/usr/share/doc/libsnmp-session-perl/snmp-session.html

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  <H1>SNMP support for Perl 5</H1>

  <p> Copyright (c) 1995-2008, Simon Leinen<br>
  All rights reserved </p>

  <em> This program is free software; you can redistribute it under
  the <a href="dist/Artistic">"Artistic License 2.0"</a> included in
  this distribution. </em>

  <p>Author: <A HREF="http://www.switch.ch/misc/leinen/">Simon
Leinen</A> &lt;<A
HREF="mailto:simon.leinen@switch.ch">simon.leinen@switch.ch</A>&gt;</p>

</DIV>

<p> This package contains Perl 5 modules <tt>SNMP_Session.pm</tt>,
<tt>BER.pm</tt>, and <tt>SNMP_util.pm</tt> which, when used together,
provide rudimentary access to remote <a
href="http://www.switch.ch/misc/leinen/snmp/">SNMP</a> (v1/v2)
agents. </p>

<p> <b> Download it from <a href="http://www.switch.ch/misc/leinen/snmp/perl/dist/"><tt>http://www.switch.ch/misc/leinen/snmp/perl/dist/</tt></a></b> </p>

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<p> The library is featured in the book <em><a
href="http://www.oreilly.com/catalog/esnmp/">Essential SNMP</a></em>
by Douglas R. Mauro and Kevin J. Schmidt, July 2001, O'Reilly &amp;
Associates, ISBN: 0-59600020-0.  You can buy it on-line at <a
href="http://www.amazon.com/exec/obidos/ASIN/0596000200/simonleinensbook/ref=nosim/"><img
border=0 width=90 height=29 align="middle"
src="http://www.switch.ch/misc/leinen/images/amazon/90X29-w-logo.gif"
alt="Amazon.com"></a> <a
href="http://www.amazon.de/exec/obidos/ASIN/0596000200/simonleinensbo0d/ref=nosim/"><img
align="middle"
src="http://www.switch.ch/misc/leinen/images/amazon/w_90x31.gif"
width=90 height=31 border="0" alt="In Partnerschaft mit
Amazon.de"></a>. </p>

<H2>Features</H2>

<P> This module differs from existing SNMP packages in that it is
 completely stand-alone, i.e. you don't need to have another SNMP
 package such as Net-SNMP.  It is also written entirely in Perl, so
 you don't have to compile any C modules.  It uses the Perl 5
 <SAMP>Socket.pm</SAMP> module and should therefore be very portable,
 even to non-Unix systems. </P>

<p> Note: For the development of new scripts, I strongly recommend to
use the higher-level programming interface provided by
<tt>SNMP_util.pm</tt>.  Its use is described in <a
href="dist/README.SNMP_util"><tt>README.SNMP_util</tt></a>.  The
remainder of this page desribes the low-level API in
<tt>SNMP_Session.pm</tt>, which you normally shouldn't use. </p>

<p> The SNMP operations currently supported are "get", "get-next",
 "get-bulk" and "set", as well as trap generation and reception. </p>

<p> For an excellent example of the type of application this is useful
 for, see Tobias
 Oetiker's <a href="URL:http://oss.oetiker.ch/mrtg/">``mrtg'' (Multi
 Router Traffic Grapher)</a> tool.  Another application that uses this
 library is <a href="http://www.dynw.com/iog/">IOG</a> (Input/Output
 Grapher). </p>

<h2> Recent Changes: </h2>

<p> For a list of changes, see the <samp><a
href="changes.html">changes.html</a></samp> file packaged with this
system. </p>

<H2>Usage</H2>

<P> The basic usage of these routines works like this: </P>

<PRE>
use BER;
require 'SNMP_Session.pm';

# Set $host to the name of the host whose SNMP agent you want
# to talk to.  Set $community to the community name under
# which you want to talk to the agent.	Set port to the UDP
# port on which the agent listens (usually 161).

