AGENTX(3) | Library Functions Manual | AGENTX(3)

# NAME

`agentx_log_fatal`, `agentx_log_warn`, `agentx_log_info`, `agentx_log_debug`, `agentx`, `agentx_connect`, `agentx_retry`, `agentx_read`, `agentx_write`, `agentx_wantwrite`, `agentx_free`, `agentx_session`, `agentx_session_free`, `agentx_context`, `agentx_context_object_find`, `agentx_context_object_nfind`, `agentx_context_uptime`, `agentx_context_free`, `agentx_region`, `agentx_region_free`, `agentx_agentcaps`, `agentx_agentcaps_free`, `agentx_index_integer_new`, `agentx_index_integer_any`, `agentx_index_integer_value`, `agentx_index_integer_dynamic`, `agentx_index_string_dynamic`, `agentx_index_nstring_dynamic`, `agentx_index_oid_dynamic`, `agentx_index_noid_dynamic`, `agentx_index_ipaddress_dynamic`, `agentx_index_free`, `agentx_object`, `agentx_object_free`, `agentx_varbind_integer`, `agentx_varbind_string`, `agentx_varbind_nstring`, `agentx_varbind_printf`, `agentx_varbind_null`, `agentx_varbind_oid`, `agentx_varbind_object`, `agentx_varbind_index`, `agentx_varbind_ipaddress`, `agentx_varbind_counter32`, `agentx_varbind_gauge32`, `agentx_varbind_unsigned32`, `agentx_varbind_timeticks`, `agentx_varbind_opaque`, `agentx_varbind_counter64`, `agentx_varbind_notfound`, `agentx_varbind_error`, `agentx_varbind_request`, `agentx_varbind_get_index_integer`, `agentx_varbind_get_index_string`, `agentx_varbind_get_index_oid`, `agentx_varbind_get_index_ipaddress`, `agentx_varbind_set_index_integer`, `agentx_varbind_set_index_string`, `agentx_varbind_set_index_nstring`, `agentx_varbind_set_index_oid`, `agentx_varbind_set_index_object`, `agentx_varbind_set_index_ipaddress` — manage an interface to an agentx master

# SYNOPSIS

library “libagentx”
`#include <agentx.h>`

extern void
`(*agentx_log_fatal)`(const char *fmt, ...);

extern void
`(*agentx_log_warn)`(const char *fmt, ...);

extern void
`(*agentx_log_info)`(const char *fmt, ...);

extern void
`(*agentx_log_debug)`(const char *fmt, ...);

struct agentx *
`agentx`(void (*nofd)(struct agentx *, void *, int), void *cookie);

void
`agentx_connect`(struct agentx *sa, int fd);

void
`agentx_retry`(struct agentx *sa);

void
`agentx_read`(struct agentx *sa);

void
`agentx_write`(struct agentx *sa);

extern void
`(*agentx_wantwrite)`(struct agentx *sa, int fd);

void
`agentx_free`(struct agentx *sa);

struct agentx_session *
`agentx_session`(struct agentx *sa, uint32_t oid[], size_t oidlen, const char *descr, uint8_t timeout);

void
`agentx_session_free`(struct agentx_session *sas);

struct agentx_context *
`agentx_context`(struct agentx_session *sas, const char *name);

struct agentx_object *
`agentx_context_object_find`(struct agentx_context *sac, const uint32_t oid[], size_t oidlen, int active, int instance);

struct agentx_object *
`agentx_context_object_nfind`(struct agentx_context *, const uint32_t oid[], size_t oidlen, int active, int inclusive);

uint32_t
`agentx_context_uptime`(struct agentx_context *sac);

void
`agentx_context_free`(struct agentx_context *sac);

struct agentx_agentcaps *
`agentx_agentcaps`(struct agentx_context *sac, uint32_t oid[], size_t oidlen, const char *descr);

void
`agentx_agentcaps_free`(struct agentx_agentcaps *saa);

struct agentx_region *
`agentx_region`(struct agentx_context *sac, uint32_t oid[], size_t oidlen, uint8_t timeout);

void
`agentx_region_free`(struct agentx_region *sar);

struct agentx_index *
`agentx_index_integer_new`(struct agentx_region *sar, uint32_t oid[], size_t oidlen);

struct agentx_index *
`agentx_index_integer_any`(struct agentx_region *sar, uint32_t oid[], size_t oidlen);

