C.3. Net-SNMP Command-Line Tools
This section briefly
describes each of the Net-SNMP tools. By default, installing Net-SNMP
places all these commands in /usr/local/bin. All
the examples in this section assume that
/usr/local/bin is in your path.
C.3.1. snmpwalk
snmpwalk
performs the
get-next operation. We've used it
throughout the book, so it should be familiar; in this section,
we'll use it to demonstrate some of the options introduced in
Table C-1.
Let's say you want to perform an snmpwalk
against a Cisco router. If you don't have any Cisco MIBs
installed, here's what you will see:
$ snmpwalk cisco.ora.com public .1.3.6.1.4.1.9
enterprises.9.2.1.1.0 = "..System Bootstrap, Version 11.2(17)GS2, [htseng 180]
EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)..Copyright (c) 1999 by Cisco Systems,
Inc..."
enterprises.9.2.1.2.0 = "reload"
enterprises.9.2.1.3.0 = "cisco"
enterprises.9.2.1.4.0 = "ora.com"
enterprises.9.2.1.5.0 = IpAddress: 127.45.23.1
enterprises.9.2.1.6.0 = IpAddress: 0.0.0.0
enterprises.9.2.1.8.0 = 131890952
enterprises.9.2.1.9.0 = 456
enterprises.9.2.1.10.0 = 500
enterprises.9.2.1.11.0 = 17767568
enterprises.9.2.1.12.0 = 0
enterprises.9.2.1.13.0 = 0
enterprises.9.2.1.14.0 = 104
enterprises.9.2.1.15.0 = 600
...
Recall that .1.3.6.1.4.1 is
.iso.org.dod.internet.private.enterprises, and 9
is Cisco's private enterprise number. Therefore, the previous
command is walking the entire Cisco subtree, which is very large;
we've deleted most of its output. The output you see
isn't very readable because we haven't yet installed the
Cisco MIBs, so the snmpwalk command has no way of
providing human-readable object names. We just have to guess what
these objects are.
This problem is easy to solve. Copy
the Cisco MIBs[79] to the main Net-SNMP repository
( /usr/local/share/snmp/mibs) and use the
-m ALL command-line option. With this option,
snmpwalk parses all the files in the MIB
repository. As a result we get the object IDs in string
(human-readable) form, and we can walk the cisco
subtree by name rather than specifying its complete numeric object ID
( .1.3.6.1.4.1.9):
$ snmpwalk -m ALL cisco.ora.com public cisco
enterprises.cisco.local.lcpu.1.0 = "..System Bootstrap, Version 11.2(17)GS2,
[htseng 180] EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)..Copyright (c) 1999 by Cisco
Systems, Inc..."
enterprises.cisco.local.lcpu.2.0 = "reload"
enterprises.cisco.local.lcpu.3.0 = "cisco"
enterprises.cisco.local.lcpu.4.0 = "ora.com"
enterprises.cisco.local.lcpu.5.0 = IpAddress: 127.45.23.1
enterprises.cisco.local.lcpu.6.0 = IpAddress: 0.0.0.0
enterprises.cisco.local.lcpu.8.0 = 131888844
enterprises.cisco.local.lcpu.9.0 = 456
enterprises.cisco.local.lcpu.10.0 = 500
enterprises.cisco.local.lcpu.11.0 = 17767568
enterprises.cisco.local.lcpu.12.0 = 0
enterprises.cisco.local.lcpu.13.0 = 0
enterprises.cisco.local.lcpu.14.0 = 104
enterprises.cisco.local.lcpu.15.0 = 600
...
