wireshark - tcp_dissect_pdus() sometimes doesn't work

Question

I'm wrote a wireshark dissector named PLUGIN.

Now when I'm testing it, for some reason a packet of type X can be seen on the wireshark as PLUGIN (like it should), and some other packets afterwards, of the same type X cannot be seen as PLUGIN.

The other packets can be found in the .pcap as [TCP segment of a reassembled PDU]

The question is: why doesn't WireShark dissect other packets of type X like the first packet of type X and shows it to me as PLUGIN ?

Why does the dissection works only for the first time?

I use functions for assembling fragments of chopped packets:

tcp_dissect_pdus()
get_PLUGIN_message_len()

as written in "9.4.2. How to reassemble split TCP Packets"

in "http://www.wireshark.org/docs/wsdg_html_chunked/ChDissectReassemble.html#TcpDissectPdus"

This is the function of dissection: (FRAME_HEADER_LEN = 8)

static void
dissect_PROTOC(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    //Reassembling TCP fragments
    tcp_dissect_pdus(tvb, pinfo, tree, TRUE, FRAME_HEADER_LEN,
                     get_PROTOC_message_len, dissect_PROTOC_message);

}

static guint get_PROTOC_message_len(packet_info *pinfo, tvbuff_t *tvb, int offset)
{
    /* the packet's size is "length" + 4bytes of TYPESIZE + 4bytes of LENGTHSIZE + 256bytes of CONTEXTIDSIZE */
    return (guint)(tvb_get_ntohl(tvb, offset + 4) + CONTEXT_ID_SIZE + TYPE_SIZE + LENGTH_SIZE); /* e.g. length is at offset 4 */
}

static void dissect_PROTOC_message(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    /* my dissecting code */
    guint32 packet_type = tvb_get_ntohl(tvb, 0);

    col_set_str(pinfo->cinfo, COL_PROTOCOL, "PROTOC");
    /* Clear out stuff in the info column */
    col_clear(pinfo->cinfo,COL_INFO);
    col_add_fstr(pinfo->cinfo, COL_INFO, "%d > %d [%s]",pinfo->srcport, pinfo->destport,
             val_to_str(packet_type, packettypenames, "Unknown (0x%02x)"));

    if (tree) { /* we are being asked for details */
        proto_item *ti              = NULL;
        proto_tree *PROTOC_tree         = NULL;
        proto_item *PROTOC_data         = NULL;
        proto_tree *PROTOC_data_tree    = NULL;
        guint32 type    = 0;
        guint32 length  = 0;
        gint offset     = 0;

        ti = proto_tree_add_item(tree, proto_PROTOC, tvb, 0, -1, ENC_NA);
        proto_item_append_text(ti, ", Type: %s",
            val_to_str(packet_type, packettypenames, "Unknown (0x%02x)"));
        PROTOC_tree = proto_item_add_subtree(ti, ett_PROTOC);

        //getting type
        type = tvb_get_ntohl(tvb, offset);
        proto_tree_add_item(PROTOC_tree, hf_PROTOC_pdu_type, tvb, 0, TYPE_SIZE, ENC_BIG_ENDIAN);
        offset += TYPE_SIZE;

        //getting length for the data length
        length = tvb_get_ntohl(tvb, offset);
        proto_tree_add_item(PROTOC_tree, hf_PROTOC_len, tvb, offset, LENGTH_SIZE, ENC_BIG_ENDIAN);
        offset += LENGTH_SIZE;
        proto_tree_add_item(PROTOC_tree, hf_PROTOC_contextid, tvb, offset, CONTEXT_ID_SIZE, ENC_BIG_ENDIAN);
        offset += CONTEXT_ID_SIZE;
        PROTOC_data = proto_tree_add_item(PROTOC_tree, hf_PROTOC_data, tvb, offset, length, FALSE);
        PROTOC_data_tree = proto_item_add_subtree(PROTOC_data, ett_PROTOC_data);
        offset += length;

    }
}

more information:

1. I opened the file.pcap on some hex editor and I can see my packets that the wireshark doesn't dissect...

2. Inside wireshark I can find a packet that wireshark doesn't dissect, with the info of: "[TCP segment of a reassembled PDU]", but it doesn't say to which reassemble packet it belong, and I can't find it anywhere...

Answer 1

I would speculate that your calculated length does not match the reality. If wireshark does not think it has all the frames it needs to complete the PDU, then I guess it might not call dissect_PROTOC_message()

You could try to print your calculated lengths and then verify that the pcap file contains that many frames:

static guint get_PROTOC_message_len(packet_info *pinfo, tvbuff_t *tvb, int offset)
{
    /* the packet's size is "length" + 4bytes of TYPESIZE + 4bytes of LENGTHSIZE + 256bytes of CONTEXTIDSIZE */
    guint len = (guint)(tvb_get_ntohl(tvb, offset + 4) + CONTEXT_ID_SIZE + TYPE_SIZE + LENGTH_SIZE); /* e.g. length is at offset 4 */
    g_warning("frame=%d PDU len=%d", pinfo->fd->num, len);
    return len;
}