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Greetings, I am building a plugin dissector that decompresses packets from a conversation for which a compression flag can be set either via a byte set on the first compressed subsequent segment or a string definition on its first segment. Subsequent segments afterwards do not have the byte set that flags a compressed stream. I've tried either finding the conversation or creating one if not found, but I need a mechanism for depicting if a packet found from a conversation has been set to "Compressed". Here is my code:

        /* First check that data is compressed */
    tvbuff_t *newBuf;
    guint8 bits1 = tvb_get_guint8(tvb, offset);
    guint8 bits2 = tvb_get_guint8(tvb, offset + 1);
    if ((bits1 == 200) && (bits2 == 300)) {
        m_IsCompressed = TRUE;

    /* Next check if this is a new stream or it's part of a CPOF conversation. */
    conversation_t *conversation = find_conversation(pinfo->fd->num, &pinfo->src, &pinfo->dst,
        pinfo->ptype, pinfo->srcport, pinfo->destport, 0);

    // first segment to process for this side of the stream?
    if (conversation == NULL){
            // Create new conversation to track
        conversation = conversation_new(pinfo->fd->num, &pinfo->src, &pinfo->dst,
            pinfo->ptype, pinfo->srcport, pinfo->destport, 0);

    /* Compression flag also gets set when "Compressing" string is found.  Other systems in conversation can also converse compressed, yet not have bytes-set flag. */
        gint newLen = tvb_reported_length_remaining(tvb, offset);
        guint8* buff_str = tvb_get_string_enc(pool, tvb, offset, newLen, ENC_ASCII | ENC_NA);
        gchar* str = convert(buff_str, strlen);
        newBuf = tvb_new_subset_remaining(tvb, offset);
                    if (m_IsCompressed)
                           conversation_add_proto_data(conversation, proto_tag, "Compressed");
    else {
                    char* state = (char*) conversation_get_proto_data(conversation, proto_tag);
                    IsCompressed = m_IsCompressed || (state || strncmp(state, "Compressed", 10) == 0);
        if (m_IsCompressed) {
            /* the remainder of the packet is compressed, decompress the payload */
            newBuf = tvb_uncompress(tvb, offset, tvb_captured_length_remaining(tvb, offset + 2));

            /* Check the return code and add a expert info warning if an error
            * occurred. The dissector continues trying to add the payload,
            * however the returned size should be 0. */
            if (newBuf) {
                /* Now re-setup the tvb buffer to have the new data */
                tvb_set_child_real_data_tvbuff(tvb, newBuf);
                add_new_data_source(pinfo, newBuf, "Decompressed Data");
            else {
                /* attempt to process it as uncompressed */
                newBuf = tvb_new_subset_remaining(tvb, offset);
        else {
                /* process new uncompressed buffer */
            newBuf = tvb_new_subset_remaining(tvb, offset);
    /* process newBuf from here on */
    offset = 0;

Could I store a Boolean into a conversation that I can access and use to determine if associated/follow-on packets need decompression? A single static boolean doesn't seem to do the trick for me.

Thanks in advanced.

asked 04 Apr '16, 20:08

coloncm's gravatar image

accept rate: 66%

edited 07 Apr '16, 12:42

conversation_new() should be inside the if (conversation == NULL) block, otherwise the conversation will never exist.

permanent link

answered 05 Apr '16, 04:46

Jaap's gravatar image

Jaap ♦
accept rate: 14%

Changed the question after suggested fix did not change outcome to the underlying problem.

(05 Apr '16, 16:35) coloncm

See README.dissector The conversation_add_proto_data function.

(06 Apr '16, 04:33) Jaap ♦

Thanks, Jaap. The mechanism seems to work except I'm not sure if I'm getting back what I'm putting in form a found conversation, e.g., gboolen values appear to be random positive or negative values instead of 1 or 0 as previously set. But, that's another question, I guess, and your comment surely answers this question.

(06 Apr '16, 18:31) coloncm
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question asked: 04 Apr '16, 20:08

question was seen: 1,843 times

last updated: 07 Apr '16, 12:42

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