[Pysnmp-dev] RE: Valid packets undecodable by pysnmp

[Ilya E. <il...@gl...>]

Once again, sorry for delaying so much.

I've tried to decode a few of the octet-streams you refer to so they
all seem to have the same problem of malformed variables-bindings
in PDU:

>>>v1.decode('0^\002\001\000\004\006public\244Q\006\011+\006\001\004\001\203\004\001\010@\004\202\327\250\007\002\001\006\002\001fC\003\004\344]0301\006\015+\006\001\004\001\203\004\001\010\001\001\004\000\004\006\000\240\370\241\277\3610\030\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X\326zj=\003\307\000\000\212\000\000\000\212\000\000\000\000\006[\312\226D\000\340{\212\012\013\010\000E\000\000|\002\230\000\000>\021\003\013\202\327\250\007\202\327\311\030\005\217\000\242\000h\373\230')
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "v1.py", line 843, in decode
    rest = msg.decode(input)
  File "v1.py", line 590, in decode
    if self.bindings.decode(self.pdu['bindings']):
  File "v1.py", line 192, in decode
    return self._decode(input)
  File "v1.py", line 369, in _decode
    raise TypeError('Trailing garbage in binding: %s' % repr(binding))
v1.TypeError: Trailing garbage in
binding: '0\030\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X'
>>>asn1.decode('0\030\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X''0\030\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X')
(SEQUENCE('\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X'),
'0\030\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X')
>>>asn1.decode('0\030\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X')
(SEQUENCE('\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X'),
'')
>>>asn1.decode('\006\016+\006\001\004\001\203\004\001\010\002\010\004\001\002\004\006\000\240\370\217}X')
(OBJECTID('.1.3.6.1.4.1.388.1.8.2.8.4.1.2'), '\004\006\000\240\370\217}X')
>>> asn1.decode('\004\006\000\240\370\217}X')
(OCTETSTRING('\000\240\370\217}X'), '')
>>>

So, this looks like this trap message produced by your device has *two*
variables-bindings with a single var-bind in each instead of a single
variables-bindings with two subsequent var-binds. The first var-bind 
carries the .1.3.6.1.4.1.388.1.8.1.1.4.0 OID and value (not shown
above) while the second has .1.3.6.1.4.1.388.1.8.2.8.4.1.2 OID and
value (see above).

This structure seems to be invalid according to RFC1157 (only one
variables-bindings structure is allowed in PDU) so that's why pysnmp can't
decode it.

A more relaxed approach would be to ignore the second variables-bindings
in PDU (does ethereal work this way?). This can be easily implemented
by subclassing and tweaking v1.BINDINGS() decode method. Please, let me
know if you need any assistance on this.

-ilya

>From: Frank Sweetser <fes@us...>
>Subject: Valid packets undecodable by pysnmp  
>Date: 2002-08-26 12:16
>
> I've got a device (specifically, a Symbol 4131 wireless access
>point) that
> I'm trying to manage via pysnmp.  The AP is sending out traps that
>pysnmp
> seems to barf on, but ethereal declares perfectly valid.
> 
> Help!
> 
> I've put a sample of the offending traps up at
> http://erwin.wpi.edu/~fs/4131.cap