221 lines
6.9 KiB
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221 lines
6.9 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of VARIABLES</TITLE>
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</HEAD><BODY>
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<H1>VARIABLES</H1>
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Section: Net-SNMP (5)<BR>Updated: 01 Oct 2010<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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<BR> variables - Format of specifying variable names to SNMP tools.
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<A NAME="lbAC"> </A>
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<H2>DESCRIPTION</H2>
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The syntax and semantics of management information in SNMP is
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given by the definitions of MIB objects, loaded from one or more
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MIB files (or "MIB modules"). These definitions are not strictly
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required for the SNMP protocol to operate correctly, but are typically
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needed by SNMP client applications to display information in a
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meaningful manner.
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<P>
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The MIB file also serves as a design document when developing an SNMP
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agent (or sub-agent) that provides this information, and ensures that
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client and server share a common understanding about what management
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information represents.
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<P>
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<A NAME="lbAD"> </A>
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<H2>OIDs</H2>
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MIB objects are specified using Object Identifiers (OIDs), which can
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take a number of forms. Note that all of the examples in this section
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refer to the same MIB object.
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<A NAME="lbAE"> </A>
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<H3>Numeric OIDs</H3>
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The fundamental format of an OID is a sequence of integer values
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(or "subidentifiers"), typically written using dots to separate
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the individual subidentifiers.
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<DL COMPACT><DT id="1"><DD>
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<BR> .1.3.6.1.2.1.1.1
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</DL>
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This is the format that is used within the SNMP protocol itself,
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in the packets that are sent over the network.
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<P>
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This form of representing an OID does not require MIB files or MIB
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object definitions to be available. However it does rely on the
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client application and/or network administrator knowing what a
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given numeric OID refers to. As such, it is not a particularly
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helpful representation to anyone just starting out with SNMP.
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<P>
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This format can be obtained by giving the command-line option
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-On
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to most Net-SNMP commands.
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<P>
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<A NAME="lbAF"> </A>
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<H3>Full OID path</H3>
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A similar (but somewhat more informative) format uses the same
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dotted list representation, but with the numeric subidentifiers
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replaced by names, taken from the relevant MIB file(s).
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<DL COMPACT><DT id="2"><DD>
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<BR> .iso.org.dod.internet.mgmt.mib-2.system.sysDescr
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</DL>
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This uniquely identifies a particular MIB object (as with the numeric
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OID), but the list of names should hopefully give some indication as
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to what information this object represents. However it does rely on
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the relevant MIB files being available (as do all formats other than
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the purely numeric OID). Such OIDs also tend to be fairly long!
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<P>
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This format can be obtained by giving the command-line option
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-Of
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to most Net-SNMP commands.
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<P>
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A variant of this (typically used when writing OIDs in descriptive
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text, rather than running programs), is to combine the name and
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numeric subidentifier:
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<DL COMPACT><DT id="3"><DD>
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<BR> <A HREF="/cgi-bin/man/man2html?1+.iso">.iso</A>(1)<A HREF="/cgi-bin/man/man2html?3+.org">.org</A>(3)<A HREF="/cgi-bin/man/man2html?6+.dod">.dod</A>(6)<A HREF="/cgi-bin/man/man2html?1+.internet">.internet</A>(1)<A HREF="/cgi-bin/man/man2html?2+.mgmt">.mgmt</A>(2)<A HREF="/cgi-bin/man/man2html?1+.mib-2">.mib-2</A>(1)<A HREF="/cgi-bin/man/man2html?1+.system">.system</A>(1)
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<BR> <A HREF="/cgi-bin/man/man2html?1+.sysDescr">.sysDescr</A>(1)
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</DL>
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<P>
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<A NAME="lbAG"> </A>
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<H3>Module-qualified OIDs</H3>
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An alternative way to (more-or-less) uniquely specify an OID,
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is to give the name of the MIB object, together with the MIB
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module where it is defined.
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<DL COMPACT><DT id="4"><DD>
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SNMPv2-MIB::sysDescr
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</DL>
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MIB object names are unique within a given module, so as long
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as there are not two MIB modules with the same name (which is
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unusual, though not unheard of), this format specifies the
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desired object in a reasonably compact form. It also makes
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it relatively easy to find the definition of the MIB object.
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<P>
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This is the default format for displaying OIDs in Net-SNMP applications.
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It can also be specified explicitly by giving the command-line option
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-OS
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to most Net-SNMP commands.
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<P>
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<A NAME="lbAH"> </A>
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<H3>Object name</H3>
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Possibly the most common form for specifying MIB objects is
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using the name of the object alone - without the full path or
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the name of the module that defines it.
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<DL COMPACT><DT id="5"><DD>
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sysDescr
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</DL>
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This is by far the shortest and most convenient way to refer to
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a MIB object. However the danger is that if two MIB modules each
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define a MIB object with the same name (which is perfectly legal
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in some circumstances), then it's not necessarily clear which MIB
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object is actually meant.
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For day-to-day use, particularly when using standard MIB objects,
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this is <I>probaby</I> safe.
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But it's important to be aware of the potential ambiguities.
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<P>
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This format can be obtained by giving the command-line option
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-Os
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to most Net-SNMP commands.
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<P>
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<A NAME="lbAI"> </A>
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<H3>UCD-format</H3>
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Previous versions of the code (UCD v4.x and earlier) used a
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simple approach to shortening the way OIDs were specified.
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If the full path of the OID began with
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.iso.org.dod.internet.mgmt.mib-2
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then this prefix was removed from the OID before displaying it.
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All other OIDs were displayed in full.
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<P>
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Similarly, if an OID was passed to the UCD library that did
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not begin with a dot (and wasn't in the module::name format),
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then the same prefix was prepended. The example OID from the
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formats listed above would therefore be given or displayed as
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<DL COMPACT><DT id="6"><DD>
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system.sysDescr
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</DL>
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The inconsistent handling of OIDs, depending on their location
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within the OID tree, proved to be more trouble than it was worth,
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and this format is no longer recommended.
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<P>
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The previous behaviour can be obtained by giving the command-line
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option
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-Ou
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(for displaying output), or
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-Iu
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(for interpreting input OIDs without a leading dot)
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to most Net-SNMP commands.
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<A HREF="/cgi-bin/man/man2html?1+snmpcmd">snmpcmd</A>(1)
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<A NAME="lbAK"> </A>
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<H2>BUGS</H2>
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The parser of the MIB files file is not expected to handle bizarre
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(although correct) interpretations of the ASN.1 notation.
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<P>
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<P>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="7"><A HREF="#lbAB">NAME</A><DD>
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<DT id="8"><A HREF="#lbAC">DESCRIPTION</A><DD>
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<DT id="9"><A HREF="#lbAD">OIDs</A><DD>
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<DL>
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<DT id="10"><A HREF="#lbAE">Numeric OIDs</A><DD>
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<DT id="11"><A HREF="#lbAF">Full OID path</A><DD>
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<DT id="12"><A HREF="#lbAG">Module-qualified OIDs</A><DD>
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<DT id="13"><A HREF="#lbAH">Object name</A><DD>
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<DT id="14"><A HREF="#lbAI">UCD-format</A><DD>
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</DL>
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<DT id="15"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="16"><A HREF="#lbAK">BUGS</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:06 GMT, March 31, 2021
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