variables

VARIABLES(5)                        Net-SNMP                        VARIABLES(5)



NAME
           variables - Format of specifying variable names to SNMP tools.

DESCRIPTION
       The syntax and semantics of management information in SNMP is given by
       the definitions of MIB objects, loaded from one or more MIB files (or
       "MIB modules").  These definitions are not strictly required for the SNMP
       protocol to operate correctly, but are typically needed by SNMP client
       applications to display information in a meaningful manner.

       The MIB file also serves as a design document when developing an SNMP
       agent (or sub-agent) that provides this information, and ensures that
       client and server share a common understanding about what management
       information represents.


OIDs
       MIB objects are specified using Object Identifiers (OIDs), which can take
       a number of forms.   Note that all of the examples in this section refer
       to the same MIB object.

   Numeric OIDs
       The fundamental format of an OID is a sequence of integer values (or
       "subidentifiers"), typically written using dots to separate the
       individual subidentifiers.
               .1.3.6.1.2.1.1.1
       This is the format that is used within the SNMP protocol itself, in the
       packets that are sent over the network.

       This form of representing an OID does not require MIB files or MIB object
       definitions to be available.  However it does rely on the client
       application and/or network administrator knowing what a given numeric OID
       refers to.  As such, it is not a particularly helpful representation to
       anyone just starting out with SNMP.

       This format can be obtained by giving the command-line option -On to most
       Net-SNMP commands.


   Full OID path
       A similar (but somewhat more informative) format uses the same dotted
       list representation, but with the numeric subidentifiers replaced by
       names, taken from the relevant MIB file(s).
               .iso.org.dod.internet.mgmt.mib-2.system.sysDescr
       This uniquely identifies a particular MIB object (as with the numeric
       OID), but the list of names should hopefully give some indication as to
       what information this object represents.  However it does rely on the
       relevant MIB files being available (as do all formats other than the
       purely numeric OID).  Such OIDs also tend to be fairly long!

       This format can be obtained by giving the command-line option -Of to most
       Net-SNMP commands.

       A variant of this (typically used when writing OIDs in descriptive text,
       rather than running programs), is to combine the name and numeric
       subidentifier:
               .iso(1).org(3).dod(6).internet(1).mgmt(2).mib-2(1).system(1)
               .sysDescr(1)


   Module-qualified OIDs
       An alternative way to (more-or-less) uniquely specify an OID, is to give
       the name of the MIB object, together with the MIB module where it is
       defined.
              SNMPv2-MIB::sysDescr
       MIB object names are unique within a given module, so as long as there
       are not two MIB modules with the same name (which is unusual, though not
       unheard of), this format specifies the desired object in a reasonably
       compact form.  It also makes it relatively easy to find the definition of
       the MIB object.

       This is the default format for displaying OIDs in Net-SNMP applications.
       It can also be specified explicitly by giving the command-line option -OS
       to most Net-SNMP commands.


   Object name
       Possibly the most common form for specifying MIB objects is using the
       name of the object alone - without the full path or the name of the
       module that defines it.
              sysDescr
       This is by far the shortest and most convenient way to refer to a MIB
       object.  However the danger is that if two MIB modules each define a MIB
       object with the same name (which is perfectly legal in some
       circumstances), then it's not necessarily clear which MIB object is
       actually meant.  For day-to-day use, particularly when using standard MIB
       objects, this is probaby safe.  But it's important to be aware of the
       potential ambiguities.

       This format can be obtained by giving the command-line option -Os to most
       Net-SNMP commands.


   UCD-format
       Previous versions of the code (UCD v4.x and earlier) used a simple
       approach to shortening the way OIDs were specified.  If the full path of
       the OID began with .iso.org.dod.internet.mgmt.mib-2 then this prefix was
       removed from the OID before displaying it.  All other OIDs were displayed
       in full.

       Similarly, if an OID was passed to the UCD library that did not begin
       with a dot (and wasn't in the module::name format), then the same prefix
       was prepended.   The example OID from the formats listed above would
       therefore be given or displayed as
              system.sysDescr
       The inconsistent handling of OIDs, depending on their location within the
       OID tree, proved to be more trouble than it was worth, and this format is
       no longer recommended.

       The previous behaviour can be obtained by giving the command-line option
       -Ou (for displaying output), or -Iu (for interpreting input OIDs without
       a leading dot) to most Net-SNMP commands.

SEE ALSO
       snmpcmd(1)

BUGS
       The parser of the MIB files file is not expected to handle bizarre
       (although correct) interpretations of the ASN.1 notation.





V5.9                               01 Oct 2010                      VARIABLES(5)