3 The TAP system in Wireshark is a powerful and flexible mechanism to get event
4 driven notification on packets matching certain protocols and/or filters.
5 In order to use the tapping system, very little knowledge of Wireshark
6 internals are required.
8 As examples on how to use the tap system see the implementation of
9 tap-rpcstat.c (tshark version)
10 gtk/rpc_stat.c (gtk-wireshark version)
12 If all you need is to keep some counters, there's the stats_tree API,
13 which offers a simple way to make a GUI and tshark tap-listener; see
14 README.stats_tree. However, keep reading, as you'll need much of what's
17 The tap system consists of two parts:
18 1, code in the actual dissectors to allow tapping data from that particular
19 protocol dissector, and
20 2, event driven code in an extension such as tap-rpcstat.c that registers
21 a tap listener and processes received data.
25 So you want to hack together a tap application?
30 First you must decide which protocol you are interested in writing a tap
31 application for and check if that protocol has already got a tap installed
33 If it already has a tap device installed then you don't have to do anything.
34 If not, then you have to add a tap but don't worry, this is extremely easy to
35 do and is done in four easy steps;
36 (see packet-rpc.c and search for tap for an example)
38 1, We need tap.h so just add '#include "tap.h"' to the includes.
40 2, We need a tap handler so just add 'static int <protocol>_tap = -1;'
42 3, Down in proto_register_<protocol>() you need to add
43 '<protocol>_tap = register_tap("<protocol>");'
45 4, In the actual dissector for that protocol, after any child dissectors
46 have returned, just add 'tap_queue_packet(<protocol>_tap, pinfo, <pointer>);'
48 <pointer> is used if the tap has any special additional data to provide to the
49 tap listeners. What this points to is dependent on the protocol that is tapped,
50 or if there are no useful extra data to provide just specify NULL. For
51 packet-rpc.c what we specify there is the persistent structure 'rpc_call' which
52 contains lots of useful information from the rpc layer that a listener might
59 (see tap-rpcstat.c as an example)
60 Interfacing your application is not that much harder either.
61 Only 3 callbacks and two functions.
64 The two functions to start or stop tapping are
66 register_tap_listener(const char *tapname, void *tapdata, const char *fstring,
68 void (*reset)(void *tapdata),
69 gboolean (*packet)(void *tapdata, packet_info *pinfo, epan_dissect_t *edt, const void *<pointer>),
70 void (*draw)(void *tapdata));
72 remove_tap_listener(void *tapdata);
75 remove_tap_listener(void *tapdata)
76 This function is used to deregister and stop a tap listener.
78 register_tap_listener() is used to register an instance of a tap application
82 is the name of the tap we want to listen to. I.e. the name used in
86 is the instance identifier. The tap system uses the value of this
87 pointer to distinguish between different instances of a tap.
88 Just make sure that it is unique by letting it be the pointer to a struct
89 holding all state variables. If you want to allow multiple concurrent
90 instances, just put ALL state variables inside a struct allocated by
91 g_malloc() and use that pointer.
92 (tap-rpcstat.c use this technique to allow multiple simultaneous instances)
95 is a pointer to a filter string.
96 If this is NULL, then the tap system will provide ALL packets passing the
97 tapped protocol to your listener.
98 If you specify a filter string here the tap system will first try
99 to apply this string to the packet and then only pass those packets that
100 matched the filter to your listener.
101 The syntax for the filter string is identical to normal display filters.
103 NOTE: Specifying filter strings will have a significant performance impact
104 on your application and Wireshark. If possible it is MUCH better to take
105 unfiltered data and just filter it yourself in the packet-callback than
106 to specify a filter string.
107 ONLY use a filter string if no other option exist.
110 is a set of flags for the tap listener. The flags that can be set are:
112 TL_REQUIRES_PROTO_TREE
114 set if your tap listener "packet" routine requires a protocol
115 tree to be built. It will require a protocol tree to be
118 1) it looks at the protocol tree in edt->tree
122 2) the tap-specific data passed to it is constructed only if
123 the protocol tree is being built.
127 set if your tap listener "packet" routine requires the column
128 strings to be constructed.
