Ignore NoSuchInstance in Session._check_exception (multiple OIDs get())

[urban-1]

Hi all,

This is a feature request to optionally ignore NoSuchInstance in Session._check_exception when more than one oids are queried via get(*oids). Minor changes are also needed in Session._convert to be able to handle this.

Why??

I am using the Session directly and I have been told that this is not the way snimpy is designed... so feel free to close this as invalid - I just thought I 'd ask...

What I need to do is query all the metrics of specific interface (I know the ifIndex) without fetching the whole ifTable. I am constructing all the OIDs and passing them directly to Session.get()

The problem is that different vendors support different OIDs! or they sometimes think is a good idea to randomly skip some... This causes get() to raise an exception while checking pysnmp's errorIndication.

Workaround

At the moment I am replacing the methods _check_exception and _convert. On the first I only remove the check for NoSuchInstance and on the second I add 2 lines:

   ...
            if isinstance(value, cl):
                return fn(value)

            # ... check for no instance ...
            if isinstance(value, rfc1905.NoSuchInstance):
                return "NoSuchInstance"

        # back to the original code    
        self._check_exception(value)

The output for a Cisco I test here:

ifOutErrors              : 0
ifAdminStatus            : 1
ifInOctets               : 0
ifInDiscards             : 0
ifOutDiscards            : 0
ifInUnknownProtos        : 0
ifDescr                  : b'Vlan500'
ifSpeed                  : 1000000000
ifOutNUcastPkts          : NoSuchInstance
ifMtu                    : 1998
ifInUcastPkts            : 0
ifLastChange             : 15925
ifInNUcastPkts           : NoSuchInstance
ifSpecific               : NoSuchInstance
ifType                   : 53
ifOutUcastPkts           : 65
ifOutQLen                : NoSuchInstance
ifOutOctets              : 4160
ifOperStatus             : 1
ifIndex                  : 500
ifPhysAddress            : b'blah'
ifInErrors               : 0

I think that would be a nice feature to have (disabled by default) that we can enable via Session.set_relax_no_instance(True) or something similar.

Please let me know if there an easier/other way of doing the same query with snimpy without working directly with pysnmp (I really like your Session :) ) and sorry for abusing snimpy again...

Andreas

[vincentbernat]

When using the high-level API, you can create a manager with none=True to get the behaviour you expect. Of course, this is of no use for you as you want to GET several OID at once.

We can add a similar argument to the Session it's tiny enough to not bother me that much.

[vincentbernat]

I didn't implement that for SNMPv1 as I would need to investigate a bit to see what it means about the global error message we get in this version.

[urban-1]

Amazingly fast response thanks a lot!!!