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Wireless sensors on Openwrt need an agent. The agent runs on the Openwrt side. It returns the wireless sensor information for the SNMP queries from LibreNMS.

Installation

Openwrt

The Openwrt side needs two items. The first item is a set of scripts that generate the information for the SNMP replies. The second item is an update to the SNMP extend configuration. This update returns the information for each query.

1: Install the scripts:

Copy the scripts from the librenms-agent repository into /etc/librenms on Openwrt. Then add this directory to /etc/sysupgrade.conf, so that it survives a firmware update: 

wget -O /etc/librenms/wlClients.sh https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/Openwrt/wlClients.sh
wget -O /etc/librenms/wlFrequency.sh https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/Openwrt/wlFrequency.sh
wget -O /etc/librenms/wlInterfaces.txt https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/Openwrt/wlInterfaces.txt
wget -O /etc/librenms/wlNoiseFloor.sh https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/Openwrt/wlNoiseFloor.sh
wget -O /etc/librenms/wlRate.sh https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/Openwrt/wlRate.sh
wget -O /etc/librenms/wlSNR.sh https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/Openwrt/wlSNR.sh
wget -O /etc/librenms/distro https://raw.githubusercontent.com/librenms/librenms-agent/master/snmp/distro
chmod +x /etc/librenms/*.sh
chmod +x /etc/librenms/distro

Only one file needs an edit. The file wlInterfaces.txt maps each wireless interface to its display name in LibreNMS. For example: 

wlan0,wl-2.4G
wlan1,wl-5.0G

2: Update the Openwrt SNMP configuration. Add extend support for the OS detection and the wireless sensor queries:

Run vi /etc/config/snmpd and add the entries below. These entries assume executable scripts in /etc/librenms. Change the network interfaces to match your hardware.

config extend
        option name distro
        option prog '/etc/librenms/distro'
config extend
        option name hardware
        option prog '/bin/cat'
        option args '/sys/firmware/devicetree/base/model'
config extend
        option name     interfaces
        option prog     "/bin/cat /etc/librenms/wlInterfaces.txt"
config extend
        option name     clients-wlan0
        option prog     "/etc/librenms/wlClients.sh wlan0"
config extend
        option name     clients-wlan1
        option prog     "/etc/librenms/wlClients.sh wlan1"
config extend
        option name     clients-wlan
        option prog     "/etc/librenms/wlClients.sh"
config extend
        option name     frequency-wlan0
        option prog     "/etc/librenms/wlFrequency.sh wlan0"
config extend
        option name     frequency-wlan1
        option prog     "/etc/librenms/wlFrequency.sh wlan1"
config extend
        option name     rate-tx-wlan0-min
        option prog     "/etc/librenms/wlRate.sh wlan0 tx min"
config extend
        option name     rate-tx-wlan0-avg
        option prog     "/etc/librenms/wlRate.sh wlan0 tx avg"
config extend
        option name     rate-tx-wlan0-max
        option prog     "/etc/librenms/wlRate.sh wlan0 tx max"
config extend
        option name     rate-tx-wlan1-min
        option prog     "/etc/librenms/wlRate.sh wlan1 tx min"
config extend
        option name     rate-tx-wlan1-avg
        option prog     "/etc/librenms/wlRate.sh wlan1 tx avg"
config extend
        option name     rate-tx-wlan1-max
        option prog     "/etc/librenms/wlRate.sh wlan1 tx max"
config extend
        option name     rate-rx-wlan0-min
        option prog     "/etc/librenms/wlRate.sh wlan0 rx min"
config extend
        option name     rate-rx-wlan0-avg
        option prog     "/etc/librenms/wlRate.sh wlan0 rx avg"
config extend
        option name     rate-rx-wlan0-max
        option prog     "/etc/librenms/wlRate.sh wlan0 rx max"
config extend
        option name     rate-rx-wlan1-min
        option prog     "/etc/librenms/wlRate.sh wlan1 rx min"
config extend
        option name     rate-rx-wlan1-avg
        option prog     "/etc/librenms/wlRate.sh wlan1 rx avg"
config extend
        option name     rate-rx-wlan1-max
        option prog     "/etc/librenms/wlRate.sh wlan1 rx max"
config extend
        option name     noise-floor-wlan0
        option prog     "/etc/librenms/wlNoiseFloor.sh wlan0"
config extend
        option name     noise-floor-wlan1
        option prog     "/etc/librenms/wlNoiseFloor.sh wlan1"
config extend
        option name     snr-wlan0-min
        option prog     "/etc/librenms/wlSNR.sh wlan0 min"
config extend
        option name     snr-wlan0-avg
        option prog     "/etc/librenms/wlSNR.sh wlan0 avg"
config extend
        option name     snr-wlan0-max
        option prog     "/etc/librenms/wlSNR.sh wlan0 max"
config extend
        option name     snr-wlan1-min
        option prog     "/etc/librenms/wlSNR.sh wlan1 min"
config extend
        option name     snr-wlan1-avg
        option prog     "/etc/librenms/wlSNR.sh wlan1 avg"
config extend
        option name     snr-wlan1-max
        option prog     "/etc/librenms/wlSNR.sh wlan1 max"

NOTE: to test a script above, run its command.

NOTE: to test the output of an extension, run this command on the LibreNMS machine:

snmpwalk -v 2c -c public -Osqnv <openwrt-host> 'NET-SNMP-EXTEND-MIB::nsExtendOutputFull."frequency-wlan0"'

NOTE: the LibreNMS machine needs snmp-mibs-downloader.

NOTE: the Openwrt machine needs distro. LibreNMS then detects the OS correctly.

3: Restart the snmp service on Openwrt:

service snmpd restart

Then wait for the discovery and the polling in LibreNMS.