Ethereal-dev: Re: [Ethereal-dev] Re: SNMP Capture Data inquiry

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From: Guy Harris <gharris@xxxxxxxxx>
Date: Wed, 20 Jul 2005 02:24:18 -0700
Jose Vicente Quinto wrote:
Can anyone explain to me what are the purpose of the initial values before the actual ObjectID (before '2b 06 01 ... 01 05 00')? 

Is it correct that "43" in the first 2 bytes -- '30 43', the '43' (decimal 67)
is the length of the whole SNMP data received that must be decoded, parsed
then encoded again for the sending of the RESPONSE PDU?


If I remember correctly, yes.
If you want to understand the way SNMP packets are encoded, you first need to understand Abstract Syntax Notation One, a/k/a "ASN.1", as defined in ITU-T Recommendations X.680, X.681, X.682, and X.683. You'd then need to understand the Basic Encoding Rules for ASN.1, as defined in ITU-T Recommendation X.691.
You'd then need to read the RFCs that define SNMP, such as RFC 1157, RFC 1901, RFC 1905, RFC 1906, and RFCs 2570-2576. 

Those documents might be more than you need.  You might want to Google for

	asn.1 tutorial

or look at pages such as

	http://www.et.put.poznan.pl/snmp/main/mainmenu.html