130 void (*reset)(void *tapdata)
131 This callback is called whenever Wireshark wants to inform your
132 listener that it is about to start [re]reading a capture file or a new capture
133 from an interface and that your application should reset any state it has
134 in the *tapdata instance.
136 gboolean (*packet)(void *tapdata, packet_info *pinfo, epan_dissect_t *edt, void *data)
137 This callback is used whenever a new packet has arrived at the tap and that
138 it has passed the filter (if there were a filter).
139 The *data structure type is specific to each tap.
140 This function returns an gboolean and it should return
141 TRUE, if the data in the packet caused state to be updated
142 (and thus a redraw of the window would later be required)
143 FALSE, if we don't need to redraw the window.
144 NOTE: that (*packet) should be as fast and efficient as possible. Use this
145 function ONLY to store data for later and do the CPU-intensive processing
146 or GUI updates down in (*draw) instead.
149 void (*draw)(void *tapdata)
150 This callback is used when Wireshark wants your application to redraw its
151 output. It will usually not be called unless your application has received
152 new data through the (*packet) callback.
153 On some ports of Wireshark (gtk2) (*draw) will be called asynchronously
154 from a separate thread up to once every 2-3 seconds.
155 On other ports it might only be called once when the capture is finished
156 or the file has been [re]read completely.
160 So, create three callbacks:
161 1, reset to reset the state variables in the structure passed to it.
162 2, packet to update these state variables.
163 3, draw to take these state variables and draw them on the screen.
165 then just make Wireshark call register_tap_listener() when you want to tap
166 and call remove_tap_listener() when you are finished.
169 WHEN DO TAP LISTENERS GET CALLED?
170 ===================================
171 Tap listeners are only called when Wireshark reads a new capture for
172 the first time or whenever Wireshark needs to rescan/redissect
174 Redissection occurs when you apply a new display filter or if you
175 change and Save/Apply a preference setting that might affect how
176 packets are dissected.
177 After each individual packet has been completely dissected and all
178 dissectors have returned, all the tap listeners that have been flagged
179 to receive tap data during the dissection of the frame will be called in
181 The order in which the tap listeners will be called is not defined.
182 Not until all tap listeners for the frame has been called and returned
183 will Wireshark continue to dissect the next packet.
184 This is why it is important to make the *_packet() callbacks execute as
185 quickly as possible, else we create an extra delay until the next packet
188 Keep in mind though: for some protocols, such as IP, the protocol can
189 appear multiple times in different layers inside the same packet.
190 For example, IP encapsulated over IP which will call the ip dissector
191 twice for the same packet.
192 IF the tap is going to return private data using the last parameter to
193 tap_queue_packet() and IF the protocol can appear multiple times inside the
194 same packet, you will have to make sure that each instance of
195 tap_queue_packet() is using its own instance of private struct variable
196 so they don't overwrite each other.
198 See packet-ip.c which has a simple solution to the problem. It creates
199 a unique instance of the IP header using ep_alloc().
200 Previous versions used a static struct of 4 instances of the IP header
201 struct and cycled through them each time the dissector was called. (4
202 was just a number taken out of the blue but it should be enough for most
203 cases.) This would fail if there were more than 4 IP headers in the same
204 packet, but that was unlikely.
209 Of course, there is nothing that forces you to make (*draw) draw stuff
211 You can hand register_tap_listener() NULL for both (*draw) and (*reset)
212 (well also for (*packet) but that would be a very boring extension).
214 Perhaps you want an extension that will execute a certain command
215 every time it sees a certain packet?
217 int packet(void *tapdata,...) {
223 register_tap_listener("tcp", struct, "tcp.port==57", NULL, packet, NULL);
225 Let struct contain an email address?
226 Then you have something simple that will make Wireshark send an email
227 out automagically for each and every time it dissects
228 a packet containing TCP traffic to port 57.
229 Please put in some rate limitation if you do this.
231 Let struct contain a command line and make (*packet) execute it?
232 The possibilities are rather large.
236 See tap-rpcstat.c for an example
237 See tap.c as well. It contains lots of comments and descriptions on the tap