Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help

	   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.

V5.7.3				  01 Oct 2010			  VARIABLES(5)


Want to link to this manual page? Use this URL:

home | help