Wireshark-dev: [Wireshark-dev] SNMP Dissector error when object length > 4 bytes
From: Francesco Ruta <[email protected]>
Date: Mon, 12 Jul 2010 17:39:25 -0700 (PDT)
When an SNMP packets contains an object with a length > 4 bytes, the dissector returns the following error:
Dissector bug, protocol SNMP: proto.c:1047: failed assertion "DISSECTOR_ASSERT_NOT_REACHED"

Unfortunately it is not very difficult to have values with a length greater than 4 bytes. 
As example, the following requests would generate the error described before --when the relevant SMI file is loaded--:
snmpset -v2c -d -c public 10.215.98.1 .1.3.6.1.4.1.111.1.1.0 u 4294967295
snmpset -v2c -d -c public 10.215.98.1 .1.3.6.1.4.1.111.1.1.0 u 4026531840

A SMI file could be as follow (this file must be loaded in the "SMI (MIB and PIB) modules" to see the problem or the dissector would return "Missing MIB"):

MY-MIB DEFINITIONS ::= BEGIN
IMPORTS
       MODULE-IDENTITY, Unsigned32, enterprises
               FROM SNMPv2-SMI;

myModule MODULE-IDENTITY
       LAST-UPDATED "201007120000Z"
       ORGANIZATION " "
       CONTACT-INFO 
                " "
       DESCRIPTION
               " "
       ::= { enterprises my(111) 1 }
       
myObject  OBJECT-TYPE
        SYNTAX      Unsigned32
        MAX-ACCESS  read-write
        STATUS      current
        DESCRIPTION
                "=   "
         ::= { myModule 1 }
END


The problem is visible (at least) with the following Window releases: 1.2.1 and 1.2.9. 


Frame 2 (91 bytes on wire, 91 bytes captured)
Ethernet II, Src: Motorola_24:01:9a (00:16:26:24:01:9a), Dst: Motorola_24:01:9a (00:16:26:24:01:9a)
Internet Protocol, Src: 192.168.236.162 (192.168.236.162), Dst: 10.215.98.1 (10.215.98.1)
User Datagram Protocol, Src Port: 53719 (53719), Dst Port: snmp (161)
Simple Network Management Protocol
    version: v2c (1)
    community: public
    data: set-request (3)
        set-request
            request-id: 1210029970
            error-status: noError (0)
            error-index: 0
            variable-bindings: 1 item
                VarBind
                    Object Name: 1.3.6.1.4.1.111.1.1.0 (MY-MIB::myObject.0)
                        Scalar Instance Index: 0
[Dissector bug, protocol SNMP: proto.c:1047: failed assertion "DISSECTOR_ASSERT_NOT_REACHED"]
    [Expert Info (Error/Malformed): proto.c:1047: failed assertion "DISSECTOR_ASSERT_NOT_REACHED"]
        [Message: proto.c:1047: failed assertion "DISSECTOR_ASSERT_NOT_REACHED"]
        [Severity level: Error]
        [Group: Malformed]



      

Attachment: test.pcap
Description: Binary data