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// WSUG Appendix How it Works
[#AppHowItWorks]
[appendix]
== How Wireshark Works
When using such a complex program like Wireshark, it’s sometimes useful to
understand the mechanisms and concepts behind the surface. This is an approach
to shed some light on the inner workings of Wireshark.
=== Program start
When Wireshark starts, a lot of things are done:
* Initialize the dissectors (register the protocol tree), including plugins
* Load and set values from the preferences file
* Load the capture filters from the cfilters file
* Load the display filters from the dfilters file
* Load and set the disabled protocols from the disabled_protos file
* Init libpcap/Npcap (the capturing engine)
* Process command line parameters
* Load and set the recently used GUI settings from the recent file
* Init and show the main screen
* If specified by command line, load a capture file or start capturing
=== Protocol dissectors
Each protocol has its own protocol dissector. When processing network data,
Wireshark calls the dissector that seems relevant to the packet data. The
dissector will then process the packet data and send any unprocessed data
back to Wireshark for further dissection.
So Wireshark will dissect a packet from the lowest to the highest protocol
layers.
But how does Wireshark know which dissector to use?
When Wireshark starts each dissector registers itself in one of two ways:
* _Static_. If the dissector knows a specific value of a lower layer, it can
directly register itself there (e.g., the HTTP dissector “knows”, that
typically the well-known TCP port 80 is used to transport HTTP data).
* _Heuristic_. If no such well-known way exists, the dissector
can register itself for the heuristic mechanism. If a lower-layer dissector
has to handle some packet data where no well-known way exists, it can
handover the packet to Wireshark’s heuristic mechanism. This will ask all
registered upper layer dissectors, if they “like” that data. These
dissectors typically look at the first few bytes of the packet, to see if they
contain some characteristic data of that protocol and then
decide whether or not to dissect that packet.
Let’s look at an example. We’ll assume, Wireshark loads a TCP/IP/Ethernet
packet. Wireshark will call the Ethernet dissector, which will dissect the
Ethernet related data (usually the first 6 + 6 + 2 bytes). The Ethernet
dissector then passes the rest of the data back to Wireshark.
Wireshark in turn will call the next related dissector, in our case the IP
dissector (because of the value 0x800 in the Ethernet type field). This
will continue until no more data has to be dissected, or the data is
unknown to Wireshark.
You can control the way Wireshark calls its dissectors, see
<<ChAdvProtocolDissectionSection>> for details.
// End of WSUG Appendix How it Works
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