31/08/06, Mike Varley <mvarley (AT) somanetworks (DOT) comwrote:
we are seeing an snmptable command respond with question
marks ('?') as the result for some of the column entries; and I am
wondering if this is normal behaviour if net-snmp encounters an error
while trying to interpret a response.

Not an error as such, but it's the way snmptable displays "gaps" in a table,
where a particular row is missing one or more column values.

id num ip
095 1 192.168.24.252
DE8 ? ?

I'm not exactly sure what the indexing for this table is, but assuming
it's the 'id' field,
what does a GETNEXT request for
id.095 num.095 ip.095
return?

(, replace this with the correct column names and index value
for the row immediately before the first '?' one).

also, we've noticed that when the '?' appears, the row is actually
repeated over and over again (sometimes thousands of times!) as if the
bulk-get gets stuck retreiving the same index over and over again.

That sounds as if one particular column is returning thousands of
intervening values,
before the next "complete" row.

Is this expected if the MIB-code is returning erroneous data?

It might occur if the MIB-code is returning inconsistent data - e.g.
if the table is changing while it's being retrieved. How are you
implementing the table? Which helper? Is the table data cached at
all, or is it retrieved afresh for each separate request?

Dave

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31/08/06, Mike Varley <mvarley (AT) somanetworks (DOT) comwrote:
Don't you hate it when people withhold information? :)

a) You might say that - I couldn't possibly comment
b) You get used to it after two or three years on the list :-(

So here is a more accurate sample of the output:

id(index1) num(index2) ipaddress someotherinfo
095 2 192.168.24.253 klm
DE8 ? 192.168.21.34 ?

Hmmm so the '?' appears in one of the index columns as well.
Are these indexes accessible or not?

If they are, then the original analysis probably still stands.
If not, then I'd need to have another look at the code (which is
unlikely to be before the weekend, I'm afraid).

So, as you can see, our secondary index is one of those 'gaps' -- we did
a test to see what happens if you perform a GETNEXT and provide
(a) just the 1st index (index1), or
(b) a secondary index (index1.index2) that is valid but non-existant,

No - that's not quite what I meant. Sorry, I wasn't clear enough.
Try a single GETNEXT request, containing two (valid) IDs from the
previous row of the table - one referring to the instance immediately
above one of the '?'s and one referring to the instance immediately
above a valid value in that row.
What do the results of that GETNEXT look like? In particular, do
the instance subidentifiers of the two IDs returned match each other?
Normally, a GETNEXT of two instances should walk through the table in
step with each other - returning matching instance subidentifiers each
time. The '?'-style output would normally arise if these two got out
of sync.

By 'gaps' do you mean the MIB code is replying with NULL? just an
incosistant value?

No - they'd be perfectly reasonable values returned. Just not
referring to the same row.
For example:

GETNEXT myCol myLoc
RESPNSE myCol.1 myLoc.1
GETNEXT myCol.1 myLoc.1
RESPNSE myCol.2 myLoc.2
GETNEXT myCol.2 myLoc.2
RESPNSE myCol.3 myLoc.5

That last response would result in a '?' being displayed for the
"myLoc" column in row #3. K?

Dave

PS: I may not respond for a couple of days - away visiting friends.
I'll pick up on any unfinished business when I get back.

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31/08/06, Mike Varley <mvarley (AT) somanetworks (DOT) comwrote:
problem we have with this bug is it is very difficult to reproduce;
I am trying to create some samples which will simulate the failure.

:-(
Never a good sign.

The current hypothesis is we have a buffer overrun, and its messing with
the secondary index.

possibility would to run "snmptable -d" (or "snmptable -Ddump")
which would print out the request/response packets as they are
sent/received. You could then wade through the mountains of
output(!), to try and locate an individual GETNEXT request that
results in a '?' row.

The indexes for that response might help to throw some light on the matter.

Good Luck!

Dave

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