[SNMP4J] strange exception

[Frank Fock]

Hi Imre,

I guess that the agent does not handle the case where the password
is wrong correctly. When an agent fails to decode a scoped PDU
correctly (this is sometimes hard to detect because there is no
CRC or similar mechanism to check the PDUs consistency) it has to
return a REPORT PDU with the appropriate USM counter.

In your case, the agent seems to try to reply with a "normal"
response which is not encoded with the wrong password which
leads to the parsing errors logged to the console.

Best regards,
Frank


Imre Kovács wrote:
> Hi everyone,
> 
> 
> 
> I wrote an agent application which listens for incoming PDUs, processes only
> the GET/SET methods and uses the SNMPv3 security capabilities. To test it, I
> use the Snmp4j.exe it is a really useful tool. Everything works fine, the
> program throws the unsupported types and security levels, but if I use the
> following parameters:
> 
> SNMP4J -c public -u Admin -r 0 -t 5000 -a MD5 -A goodpassword -x DES -P
> badpassword -p GET 10.20.2.101/161 "1.3.6.1.4.1.5.5.2.0"
> 
> Where the authentication password is the same as in the application and the
> privacy password is a wrong one. I get various types of exceptions, not from
> my program, but from the package and I can't catch it.  I get one exception
> at a time, but they are different every time. The application does not
> freeze, it runs alive, but every time there is these messages in the
> console. It is the only combination which causes this, if both passwords are
> wrong or just the authentication one, there is no exception.
> 
> 
> 
> Examples of these messages:
> 
> java.io.IOException: The encoded length 57 exceeds the number of bytes left
> in input at position 2 which actually is 48
> 
>         at org.snmp4j.asn1.BER.checkLength(BER.java:902)
> 
>         at org.snmp4j.asn1.BER.decodeLength(BER.java:547)
> 
>         at org.snmp4j.asn1.BER.decodeHeader(BER.java:584)
> 
>         at org.snmp4j.asn1.BER.decodeHeader(BER.java:609)
> 
>         at org.snmp4j.ScopedPDU.decodeBER(ScopedPDU.java:141)
> 
>         at org.snmp4j.mp.MPv3.prepareDataElements(MPv3.java:984)
> 
>         at org.snmp4j.MessageDispatcherImpl.dispatchMessage(
> MessageDispatcherImpl.java:240)
> 
>         at org.snmp4j.MessageDispatcherImpl.processMessage(
> MessageDispatcherImpl.java:323)
> 
>         at org.snmp4j.util.MultiThreadedMessageDispatcher$MessageTask.run(
> MultiThreadedMessageDispatcher.java:190)
> 
>         at org.snmp4j.util.ThreadPool$TaskManager.run(ThreadPool.java:236)
> 
> java.io.IOException: Cannot process extension IDs at position 1
> 
>         at org.snmp4j.asn1.BER.decodeHeader(BER.java:580)
> 
>         at org.snmp4j.asn1.BER.decodeHeader(BER.java:609)
> 
>         at org.snmp4j.ScopedPDU.decodeBER(ScopedPDU.java:141)
> 
>         at org.snmp4j.mp.MPv3.prepareDataElements(MPv3.java:984)
> 
>         at org.snmp4j.MessageDispatcherImpl.dispatchMessage(
> MessageDispatcherImpl.java:240)
> 
>         at org.snmp4j.MessageDispatcherImpl.processMessage(
> MessageDispatcherImpl.java:323)
> 
>         at org.snmp4j.util.MultiThreadedMessageDispatcher$MessageTask.run(
> MultiThreadedMessageDispatcher.java:190)
> 
>         at org.snmp4j.util.ThreadPool$TaskManager.run(ThreadPool.java:236)
> 
> java.io.IOException: Wrong ASN.1 type. Not a string: 91 at position 3
> 
>         at org.snmp4j.asn1.BER.decodeString(BER.java:704)
> 
>         at org.snmp4j.smi.OctetString.decodeBER(OctetString.java:151)
> 
>         at org.snmp4j.ScopedPDU.decodeBER(ScopedPDU.java:143)
> 
>         at org.snmp4j.mp.MPv3.prepareDataElements(MPv3.java:984)
> 
>         at org.snmp4j.MessageDispatcherImpl.dispatchMessage(
> MessageDispatcherImpl.java:240)
> 
>         at org.snmp4j.MessageDispatcherImpl.processMessage(
> MessageDispatcherImpl.java:323)
> 
>         at org.snmp4j.util.MultiThreadedMessageDispatcher$MessageTask.run(
> MultiThreadedMessageDispatcher.java:190)
> 
>         at org.snmp4j.util.ThreadPool$TaskManager.run(ThreadPool.java:236)
> 
> java.io.IOException: The encoded length 96 exceeds the number of bytes left
> in input at position 2 which actually is 48
> 
>         at org.snmp4j.asn1.BER.checkLength(BER.java:902)
> 
>         at org.snmp4j.asn1.BER.decodeLength(BER.java:547)
> 
>         at org.snmp4j.asn1.BER.decodeHeader(BER.java:584)
> 
>         at org.snmp4j.asn1.BER.decodeHeader(BER.java:609)
> 
>         at org.snmp4j.ScopedPDU.decodeBER(ScopedPDU.java:141)
> 
>         at org.snmp4j.mp.MPv3.prepareDataElements(MPv3.java:984)
> 
>         at org.snmp4j.MessageDispatcherImpl.dispatchMessage(
> MessageDispatcherImpl.java:240)
> 
>         at org.snmp4j.MessageDispatcherImpl.processMessage(
> MessageDispatcherImpl.java:323)
> 
>         at org.snmp4j.util.MultiThreadedMessageDispatcher$MessageTask.run(
> MultiThreadedMessageDispatcher.java:190)
> 
>         at org.snmp4j.util.ThreadPool$TaskManager.run(ThreadPool.java:236)
> 
> The code of my application is a bit too long to copy here, but as you see,
> there is no reference out of the snmp4j package. I use the ThreadPool and
> MultiThreadedMessageDispatcher classes to have more thread working on the
> incoming messages. I initialized them this way :
> 
> threadPool = ThreadPool.create("DispatcherPool", 2);
> mtDispatcher = new MultiThreadedMessageDispatcher(threadPool, new
> MessageDispatcherImpl());
> 
> mtDispatcher.addMessageProcessingModel(new MPv3());
> mtDispatcher.addMessageProcessingModel(new MPv2c());
> mtDispatcher.addMessageProcessingModel(new MPv1());
> 
> snmp = new Snmp(mtDispatcher, transport);
> 
> Has anyone an idea what did I wrong? The final version of my program won't
> use the console, and will work fine even with this exception, but is is a
> little embarassing.
> 
> Thanks for the help in advance!
> 
> Regards
> --
> Imre Kovacs, Hungary
> _______________________________________________
> SNMP4J mailing list
> SNMP4J at agentpp.org
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