struct agentx_index *
`agentx_index_integer_value`(struct agentx_region *sar, uint32_t oid[], size_t oidlen, int32_t value);

struct agentx_index *
`agentx_index_integer_dynamic`(struct agentx_region *sar, uint32_t oid[] , size_t, oidlen");

struct agentx_index *
`agentx_index_string_dynamic`(struct agentx_region *sar, uint32_t oid[], size_t oidlen);

struct agentx_index *
`agentx_index_nstring_dynamic`(struct agentx_region *sar, uint32_t oid[], size_t oidlen, size_t slen);

struct agentx_index *
`agentx_index_oid_dynamic`(struct agentx_region *sar, uint32_t oid[], size_t oidlen);

struct agentx_index *
`agentx_index_noid_dynamic`(struct agentx_region *sar, uint32_t oid[], size_t oidlen, size_t vlen);

struct agentx_index *
`agentx_index_ipaddress_dynamic`(struct agentx_region *sar, uint32_t oid[], size_t oidlen);

void
`agentx_index_free`(struct agentx_index *sai);

struct agentx_object *
`agentx_object`(struct agentx_region *sar, uint32_t oid[], size_t oidlen, struct agentx_index *index[], size_t indexlen, int implied, void (*getcb)(struct agentx_varbind *));

void
`agentx_object_free`(struct agentx_object *sao);

void
`agentx_varbind_integer`(struct agentx_varbind *sav, int32_t value);

void
`agentx_varbind_string`(struct agentx_varbind *sav, const char *value);

void
`agentx_varbind_nstring`(struct agentx_varbind *sav, const char *value, size_t slen);

void
`agentx_varbind_printf`(struct agentx_varbind *sav, const char *fmt, ...);

void
`agentx_varbind_null`(struct agentx_varbind *sav);

void
`agentx_varbind_oid`(struct agentx_varbind *sav, const uint32_t oid[], size_t oidlen);

void
`agentx_varbind_object`(struct agentx_varbind *sav, struct agentx_object *sao);

void
`agentx_varbind_index`(struct agentx_varbind *sav, struct agentx_index *sai);

void
`agentx_varbind_ipaddress`(struct agentx_varbind *sav, const struct in_addr *addr);

void
`agentx_varbind_counter32`(struct agentx_varbind *sav, uint32_t value);

void
`agentx_varbind_gauge32`(struct agentx_varbind *sav, uint32_t value);

void
`agentx_varbind_unsigned32`(struct agentx_varbind *sav, uint32_t value);

void
`agentx_varbind_timeticks`(struct agentx_varbind *sav, uint32_t value);

void
`agentx_varbind_opaque`(struct agentx_varbind *sav, const char *value, size_t slen);

void
`agentx_varbind_counter64`(struct agentx_varbind *sav, uint64_t value);

void
`agentx_varbind_notfound`(struct agentx_varbind *sav);

void
`agentx_varbind_error`(struct agentx_varbind *sav);

enum agentx_request_type
`agentx_varbind_request`(struct agentx_varbind *sav);

int32_t
`agentx_varbind_get_index_integer`(struct agentx_varbind *sav, struct agentx_index *sai);

const unsigned char *
`agentx_varbind_get_index_string`(struct agentx_varbind *sav, struct agentx_index *sai, size_t *slen, int *implied);

const uint32_t *
`agentx_varbind_get_index_oid`(struct agentx_varbind *sav, struct agentx_index *sai, size_t *oidlen, int *implied);

const struct in_addr *
`agentx_varbind_get_index_ipaddress`(struct agentx_varbind *sav, struct agentx_index *sai);

void
`agentx_varbind_set_index_integer`(struct agentx_varbind *sav, struct agentx_index *sai, int32_t value);

void
`agentx_varbind_set_index_string`(struct agentx_varbind *sav, struct agentx_index *sai, const unsigned char *value);

void
`agentx_varbind_set_index_nstring`(struct agentx_varbind *sav, struct agentx_index *sai, const unsigned char *value, size_t slen);

void
`agentx_varbind_set_index_oid`(struct agentx_varbind *sav, struct agentx_index *sai, const uint32_t *oid, size_t oidlen);

void
`agentx_varbind_set_index_object`(struct agentx_varbind *sav, struct agentx_index *sai, struct agentx_object *sao);

void
`agentx_varbind_set_index_ipaddress`(struct agentx_varbind *sav, struct agentx_index *sai, const struct in_addr *addr);

    enum agentx_request_type {
            AGENTX_REQUEST_TYPE_GET,
            AGENTX_REQUEST_TYPE_GETNEXT,
            AGENTX_REQUEST_TYPE_GETNEXTINCLUSIVE
    };

