SNMP MIBS Extensions and how to use

Using extend in SNMP to Expose Custom Script Output

I don’t know how many people are familiar with SNMP configuration directives, but one directive I find especially interesting is extend.

What makes extend powerful is that you can define your own script, execute it through SNMP, and expose its output under variables such as nsExtendOutput. Later, you can use these values to build convenient graphs using tools like MRTG or RRD, tailored to your needs 🙂.

Let’s start.

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1) Create a Script to Get Network Traffic Stats

The following script retrieves packets received, packets sent, and both per second from an interface (e.g., eth0) over the last 5 minutes.

#!/bin/bash

stats=$(sar -n DEV -s $(date --date='5 mins ago' +%T) | grep eth0 | tail -n2 | head -n1)

case $1 in
    received ) echo "$stats" | awk '{ print $6 }' ;;
    sent     ) echo "$stats" | awk '{ print $7 }' ;;
    both     ) echo "$stats" | awk '{ print $6 + $7 }' ;;
esac

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2) Save the Script and Update snmp.conf

• Save the script anywhere you want
• Make it executable
• Then update snmp.conf with the following extend entries:

extend networktraffic      /etc/snmp/myscript.sh both
extend networktrafficrcvd  /etc/snmp/myscript.sh received
extend networktrafficsent  /etc/snmp/myscript.sh sent

---

3) Test Using snmpwalk

If you are using SNMP v3

snmpwalk -v3 -u testuser2 -l authNoPriv -a MD5 -A "pass" localhost .1 | grep networktraffic

If you are using SNMP v2c

snmpwalk -v2c -c public localhost .1 | grep networktraffic

---

Sample Output

You should see output similar to the following (pay attention to the nsExtendOutput entries):

HOST-RESOURCES-MIB::hrSWRunParameters.12938 = STRING: "networktraffic"
NET-SNMP-EXTEND-MIB::nsExtendCommand."networktraffic" = STRING: /etc/snmp/myscript.sh
NET-SNMP-EXTEND-MIB::nsExtendCommand."networktrafficrcvd" = STRING: /etc/snmp//myscript.sh
NET-SNMP-EXTEND-MIB::nsExtendCommand."networktrafficsent" = STRING: /etc/snmp//myscript.sh
NET-SNMP-EXTEND-MIB::nsExtendArgs."networktraffic" = STRING: both
NET-SNMP-EXTEND-MIB::nsExtendArgs."networktrafficrcvd" = STRING: received
NET-SNMP-EXTEND-MIB::nsExtendArgs."networktrafficsent" = STRING: sent
NET-SNMP-EXTEND-MIB::nsExtendInput."networktraffic" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendInput."networktrafficrcvd" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendInput."networktrafficsent" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendCacheTime."networktraffic" = INTEGER: 5
NET-SNMP-EXTEND-MIB::nsExtendCacheTime."networktrafficrcvd" = INTEGER: 5
NET-SNMP-EXTEND-MIB::nsExtendCacheTime."networktrafficsent" = INTEGER: 5
NET-SNMP-EXTEND-MIB::nsExtendExecType."networktraffic" = INTEGER: exec(1)
NET-SNMP-EXTEND-MIB::nsExtendExecType."networktrafficrcvd" = INTEGER: exec(1)
NET-SNMP-EXTEND-MIB::nsExtendExecType."networktrafficsent" = INTEGER: exec(1)
NET-SNMP-EXTEND-MIB::nsExtendRunType."networktraffic" = INTEGER: run-on-read(1)
NET-SNMP-EXTEND-MIB::nsExtendRunType."networktrafficrcvd" = INTEGER: run-on-read(1)
NET-SNMP-EXTEND-MIB::nsExtendRunType."networktrafficsent" = INTEGER: run-on-read(1)
NET-SNMP-EXTEND-MIB::nsExtendStorage."networktraffic" = INTEGER: permanent(4)
NET-SNMP-EXTEND-MIB::nsExtendStorage."networktrafficrcvd" = INTEGER: permanent(4)
NET-SNMP-EXTEND-MIB::nsExtendStorage."networktrafficsent" = INTEGER: permanent(4)
NET-SNMP-EXTEND-MIB::nsExtendStatus."networktraffic" = INTEGER: active(1)
NET-SNMP-EXTEND-MIB::nsExtendStatus."networktrafficrcvd" = INTEGER: active(1)
NET-SNMP-EXTEND-MIB::nsExtendStatus."networktrafficsent" = INTEGER: active(1)
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."networktraffic" = STRING: 12145.6
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."networktrafficrcvd" = STRING: 6309.75
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."networktrafficsent" = STRING: 5835.86
NET-SNMP-EXTEND-MIB::nsExtendOutputFull."networktraffic" = STRING: 12145.6
NET-SNMP-EXTEND-MIB::nsExtendOutputFull."networktrafficrcvd" = STRING: 6309.75
NET-SNMP-EXTEND-MIB::nsExtendOutputFull."networktrafficsent" = STRING: 5835.86
NET-SNMP-EXTEND-MIB::nsExtendOutNumLines."networktraffic" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendOutNumLines."networktrafficrcvd" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendOutNumLines."networktrafficsent" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendResult."networktraffic" = INTEGER: 0
NET-SNMP-EXTEND-MIB::nsExtendResult."networktrafficrcvd" = INTEGER: 0
NET-SNMP-EXTEND-MIB::nsExtendResult."networktrafficsent" = INTEGER: 0
NET-SNMP-EXTEND-MIB::nsExtendOutLine."networktraffic".1 = STRING: 12145.6
NET-SNMP-EXTEND-MIB::nsExtendOutLine."networktrafficrcvd".1 = STRING: 6309.75
NET-SNMP-EXTEND-MIB::nsExtendOutLine."networktrafficsent".1 = STRING: 5835.86

The values you’ll typically graph are:

• NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."networktraffic"
• NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."networktrafficrcvd"
• NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."networktrafficsent"