$session = SNMP_Session-&gt;open ($host, $community, $port)
    or die "couldn't open SNMP session to $host";

# Set $oid1, $oid2... to the BER-encoded OIDs of the MIB
# variables you want to get.

if ($session-&gt;get_request_response ($oid1, $oid2, ...)) {
    ($bindings) = $session-&gt;decode_get_response ($session-&gt;{pdu_buffer});

    while ($bindings ne '') {
	($binding,$bindings) = &amp;decode_sequence ($bindings);
	($oid,$value) = &amp;decode_by_template ($binding, "%O%@");
	print &amp;pretty_print ($oid)," =&gt; ", &amp;pretty_print ($value), "\n";
    }
} else {
    die "No response from agent on $host";
}
</PRE>

<H2>Encoding OIDs</H2>

<P> In order to BER-encode OIDs, you can use the function
<B>BER::encode_oid</B>.  It takes (a vector of) numeric subids as an
argument.  For example, </P>

<PRE>
use BER;
encode_oid (1, 3, 6, 1, 2, 1, 1, 1, 0)
</PRE>

<P> will return the BER-encoded OID for the <B>sysDescr.0</B>
 (1.3.6.1.2.1.1.1.0) instance of <A
 HREF="ftp://ftp.isi.edu/in-notes/rfc1213.txt">MIB-2</A>. </P>

<H2>Decoding the results</H2>

<P> When get_request_response returns success, you must decode the
 response PDU from the remote agent.  The function
 <B>decode_get_response</B> can be used to do this.  It takes a
 get-response PDU, checks its syntax and returns the <EM>bindings</EM>
 part of the PDU.  This is where the remote agent actually returns the
 values of the variables in your query. </P>

<p> You should iterate over the individual bindings in this
 <EM>bindings</EM> part and extract the value for each variable.  In
 the example above, the returned bindings are simply printed using the
 <B>BER::pretty_print</B> function. </p>

<p> For better readability of the OIDs, you can also use the following
 idiom, where the <samp>%pretty_oids</samp> hash maps BER-encoded
 numerical OIDs to symbolic OIDs.  Note that this simple-minded
 mapping only works for response OIDs that exactly match known OIDs,
 so it's unsuitable for table walking (where the response OIDs include
 an additional row index). </p>

<pre>
<font color="#c00000">%ugly_oids = qw(sysDescr.0	1.3.6.1.2.1.1.1.0
		sysContact.0	1.3.6.1.2.1.1.4.0);
foreach (keys %ugly_oids) {
    $ugly_oids{$_} = encode_oid (split (/\./, $ugly_oids{$_}));
    $pretty_oids{$ugly_oids{$_}} = $_;
}</font>
...
if ($session-&gt;get_request_response ($ugly_oids{'sysDescr.0'},
				    $ugly_oids{'sysContact.0'})) {
    ($bindings) = $session-&gt;decode_get_response ($session-&gt;{pdu_buffer});
    while ($bindings ne '') {
	($binding,$bindings) = &amp;decode_sequence ($bindings);
	($oid,$value) = &amp;decode_by_template ($binding, "%O%@");
	print <font color="#c00000">$pretty_oids{$oid},</font>" =&gt; ",
	      &amp;pretty_print ($value), "\n";
    }
} ...
</pre>

<H2><a name="set-req-use">Set Requests</a></H2>

<p> Set requests are generated much like get or getNext requests are,
 with the exception that you have to specify not just OIDs, but also
 the values the variables should be set to.  Every binding is passed
 as a reference to a two-element array, the first element being the
 encoded OID and the second one the encoded value.  See the
 <tt>test/set-test.pl</tt> script for an example, in particular the
 subroutine <tt>snmpset</tt>. </p>

<H2><a name="map-table-use">Walking Tables</a></H2>

<p> Beginning with version 0.57 of <tt>SNMP_Session.pm</tt>, there is API
 support for walking tables.  The <tt>map_table</tt> method can be
 used for this as follows: </p>

<pre>
sub walk_function ($$$) {
  my ($index, $val1, $val3) = @_;
  ...
}

...
$columns = [$base_oid1, $base_oid3];
$n_rows = $session-&gt;map_table ($columns, \&amp;walk_function);
</pre>

<p> The <em>columns</em> argument must be a reference to a list of
 OIDs for table columns sharing the same index.  The method will
 traverse the table and call the <em>walk_function</em> for each row.
 The arguments for these calls will be:

<ol>

<li> the <em>row index</em> as a partial OID in dotted notation,
 e.g. "1.3", or "10.0.1.34".