`#define AGENTX_MASTER_PATH "/var/agentx/master"`
`#define AGENTX_OID_MAX_LEN 128`
`#define AGENTX_OID_INDEX_MAX_LEN 10`
`#define AGENTX_OID(...)`
`#define AGENTX_MIB2 1, 3, 6, 1, 2, 1`
`#define AGENTX_ENTERPRISES 1, 3, 6, 1, 4, 1`

# DESCRIPTION

The `agentx` functions allow an application to describe their MIB layout and provide an fd based interface to control the internal agentx state. `agentx` is not thread safe.

## DESCRIBING THE MIB

`agentx` is a framework to abstract away the agentx protocol from the application. For the framework to report information to the administrator, the `agentx_log_fatal`(), `agentx_log_warn`(), `agentx_log_info`() and `agentx_log_debug`() functions must be set.

When sa is created by `agentx`() or when sa detects that there is no connection to the agentx master, it calls out to nofd with itself, cookie and an integer close as arguments. If close is not set, `nofd`() is expected to set up a new fd to the agentx master. This one can usually be found at `AGENTX_MASTER_PATH`. This fd can be returned to sa at any moment via `agentx_connect`(), but must always be done as a result of a call to `nofd`(). Once `agentx_connect`() has been called, the application is responsible for retrieving data when available on fd by calling `agentx_read`(). If nonblocking writes are desirable, the `agentx_wantwrite`() pointer can be set to an application function and will be called as soon as there's data available to be written out. Once fd is ready for a write, the function `agentx_write`() should be called.

If any of the session, agentcaps, region, index, or objects failed to enable correctly (as can be seen by the admin through the logs) they can be retried through `agentx_retry`().

sa can be freed via `agentx_free`(). It will close all active sessions and free all derived objects. Once freed, no new objects can be derived from the freed objects. Once all sessions are closed, it will call out to `nofd`() with close set, indicating that the application can clean up any context related to sa.

On top of the sa connection a agentx_session must be set up. Normally there's only a single session per sa. The timeout argument specifies the maximum time in seconds the master should wait for a reply before determining we're gone. If set to 0, the agentx master determines the timeout. The oid and oidlen combination identifies the subagent and will be visible through the agentxSessionObjectID object on the agentx master. The descr is a short displaystring description of the agent and will be visible through the agentxSessionDescr object on the agentx master.

The agentx_context is the SNMPv3 context in which the objects operate and is built on top of agentx_session sas. If the default context is requested, name must be NULL.

`agentx_agentcaps`() registers an entry in the agentx master's sysORTable. The oid, oidlen combination should point to an AGENT-CAPABILITIES object which describes the capabilities of the subagent. descr should be a textual description of the capabilities. If no AGENT-CAPABILITIES object is defined, this function can be omitted.

A agentx_region indicates a region inside the object-tree for which get- and set-requests will be queried. If the OID has already been claimed by another subagent, it will try to claim it on a lower priority. The timeout parameter overrules its agentx_session counterpart.

For objects in a table one or more agentx_index elements must be supplied. `agentx_index_integer_new`(), `agentx_index_integer_any`() and `agentx_index_integer_value`() register an integer index at the agentx master. Of these `agentx_index_integer_new`() registers a new, previously unused, index; `agentx_index_integer_any`() registers the first available index; and `agentx_index_integer_value`() tries to register a specific value. If the registration of an index fails, an error will be logged and all objects using it will remain disabled. The OID where the index should be registered is documented by the MIB. These registered indices are usually used for tables where multiple subagents are registered.

For dynamic indices the agentx_index_*_dynamic functions can be used, based on the data type of the object. The data type should match the data type in the MIB at the oid object. Indices of data type string or oid with a fixed length should be created via `agentx_index_nstring_dynamic`() and `agentx_index_noid_dynamic`() respectively.

agentx_object is an object as described in the MIB. For scalar objects (without indices) the final zero must be omitted. For table entries a list of 1 or more indices must be added via index and indexlen. The list of indices must match the INDEX list on the ENTRY object in the MIB. The total length of the OID, including indices, can't be more than `AGENTX_OID_MAX_LEN` and indexlen can't be more than `AGENTX_OID_INDEX_MAX_LEN`. If implied is set, the final index must be of type OID or string and will omit the leading length indicator. This value must only be set if specified in the MIB. `getcb`() will be called for each varbind in a GET, GETNEXT or GETBULK request that matches the object.

