Wireshark-dev: Re: [Wireshark-dev] TCP dissect issue when app-level message spans multiple TCP
From: "Fernandez, Rafael" <[email protected]>
Date: Thu, 5 May 2011 15:56:30 -0500
I completely agree with your answer. That is what I expect to happen.
I was able to download wireshark-1.5.2-SVN-36997, compile against it, and the issue still happens.
I must note that there is a [TCP Previous segment lost] and a [TCP out-of-order] 10 frames apart and it is also where the problem starts occurring until the end of the capture file.
I now register a different dissector for UDP and TCP.

Nonetheless, This is my current code:

void dissect_tcp_message(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
        tcp_dissect_pdus(tvb, pinfo, tree, TRUE, 4,
                         get_tcp_message_len,
                         dissect_tcp_message);
}

void dissect_tcp_message(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
      if (check_col(pinfo->cinfo, COL_PROTOCOL))
            col_set_str(pinfo->cinfo, COL_PROTOCOL, "MyMessage");

        gint32 length, type, legacy;
        if(tvb_bytes_exist(tvb, 0, MESSAGE_HEADER_SIZE))
        {
                length = tvb_get_letohl(tvb, 0);
                type   = tvb_get_letohl(tvb, 4);
                legacy = tvb_get_letohl(tvb, 8);
        }
        else
        {
                call_dissector(data_handle, tvb, pinfo, tree);
                return;
        }

        if (check_col(pinfo->cinfo, COL_INFO))
        {
                col_add_fstr(pinfo->cinfo, COL_INFO, "[%s] ",
                               typeAsString(type));
                col_set_fence(pinfo->cinfo, COL_INFO);
        }

        if(!tree)
        {
                return;
        }

        proto_item *base_item, *header_item, *body_item;
        proto_tree *base_tree, *header_tree;
        base_item = header_item = body_item = NULL;
        base_tree = header_tree = NULL;

        base_item = proto_tree_add_item(tree, proto_mymessage, tvb, 0, -1, FALSE);
        base_tree = proto_item_add_subtree(base_item, ett_mymessage);

        header_tree = proto_item_add_subtree(base_item, ett_mymessage);
        header_item = proto_tree_add_item(base_tree, hf_header, tvb, 0, MESSAGE_HEADER_SIZE, FALSE);

        body_item = proto_tree_add_item(base_tree, hf_body, tvb, MESSAGE_HEADER_SIZE, -1, FALSE);

        header_tree = proto_item_add_subtree(header_item, ett_mymessage);
        proto_tree_add_int(header_tree, hf_header_length, tvb, 0, 4, length);
        proto_tree_add_string_format_value(header_tree, hf_header_type, tvb, 4, 4, "", "%s [%d]",
                                                        typeAsString(type), type);
        proto_tree_add_int(header_tree, hf_header_seq, tvb, 8, 4, legacy);
}

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

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Guy Harris
Sent: Thursday, May 05, 2011 2:59 PM
To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] TCP dissect issue when app-level message spans multiple TCP packets


On May 5, 2011, at 10:39 AM, Fernandez, Rafael wrote:

> The issue is the following:
>
> In epan/dissectors/packet-tcp.c-tcp_dissect_pdus():
> line 1993: get_pdu_len returns 322. Sets plen to 322.
> line 2053-2061: length_remaining is 144. Thus (length_remaining < plen) is true. Sets pinfo->desegment_offset and pinfo->desegment_len. tcp_dissect_pdus returns.

...meaning that the TCP segment handed to tcp_dissect_pdus() didn't have a complete message, so there's nothing more for it to do at that point.

> In my_dissector.c:
> 1. Once tcp_dissect_pdus returns, dissect_message returns.
> 2. dissect_message gets called soon after and calls tcp_dissect_pdus again.

...when the next segment is available.

dissect_message() should, at that point, have been handed to it by the TCP dissector a reassembled tvbuff containing:

        the 144 bytes from the TCP segment that was handed to it in the previous call;

        following it, up to 178 bytes of additional data from the next segment;

in which case...

> In packet-tcp.c-tcp_dissect_pdus:
> Line 1993: get_pdu_len gets called again. It read 4 bytes starting from the 178th (322-144) byte of the application message. Thus, it returns a garbage size.

...get_pdu_len() should be handed the reassembled tvbuff in question, with an offset argument of 0, so it should be reading the same 4 bytes that it did on the first call, *not* the first 4 bytes of the second segment.

dissect_message() is the dissector you registered with TCP as the dissector for your protocol, right?

> Don't you think there is an issue with this?

There's an issue with *something*, but, given that tcp_dissect_pdus() has worked and continues to work fine for a number of other protocols, it's not a simple or obvious issue.
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