Ignored SNMP packets¶

Some network devices do not respond to PySNMP-based management requests for particular OIDs.

$ pysnmpget -v2c -c public 10.0.0.33 1.3.6.1.2.1.2.2.1.10.3
SNMPv2-SMI::mib-2.2.2.1.10.3 = Counter32: 1519568842
$ snmpget.py -v2c -c public 10.0.0.33 1.3.6.1.2.1.2.2.1.10.4
requestTimedOut

Meanwhile, tcpcump shows request-response sequence:

13:33:30.161843 IP 10.0.0.33.snmp > 10.0.0.1.51094:
GetResponse(31)  interfaces.ifTable.ifEntry.ifInOctets.3=1532504859
13:33:30.161881 IP 10.0.0.33.snmp > 10.0.0.1.51094:
GetResponse(31)  interfaces.ifTable.ifEntry.ifInOctets.3=1532504859

In some cases, particularly when running v1arch PySNMP code, the
following exception may be thrown on response processing:

 Traceback (most recent call last):
 ....
 File "build/bdist.linux-i686/egg/pyasn1/type/base.py", line 64, in
__init__
 File "build/bdist.linux-i686/egg/pyasn1/type/base.py", line 32, in _verifySubtypeSpec
 File "build/bdist.linux-i686/egg/pyasn1/type/constraint.py", line 33, in __call__
 pyasn1.type.error.ValueConstraintError: ConstraintsIntersection(ConstraintsIntersection(), ValueRangeConstraint(0, 4294967295)) failed at: ValueRangeConstraint(0, 4294967295) failed at: -1413698940

This appears to be a [widespread] bug in BER integer encoders. It usually gets noticed on Counter values as they are constrained to be positive while wrong encoding yields them negative.

Here’s broken encoding:

>>> decoder.decode('A\x04\xab\xbc\xaa\x84', asn1Spec=rfc1155.Counter())
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError: ConstraintsIntersection(ConstraintsIntersection(), ValueRangeConstraint(0, 4294967295)) failed at: ValueRangeConstraint(0, 4294967295) failed at: -1413698940

And here’s a good one:

>>> decoder.decode('A\x05\x00\xab\xbc\xaa\x84',
>>> asn1Spec=rfc1155.Counter())
(Counter('2881268356'), '')

Notice the third octet -- positive values must have its highest bit set
to zero.

Here's an example hack that converts negated values into their positive
complimentaries for Counter type.

from pysnmp.proto import rfc1155, rfc1902, api
from pyasn1.codec.ber import encoder, decoder

# --- hack Counter type

def counterCloneHack(self, *args):
    if args and args[0] < 0:
        args = (0xffffffff+args[0]-1,) + args[1:]

    return self.__class__(*args)

rfc1155.Counter.clone = counterCloneHack
rfc1902.Counter32.clone = counterCloneHack

Execute this hack before any SNMP message processing occurs in your app.

The bad news is that if this BER encoding bug also affects Integer values,
in that case it is theoretically impossible to fix because, unlike Counter,
Integer values may legally be negative so they could not unconditionally be
converted into positives.

Therefore the best solution would be to get vendors fixing their
BER encoders.