<li> the values of the requested table columns in that row, in
 BER-encoded form.  If you want to use the standard
 <tt>pretty_print</tt> subroutine to decode the values, you can use
 the following idiom:

<pre>
  grep (defined $_ &amp;&amp; ($_=pretty_print $_), ($val1, $val3));
</pre>

</ol>

<H3><a name="map-table-4-use">Walking Tables With
<tt>get-bulk</tt></a></H3>

<p> Since version 0.67, <tt>SNMP_Session</tt> uses a different
<tt>get_table</tt> implementation for <tt>SNMPv2c_Session</tt>s.  This
version uses the ``powerful <tt>get-bulk</tt> operator'' to retrieve
many table rows with each request.  In general, this will make table
walking much faster under SNMPv2c, especially when round-trip times to
the agent are long. </p>

<p> There is one difficulty, however: With <tt>get-bulk</tt>, a
management application can specify the maximum number of rows to
return in a single response.  <tt>SNMP_Session.pm</tt> provides a new
function, <tt>map_table_4</tt>, in which this <tt>maxRepetitions</tt>
value can be specified explicitly. </p>

<p> For maximum efficiency, it should be set to a value that is one
greater than the number of rows in the table.  If it is smaller, then
<tt>map_table</tt> will use more request/response cycles than
necessary; if it is bigger, the agent will have to compute variable
bindings beyond the end of the table (which <tt>map_table</tt> will
throw away). </p>

<p> Of course it is usually impossible to know the size of the table
in advance.  If you don't specify <tt>maxRepetitions</tt> when walking
a table, then <tt>map_table</tt> will use a per-session default
(<tt>$session-&gt;default_max_repetitions</tt>).  The default value for
this default is 12. </p>

<p> If you walk a table multiple times, and the size of the table is
relatively stable, you should use the return value of
<tt>map_table</tt> (which is the number of rows it has encountered) to
compute the next value of <tt>maxRepetitions</tt>.  Remember to add
one so that <tt>map_table</tt> notices when the table is finished!
</p>

<p> Note that for really big tables, this doesn't make a big
difference, since the table won't fit in a single response packet
anyway. </p>

<H2><a name="trap-send">Sending Traps</a></H2>

<p> To send a trap, you have to open an SNMP session to the trap
 receiver.  Usually this is a process listening to UDP port 162 on a
 network management station.  Then you can use the
 <tt>trap_request_send</tt> method to encode and send SNMPv1 traps.
 There is no way to find out whether the trap was actually received at
 the management station - SNMP traps are fundamentally unreliable.
</p>

<p> When constructing an SNMPv1 trap, you must provide

<ul>

<li> the "enterprise" Object Identifier for the entity that generates
 the trap

<li> your IP address

<li> the generic trap type

<li> the specific trap type

<li> the sysUpTime at the time of trap generation

<li> a sequence (may be empty) of variable bindings further describing 
 the trap.

