1// WSUG Appendix How it Works
2
3[[AppHowItWorks]]
4
5[appendix]
6== How Wireshark Works
7
8When using such a complex program like Wireshark, it’s sometimes useful to
9understand the mechanisms and concepts behind the surface. This is an approach
10to shed some light on the inner workings of Wireshark.
11
12=== Program start
13
14When Wireshark starts, a lot of things are done:
15
16* Initialize the dissectors (register the protocol tree), including plugins
17
18* Load and set values from the preferences file
19
20* Load the capture filters from the cfilters file
21
22* Load the display filters from the dfilters file
23
24* Load and set the disabled protocols from the disabled_protos file
25
26* Init libpcap/Npcap (the capturing engine)
27
28* Process command line parameters
29
30* Load and set the recently used GUI settings from the recent file
31
32* Init and show the main screen
33
34* If specified by command line, load a capture file or start capturing
35
36=== Protocol dissectors
37
38Each protocol has its own protocol dissector. When processing network data,
39Wireshark calls the dissector that seems relevant to the packet data. The
40dissector will then process the packet data and send any unprocessed data
41back to Wireshark for further dissection.
42
43So Wireshark will dissect a packet from the lowest to the highest protocol
44layers.
45
46But how does Wireshark know which dissector to use?
47
48When Wireshark starts each dissector registers itself in one of two ways:
49
50* _Static_. If the dissector knows a specific value of a lower layer, it can
51  directly register itself there (e.g. the HTTP dissector “knows”, that
52  typically the well known TCP port 80 is used to transport HTTP data).
53
54* _Heuristic_. If no such well known way exists, the dissector
55  can register itself for the heuristic mechanism. If a lower layer dissector
56  has to handle some packet data where no well known way exists, it can
57  handover the packet to Wireshark’s heuristic mechanism. This will ask all
58  registered upper layer dissectors, if they “like” that data. These
59  dissectors typically look at the first few bytes of the packet, to see if they
60  contain some characteristic data of that protocol and then
61  decide whether or not to dissect that packet.
62
63Let’s look at an example. We’ll assume, Wireshark loads a TCP/IP/Ethernet
64packet. Wireshark will call the Ethernet dissector, which will dissect the
65Ethernet related data (usually the first 6 + 6 + 2 bytes). The Ethernet
66dissector then passes the rest of the data back to Wireshark.
67Wireshark in turn will call the next related dissector, in our case the IP
68dissector (because of the value 0x800 in the Ethernet type field). This
69will continue until no more data has to be dissected, or the data is
70unknown to Wireshark.
71
72You can control the way Wireshark calls its dissectors, see
73<<ChAdvProtocolDissectionSection>> for details.
74
75// End of WSUG Appendix How it Works
76