Now let's trim the output by
adding the -Os option, which omits the initial
part of each OID:
$ snmpwalk -m ALL -Os cisco.ora.com public cisco
lcpu.1.0 = "..System Bootstrap, Version 11.2(17)GS2, [htseng 180] EARLY
DEPLOYMENT RELEASE SOFTWARE (fc1)..Copyright (c) 1999 by Cisco Systems, Inc..."
lcpu.2.0 = "reload"
lcpu.3.0 = "cisco"
lcpu.4.0 = "ora.com"
lcpu.5.0 = IpAddress: 127.45.23.1
lcpu.6.0 = IpAddress: 0.0.0.0
lcpu.8.0 = 131888844
lcpu.9.0 = 456
lcpu.10.0 = 500
lcpu.11.0 = 17767568
lcpu.12.0 = 0
lcpu.13.0 = 0
lcpu.14.0 = 104
lcpu.15.0 = 600
...
This output is a little easier to read, since it cuts off the
redundant part of each OID. Let's take this command one step
further:
$ snmpwalk -OsS cisco.ora.com public system
RFC1213-MIB::sysDescr.0 = "Cisco Internetwork Operating System Software ..IOS (tm)
GS Software (GSR-K4P-M), Version 12.0(15)S, EARLY DEPLOYMENT RELEASE SOFTWARE
(fc1)..TAC Support: http://www.cisco.com/cgi-bin/ibld/view.pl?i=support..
Copyright (c) 1986-2001 by Cisco Systems, Inc..."
RFC1213-MIB::sysObjectID.0 = OID: DTRConcentratorMIB::catProd.182
EXPRESSION-MIB::sysUpTimeInstance = Timeticks: (344626986) 39 days, 21:17:49.86
RFC1213-MIB::sysContact.0 = "O'Reilly Data Center"
RFC1213-MIB::sysName.0 = "cisco.ora.com"
RFC1213-MIB::sysLocation.0 = "Atlanta, GA"
RFC1213-MIB::sysServices.0 = 6
RFC1213-MIB::system.8.0 = Timeticks: (0) 0:00:00.00
This command walks the system subtree. Since the
system group falls under
mib-2, there is no need to use -m
ALL; mib-2 is one of the MIBs the
Net-SNMP tools load automatically. Adding S to the
-O option instructs the command to prefix each
line of output with the name of the MIB file; we see that each line
begins with RFC1213-MIB, which is the name of
the file that defines mib-2.
C.3.7. snmpdelta
The snmpdelta command
monitors OIDs and tracks changes in OID values over time. Its syntax
is:
snmpdelta options hostname community objectID...
snmpdelta requires you to specify the OID of an
integer-valued scalar object -- it can't monitor tables. For
example, if you want to want to watch the octets arriving on an
interface, you can't just specify
ifInOctets; you must specify the interface
number in addition to the object name (e.g.,
ifInOctets.3). By default,
snmpdelta polls the given object every second.
Table C-3 lists some of the
snmpdelta-specific options. There are many
problems with the documentation for this command, but if you stick to
the options listed below you should be on firm ground.
Table C-3. snmpdelta Options
Option
|
Description
|
-t
|
The documentation says "Determine time interval from the
monitored entity." It's not clear what this means, but
you seem to need this entry to get nonzero readings.
|
-s
|
Display a timestamp with every set of results.
|
-m
|
Print the maximum value obtained.
|
-l
|
Write the output to a file. The filename is in the form
hostname-OID. For example, if you want to
monitor the variables ifInOctets.3 and
ifOutOctets.3 on the host router, the
-l option will create two files,
hostname-ifInOctets.3 and
hostname-ifOutOctets.3, where the output of
snmpdelta will be written. (Note that this output
has no apparent connection to the configuration, as the documentation
claims.)
|
-p
|
Specify the polling interval (the default is 1 second).
|
-T
|
Print output in tabular format.
|
C.3.15. snmptranslate
The Net-SNMP package comes
with a handy tool called snmptranslate that
translates between numerical and human-readable object names. More
generally, it can be used to look up information from MIB files. Its
syntax is:
snmptranslate options objectID
snmptranslate does not perform queries against any
device, so it doesn't need the hostname or
community parameters. Its sole purpose is to
read MIB files and produce output about specific objects. Before
looking at examples, it's worth noting that
snmptranslate's interpretations of the
-O options are, to be kind, interesting. To speak
more plainly, they're just plain wrong. The following examples
show what actually happens when you use these
options -- we'll leave the rationalization to you. We expect
these problems to be fixed in some later version of Net-SNMP.
