Robert Naumann wrote:
> i'm new to the dissector developement for wireshark. I've read that it
> should be actually possible to dissect multiple PDUs from one TCP
> Segment with tcp_dissect_pdus(), but i don't get it working.
>
> For example: i've got 2 different PDUs with different lengths (5 and 6).
> I already got the pdu lengths. The Length of the TCP Datasegment is 11.
>
> So for the first PDU the function:
>
> tcp_dissect_pdus(tvb, pinfo, tree, TRUE, min_pdu_size,
> *first_pdu_size*, dissect_pdu_sc);
tcp_dissect_pdus() isn't called for every PDU; it's called *once* in the
dissector for your protocol, and the argument you mark as "first pdu
size" is a function that gets the PDU size of a PDU.
"min_pdu_size" should be the minimum amount of packet data needed to
calculate the PDU size. For example, if every PDU starts with a 2-byte
PDU type and a 2-byte PDU length, in that order, "min_pdu_size" should
be 4, to include the PDU length. If the length comes *before* the type,
it'd be 2.
If the value of the length field in the PDU includes the lengths of the
type and length fields, and the fields are in big-endian order, then:
In the case where there's a type and length, in that order, the function
to get the PDU size would be
static gint
get_sc_pdu_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset)
{
return tvb_get_ntohs(tvb, offset + 2);
}
and in the case where the length comes before the type, it'd be
static gint
get_sc_pdu_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset)
{
return tvb_get_ntohs(tvb, offset);
}
Replace "tvb_get_ntohs()" with "tvb_get_letohs()" if the fields are in
little-endian order.
If the value of the length field in the PDU *doesn't* include the
lengths of the type and length field, you'd add 4 to the length field in
the first of those functions and add 2 to the length field in the second
of those functions.
> so the first PDU with the correct size (proved) should be passed to the
> dissector function dissect_pdu_sc.
>
> My first problem:
> =============
> At the dissection of my top level protocol the PDU is marked as
> [Malformed Packet], why? Size is correct! bitstream show the complete
> data of my PDU
So your function is returning the value 5 for the first PDU?
Is your dissector trying to dissect more than 5 bytes in the PDU? If
so, it's acting as if the packet should be longer than 5 bytes long, and
thus a 5-byte-long packet would be malformed. An attempt to fetch data
past the end of the 5-byte tvbuff your dissector is handed will cause an
exception to be thrown and your packet marked as malformed.
> My second problem:
> ===============
> The second PDU within the the TCP Segment wont be dissected and not
> shown as a part of my top level protocol. Is the second PDU
> automatically processed
Yes.
> or do i have to call the function
> tcp_dissect_pdus() again?
No. It's called "tcp_dissect_pdus()", plural, not "tcp_dissect_pdu()",
singular, because it's supposed to be handed all the data that's passed
to the disector and dissects all the PDUs in that data.
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