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Man Pages

Manual Reference Pages  -  SNMP_MIBII (3)


mibII, mibif_notify_f, mib_netsock, mib_if_set_dyn, mib_refresh_iflist, mib_find_if, mib_find_if_sys, mib_find_if_name, mib_first_if, mib_next_if, mib_register_newif, mib_unregister_newif, mib_fetch_ifmib, mib_if_admin, mib_find_ifa, mib_first_ififa, mib_next_ififa, mib_ifstack_create, mib_ifstack_delete, mib_find_rcvaddr, mib_rcvaddr_create, mib_rcvaddr_delete, mibif_notify, mibif_unnotify - mib-2 module for bsnmpd.


     Direct Network Access
     Network Interfaces
     Interface Events
     Interface Addresses
     Interface Receive Addresses
     Interface Stack Table
See Also


(begemotSnmpdModulePath. mibII = /usr/lib/


.In bsnmp/snmpmod.h
.In bsnmp/snmp_mibII.h typedef void (*mibif_notify_f) struct mibif *ifp enum mibif_notify event void *uarg
.Vt extern int mib_netsock ; void mib_if_set_dyn const char *ifname void mib_refresh_iflist void struct mibif * mib_find_if u_int ifindex struct mibif * mib_find_if_sys u_int sysindex struct mibif * mib_find_if_name const char *ifname struct mibif * mib_first_if void struct mibif * mib_next_if const struct mibif *ifp int mib_register_newif int (*func)(struct mibif *) const struct lmodule *mod void mib_unregister_newif const struct lmodule *mod int mib_fetch_ifmib struct mibif *ifp int mib_if_admin struct mibif *ifp int up struct mibifa * mib_find_ifa struct in_addr ipa struct mibifa * mib_first_ififa const struct mibif *ifp struct mibifa * mib_next_ififa struct mibifa *ifa int mib_ifstack_create const struct mibif *lower const struct mibif *upper void mib_ifstack_delete const struct mibif *lower const struct mibif *upper struct mibrcvaddr * mib_find_rcvaddr u_int ifindex const u_char *addr size_t addrlen struct mibrcvaddr * mib_rcvaddr_create struct mibif *ifp const u_char *addr size_t addrlen void mib_rcvaddr_delete struct mibrcvaddr *addr void * mibif_notify struct mibif *ifp const struct lmodule *mod mibif_notify_f func void *uarg void mibif_unnotify void *reg


The snmp_mibII module implements parts of the internet standard MIB-2. Most of the relevant MIBs are implemented. Some of the tables are restricted to be read-only instead of read-write. The exact current implementation can be found in /usr/share/snmp/defs/mibII_tree.def. The module also exports a number of functions and global variables for use by other modules, that need to handle network interfaces. This man page describes these functions.


The mibII module opens a socket that is used to execute all network related ioctl(2) functions. This socket is globally available under the name mib_netsock.


The mibII module handles a list of all currently existing network interfaces. It allows other modules to handle their own interface lists with special information by providing a mechanism to register to events that change the interface list (see below). The basic data structure is the interface structure:
struct mibif {
        TAILQ_ENTRY(mibif) link;
        u_int           flags;
        u_int           index;  /* logical ifindex */
        u_int           sysindex;
        char            name[IFNAMSIZ];
        char            descr[256];
        struct ifmibdata mib;
        uint64_t        mibtick;
        void            *specmib;
        size_t          specmiblen;
        u_char          *physaddr;
        u_int           physaddrlen;
        int             has_connector;
        int             trap_enable;
        uint64_t        counter_disc;
        mibif_notify_f  xnotify;
        void            *xnotify_data;
        const struct lmodule *xnotify_mod;
        struct asn_oid  spec_oid;

