok, the daemon merges the module’s MIB tree into its current global tree, calls the modules init function. If this function returns an error, the modules MIB tree is removed from the global one and the module is unloaded. If initialisation is successful, the modules start function is called. After it returns the loaded functions of all modules (including the loaded one) are called.

When the module is unloaded, its MIB tree is removed from the global one, the communities, request id ranges, running timers and selected file descriptors are released, the fini function is called, the module file is unloaded and the loaded functions of all other modules are called.


There are a number of macros designed to help implementing SNMP tables. A problem while implementing a table is the support for the GETNEXT operator. The GETNEXT operation has to find out whether, given an arbitrary OID, the lessest table row, that has an OID higher than the given OID. The easiest way to do this is to keep the table as an ordered list of structures each one of which contains an OID that is the index of the table row. This allows easy removal, insertion and search.

The helper macros assume, that the table is organized as a TAILQ (see queue(3) and each structure contains a
.Vt struct asn_oid that is used as index. For simple tables with only a integer or unsigned index, an alternate form of the macros is available, that presume the existence of an integer or unsigned field as index field.

The macros have name of the form

The INSERT_* macros are used in the SET operation to insert a new table row into the table. The FIND_* macros are used in the GET operation to find a specific row in the table. The NEXT_* macros are used in the GETNEXT operation to find the next row in the table. The last two macros return a pointer to the row structure if a row is found, NULL otherwise. The macros *_OBJECT_OID_* assume the existence of a
.Vt struct asn_oid that is used as index, the macros *_OBJECT_INT_* assume the existance of an unsigned integer field that is used as index.

The macros *_INDEX allow the explicit naming of the index field in the parameter INDEX, whereas the other macros assume that this field is named index. The macros *_LINK_* allow the explicit naming of the link field of the tail queues, the others assume that the link field is named link. Explicitely naming the link field may be necessary if the same structures are held in two or more different tables.

The arguments to the macros are as follows:

PTR A pointer to the new structure to be inserted into the table.
LIST A pointer to the tail queue head.
LINK The name of the link field in the row structure.
INDEX The name of the index field in the row structure.
OID Must point to the var field of the value argument to the node operation callback. This is the OID to search for.
SUB This is the index of the start of the table index in the OID pointed to by OID. This is usually the same as the sub argument to the node operation callback.


The variable this_tick contains the tick (there are 100 SNMP ticks in a second) when the current PDU processing was started. The variable start_tick contains the tick when the daemon was started. The function get_ticks returns the current tick. The number of ticks since the daemon was started is
get_ticks() - start_tick


The scalar fields of the system group are held in the global variable systemg:
struct systemg {
struct asn_oid object_id;
u_int32_t services;
u_int32_t or_last_change;


The SNMP daemon implements a community table. On recipte of a request message the community string in that message is compared to each of the community strings in that table, if a match is found, the global variable community is set to the community identifier for that community. Community identifiers are unsigned integers. For the three standard communities there are three constants defined:
#define COMM_READ 1
#define COMM_WRITE 2

community is set to COMM_INITIALIZE while the assignments in the configuration file are processed. To COMM_READ or COMM_WRITE when the community strings for the read-write or read-only community are found in the incoming PDU.

Modules can define additional communities. This may be necessary to provide transport proxying (a PDU received on one communication link is proxied to another link) or to implement non-UDP access points to SNMP. A new community is defined with the function comm_define. It takes the following parameters:

priv This is an integer identifying the community to the module. Each module has its own namespace with regard to this parameter. The community table is indexed with the module name and this identifier.
descr This is a string providing a human readable description of the community. It is visible in the community table.
mod This is the module defining the community.
str This is the initial community string.

The function returns a globally unique community identifier. If a PDU is received who’s community string matches, this identifier is set into the global community.

The function comm_string returns the current community string for the given community.

All communities defined by a module are automatically released when the module is unloaded.


The global variable oid_zeroDotZero contains the OID 0.0.


For modules that implement SNMP client functions besides SNMP agent functions it may be necessary to identify SNMP requests by their identifier to allow easier routing of responses to the correct sub-system. Request id ranges provide a way to aquire globally non-overlapping sub-ranges of the entire 31-bit id range.

A request id range is allocated with reqid_allocate. The arguments are: the size of the range and the module allocating the range. For example, the call
id = reqid_allocate(1000, module);

allocates a range of 1000 request ids. The function returns the request id range identifier or 0 if there is not enough identifier space. The function reqid_base returns the lowest request id in the given range.

Request id are allocated starting at the lowest one linear throughout the range. If the client application may have a lot of outstanding request the range must be large enough so that an id is not reused until it is really expired. reqid_next returns the sequentially next id in the range.

The function reqid_istype checks whether the request id reqid is withing the range identified by type. The function reqid_type returns the range identifier for the given reqid or 0 if the request id is in none of the ranges.


The SNMP daemon supports an arbitrary number of timers with SNMP tick granularity. The function timer_start arranges for the callback func to be called with the argument uarg after ticks SNMP ticks have expired. mod is the module that starts the timer. Timers are one-shot, they are not restarted. The function returns a timer identifier that can be used to stop the timer via timer_stop. If a module is unloaded all timers started by the module that have not expired yet are stopped.


A module may need to get input from socket file descriptors without blocking the daemon (for example to implement alternative SNMP transports).

