Wireshark-dev: Re: [Wireshark-dev] Register dissector to MAC address
From: "Maynard, Chris" <[email protected]>
Date: Mon, 12 Nov 2007 17:13:08 -0500
At first glance, packet-eth.c seems to have heuristic support, but it doesn't appear to work, at least not how I expected it to.  For example, originally for the WOL dissector, I registered as I do for UDP, namely:
    heur_dissector_add("eth", dissect_wol, proto_wol);
 
But registering it that way didn't work for me, so it's been changed to:
    dissector_add("ethertype", ETHERTYPE_WOL, wol_handle);
 
I didn't dig too deeply into why it failed since I had a reasonable alternative, but I suppose I should have.  It now seems to me to be a bug in packet-eth.c, but I'm not entirely sure, based on the comments in the code.
 
First, compare the way a dissector like packet-udp.c tries the heuristic dissectors, using the "next_tvb":
    next_tvb = tvb_new_subset(tvb, offset, len, reported_len);
    if (dissector_try_heuristic(heur_subdissector_list, next_tvb, pinfo, tree))

Now look at how packet-eth.c does it:
    if (dissector_try_heuristic(heur_subdissector_list, tvb, pinfo, parent_tree))
        goto end_of_eth;

Notice that there's no "next_tvb".  I assumed that this was intentional when I looked at it before, but now I'm not so sure.  A bug?  It now sure looks like it to me.  I couldn't find any other dissectors that try to heuristically register to "eth" as I tried above.  Perhaps because it doesn't work?  If it is a bug, then once that's corrected, then that would be the better way to register both WOL and the original poster's dissector - heuristically.
 
- Chris

________________________________

From: [email protected] on behalf of Stephen Fisher
Sent: Mon 11/12/2007 12:50 PM
To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] Register dissector to MAC address



On Mon, Nov 12, 2007 at 12:37:10PM -0500, Maynard, Chris wrote:

> Can anyone think of a reason NOT to add heuristic dissection support
> to packet-eth.c?  Or does anyone have a better/alternate way to solve
> this?

My first thought is that the original poster's dissector could be a
heuristic that checks against the MAC address when deciding whether to
acccept the packet or not.  Does this need changes to packet-eth.c?  I'm
not sure, but could find out by researching the code a bit.


Steve

_______________________________________________
Wireshark-dev mailing list
[email protected]
http://www.wireshark.org/mailman/listinfo/wireshark-dev





-----------------------------------------
This email may contain confidential and privileged material for the
sole use of the intended recipient(s). Any review, use, retention,
distribution or disclosure by others is strictly prohibited. If you
are not the intended recipient (or authorized to receive for the
recipient), please contact the sender by reply email and delete all
copies of this message. Also, email is susceptible to data
corruption, interception, tampering, unauthorized amendment and
viruses. We only send and receive emails on the basis that we are
not liable for any such corruption, interception, tampering,
amendment or viruses or any consequence thereof.

<<winmail.dat>>