Wireshark-dev: Re: [Wireshark-dev] Exported PUD proto_name
From: Dario Lombardo <[email protected]>
Date: Fri, 26 Aug 2016 10:44:37 +0200
Ok, we'll wait for some clarifications from Alexis.

On Fri, Aug 26, 2016 at 10:25 AM, Pascal Quantin <[email protected]> wrote:


2016-08-26 10:14 GMT+02:00 Dario Lombardo <[email protected]>:
It seems that it prevents the dissection of exported payloads if proto_name is disaligned. Alexis is experiencing that during the testing of udpdump. Maybe he can clarify where the problem is so we can work on it. AFAIK, he's using aruba_erm as proto_name in udpdump, but this is changed into "aruba_erm\x00\x00\x00" that is not matched by the dissectors table. Alexis, did I get the point?

I'm using exported PDU with a protocol name that is not a multiple of 4 bytes everyday without any issue.
I'm nor surprised that it does not work with "aruba_erm" as this dissector is not registered by name.
 Exported PDU can support dissectors registered by name, in a dissector table, or heuristic ones (see exported_pdu.h fiel for details).



On Fri, Aug 26, 2016 at 10:05 AM, Pascal Quantin <[email protected]> wrote:
Hi Dario,

2016-08-26 10:02 GMT+02:00 Dario Lombardo <[email protected]>:
Hi,
I'm looking into the code of exported_pdu.c and specifically into export_pdu_create_tags(). The first tag it creates is the tag with proto_name. The piece of code that I don't understand is

        /* Start by computing size of protocol name as a tag */
proto_str_len = (int)strlen(proto_name);

/* Ensure that tag length is a multiple of 4 bytes */
proto_tag_len = ((proto_str_len + 3) & 0xfffffffc);

/* Add Tag + length */
tag_buf_size += (proto_tag_len + 4);

       [...]

exp_pdu_data->tlv_buffer = (guint8 *)g_malloc0(tag_buf_size);

Basically, the buffer to store the proto_name tag must be multiple of 4 bytes. This means that if I use "data", I have "data", but if I use "data1" I have "data1\x00\x00\x00". What's the rationale behind this? Why is the alignment to 4 bytes required? 

This is inspired by the pcapng specification. You have a complete description of the TLV format used in epan/exported_pdu.h

Pascal.


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