/** \mainpage Examples
\ref sniffer
*/
/** \page sniffer Sniffer: A simple example application
\dontinclude Sniffer.cc
The Sniffer application is a simple command line network sniffer like \c tcpdump or \c
tethereal. The application uses a packet socket to read Ethernet packets from the \c eth0
interface and dumps the parsed packets out to the standard output.
To try out the example application, check out the library, go to the \c Sniffer
directory and execute
# scons -u
# ./sniffer loop
< Hit Ctrl-C when you've seen enough >
# ./sniffer scheduler
< Hit Ctrl-C when you've seen enough >
We will now look at the code which is found in \c Sniffer.cc in the \c Sniffer directory. The
code starts out by including the necessary headers
\skip // Custom includes
\until membind
(The additional includes found in the source but not shown here are part of a short-time fix
which will be removed as soon as possible). The example application now contains a helper
routine to produce a packet hexdump. We will skip this routine here. The example includes two
implementations, one using blocking calls and a while loop, the other using the senf::Scheduler
for asynchronous event notification. They are implemented in \c loop_main() and \c
scheduler_main(). They will be documented below. For now, we skip these implementations and go
straight to the \c main() function
\skip int main(
\until return 1;
\until }
This routine simply interprets the first command line argument and dispatches to the required
implementation.
Now lets go back and study each implementation in detail.
\dontinclude Sniffer.cc
\section example_loop A Blocking Implementation
This implementation is found in the \c loop_main function.
\skip loop_main
\until try
We catch all exceptions in a \c try block. This is good for a deliverable binary. When debugging
the application, it might be better to let the exception \c abort the execution so you can get a
backtrace of the exception origin in the debugger.
We now create a packet socket and bind it to the \c eth0 interface. A packet socket is a linux
specific type of socket which returns ethernet packets directly from the network wire. By
uncommenting the last line, you may switch the interface into promiscuous mode.
\until //
We will now read packets from the socket forever, that is until the user hits Ctrl-C
\skip while
\until read
The next step is, to parse the data read from the socket as an Ethernet packet
\until ;
Lets digest this line step by step: We declare a variable named \c packet as a smart pointer to
an \c EthernetPacket instance. \c ptr is a typedef member of all Packet classes for the
corresponding smart pointer type. We then initialize this pointer with a call to the static \c
create member of the \c Packet class. This member takes the type of Packet to parse as a
template argument. We pass \c EthernetPacket here. The function takes an iterator range as an
argument, and we pass it the complete packet just read by giving the range \c begin() to \c
end() of our just read \c data string.
The next step is to write out the packet to the standard output
\until \n\n
The \c dump call will write out a complete representation of the parsed packet data. The Packet
library will \e not try to interpret payload data as long as no exact indication of the payload
type is available (example: A UDP Payload is not parsed further unless you explicitly tell the
library, how to parse it). Tools like \c tethereal guess the payload type by checking port
numbers and the payload data, however this is not advisable for a general purpose packet
library.
The next line, \c hexdump, will write out the \e last packet component. Packets are managed as a
chain of headers. The last header is normally a \c DataPacket holding the payload data.
That's it. We finish of by catching the exception and giving as much detail as possible if an
exception is caught
\until ;
\until }
\until }
The \c prettyName function from the \c Utils library is used, to get a nice, printable
representation of the \e dynamic type of the exception instance. It is an interface to the g++
demangler. This is necessary since the \c name member of the C++ \c type_info instance is a
mangled name in \c g++.
That's it for the simple blocking implementation.
\section example_scheduler Using the Scheduler
However, we have another one which uses the Scheduler. We do this as it will be most of the
time: We define a class which manages reading the packets and dumping them out.
\until }
The class constructor binds the socket defined as a data member to the correct interface.
\until add
The public \c run() member is called to run the sniffer. It first adds the socket to the
Scheduler. The \c add() call takes two Arguments, the socket to bind to (which can be a lot of
things and must not necessarily be a socket instance) and callback to call, whenever there is an
event on that socket. A third argument may be specified to restrict the events, on which the
function is called, here we have left out this argument which defaults to
senf::Scheduler::EV_ALL.
The callback is specified as a Boost.Function object. We use the \c
senf::membind helper from the Utils library to build such a function object. This helper takes
an arbitrary class member and binds it to a specific instance.
\until }
Calling the Schedulers \c process() method will start the event loop. This call does not return
(ok, it does return in special cases if \c senf::Scheduler::terminate() is called which does not
apply here).
\until {
The \c dumpPacket() member is called by the scheduler whenever an event on the socket is
encountered. The scheduler always passes two arguments: The socket and an event id which
identifies the type of event which triggered the call.
\until };
The body is absolutely identical to the body of the \c while loop of the blocking
implementation. However, the scheduler guarantees, that a read on the socket will not block if
the socket is triggered to be readable (even if the socket is not set to non-blocking mode).
We now only need to provide the \c scheduler_main() function to run this code
\until 0;
\until }
This function is straight forward. The exception handling is the same as in \c loop_main(). The
code then just creates a \c Sniffer instance and calls it's \c run() member.
\see \ref components \n
\ref build \n
libSocket API reference \n
libPackets API reference \n
libUtils API reference
*/
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