## HANDLING GET REQUESTS

A call to `getcb`() must eventually result in a call to one of the following functions:

`agentx_varbind_integer`()
  Set the return value to an int32_t value.
`agentx_varbind_string`()
  A C string wrapper around `agentx_varbind_nstring`().
`agentx_varbind_nstring`()
  Set the return value to an octetstring.
`agentx_varbind_printf`()
  A printf wrapper around `agentx_varbind_nstring`().
`agentx_varbind_null`()
  Set the return value to null.
`agentx_varbind_oid`()
  Set the return value to an OID value.
`agentx_varbind_object`()
  An agentx_object wrapper around `agentx_varbind_oid`().
`agentx_varbind_index`()
  An agentx_index wrapper around `agentx_varbind_oid`().
`agentx_varbind_ipaddress`()
  Set the return value to ipaddress.
`agentx_varbind_counter32`()
  Set the return value to an uint32_t of type counter32.
`agentx_varbind_gauge32`()
  Set the return value to an uint32_t of type gauge32.
`agentx_varbind_unsigned32`()
  A wrapper around agentx_varbind_gauge32.
`agentx_varbind_timeticks`()
  Set the return value to an uint32_t of type timeticks.
`agentx_varbind_opaque`()
  Set the return value to an opaque value.
`agentx_varbind_counter64`()
  Set the return value to an uint64_t of type counter64.
`agentx_varbind_notfound`()
  When the request is of type GET, return a nosuchinstance error. When the request is of type GETNEXT or GETBULK, return an endofmibview error. On endofmibview the next object is queried. This function can only be called on objects that contain one or more *_dynamic indices.
`agentx_varbind_error`()
  Returns a GENERR error to the client.

For objects containing *_dynamic indices the following support functions are to be used:

`agentx_varbind_request`()
  Returns whether the request is of type GET, GETNEXT or GETNEXTINCLUSIVE.
`agentx_varbind_get_index_integer`()
  Retrieve a single int32_t index value.
`agentx_varbind_get_index_string`()
  Retrieve an octetstring index value. slen is the length of the string and implied indicates if the next value for this index should be length sorted before alphabetically sorted.
`agentx_varbind_get_index_oid`()
  Retrieve an oid index value. oidlen is the length of the oid and implied indicates if the next value for this index should be length sorted before alphabetically sorted.
`agentx_varbind_get_index_ipaddress`()
  Retrieve an ipaddress index value.
`agentx_varbind_set_index_integer`()
  Sets a single int32_t index value.
`agentx_varbind_set_index_string`()
  A C string wrapper around `agentx_varbind_set_index_nstring`().
`agentx_varbind_set_index_nstring`()
  Set an octetstring index value.
`agentx_varbind_set_index_oid`()
  Set an oid index value.
`agentx_varbind_set_index_object`()
  A agentx_object wrapper around `agentx_varbind_set_index_oid`().
`agentx_varbind_set_index_ipaddress`()
  Set an ipaddress index value.

For these functions sai must be part of the object the request is performed on. The function type must also match the data type of sai.

Other functions that can retrieve information from the agentx context are:

`agentx_context_object_find`()
  Find an agentx_object created inside agentx_context sac based on oid and oidlen. If active is set the object must be reachable from the agentx master, else NULL is returned. If oid can be an instance, find its parent object.
`agentx_context_object_nfind`()
  Find the next agentx_object created inside agentx_context sac based on oid and oidlen. If active is set the object must be reachable from the agentx master, else NULL is returned. If inclusive is set, the object returned may also exactly match oid.
`agentx_context_uptime`()
  Returns the sysuptime in seconds for sac in timeticks.

# SEE ALSO

snmp(1), snmpd(8)

# STANDARDS

M. Daniele, B. Wijnen, M. Ellison, Ed., and D. Francisco, Ed., Agent Extensibility (AgentX) Protocol Version 1, RFC 2741, January 2000.

L. Heintz, S. Gudur, and M. Ellison, Ed., Definitions of Managed Objects for Extensible SNMP Agents, RFC 2742, January 2000.

# HISTORY

The `agentx` API first appeared in OpenBSD 6.9.

# AUTHORS

Martijn van Duren <martijn@openbsd.org>

June 5, 2025 | Debian