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Introduction

An exploration of implementing a custom MIB as a sub agent of net-snmp

This implements the NET-SNMP-EXAMPLES-MIB IETF working group table example, and allows the entries in that table to be updated via MQTT messages.

The very basic steps of starting this project were

  1. mib2c -c mib2c.table_data.conf netSnmpIETFWGTable
  2. grab the subagent/agent code and makefile from mib2c.mfd.conf or the web
  3. Add in mqtt, json parsing and the table update code.

This example project uses the "tdata" style, even though the mib2c config file implies table_data. net-snmp provides many ways of working with tables! This is the way I found it easiest to understand and get started, but it is by no means the only way to do this! (I'm a complete net-snmp novice!)

Dependencies

  • netsnmp (obviously, tested with net-snmp 5.7.2)
  • libmosquitto (Tested with mosquitto 1.2.3 and 1.3.1)
  • json-c (Tested with 0.11)

Examples:

Building the SNMP Agent

$ make
# run in foreground, as a master agent, with some logging, on an unprivileged port
$ ./netSnmpIETFWGTable -f -M -L -Dmosquitto,verbose,snmp-mqtt-ietfwg udp:localhost:1161

Query the default entries

$ snmptable  -v 2c -c public -C i udp:localhost:1161 NET-SNMP-EXAMPLES-MIB::netSnmpIETFWGTable
SNMP table: NET-SNMP-EXAMPLES-MIB::netSnmpIETFWGTable

                 index  nsIETFWGChair1   nsIETFWGChair2
           "second wg"      "Andy Awe"     "Bobby Bush"
       "make it three" "Alan Aardvark" "Bruce Bullwhip"
"karls working group1"  "Albert Apple"    "Bill Braggy"

Delete an entry:

$ mosquitto_pub -t "test/ietfwg/second wg/delete" -m body_ignored
$ snmptable  -v 2c -c public -C i udp:localhost:1161 NET-SNMP-EXAMPLES-MIB::netSnmpIETFWGTable
SNMP table: NET-SNMP-EXAMPLES-MIB::netSnmpIETFWGTable

                 index  nsIETFWGChair1   nsIETFWGChair2
       "make it three" "Alan Aardvark" "Bruce Bullwhip"
"karls working group1"  "Albert Apple"    "Bill Braggy"

Add a new working group

$ mosquitto_pub -t "test/ietfwg/some_new_wg/update" -m '{"chair1" : "karlos", "chair2": "second karlos"}'
$ snmptable  -v 2c -c public -C i udp:localhost:1161 NET-SNMP-EXAMPLES-MIB::netSnmpIETFWGTableSNMP
table: NET-SNMP-EXAMPLES-MIB::netSnmpIETFWGTable

                 index  nsIETFWGChair1   nsIETFWGChair2
         "some_new_wg"        "karlos"  "second karlos"
       "make it three" "Alan Aardvark" "Bruce Bullwhip"
"karls working group1"  "Albert Apple"    "Bill Braggy"

Update an existing group

This is the same as update, just using a working group name that already exists