Let's say you want to know the enterprise OID for Cisco
Systems. The following command does the trick:
$ snmptranslate -m ALL -IR -Of cisco
.1.3.6.1.4.1.9
This tells us that Cisco's enterprise OID is
.1.3.6.1.4.9. Note the use of the -IR
option, which tells snmptranslate to do
a random-access search for an object named
cisco. If you leave this option out,
snmptranslate will fail because it will try to
locate cisco under the
mib-2 tree.
Let's say you want to take .1.3.6.1.4.1.9
and convert it to its full symbolic name. That's easy:
$ snmptranslate -m ALL -Ofn .1.3.6.1.4.1.9
.iso.org.dod.internet.private.enterprises.cisco
In this case, -IR
isn't needed because we're not performing a random-access
search. -Ofn ensures that we print the full object
ID, in symbolic (text) form. Here's what happens if we use
-Of by itself:
$ snmptranslate -m ALL -Of .1.3.6.1.4.1.9
enterprises.cisco
As we said earlier, this is not how you'd expect
-Ofn and -Of to behave. If
you're writing scripts, you shouldn't count on this
behavior staying the same in future versions.
Now, let's say you want to know
a little bit more information about a particular object.
The-Td option displays the
object's definition as it appears in the MIB file:
$ snmptranslate -Td system.sysLocation
.1.3.6.1.2.1.1.6
sysLocation OBJECT-TYPE
-- FROM SNMPv2-MIB, RFC1213-MIB
-- TEXTUAL CONVENTION DisplayString
SYNTAX OCTET STRING (0..255)
DISPLAY-HINT "255a"
MAX-ACCESS read-write
STATUS current
DESCRIPTION "The physical location of this node (e.g., 'telephone
closet, 3rd floor'). If the location is unknown, the
value is the zero-length string."
::= { iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1) system(1) 6 }
-Td can save you a
lot of work poking through MIB files to find an appropriate
definition, particularly when combined with -IR.
Furthermore, the last line shows you the entire object ID in both
numeric and string forms, not just the object's parent. Note
that the other Net-SNMP commands have an unrelated
-T option; don't get confused.
-T is meaningless for this command, because
snmptranslate only looks up a local file and
doesn't need to access the network.
The -Tp option
prints an entire OID tree. The best way to understand this is to see
it:
$ snmptranslate -Tp system
+--system(1)
|
+-- -R-- String sysDescr(1)
| Textual Convention: DisplayString
| Size: 0..255
+-- -R-- ObjID sysObjectID(2)
+-- -R-- TimeTicks sysUpTime(3)
+-- -RW- String sysContact(4)
| Textual Convention: DisplayString
| Size: 0..255
+-- -RW- String sysName(5)
| Textual Convention: DisplayString
| Size: 0..255
+-- -RW- String sysLocation(6)
| Textual Convention: DisplayString
| Size: 0..255
+-- -R-- Integer sysServices(7)
+-- -R-- TimeTicks sysORLastChange(8)
| Textual Convention: TimeStamp
|
+--sysORTable(9)
|
+--sysOREntry(1)
|
+-- ---- Integer sysORIndex(1)
+-- -R-- ObjID sysORID(2)
+-- -R-- String sysORDescr(3)
| Textual Convention: DisplayString
| Size: 0..255
+-- -R-- TimeTicks sysORUpTime(4)
Textual Convention: TimeStamp
We displayed the system subtree because
it's fairly short. From this output it's relatively easy
to see all the objects underneath system,
together with their types and textual conventions. This is a great
way to see what objects are defined in a MIB, as well as their
relationships to other objects. The output can be voluminous, but
it's still a convenient way to get a map and figure out what
objects are likely to be useful.
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