Wireshark-dev: Re: [Wireshark-dev] TCP dissect issue when app-level message spans multiple TCP
From: Graham Bloice <[email protected]>
Date: Thu, 05 May 2011 16:12:24 +0100
On 05/05/2011 16:00, Chris Maynard wrote:
Fernandez, Rafael <[email protected]> writes:

I am writing a dissector plugin and I am using tcp_dissect_pdus in order to
reassemble packets. However, I
am experiencing issues when there are multiple application layer messages in
one packet and the last one
is not complete. Specifically, I have 5 messages in one TCP packet. There are
5 calls to the get_pdu_len
function and four calls to the dissect_pdu function. Since there is not enough
data in the current
tvbuff_t, tcp_dissect_pdus returns. When dissect_message is called again, the
beginning of the
message is lost. When get_pdu_len gets called again it reads a garbage size.

Per https://www.wireshark.org/docs/wsdg_html_chunked/ChDissectReassemble.html,
I thought that
tcp_dissect_pdus is supposed to handle this. I can't find example code in any
dissector in
/epan/dissectors/* that handles this unlike I am. Am I missing something?

This is part of my code:

void dissect_message(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
        if(pinfo->tcp_tree != NULL) {
                tcp_dissect_pdus(tvb, pinfo, tree, TRUE, MESSAGE_HEADER_SIZE,
                                 get_message_tcpmessage_len,
dissect_message_tcpmessage);
        }
        else {
                // For UDP just print the data for now.
                call_dissector(data_handle, tvb, pinfo, tree);
        }
}

guint get_message_tcpmessage_len(packet_info *pinfo, tvbuff_t *tvb, int offset)
{
        if(tvb_length_remaining(tvb, offset) <= 8)
                return 0;

        gint32 size = tvb_get_letohl(tvb, offset)+MESSAGE_HEADER_SIZE;
        gint32 type = tvb_get_letohl(tvb, offset+4);
        printf("Len: %u type %d\n", size, type);
        if(strcmp(typeAsString(type), "Unknown") == 0)
        {
                return tvb_length_remaining(tvb, offset);
        }
        return size;
}
I don't think your get_message_tcpmessage_len() function is coded correctly.  It
looks like you're trying to do some heuristics in there when all you really
should be doing is returning the length of your PDU.  In your case, it looks
like the length field is the 1st 4-bytes of the header, so I think you should
probably just be doing something like this:

    if(pinfo->tcp_tree != NULL) {
        tcp_dissect_pdus(tvb, pinfo, tree, TRUE, 4, get_message_tcpmessage_len, 
                         dissect_message_tcpmessage);
    }

(NOTE: Here, you could use MESSAGE_HEADER_SIZE instead of 4, but you really only
need the 1st 4 bytes to determine the length of the entire PDU.)

guint get_message_tcpmessage_len(packet_info *pinfo, tvbuff_t *tvb, int offset)
{
    return tvb_get_letohl(tvb, offset) + MESSAGE_HEADER_SIZE;
}



The get_message_tcpmessage_len function should also check that there are enough bytes in the passed in tvb to call tvb_get_letohl(), as the OP's code did.

-- 
Regards,

Graham Bloice