variables - Format of specifying variable names to SNMP tools.


   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.


   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.
   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).
   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

   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.
   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.
   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.

   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 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
   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.




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


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