Wireshark-dev: Re: [Wireshark-dev] How does dumpcap.c communicate linktype when pushing packets
From: Guy Harris <[email protected]>
Date: Thu, 10 May 2012 19:22:12 -0700
On May 10, 2012, at 7:11 PM, Richard Sharpe wrote:

> On Thu, May 10, 2012 at 7:05 PM, Guy Harris <[email protected]> wrote:
> 
>> The messages from dumpcap to Wireshark on the sync pipe just say things such as "there
>> are N more packets to read from the capture file" or "I've stopped writing to that capture
>> file and am now writing to a capture file with this pathname"; they do not say "this capture
>> has link-layer header type XXX", or even "this capture has a new interface with link-layer
>> header type XXX" (given that "this capture has link-layer type XXX" is insufficient to fully
>> support capturing on multiple interfaces, which 1.7.x supports).
> 
> It would seem that we need to say "this packet has link-layer type
> XXX" and the pcap-opts that is passed some of the way in supports
> that, it seems.

The way that we say "this packet has link-layer type XXX" is that wtap_read() fills in a structure that includes the packet's WTAP_ENCAP value.  How the WTAP_ENCAP value is determined is dependent on the file type - all packets in a pcap file have the WTAP_ENCAP value from the file header, packets from a pcap-ng file have an interface ID that refers to an already-seen IDB that gives the WTAP_ENCAP value for packets on that interface, etc.

That's independent of the capture options.  When capturing on an interface supported by libpcap/WinPcap, the option that's passed in for that interface has a DLT_ value to pass to pcap_set_datalink() after that interface is opened (for most interfaces, there's only one possible DLT_ value, and that's the one that's passed in); when capturing on a pipe, the link-layer header type cannot be set, as the packets being written to the pipe have whatever header they have.