The CAN ID is passed as the "data" parameter to the subdissector from the CAN
dissector. The C code of a subdissector to handle it would be as follows:
struct can_identifier
{
guint32 id;
};
static int subdissector_function(tvbuff_t *tvb, packet_info *pinfo, proto_tree
*tree, void* data)
{
struct can_identifier can_id;
can_id = *((struct can_identifier*)data);
if (can_id.id != 42)
{
return 0;
}
...
}
I'm not as familiar with Lua to know how to setup the equivalent, but you
shouldn't need to "override" the CAN dissector to get the CAN ID.
-----Original Message-----
From: Sebastian Schildt <[email protected]>
To: wireshark-dev <[email protected]>
Sent: Tue, Dec 15, 2015 5:37 am
Subject: [Wireshark-dev] Chained CAN dissecector: Can not get reference for CAN
dissector
Hello Wiresharkers,
I have a problem (obviously :) ) . I want to create a CAN dissector (in Lua).
What I already achieved is creating a Can subdissector: So my dissector gets
called for CAN payload. However, I need access to the CAN identfier, so I
figured my dissector needs to be on the same "level" as the CAN dissector
(right?)
Therefore I tried to use the "chained dissector" pattern, where my dissector
would call the CAN dissector and then does some further processing. For this my
dissector must call the CAN dissector, but I do not know how to get a reference
to it.
I tried
original_can_dissector = Dissector.get("CAN")
or
original_can_dissector = Dissector.get("can")
that did not work, then I used lua to print
for key,value in pairs(Dissector.list()) do print(key,value) end
but I did not find any reference to CAN. Is the CAN dissector hidden in some
DissectorTable? But then which one? I couldn't find it.
The relevant lines in the C code seem to be
proto_can = proto_register_protocol(
"Controller Area Network", /* name */
"CAN", /* short name */
"can" /* abbrev */
);
And
dissector_handle_t can_handle;
can_handle = create_dissector_handle(dissect_socketcan, proto_can);
dissector_add_uint("wtap_encap", WTAP_ENCAP_SOCKETCAN, can_handle);
dissector_add_uint("sll.ltype", LINUX_SLL_P_CAN, can_handle);
but I am not very familiar with Wireshark source, so I am guestimating here.
Also, after solving the problem of getting a reference to the original CAN
dissector I guess I need to replicate the add_uint calls in Lua to register my
own dissector. If anyone has the correct syntax up his sleeve I'd also
appreciate it :)
Thanks
Sebastian
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