Wireshark physical packet

How does wireshark interpret physical packets? As far as I know, all p开发者_JS百科ackets look to be the same, so how does it decode them to pass to next higher protocol?

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When it's used to capture live traffic it knows the type of the interface and therefore the L2 encapsulation of packets, and when it reads a pcap file, the file has a field in the header indicating network type.

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There are probably a number of different mechanisms. You can download the dissectors and study the source to find out the various methods.

I wrote a dissector for a network sniffer and ported it to Ethereal and then Wireshark (or maybe someone else ported it; I don't remember). But the basic logic is that the dissector gets added to the list of possible dissectors. Wireshark calls a dissector and it decodes the packet if it can. If not, it calls the next one in the chain.

In the code I wrote, I simply analyzed the packet (UDP in my situation) to determine if it fit the profile of the desired packet using checksums and known data in the packet. If it decided it was the packet I was interested in I just extracted the various pieces of interesting data from the packet. The function tvb_get_ptr returns a pointer to the start of the data.