VARIABLES(5)                       Net-SNMP                       VARIABLES(5)

           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 indi-
       vidual 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 ob-
       ject 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  representa-
       tion 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 reason-
       ably compact form.  It also makes it relatively easy to find the  defi-
       nition of the MIB object.

       This  is  the  default  format for displaying OIDs in Net-SNMP applica-
       tions.  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 circum-
       stances), then it's not necessarily clear which MIB object is  actually
       meant.   For  day-to-day  use, particularly when using standard MIB ob-
       jects, this is probaby safe.  But it's important to be aware of the po-
       tential 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  ap-
       proach  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 pre-
       fix 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 op-
       tion -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 (al-
       though correct) interpretations of the ASN.1 notation.

V5.8                              01 Oct 2010                     VARIABLES(5)
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