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MODULE LOADING AND UNLOADING

Modules are loaded by writing to the module table. This table is indexed by a string, that identifies 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 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 whose 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 initialization 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.

IMPLEMENTING TABLES

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 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

{INSERT,FIND,NEXT}_OBJECT_{OID,INT}[_LINK[_INDEX]]

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 struct asn_oid that is used as index, the macros *_OBJECT_INT_*() assume the existence 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. Explicitly 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.

DAEMON TIMESTAMPS

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 SYSTEM GROUP

The scalar fields of the system group are held in the global variable systemg:

struct systemg {
	u_char		*descr;
	struct asn_oid	object_id;
	u_char		*contact;
	u_char		*name;
	u_char		*location;
	uint32_t	services;
	uint32_t	or_last_change;
};

COMMUNITIES

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_INITIALIZE	0
#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 SNMPv1 or SNMPv2 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 USER-BASED SECURITY GROUP

The scalar statistics of the USM group are held in the global variable snmpd_usmstats:

struct snmpd_usmstat {
	uint32_t	unsupported_seclevels;
	uint32_t	not_in_time_windows;
	uint32_t	unknown_users;
	uint32_t	unknown_engine_ids;
	uint32_t	wrong_digests;
	uint32_t	decrypt_errors;
};

A global list of configured USM users is maintained by the daemon.

struct usm_user {
	struct snmp_user		suser;
	uint8_t				user_engine_id[SNMP_ENGINE_ID_SIZ];
	uint32_t			user_engine_len;
	char				user_public[SNMP_ADM_STR32_SIZ];
	uint32_t			user_public_len;
	int32_t				status;
	int32_t				type;
	SLIST_ENTRY(usm_user)		up;
};

THE MANAGEMENT TARGET GROUP

The Management Target group holds target address information used when sending SNMPv3 notifications.

The scalar statistics of the Management Target group are held in the global variable snmpd_target_stats:

struct snmpd_target_stats {
	uint32_t			unavail_contexts;
	uint32_t			unknown_contexts;
};

Three global lists of configured management target addresses, parameters and notifications respectively are maintained by the daemon.

struct target_address {
	char				name[SNMP_ADM_STR32_SIZ];
	uint8_t				address[SNMP_UDP_ADDR_SIZ];
	int32_t				timeout;
	int32_t				retry;
	char				taglist[SNMP_TAG_SIZ];
	char				paramname[SNMP_ADM_STR32_SIZ];
	int32_t				type;
	int32_t				socket;
	int32_t				status;
	SLIST_ENTRY(target_address)	ta;
};

struct target_param {
	char				name[SNMP_ADM_STR32_SIZ];
	int32_t				mpmodel;
	int32_t				sec_model;
	char				secname[SNMP_ADM_STR32_SIZ];
	enum snmp_usm_level		sec_level;
	int32_t				type;
	int32_t				status;
	SLIST_ENTRY(target_param)	tp;
};

struct target_notify {
	char				name[SNMP_ADM_STR32_SIZ];
	char				taglist[SNMP_TAG_SIZ];
	int32_t				notify_type;
	int32_t				type;
	int32_t				status;
	SLIST_ENTRY(target_notify)	tn;
};

The daemon does not create or remove entries in the Management Target group lists, it gives an interface to external loadable module(s) to manage the lists. target_new_address() adds a target address entry, and target_delete_address() deletes an existing entry from the target address list. target_activate_address() creates a socket associated with the target address entry so that SNMP notifications may actually be send to that target address. target_first_address() will return a pointer to the first target address entry in the list, while target_next_address() will return a pointer to the next target address of a given entry if one exists. target_new_param() adds a target parameters' entry, and target_delete_param() deletes an existing entry from the target parameters list. target_first_param() will return a pointer to the first target parameters' entry in the list, while target_next_param() will return a pointer to the next target parameters of a given entry if one exists. target_new_notify() adds a notification target entry, and target_delete_notify() deletes an existing entry from the notification target list. target_first_notify() will return a pointer to the first notification target entry in the list, while target_next_notify() will return a pointer to the next notification target of a given entry if one exists. target_flush_all() is used to remove all configured data from the three global Management Target Group lists.

WELL KNOWN OIDS

The global variable oid_zeroDotZero contains the OID 0.0. The global variables oid_usmUnknownEngineIDs oid_usmNotInTimeWindows contains the OIDs 1.3.6.1.6.3.15.1.1.4.0 and 1.3.6.1.6.3.15.1.1.2.0 used in the SNMPv3 USM Engine Discovery.

REQUEST ID RANGES

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 acquire 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 within 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.

TIMERS

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. These timers are one-shot, they are not restarted. Repeatable timers are started with timer_start_repeat() which takes an additional argument repeat_ticks. The argument ticks gives the number of ticks until the first execution of the callback, while repeat_ticks is the number of ticks between invocations of the callback. Note, that currently the number of initial ticks silently may be set identical to the number of ticks between callback invocations. 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.

FILE DESCRIPTOR SUPPORT

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 read 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 synchronous input on a selected socket. The effect of fd_suspend() can be undone with fd_resume().

OBJECT RESOURCES

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.

TRANSMIT AND RECEIVE BUFFERS

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().