Check snmp

Overview

This check utilizes snmpget to query for information on a network entity, so it provides all that snmpget is capable of, for example SNMP v2c and SNMP v3.

Bemerkung

Only use SNMP if there is no other way. SNMP puts much strain on the target system and the monitoring software.

If you can use an agent of your monitoring software for monitoring, for example Icinga, and one of our plugins, do it.
If you can’t install an agent, but there is a good (REST-)API available (and maybe one of our plugins), use that.
If you can’t install an agent on a device and there is no (good) API, then use SNMP.
If possible, use a SNMP specialized solution like LibreNMS instead of this check.
Prefer SNMPv2. Although completely insecure, it is fast and keeps the load on your appliance low.

Fact Sheet

Check Plugin Download	https://github.com/Linuxfabrik/monitoring-plugins/tree/main/check-plugins/snmp
Check Interval Recommendation	Every 5 minutes
Can be called without parameters	No
Compiled for	Linux
Requirements	`snmpget` from `net-snmp-utils`
Uses SQLite DBs	`$TEMP/linuxfabrik-monitoring-plugins-snmp.db`

Help

usage: snmp [-h] [-V] [--community COMMUNITY] [--device DEVICE] [--hide-ok]
            -H HOSTNAME [--mib MIB] [--mib-dir MIBDIR]
            [--snmpversion {1,2c,3}] [-t TIMEOUT] [--v3bootstime V3BOOTSTIME]
            [--v3context V3CONTEXT] [--v3contextengineid V3CONTEXTENGINEID]
            [--v3securityengineid V3SECURITYENGINEID]
            [--v3level {noAuthNoPriv,authNoPriv,authPriv}]
            [--v3authprotpassword V3AUTHPROTPASSWORD]
            [--v3privprotpassword V3PRIVPROTPASSWORD]
            [--v3authprot {MD5,SHA,SHA-224,SHA-256,SHA-384,SHA-512}]
            [--v3privprot {DES,AES,AES-192,AES-256}]
            [--v3username V3USERNAME] [--test TEST]

This check is a SNMP application that uses the SNMP GET request to query for
information on a network entity. The object identifiers (OIDs) of interest
have to be defined in a CSV file, including optional WARN and CRIT parameters.

optional arguments:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit
  --community COMMUNITY
                        SNMP Version 1 or 2c specific. Set the community
                        string. Default: public.
  --device DEVICE       The name of a device file containing the SNMP-OIDs,
                        located under `./device-oids`, for example `switch-
                        fs-s3900.csv` or `printer-brother-mfcj5720dw.csv`.
                        Default: any-any-any.csv.
  --hide-ok             Don't print OIDs with OK state. Default: False.
  -H HOSTNAME, --hostname HOSTNAME
                        SNMP Appliance address.
  --mib MIB             Load given list of MIBs, for example `+FS-MIB` or `FS-
                        MIB:BROTHER-MIB`. Behaves like the `-m` option of
                        `snmpget`.
  --mib-dir MIBDIR       Look in given list of directories for MIBs. Behaves
                        like the `-M` option of `snmpget`. Default:
                        $HOME/.snmp/mibs:/usr/share/snmp/mibs.
  --snmpversion {1,2c,3}
                        Specifies SNMP version to use. Default: 2c.
  -t TIMEOUT, --timeout TIMEOUT
                        Network timeout in seconds. Default: 7 (seconds).
  --v3bootstime V3BOOTSTIME
                        SNMP Version 3 specific. Set destination engine
                        boots/time.
  --v3context V3CONTEXT
                        SNMP Version 3 specific. Set context name (e.g.
                        bridge1).
  --v3contextengineid V3CONTEXTENGINEID
                        SNMP Version 3 specific. Set context engine ID (e.g.
                        800000020109840301).
  --v3securityengineid V3SECURITYENGINEID
                        SNMP Version 3 specific. Set security engine ID (e.g.
                        800000020109840301).
  --v3level {noAuthNoPriv,authNoPriv,authPriv}
                        SNMP Version 3 specific. Set security level.
  --v3authprotpassword V3AUTHPROTPASSWORD
                        SNMP Version 3 specific. Set authentication protocol
                        pass phrase.
  --v3privprotpassword V3PRIVPROTPASSWORD
                        SNMP Version 3 specific. Set privacy protocol pass
                        phrase.
  --v3authprot {MD5,SHA,SHA-224,SHA-256,SHA-384,SHA-512}
                        SNMP Version 3 specific. Set authentication protocol.
  --v3privprot {DES,AES,AES-192,AES-256}
                        SNMP Version 3 specific. Set privacy protocol.
  --v3username V3USERNAME
                        SNMP Version 3 specific. Set security name (e.g.
                        bert).
  --test TEST           For unit tests. Needs "path-to-stdout-file,path-to-
                        stderr-file,expected-retc".

