Re: pcap FCS length and LT_FCS_DATALINK_EXT()

[Guy Harris <guy () alum mit edu>]

On May 5, 2013, at 3:49 PM, Stephen Donnelly <Stephen.Donnelly () emulex com> wrote:

> In 2007 in libpcap afbb1ce7 you committed some code (possibly from Florent Drouin) adding the LT_FCS_DATALINK_EXT 
> mechanism to record whether the capture includes information about captured FCS length, and if so what length it is.

Yes, that was from a thread with the subject "Request for a new DLT for MTP2 with FCS" from 2007, in which Florent, 
you, and I were the participants.

> I believe that currently only the DAG capture code supports this, and as implemented the LT_FCS_LENGTH is in units of 
> 16-bits, e.g. 16-bit FCS returns 1, and 32-bit FCS returns 2?

Units of bits won't work, as the field is only 4 bits long, so it can't even handle 16 bits.  As units of 16 bits is 
already being used, we might as well go with it.

> I don't think Wireshark's pcap wiretap code currently uses this mechanism to check for FCS length, does it make sense 
> to add it?

Yes, it does.  If you have libpcap 1.0 or later, you have pcap_datalink_ext(), so tcpdump could use it as well.
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