VARIABLES(5)              (01 Oct 2010)              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

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          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.

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        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 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.

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