</ul>

<p> For SNMPv2 traps, you need:

<ul>

<li> the trap's OID

<li> the sysUpTime at the time of trap generation

<li> the bindings list as above

</ul>

<p> For SNMPv2 traps, the uptime and trap OID are encoded as bindings
which are added to the front of the other bindings you provide. </p>

<p> Here is a short example: </p>

<pre>
my $trap_receiver = "netman.noc";
my $trap_community = "SNMP_Traps";
my $trap_session = $version eq '1'
    ? SNMP_Session-&gt;open ($trap_receiver, $trap_community, 162)
    : SNMPv2c_Session-&gt;open ($trap_receiver, $trap_community, 162);
my $myIpAddress = ...;
my $start_time = time;

...

sub link_down_trap ($$) {
  my ($if_index, $version) = @_;
  my $genericTrap = 2;		# linkDown
  my $specificTrap = 0;
  my @ifIndexOID = ( 1,3,6,1,2,1,2,2,1,1 );
  my $upTime = int ((time - $start_time) * 100.0);
  my @myOID = ( 1,3,6,1,4,1,2946,0,8,15 );

  warn "Sending trap failed"
    unless ($version eq '1')
	? $trap_session-&gt;trap_request_send (encode_oid (@myOID),
					    encode_ip_address ($myIpAddress),
					    encode_int ($genericTrap),
					    encode_int ($specificTrap),
					    encode_timeticks ($upTime),
					    [encode_oid (@ifIndex_OID,$if_index),
					     encode_int ($if_index)],
					    [encode_oid (@ifDescr_OID,$if_index),
					     encode_string ("foo")])
	    : $trap_session-&gt;v2_trap_request_send (\@linkDown_OID, $upTime,
						   [encode_oid (@ifIndex_OID,$if_index),
						    encode_int ($if_index)],
						   [encode_oid (@ifDescr_OID,$if_index),
						    encode_string ("foo")]);
}
</pre>

<H2><a name="trap-recv">Receiving Traps</a></H2>

<p> Since version 0.60, SNMP_Session.pm supports the receipt and
 decoding of SNMPv1 trap requests.  Since version 0.75, SNMPv2 Trap
 PDUs are also recognized. </p>

<p> To receive traps, you have to create a special SNMP session that
 passively listens on the SNMP trap transport address (usually UDP
 port 162).  Then you can receive traps (actually, SNMPv1 traps,
 SNMPv2 traps, and SNMPv2 informs) using the <tt>receive_trap</tt>
 method and decode them using <tt>decode_trap_request</tt>.  The
 <em>enterprise</em>, <em>agent</em>, <em>generic</em>,
 <em>specific</em> and <em>sysUptime</em> return values are only
 defined for SNMPv1 traps.  In SNMPv2 traps and informs, the
 equivalent information is contained in the bindings.
</p>

<pre>
my $trap_session = SNMPv1_Session-&gt;open_trap_session ()
  or die "cannot open trap session";
my ($trap, $sender_addr, $sender_port) = $trap_session-&gt;receive_trap ()
  or die "cannot receive trap";
my ($community, $enterprise, $agent,
    $generic, $specific, $sysUptime, $bindings)
  = $trap_session-&gt;decode_trap_request ($trap)
    or die "cannot decode trap received"
...
my ($binding, $oid, $value);
while ($bindings ne '') {
    ($binding,$bindings) = &amp;decode_sequence ($bindings);
    ($oid, $value) = decode_by_template ($binding, "%O%@");
    print BER::pretty_oid ($oid)," =&gt; ",pretty_print ($value),"\n";
}
</pre>

<H2>Future Plans</H2>

<H3>SNMPv3 Support</H3>

<P> The code could first be restructured to follow the modularization
 proposed in <a
 href="ftp://sunsite.cnlab-switch.ch/doc/standard/rfc/22xx/2271">RFC
 2271</a> (An Architecture for Describing SNMP Management Frameworks).
 The existing SNMPv1 and SNMPv2c support must somehow be retrofitted
 to this framework.  Later, one could add support for SNMPv3 PDU
 formats and for user-based security. </p>

<H3>Higher-Level APIs</H3>

<P> The current programming interface is very close to the level of
 SNMP operations and PDUs.  For actual management applications, there
 are probably more convenient interfaces that could be defined. </P>

<HR>
<ADDRESS>
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20080402
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<A HREF="http://www.switch.ch/misc/leinen/">
 Simon Leinen &lt;simon.leinen@switch.ch&gt;</A>

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