> On Aug 31, 2021, at 10:37 PM, Ariel Burbaickij <ariel.burbaickij@xxxxxxxxx> wrote:
>
> Hello Christopher, all,
> as I wrote "... to write Lua dissector...", so instructions what and how to do on command line do not apply in this case. Meanwhile, I figured out by myself how this is supposed to work:
>
> local udlt = DissectorTable.get("wtap_encap")
> udlt:add(wtap.USER1, ypp)
>
> why not to stick to one naming convention of user_dlt
An explanation of various link-layer type indicators:
Wireshark can read several file formats; they do not all use the same numerical values for any given link-layer type.
pcap and pcapng files use the LINKTYPEs specified on
https://www.tcpdump.org/linktypes.html
The numerical values in that file appear in the headers of pcap files and the Interface Description Blocks of pcapng files.
libpcap uses DLTs in its APIs. DLTs are *not* guaranteed to have the same numerical values on all platforms; historically, various OSes have given some DLTs different values on different OSes, so no program should depend on the numerical value; libpcap preserves that, for binary compatibility.
The LINKTYPEs were created to provide values that *would* be guaranteed to be the same, no matter what platform the file is written on; libpcap maps between LINKTYPEs and DLTs.
No current libpcap API uses LINKTYPEs.
Wireshark reads more than just pcap and pcapng files, and some of the files it reads have link-layer types for which there is no corresponding LINKTYPE_ value. Therefore, it has its *own* set of link-layer types - those are the WTAP_ENCAPs.
There is no guarantee that a WTAP_ENCAP that corresponds to a given LINKTYPE has the same numerical value, and there never will be such a guarantee - we don't even guarantee that the numerical values of WTAP_ENCAPs will remain the same from one Wireshark major release to another.
The APIs Wireshark offers to plugins, whether they're for C or Lua plugins, use WTAP_ENCAPs, not LINKTYPEs. There is, therefore, no guarantee that 148 will work as a way to refer to WTAP_ENCAP_USER1, even though the numerical value of LINKTYPE_USER1 is 148. The same applies for all other USERn values from USER0 to USER15 - use WTAP_ENCAP_USERn, not the numerical value for LINKTYPE_USERn, in libwiretap and libwireshark APIs.
The naming convention we use is that, when registering in the "wtap_encap" dissector table with the Wireshark encapsulation value WTAP_ENCAP_xxx, you use WTAP_ENCAP_xxx in C code and wtap.xxx in Lua code.