The mibII module tries to implement the semantic if ifIndex as described in RFC-2863. This RFC states, that an interface indexes may not be reused. That means, for example, if tun is a synthetic interface type and the system creates the interface tun0, destroys this interfaces and again creates a tun 0, then these interfaces must have different interface indexes, because in fact they are different interfaces. If, on the other hand, there is a hardware interface xl0 and this interface disappears, because its driver is unloaded and appears again, because the driver is loaded again, the interface index must stay the same. mibII implements this by differentiating between real and synthetic (dynamic) interfaces. An interface type can be declared dynamic by calling the function mib_if_set_dyn with the name if the interface type (for example "tun )." For real interfaces, the module keeps the mapping between the interface name and its ifIndex in a special list, if the interface is unloaded. For dynamic interfaces a new ifIndex is generated each time the interface comes into existence. This means, that the interface index as seen by SNMP is not the same index as used by the system. The SNMP ifIndex is held in field index, the system’s interface index is sysindex.

A call to mib_refresh_iflist causes the entire interface list to be re-created.

The interface list can be traversed with the functions mib_first_if and mib_next_if. Be sure not to change the interface list while traversing the list with these two calls.

There are three functions to find an interface by name or index. mib_find_if finds an interface by searching for an SNMP ifIndex, mib_find_if_sys finds an interface by searching for a system interface index and mib_find_if_name finds an interface by looking for an interface name. Each of the function returns NULL if the interface cannot be found.

The function mib_fetch_ifmib causes the interface MIB to be refreshed from the kernel.

The function mib_if_admin can be used to change the interface administrative state to up (argument is 1) or down (argument is 0).


A module can register itself to receive a notification when a new entry is created in the interface list. This is done by calling mib_register_newif. A module can register only one function, a second call to mib_register_newif causes the registration to be overwritten. The registration can be removed with a call to mib_unregister_newif. It is unregistered automatically, when the registering module is unloaded.

A module can also register to events on a specific interface. This is done by calling mibif_notify. This causes the given callback func to be called with the interface pointer, a notification code and the user argument uarg when any of the following events occur:
  The interface is destroyed.

This mechanism can be used to implement interface type specific MIB parts in other modules. The registration can be removed with mib_unnotify which the return value from mib_notify. Any notification registration is removed automatically when the interface is destroyed or the registering module is unloaded. Note that only one module can register to any given interface.


The mibII module handles a table of interface IP-addresses. These addresses are held in a
struct mibifa {
        TAILQ_ENTRY(mibifa) link;
        struct in_addr  inaddr;
        struct in_addr  inmask;
        struct in_addr  inbcast;
        struct asn_oid  index;
        u_int           ifindex;
        u_int           flags;

The (ordered) list of IP-addresses on a given interface can be traversed by calling mib_first_ififa and mib_next_ififa. The list should not be considered read-only.


The internet MIB-2 contains a table of interface receive addresses. These addresses are handled in:
struct mibrcvaddr {
        TAILQ_ENTRY(mibrcvaddr) link;
        struct asn_oid  index;
        u_int           ifindex;
        u_char          addr[ASN_MAXOIDLEN];
        size_t          addrlen;
        u_int           flags;
enum {
        MIBRCVADDR_VOLATILE     = 0x00000001,
        MIBRCVADDR_BCAST        = 0x00000002,
        MIBRCVADDR_HW           = 0x00000004,

Note, that the assignment of MIBRCVADDR_BCAST is based on a list of known interface types. The flags should be handled by modules implementing interface type specific MIBs.

A receive address can be created with mib_rcvaddr_create and deleted with mib_rcvaddr_delete. This needs to be done only for addresses that are not automatically handled by the system.

A receive address can be found with mib_find_rcvaddr.


The mibII module maintains also the interface stack table. Because for complex stacks, there is no system supported generic way of getting this information, interface type specific modules need to help setting up stack entries. The mibII module handles only the top and bottom entries.

A table entry is created with mib_ifstack_create and deleted with mib_ifstack_delete. Both functions need the pointers to the interfaces. Entries are automatically deleted if any of the interfaces of the entry is destroyed. The functions handle both the stack table and the reverse stack table.


/usr/share/snmp/defs/mibII_tree.def The description of the MIB tree implemented by mibII.
  The various internet MIBs.


gensnmptree(1), snmpmod(3)


This implementation conforms to the applicable IETF RFCs.


.An Hartmut Brandt Aq
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