9.5. How to tap protocols
Adding a Tap interface to a protocol allows it to do some useful things.
In particular you can produce protocol statistics from the tap interface.
A tap is basically a way of allowing other items to see whats happening as
a protocol is dissected. A tap is registered with the main program, and
then called on each dissection. Some arbitrary protocol specific data
is provided with the routine that can be used.
To create a tap, you first need to register a tap.
A tap is registered with an integer handle, and registered
with the routine register_tap. This takes a string name
with which to find it again.
Example 9.19. Initialising a tap
#include <epan/tap.h>
static int foo_tap = -1;
struct FooTap {
gint packet_type;
gint priority;
...
};
...
foo_tap = register_tap("foo");
Whilst you can program a tap without protocol specific data, it
is generally not very useful. Therefore it's a good idea
to declare a structure that can be passed through the tap.
This needs to be a static structure as it will be used after the
dissection routine has returned. Its generally best to pick out some
generic parts of the protocol you are dissecting into the tap data.
A packet type, a priority, a status code maybe.
The structure really needs to be included in a header file so
that it can be included by other components that want to listen in
to the tap.
Once you have these defined, it's simply a case of populating the
protocol specific structure and then calling tap_queue_packet probably
as the last part of the dissector.
Example 9.20. Calling a protocol tap
static struct FooTap pinfo;
pinfo.packet_type = tvb_get_guint8(tvb, 0);
pinfo.priority = tvb_get_ntohs(tvb, 8);
...
tap_queue_packet(foo_tap, pinfo, &pinfo);
This now enables those interested parties to listen in on the details
of this protocol conversation.
