Wireshark · Wireshark-dev: Re: [Wireshark-dev] Chained CAN dissecector: Can not get reference for CAN dissector

[Pascal Quantin <pascal.quantin@xxxxxxxxx>]

2015-12-15 11:37 GMT+01:00 Sebastian Schildt <sebastian@xxxxxxxxxxxxxx>:

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 :)

Hi Sebastian,

The CAN dissector does not register itself by name (no call to register_dissector() function), so as you noticed you will not find it on the list.

The best way to move forward would be be to contribute to Wireshark so as to add the missing register_dissector function call. But it will be only added to the development tree (Wireshark 2.1) and not backported to stable releases as I do not think it will be considered as a fix, but an enhancement.

Regards,

Pascal.