The function fd_select causes the callback function func to be called with the file descriptor fd and the user argument uarg whenever the file descriptor fd can be red or has a close condition. If the file descriptor is not in non-blocking mode, it is set to non-blocking mode. If the callback is not needed anymore, fd_deselect may be called with the value returned from fd_select. All file descriptors selected by a module are automatically deselected when the module is unloaded.

To temporarily suspend the file descriptor registration fd_suspend can be called. This also causes the file descriptor to be switched back to blocking mode if it was blocking prior the call to fd_select. This is necessary to do synchronuous input on a selected socket. The effect of fd_suspend can be undone with fd_resume.


The system group contains an object resource table. A module may create an entry in this table by calling or_register with the oid to be registered, a textual description in str and a pointer to the module mod. The registration can be removed with or_unregister. All registrations of a module are automatically removed if the module is unloaded.


A buffer is allocated via buf_alloc. The argument must be 1 for transmit and 0 for receive buffers. The function may return NULL if there is no memory available. The current buffersize can be obtained with buf_size.


For modules that need to do their own PDU processing (for example for proxying) the following functions are available:

Function snmp_input_start decodes the PDU, searches the community, and sets the global this_tick. It returns one of the following error codes:

SNMPD_INPUT_OK Everything ok, continue with processing.
SNMPD_INPUT_FAILED The PDU could not be decoded, has a wrong version or an unknown community string.
SNMPD_INPUT_VALBADLEN A SET PDU had a value field in a binding with a wrong length field in an ASN.1 header.
SNMPD_INPUT_VALRANGE A SET PDU had a value field in a binding with a value that is out of range for the given ASN.1 type.
SNMPD_INPUT_VALBADENC A SET PDU had a value field in a binding with wrong ASN.1 encoding.
SNMPD_INPUT_TRUNC The buffer appears to contain a valid begin of a PDU, but is too short. For streaming transports this means that the caller must save what he already has and trying to obtain more input and reissue this input to the function. For datagram transports this means that part of the datagram was lost and the input should be ignored.

The function snmp_input_finish does the other half of processing: if snmp_input_start did not return OK, tries to construct an error response. If the start was OK, it calls the correct function from bsnmpagent to execute the request and depending on the outcome constructs a response or error response PDU or ignores the request PDU. It returns either SNMPD_INPUT_OK or SNMPD_INPUT_FAILED. In the first case a response PDU was constructed and should be sent.

The function snmp_output takes a PDU and encodes it.

The function snmp_send_port takes a PDU, encodes it and sends it through the given port (identified by the transport and the index in the port table) to the given address.

The function snmp_send_trap sends a trap to all trap destinations. The arguments are the oid identifying the trap and a NULL-terminated list of
.Vt struct snmp_value pointers that are to be inserted into the trap binding list.


For simple scalar variables that need no dependencies a number of support functions is available to handle the set, commit, rollback and get.

The following functions are used for OCTET STRING scalars, either NUL terminated or not:

should be called for SNMP_OP_SET. value and ctx are the resp. arguments to the node callback. valp is a pointer to the pointer that holds the current value and req_size should be -1 if any size of the string is acceptable or a number larger or equal zero if the string must have a specific size. The function saves the old value in the scratch area (note, that any initial value must have been allocated by malloc 3 ), allocates a new string, copies over the new value, NUL-terminates it and sets the new current value.
simply frees the saved old value in the scratch area.
frees the new value, and puts back the old one.
is used for GET or GETNEXT. If len is -1, the length is computed via strlen(3) from the current string value. If the current value is NULL, a OCTET STRING of zero length is returned.
must be called if either rollback or commit fails to free the saved old value.

The following functions are used to process scalars of type IP-address:

ip_save Saves the current value in the scratch area and sets the new value from valp.
ip_commit Does nothing.
Restores the old IP address from the scratch area.
ip_get Retrieves the IP current address.

The following functions handle OID-typed variables:

oid_save Saves the current value in the scratch area by allocating a
.Vt struct asn_oid with malloc(3) and sets the new value from oid.
Frees the old value in the scratch area.
Restores the old OID from the scratch area and frees the old OID.
oid_get Retrieves the OID


The following functions help in handling table indexes:
Decodes the index part of the OID. The parameter oid must be a pointer to the var field of the value argument of the node callback. The sub argument must be the index of the start of the index in the OID (this is the sub argument to the node callback). code is the index expression (parameter idx to the node callback). These parameters are followed by parameters depending on the syntax of the index elements as follows:
.Vt int32_t * expected as argument.
.Vt u_int64_t * expected as argument. Note, that this syntax is illegal for indexes.
.Vt u_char ** and a
.Vt size_t * expected as arguments. A buffer is allocated to hold the decoded string.
.Vt struct asn_oid * is expected as argument.
.Vt u_int8_t * expected as argument that points to a buffer of at least four byte.
.Vt u_int32_t expected.
NULL No argument expected.
index_compare compares the current variable with an OID. oid1 and sub come from the node callback arguments value->var and sub resp. oid2 is the OID to compare to. The function returns -1, 0, +1 when the variable is lesser, equal, higher to the given OID. oid2 must contain only the index part of the table column.
index_compare_off is equivalent to index_compare except that it takes an additional parameter off that causes it to ignore the first off components of both indexes.
index_append appends OID src beginning at position sub to dst.
index_append_off appends OID src beginning at position off to dst beginning at position sub + off.


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



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