[mrtg] ifHC fallback with snmp v3

Daniel J McDonald dan.mcdonald at austinenergy.com
Thu Dec 28 15:39:33 MET 2006

In the snmp v2 world, when the ifHC counters fail, mrtg automagically
switches to snmp v1 and polls the plain if counters.  When running snmp
v3, you can't fall back to snmp v1 since the credentials won't work.

My workaround has been in my iftemplate file:
if ($if_ok and $if_snmp_descr !~ /unrouted/ and $if_snmp_descr !~ /Null/i) { 
my $test_hc = snmpget($router,$v3opt,'ifHCInOctets.'.$if_index);
if ($test_hc =~ /nosuchinstance/i) {
        $target_lines .= <<LOWSPEED
Target[$target_name]: ifInOctets$if_ref&ifOutOctets$if_ref:$router_connect
} else {
        $target_lines .= <<HIGHSPEED
Target[$target_name]: $if_ref:$router_connect

But, I've recently noticed that the snmp v3 attributes were being
stripped around line 887 in cfgmaker.

I can fix cfgmaker to always spit out snmp v3 credentials, but then
there is no ifHC fallback automatically provided by cfgmaker, and
relying on everyone else to write a custom iftemplate is not a terribly
supportable solution.  

Should I adjust cfgmaker to do the explicit OID hack for fallback from
snmp v3, or should I fiddle with the polling process to trigger the
fallback some other way?  If the latter, what should the syntax look
like in the .cfg file to signify a fallback?

Daniel J McDonald, CCIE # 2495, CISSP # 78281, CNX
Austin Energy

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