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The TAP system in ethereal is a powerful and flexible mechanism to get event
driven notification on packets matching certain protocols and/or filters.
In order to use the tapping system, very little knowledge of ethereal
internals are required.
As examples on how to use the tap system see the implementation of
tap-rpcstat.c (tethereal version)
gtk2/gtk2-rpcstat.c (gtk2-ethereal version)
The tap system consists of two parts:
1, code in the actual dissectors to allow tapping data from that particular
protocol dissector, and
2, event driven code in an extension such as tap-rpcstat.c that registers
a tap listener and processes received data.
So you want to hack together a tap application?
TAP
===
First you must decide which protocol you are interested in writing a tap
application for and check if that protocol has already has got a tap installed
in it.
If it already has a tap device installed then you dont have to do anything.
If not, then you have to add a tap but dont worry, this is extremely easy to
do and is done in four easy steps;
(see packet-rpc.c and search for tap for an example)
1, We need tap.h so just add '#include "tap.h"' to the includes.
2, We need a tap handler so just add 'static int <protocol>_tap = -1;'
3, Down in proto_register_<protocol>() you need to add
'<protocol>_tap = register_tap("<protocol>");'
4, In the actual dissector for that protocol, after any child dissectors
have returned, just add 'tap_queue_packet(<protocol>_tap, pinfo, <pointer>);'
<pointer> is used if the tap has any special additional data to provide to
the tap listeners. What this points to is dependant on the protocol that
is tapped, or if there are no useful extra data to provide just specify NULL.
For packet-rpc.c what we specify there is the persistant structure 'rpc_call'
which contains lots of useful information the rpc layer that a listener might
need.
TAP LISTENER
============
(see tap-rpcstat.c as an example)
Interfacing your application is not that much harder either.
Only 3 callbacks and two functions.
The two functions to start or stop tapping are
register_tap_listener(char *tapname, void *tapdata, char *fstring,
void (*reset)(void *tapdata),
int (*packet)(void *tapdata, packet_info *pinfo, epan_dissect_t *edt, void *<pointer>),
void (*draw)(void *tapdata),
remove_tap_listener(void *tapdata);
remove_tap_listener(void *tapdata)
This function is used to deregister and stop a tap listener.
register_tap_listeners() is used to register an instance of a tap application
to the tap system.
*tapname
is the name of the tap we want to listen to. I.e. the name used in
step 3 above.
*tapdata
is the instance identifier. The tap system uses the value of this
pointer to distinguish between different instances of a tap.
Just make sure that it is unique by letting it be the pointer to a struct
holding all state variables. If you want to allow multiple concurrent
instances, just put ALL state variables inside a struct allocated by
g_malloc() and use that pointer.
(tap-rpcstat.c use this technique to allow multiple simultaneous instances)
*fstring
is a pointer to a filter string.
If this is NULL, then the tap system will provide ALL packets passing the
tapped protocol to your listener.
If you specify a filter string here the tap system will first try
to apply this string to the packet and then only pass those packets that
matched the filter to your listener.
The syntax for the filter string is identical to normal display filters.
NOTE: Specifying filter strings will have a significant performance impact
on your application and ethereal. If possible it is MUCH better to take
unfiltered data and just filter it yourself in the packet-callback than
to specify a filter string.
ONLY use a filter string if no other option exist.
void (*reset)(void *tapdata)
This callback is called whenever ethereal wants to inform your
listener that it is about to start [re]reading a capture file or a new capture
from an interface and that your application should reset any state it has
in the *tapdata instance.
int (*packet)(void *tapdata, packet_info *pinfo, epan_dissect_t *edt, void *data)
This callback is used whenever a new packet has arrived at the tap and that
it has passed the filter (if there were a filter).
The *data structure type is specific to each tap.
This function returns an int and it should return
1, if the data in the packet aused state to be updated
(asnd thus a redraw of the window would later be required)
0, if we dont need to redraw the window.
NOTE: that (*packet) should be as fast and efficient as possible. Use this
function ONLY to store data for later and do the cpu-intensive processing
or GUI updates down in (*draw) instead.
void (*draw)(void *tapdata)
This callback is used when ethereal wants your application to redraw its
output. It will usually not be called unless your application has received
new data through the (*packet) callback.
On some ports of ethereal (gtk2) (*draw) will be called asynchronously
from a separate thread up to once every 2-3 seconds.
On other ports it might only be called once when the capture is finished
or the file has been [re]read completely.
So, create three callbacks:
1, reset to reset the state variables in the structure passed to it.
2, packet to update these state variables.
3, draw to take these state variables and draw them on the screen.
then just make ethereal call register_tap_listener() when you want to tap
and call remove_tap_listener() when you are finished.
TIPS
====
Of course, there is nothing that forces you to make (*draw) draw stuff
on the screen.
You can hand register_tap_listener() NULL for both (*draw) and (*reset)
(well also for (*packet) but that would be a very boring extension).
Perhaps you want an extension that will execute a certain command
everytime it sees a certain packet?
Well, try this :
int packet(void *tapdata,...) {
...
system("mail ...");
return0;
}
register_tap_listener("tcp", struct, "tcp.port==57", NULL, packet, NULL);
Let struct contain an email address?
Then you have something simple that will make ethereal send an email
out automagically for each and every time it dissects
a packet containing TCP traffic to port 57.
Please put in some rate limitation if you do this.
Let struct contain a command line and make (*packet) execute it?
The possibilities are rather large.
See tap-rpcstat.c for an example
See tap.c as well. It contains lots of comments and descriptions on the tap
system.
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