phpman > man > variables(5)

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 understand‐
       ing 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"), typi‐
       cally 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 know‐
       ing  what a given numeric OID refers to.  As such, it is not a particularly helpful represen‐
       tation 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  mod‐
       ules  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  per‐
       fectly  legal in some circumstances), then it's not necessarily clear which MIB object is ac‐
       tually 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.inter‐
       net.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  com‐
       mands.

SEE ALSO
       snmpcmd(1)

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





V5.9.1                                       01 Oct 2010                                VARIABLES(5)