Installation

Install snmpget:

# on CentOS:
yum -y install net-snmp-utils

Plugin Directory Strcuture

/usr/lib64/nagios/plugins/
├── device-mibs
│   ├── printer-...
│   ├── ...
│   └── switch-...
└── device-oids

Handling MIBs

If needed, get any MIB files ready. Copy them to $HOME/.snmp/mibs or /usr/share/snmp/mibs. If you prefer other locations, provide the paths using the --mib-dir parameter (same syntax as the -M parameter of snmpget). The checks comes with some predefined, device-dependend MIBs located at /usr/lib64/nagios/plugins/device-mibs/.

Create an OID list in /usr/lib64/nagios/plugins/device-oids/... using CSV format. For details, have a look at „Defining a Device“ within this document.

Usage Examples

A minimal command call:

./snmp --hostname 10.80.32.109

Calling this the check…

fetches a set of most common SNMP OIDs like Contact or Uptime, defined in device-oids/any-any-any.csv,
calls snmpget -v 2c -c public -r 0 -t 7 -OSqtU -M $HOME/.snmp/mibs:/usr/share/snmp/mibs 10.80.32.109 OID1 OID2 ...,
parses the output,
interprets the result and calculates the return state.

Other example using a more specific OID list and an additional MIB directory:

/usr/lib64/nagios/plugins/snmp \
    --device switch-fs-s3900.csv \
    --mib-dir +/usr/lib64/nagios/plugins/device-mibs/switch-fs-s3900 \
    --hide-ok \
    --hostname 10.80.32.109

Defining a Device

If you want to define a device-specific list of OIDs, including any calculations, warning and critical thresholds, create a CSV file located at device-oids, using , as delimiter and " as quoting character. A minimal example for nearly any device:

OID	Name	Re-Calc	Unit Label	WARN	CRIT	Show in 1st Line	Report Change as
SNMPv2-MIB::sysName.0	Name
SNMPv2-MIB::sysLocation.0	Location						WARN
SNMPv2-MIB::sysUpTime.0	Uptime	int(value) / 100	s	value > 436524*3600	value > 536524*3600	True

The columns in detail:

OID

The Object-Identifier from any of your MIB files.
Name

If provided, the check prints this instead of the OID.
Re-Calc

Feel free to use any Python Code based on the variables value and values, which contain the result of the SNMPGET operation on the given OID.
Unit

This is the „Unit of Measurement“, case-insensitiv.
- s - seconds (also us, ms)
- % - percentage
- B - bytes (also KB, MB, TB, …)
- bps - bits per second (also Kbps, Mbps, …)
- c - a continous counter (such as bytes transmitted on an interface)
If you provide two comma-separated units, for example „b,c“, the first one will be used to display a human-readable format („Bytes“), and the second one is used to suffix the perfdata („continous counter“).

For output, the following units will always be converted to a human-friendly format:
- s - seconds
- b - bytes
- bps - bits per second
WARN

The warning threshold for the re-calculated or raw value.
CRIT

The critical threshold for the re-calculated or raw value.
Show in first line

Should value be printed in the first line of the check output?
Report Change as

Should a change of value be reported as WARN or CRIT? The check stores the initial values on the first run in $TEMP/linuxfabrik-monitoring-plugins-snmp.db.

The output would be something like this

Uptime: 5M 1W

Key         Value           State
---         -----           -----
Name        BRW38B1DB3B30F4 [OK]
Location    Office          [OK]
Contact     The Printer Man [OK]
Description Brother NC-350w [OK]
Uptime      5M 1W           [OK]

The check divides the OID list automatically into blocks of 25 OIDs per SNMPGET request.

