pysnmp-从NET-SNMP-EXTEND-MIB扩展脚本获取输出？

Question

How can I properly invoke an SNMP extend script by MIB with pysnmp 4.3?

I have this entry in the remote server's snmpd.conf file:

extend check_fd_wap /app/users/nagios_checks/check_fd_wap.sh

which in turn can be invoked by:

snmpwalk -t 60 -v2c -c greendale remoteserver 'NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."check_fd_wap"'
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."check_fd_wap" = STRING: { "sys_inuse" : 32640, "proc_data" : {  "ssl-mmsib" : { "proc_used" : 22, "proc_limit" : 200000 } , (...) }

In pysnmp I'm trying to invoke MIB NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."check_fd_wap" using the sample code:

errorIndication, errorStatus, errorIndex, varBinds = next(
    getCmd(SnmpEngine(),
        CommunityData(community, mpModel=1),
        UdpTransportTarget((hostname, port)),
        ContextData(),
        ObjectType(ObjectIdentity('NET-SNMP-EXTEND-MIB','nsExtendOutput1Line','check_fd_wap')))
)

for varBind in varBinds:
    print(' = '.join([x.prettyPrint() for x in varBind]))

However, this fails with the 'DisplayString' failed to cast value OctetString error message:

raise SmiError('MIB object %r having type %r failed to cast value %r: %s' % (self.__args[0].prettyPrint(), self.__args[0].getMibNode().getSyntax().__class__.__name__, self.__args[1], sys.exc_info()[1]))
;SmiError: MIB object u'NET-SNMP-EXTEND-MIB::nsExtendOutput1Line.check_fd_wap' having type 'DisplayString' failed to cast value OctetString('{ "sys_inuse" (...)"" at DisplayString

However, calling it with the translated OID works fine ( '.1.3.6.1.4.1.8072.1.3.2.3.1.1.12.99.104.101.99.107.95.102.100.95.119.97.112' ).

SNMPv2-SMI::enterprises.8072.1.3.2.3.1.1.12.99.104.101.99.107.95.102.100.95.119.97.112 = { "sys_inuse" : 31110, "proc_data" : {  "ssl-mmsib" : { "proc_used" : 19, "proc_limit" : 200000 } } }

What is the reason for this error?

Also, if I invoke the getCmd with the full MIB NET-SNMP-EXTEND-MIB::nsExtendOutput1Line.check_fd_wap as one argument, I get the error:

pysnmp.smi.error.NoSuchObjectError: NoSuchObjectError({'str': 'Can\'t resolve node name ::(\'NET-SNMP-EXTEND-MIB::nsExtendOutput1Line\', \'"check_fd_wap"\') at <pysnmp.smi.view.MibViewController instance at 0x2989638>'})

Is it possible to invoke the SNMP get with the full MIB in one string?

Answer 1

This failure:

'DisplayString' failed to cast value OctetString

happens when pysnmp receives a response from SNMP Agent for requested OID

1.3.6.1.4.1.8072.1.3.2.3.1.1.12.99.104.101.99.107.95.102.100.95.119.97.112

(which corresponds to NET-SNMP-EXTEND-MIB::nsExtendOutput1Line.check_fd_wap MIB object) and then it tried to convert response value ( { "sys_inuse" : 32640, "proc... ) into human-friendly DisplayString representation. At that point pysnmp failed but the exact cause is not included in your report for some reason.

I am guessing that response string sent by your SNMP Agent may accidentally exceed the length of 255 characters (which is a constraint of DisplayString type ).

Formally correct solution is to make SNMP Agent response for that MIB object fitting 255 ASCII characters to satisfy DisplayString type limits.

Alternatively, you could disable MIB lookup for response values in pysnmp by passing lookupMib=False keyword argument to getCmd() function.