PySNMP can not recognize response

Question

i am using the following simple script:

from pysnmp.entity.rfc3413.oneliner import cmdgen

errorIndication, errorStatus, errorIndex, \
varBindTable = cmdgen.CommandGenerator().bulkCmd(
            cmdgen.CommunityData('test-agent', 'public'),
            cmdgen.UdpTransportTarget(('IP.IP.IP.IP', 161)),
            0,
            1,
            (1,3,6,1,2,1,4,24,4,1,2,169,254)
        )

if errorIndication:
   print errorIndication
else:
    if errorStatus:
        print '%s at %s\n' % (
            errorStatus.prettyPrint(),
            errorIndex and varBindTable[-1][int(errorIndex)-1] or '?'
            )
    else:
        for varBindTableRow in varBindTable:
            for name, val in varBindTableRow:
                print '%s = %s' % (name.prettyPrint(), val.prettyPrint())

Using snmpwalk from command line to this device returns expected result. But script returns No SNMP response received before timeout. If i omit this OID then everything works fine. So the problem is in this OID

Here tcpdump stats:

/usr/sbin/tcpdump -nn -vv -s0 -A host HOST and udp

tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes

12:15:31.494920 IP (tos 0x0, ttl  64, id 0, offset 0, flags [DF], proto: UDP (17), length: 77) IP.IP.IP.IP.47911 > IP.IP.IP.IP.161: [bad udp cksum 4b7d!]  { SNMPv2c { GetBulk(34) R=8993731  N=0 M=1 .1.3.6.1.2.1.4.24.4.1.2.169.254 } }
E..M..@[email protected]..]<..]</.'...9.S0/.....public."....;.......0.0...+..........).~..

12:15:31.495666 IP (tos 0x0, ttl  64, id 0, offset 0, flags [DF], proto: UDP (17), length: 98) IP.IP.IP.IP.161 > IP.IP.IP.IP.47911: [udp sum ok]  { SNMPv2c { GetResponse(55) R=8993731  .1.3.6.1.2.1.4.24.4.1.2.169.254.0.0.0.0.255.255.0.0.0.0.0=[inetaddr len!=4]0.0.255.255.0.0.0.0 } }
E..b..@[email protected]..]</.]<....'.N.\0D.....public.7....;.......0)0'..+..........).~.............@.........

12:15:32.500226 IP (tos 0x0, ttl  64, id 0, offset 0, flags [DF], proto: UDP (17), length: 77) IP.IP.IP.IP.47911 > IP.IP.IP.IP.161: [bad udp cksum 4b7d!]  { SNMPv2c { GetBulk(34) R=8993731  N=0 M=1 .1.3.6.1.2.1.4.24.4.1.2.169.254 } }
E..M..@[email protected]..]<..]</.'...9.S0/.....public."....;.......0.0...+..........).~..

12:15:32.500624 IP (tos 0x0, ttl  64, id 0, offset 0, flags [DF], proto: UDP (17), length: 98) IP.IP.IP.IP.161 > IP.IP.IP.IP.47911: [udp sum ok]  { SNMPv2c { GetResponse(55) R=8993731  .1.3.6.1.2.1.4.24.4.1.2.169.254.0.0.0.0.255.255.0.0.0.0.0=[inetaddr len!=4]0.0.255.255.0.0.0.0 } }
E..b..@[email protected]..]</.]<....'.N.\0D.....public.7....;.......0)0'..+..........).~.............@.........

As we can see, device returns response .1.3.6.1.2.1.4.24.4.1.2.169.254.0.0.0.0.255.255.0.0.0.0.0=[inetaddr len!=4]0.0.255.255.0.0.0.0, but nothing happens and pysnmp just continue to try the value of this OID again and again.. snmpwalk recognizes this response as IP ADDRESS 0.0.255.255

Can you guys help me? Thanks in advance and sorry my english.

Answer 1

Your SNMP Agent seems to produce broken SNMP messages. While IPv4 address is four-octets long, your Agent reports eight-octets value.

As per SNMP RFCs, pysnmp drops malformed SNMP messages and retries original request a few times in hope to get correct response.

To make pysnmp working with specifically malformed IP address values you could patch its IpAddress class at runtime to make it taking just the four leading octets from a possibly longer initializer:

>>> def ipAddressPrettyIn(self, value):
...   return origIpAddressPrettyIn(self, value[:4])
...
>>> origIpAddressPrettyIn = v2c.IpAddress.prettyIn
>>> v2c.IpAddress.prettyIn = ipAddressPrettyIn
>>>
>>> msg, rest = decoder.decode(wholeMsg, asn1Spec=v2c.Message())
>>> print msg.prettyPrint()
Message:
version='version-2'
 community=public
 data=PDUs:
 response=ResponsePDU:
  request-id=6564368
  error-status='noError'
  error-index=0
  variable-bindings=VarBindList:
   VarBind:
    name=1.3.6.1.2.1.4.24.4.1.2.169.254.0.0.0.0.255.255.0.0.0.0.0
    =_BindValue:
     value=ObjectSyntax:
       application-wide=ApplicationSyntax:
       ipAddress-value=0.0.255.255