1VARIABLES(5)                       Net-SNMP                       VARIABLES(5)
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NAME

6           variables - Format of specifying variable names to SNMP tools.
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DESCRIPTION

9       The  syntax and semantics of management information in SNMP is given by
10       the definitions of MIB objects, loaded from one or more MIB  files  (or
11       "MIB  modules").   These  definitions are not strictly required for the
12       SNMP protocol to operate correctly, but are typically  needed  by  SNMP
13       client applications to display information in a meaningful manner.
14
15       The  MIB  file also serves as a design document when developing an SNMP
16       agent (or sub-agent) that provides this information, and  ensures  that
17       client  and  server  share a common understanding about what management
18       information represents.
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20

OIDs

22       MIB objects are specified using Object Identifiers  (OIDs),  which  can
23       take a number of forms.   Note that all of the examples in this section
24       refer to the same MIB object.
25
26   Numeric OIDs
27       The fundamental format of an OID is a sequence of  integer  values  (or
28       "subidentifiers"),  typically  written using dots to separate the indi‐
29       vidual subidentifiers.
30               .1.3.6.1.2.1.1.1
31       This is the format that is used within the SNMP protocol itself, in the
32       packets that are sent over the network.
33
34       This  form  of  representing  an  OID does not require MIB files or MIB
35       object definitions to be available.  However it does rely on the client
36       application  and/or  network administrator knowing what a given numeric
37       OID refers to.  As such, it is not a particularly  helpful  representa‐
38       tion to anyone just starting out with SNMP.
39
40       This  format  can  be obtained by giving the command-line option -On to
41       most Net-SNMP commands.
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43
44   Full OID path
45       A similar (but somewhat more informative) format uses the  same  dotted
46       list  representation,  but  with the numeric subidentifiers replaced by
47       names, taken from the relevant MIB file(s).
48               .iso.org.dod.internet.mgmt.mib-2.system.sysDescr
49       This uniquely identifies a particular MIB object (as with  the  numeric
50       OID), but the list of names should hopefully give some indication as to
51       what information this object represents.  However it does rely  on  the
52       relevant  MIB  files  being available (as do all formats other than the
53       purely numeric OID).  Such OIDs also tend to be fairly long!
54
55       This format can be obtained by giving the command-line  option  -Of  to
56       most Net-SNMP commands.
57
58       A  variant  of  this  (typically  used when writing OIDs in descriptive
59       text, rather than running programs), is to combine the name and numeric
60       subidentifier:
61               .iso(1).org(3).dod(6).internet(1).mgmt(2).mib-2(1).system(1)
62               .sysDescr(1)
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64
65   Module-qualified OIDs
66       An  alternative  way  to  (more-or-less) uniquely specify an OID, is to
67       give the name of the MIB object, together with the MIB module where  it
68       is defined.
69              SNMPv2-MIB::sysDescr
70       MIB  object names are unique within a given module, so as long as there
71       are not two MIB modules with the same name (which  is  unusual,  though
72       not  unheard of), this format specifies the desired object in a reason‐
73       ably compact form.  It also makes it relatively easy to find the  defi‐
74       nition of the MIB object.
75
76       This  is  the  default  format for displaying OIDs in Net-SNMP applica‐
77       tions.  It can also be specified explicitly by giving the  command-line
78       option -OS to most Net-SNMP commands.
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80
81   Object name
82       Possibly  the  most common form for specifying MIB objects is using the
83       name of the object alone - without the full path or  the  name  of  the
84       module that defines it.
85              sysDescr
86       This  is  by far the shortest and most convenient way to refer to a MIB
87       object.  However the danger is that if two MIB modules  each  define  a
88       MIB object with the same name (which is perfectly legal in some circum‐
89       stances), then it's not necessarily clear which MIB object is  actually
90       meant.   For  day-to-day  use,  particularly  when  using  standard MIB
91       objects, this is probaby safe.  But it's important to be aware  of  the
92       potential ambiguities.
93
94       This  format  can  be obtained by giving the command-line option -Os to
95       most Net-SNMP commands.
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97
98   UCD-format
99       Previous versions of the code (UCD v4.x  and  earlier)  used  a  simple
100       approach  to  shortening the way OIDs were specified.  If the full path
101       of the OID began with .iso.org.dod.internet.mgmt.mib-2 then this prefix
102       was  removed  from  the  OID before displaying it.  All other OIDs were
103       displayed in full.
104
105       Similarly, if an OID was passed to the UCD library that did  not  begin
106       with  a dot (and wasn't in the module::name format), then the same pre‐
107       fix was prepended.   The example OID  from  the  formats  listed  above
108       would therefore be given or displayed as
109              system.sysDescr
110       The  inconsistent  handling of OIDs, depending on their location within
111       the OID tree, proved to be more trouble than it  was  worth,  and  this
112       format is no longer recommended.
113
114       The  previous  behaviour  can  be  obtained  by giving the command-line
115       option -Ou (for displaying output), or -Iu (for interpreting input OIDs
116       without a leading dot) to most Net-SNMP commands.
117

SEE ALSO

119       snmpcmd(1)
120

BUGS

122       The  parser  of  the  MIB  files file is not expected to handle bizarre
123       (although correct) interpretations of the ASN.1 notation.
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129V5.8                              01 Oct 2010                     VARIABLES(5)
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