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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 applica-tions applications
       tions 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  pro-vides provides
       vides  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 avail-able. available.
       able.  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  pro-grams), programs),
       grams), 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 alonewithout
       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.





V5.6                                             08 Feb 2002                                    VARIABLES(5)