Writable SNMP

Keegan Holley keegan.holley at sungard.com
Wed Dec 7 16:29:46 UTC 2011

> > I can see the other comments about interactive commands and bulk
> > read/writes, but what's the harm of doing it on internet connected boxes
> vs.
> > non-internet boxes.  Just about everyone uses snmp reads in the interwebs
> I think the general feeling is that snmp is udp so it's spoofable and
> your perimeter acls will never be 100% (or rather, are you willing to
> risk them not being 100%?)

That's a given even though there's still the string to deny the spoofed
traffic someone could cause a fair amount of trouble if they knew what your
acl's look like.  That being said I don't think I've ever seen a company
that doesn't at least use SNMP for billing and basic monitoring and many
don't think to block it at their edge.  It's hard to convince someone to
change something that's been around for years though.  SNMP is flawed
though and enabling writes just makes it worse.

> > and as such filters it at their edges and hopefully on each platform.
> You'd
> > secure it the same way you'd secure readable SNMP I assume.
> read is 'painful', write can be downright deadly (to your SLAs).
> >>
> >> Also, who tests snmp WRITE in their code? at scale? for daily
> >> operations tasks?
> >
> >
> > lol, that could be a problem.
> yea, I bet the number of people that test, at scale/operations-pace,
> snmp WRITE is countable on a single hand.
> >>
> >> ... (didn't the snmp incident in 2002 teach us
> >> something?)
> >>
> > Please elaborate.
> <
> http://www.cisco.com/warp/public/707/cisco-sa-20010227-ios-snmp-ilmi.shtml
> >
> oh, 2001... memory lag :(

I don't remember hearing about this causing issues in a production network.
 According to the article you can just filter SNMP by IP which should be in
place anyway.  It's triggered by some sort of hidden string which would
make it malicious, unless I'm missing something.

In lieu of a software upgrade, a workaround can be applied to certain IOS
releases by disabling the ILMI community or "**ilmi*" view and applying an
access list to prevent unauthorized access to SNMP. Any affected system,
regardless of software release, may be protected by filtering SNMP traffic
at a network perimeter or on individual devices.

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