Calculating and Comparing using `value` and `values`

value contains the value of the current OID, simply and always as a Python string. values is a Python dictionary containing all re-calculated (or raw) values, up to this point. The dictionary keys are based on the „Name“. If „Name“ is not set, the dictionary keys are based on the „OID“.

The value returned by snmpget for a given OID is always a string. If you want to use it for calculations or integer-based comparisons, re-calculate it by specifying int(value) in column (SNMP knows nothing about floats).

Both variables are allowed to be used in Python code in the columns „Re-Calc“, „WARN“ and „CRIT“. This enables you to even warn in the current OID depending on previous values, for example.

In the last three lines of this example we simply calculate „NIC.1 Traffic“ as a sum of „NIC.1 rx“ and „NIC.1 tx“, for which there is no SNMP OID:

OID	Name	Re-Calc	Unit Label	WARN
SNMPv2-MIB::sysUpTime.0	Uptime	int(value) / 100	s	value > 436524*3600
IF-MIB::ifSpeed.1	NIC.1 Speed	int(value)	bps
IF-MIB::ifOperStatus.1	NIC.1 Status
IF-MIB::ifOutOctets.1	NIC.1 tx	int(value)	b,c
IF-MIB::ifInOctets.1	NIC.1 rx	int(value)	b,c
<leave this empty>	NIC.1 Traffic	values[‚NIC.1 tx‘] + values[‚NIC.1 rx‘]	b,c

Parameter Mapping

`snmpget`	This check
`-v 1\|2c\|3`	`--snmpversion {1,2c,3}`
`-c COMMUNITY`	`--community COMMUNITY`
`-a PROTOCOL`	`--v3authprot {MD5,SHA,SHA-224,SHA-256,SHA-384,SHA-512}`
`-A PASSPHRASE`	`--v3authprotpassword V3AUTHPROTPASSWORD`
`-e ENGINE-ID`	`--v3securityengineid V3SECURITYENGINEID`
`-E ENGINE-ID`	`--v3contextengineid V3CONTEXTENGINEID`
`-l LEVEL`	`--v3level {noAuthNoPriv,authNoPriv,authPriv}`
`-n CONTEXT`	`--v3context V3CONTEXT`
`-u USER-NAME`	`--v3username V3USERNAME`
`-x PROTOCOL`	`--v3privprot {DES,AES,AES-192,AES-256}`
`-X PASSPHRASE`	`--v3privprotpassword V3PRIVPROTPASSWORD`
`-Z BOOTS,TIME`	`--v3bootstime V3BOOTSTIME`
`-r RETRIES`	hard-coded to `0`
`-t TIMEOUT`	`-t TIMEOUT`, `--timeout TIMEOUT`
`-m MIB[:...]`	`--mib MIB`
`-M DIR[:...]`	`--mib-dir MIBDIR`

How to fetch a list of OIDs

Example:

snmpbulkwalk -v2c \
    -c public \
    -OSt \
    -M +/usr/lib64/nagios/plugins/device-mibs/switch-netgear-xs716t \
    10.80.32.141 NETGEAR-SWITCHING-MIB::agentInfoGroup

Q & A

I get Too many object identifiers specified. Only 128 allowed in one request.: Probably your SNMP v3 parameters are incomplete or incorrect.
I get add_mibdir: strings scanned in from .snmp/mibs/.index are too large. count = ...: There seems to be a malformed, a duplicated MIB file or one with spaces in its filename within one of your MIB directories.
I get Error in packet. Reason: (tooBig) Response message would have been too large.: A „tooBig“ response simply means that the SNMP agent tried to generate a response with all requested OID’s, but the response grew too big for its buffer, resulting in this error message. To avoid this, we divide your OID list and send a maximum of 25 oids per request each.
Within Icinga, if I acknowledge a value change in WARN or CRIT state, does the plugin returns OK?: If you acknowledge a value change in Icinga, the desired WARN or CRIT state remains - due to the fact that SNMP is mostly run against hardware, and you have to check what triggered the change. If everything is fine, delete $TEMP/linuxfabrik-monitoring-plugins-snmp.db. On the next run of the plugin, it will recreate the inventory.

States

Depending on the OID definitions the check returns

OK
WARN
CRIT
UNKNOWN

Perfdata / Metrics

All numeric values are automatically returned as perfdata objects.

Credits, License

Authors: Linuxfabrik GmbH, Zurich
License: The Unlicense, see LICENSE file.