Custom GPIO/SNMP Object

A Rasp Pi GPIO pin (pin 18 in this example) is set for read/write access with:

gpio mode 18 output

You can test setting and reading this pin with the gpio read and gpio write commands:

gpio read 18
 0
 grio write 18 1
 1

The Net-SNMP agent snmpd supports the creation of custom read/write objects (OIDs). The pass-through MIB extension command in snmpd.conf allows you to call script files. Pass-through script files need to follow a few rules:

• An snmpget request passes a -g parameter.
• The snmpget response needs to be three lines: OID, data type, and value.
• An snmpset request passes a -s parameter and value as the fourth item.

Listing 2 is a Bash script file that reads pin 18 on an snmpget request (-g), and it will write to pin 18 on an snmpset (-s). This script is made executable by entering

chmod +x powerswitch

To enable SNMP calls from the powerswitch script file, it needs to be referenced in snmpd.conf. Look for the "Pass-through" section and add a line with the OID and shell you want to use and a path to the script file, such as:

#  "Pass-through" MIB extension command
#
pass .1.3.6.1.2.1.25.1.8 /bin/bash /home/pi/powerswitch

After the snmpd.conf file is changed, the snmpd service needs to be restarted. If everything is done correctly, you can use the SNMP command-line routines to test reading and writing to the newly created OID (Listing 3). As a future step you can also give your new custom OID a meaningful MIB name.

Node-RED SNMP Set Example

An exec node can be used to run command-line utilities. Figure 11 shows the logic that sets an SNMP value with an On and Off button. The button nodes are configured to pass a 1 or 0 payload. The exec node contains the snmpset command (Figure 12), which will append the 0 or 1 from the button node. My final web dashboard (Figure 13) includes a gauge to monitor the output status along with the buttons to control pin 18.

Figure 11: Set SNMP values with buttons.

Figure 12: The exec node configuration.

Figure 13: The PowerSwitch dashboard.

Summary

Network monitoring with the Raspberry Pi can be accomplished in a number of ways. With some basic SNMP configuration on the Rasp Pi, you can make data available to higher level packages or create simple standalone solutions with Node-RED.

A number of other useful GPIO projects are possible with SNMP, as well. For example, I use a PowerSwitch Tail II ($26) [4] power cord that can be enabled and disabled through I/O pins to monitor and control powered devices like lights, PCs, and dehumidifiers from SNMP.