[Click] specify DLT in FromDevice?

[Ian Rose]

When using libpcap, some drivers support multiple datalink-types.  I 
believe this is why tcpdump exposes a -y flag (to set the datalink type) 
and a -L flag (to list the available datalink types for a device).  I 
think perhaps if you do not specify one (via pcap_set_datalink) then it 
will default to the first that appears in the -L list.

For example, we are using the 'ath' FreeBSD 802.11 driver:

> ianrose at somehost 9$ sudo tcpdump -L -i ath1
> Data link types (use option -y to set):
>   EN10MB (Ethernet)
>   IEEE802_11 (802.11)
>   IEEE802_11_RADIO (802.11 plus BSD radio information header)

If I run tcpdump without -y, or if I use an unmodified FromDevice 
element, I receive ethernet packets from the device which is not what I 
want.  I just add a '-y IEEE802_11_RADIO' to make tcpdump work, but 
FromDevice cannot be made to do this with hacking it.

- Ian


Eddie Kohler wrote:
> Hi Ian --
> 
> I believe that user-level FromDevice detects the datalink type from 
> pcap, so there wouldn't be a point to the argument.  Did you find this 
> did not work?
> 
> Eddie
> 
> 
> Ian Rose wrote:
>> I perhaps should also clarify that I am talking specifically about the 
>> userlevel FromDevice element ("FromDevice.u" on the wiki).
>>
>>
>> Ian Rose wrote:
>>> Hello all -
>>>
>>> I am new to this list - apologies if this question has previously be 
>>> answered.  I am looking at the FromDevice element, but I see no way 
>>> to specify a datalinktype (i.e. the equivalent of the '-y' flag in 
>>> tcpdump).  Is there any reason that an argument for this was left 
>>> out?  It seems pretty easy for me to add, but I wanted to check first 
>>> to make sure I wasn't overlooking or misunderstanding something.
>>>
>>> thanks,
>>> Ian
>>>
>>> p.s. I am running on FreeBSD, so I am using the libpcap-based code in 
>>> FromDevice, as opposed to the Linux-based code.
>>>
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