Assignment 1
------------

Write a program for receiving and displaying (as a hexudmp) the
content of UDP packets.  This udpspy shall be controlled via a
set of command line parameters and support at least the following
options with the respective syntax:

udpspy -a <addr-spec> -i <interface> -s -l <dumplen> -t <add-spec> -D <duration>

       -a: used to specify a transport address that shall be used for
	   listening for incoming UDP packets.  Option -a may be given
	   repeatedly and, for each occurrence, a separate UDP socket 
	   shall be created for receiving packets.  The <addr-spec>
	   may use any of the following formats:

	   a.b.c.d/port         with a.b.c.d being an IPv4 address in
				dotted decimal notation.
	   hostname/port	with hostname being an hostname that can
				be resolved into an IPv4 (or IPv6)
				address by using the name resolution
				mechanisms offered by the operating system
	   /port		only specifies a port number so that
				INADDR_ANY is used as IP address; this
				implies unicast reception.
	   a:b:c:d:e:f:g:h/port (voluntary) with a:...:h representing
				an IPv6 address.

				Note: RFC2732 contains numerous
				examples for valid textual notations
				for IPv6 addresses.

	   All addresses and hostnames may be or resolve to unicast as
	   well as multicast addresses.  Remember: multicast addresses
	   are class D IP addresses and are in the range 224.0.0.0 to
	   239.255.255.255.  Just determine the address type and then
	   handle the address accordingly.

	   Note: Resolving IP address using getaddrinfo() or
	   gethostbyname (), etc. may return multiple IP addresses.
	   You only need to process one of these but you need to make
	   sure that the address family is the "right" one.

	   The IP unicast address and the port number (or just the
	   port number) but _not_ the IP multicast address need to be
	   handed to the bind() system call in the data structure
	   struct sockaddr_in (for IPv4).  For multicast reception, do
	   not forget the setsockopt() with IP_ADD_MEMBERSHIP.

       -i: Used in conjunction with multicasting only and allowed to
	   be specified at most once, this option provides the
	   local IP address of the interface that shall be used for
	   joining all multicast groups.  This information needs to go
	   along with IP_ADD_MEMBERSHIP in "struct ip_mreq".

       -s: Short form: turns off the hexdump output and thus creates
	   a single line of output per packet (<80 characters)
	   containing a timestamp with microsecond resolution, source
	   and destination IP address and port number and the packet
	   size.

	   If -s is not given, the output will also have the hexdump.

       -l: Limits the hexdump output length to the specified number of
	   bytes per packet (otherwise, the full packet contents shall
	   be dumped).

       -t: Open a listening socket for incoming TCP control
	   connections.  If -t is _not_ specified the output will be
	   to stdout.  If -t is given, a TCP socket will be created
	   for the specified address and no output will be printed to
	   stdout.  Every client that connects to the TCP port number
	   will receive a copy of the output (according to -l and -s)
	   over the TCP connection until it disconnects again.

	   Output need not be buffered.  If there is no client, the
	   output is simply discarded.  While not a good idea in
	   practice, for the purpose of this assignment, you may
	   assume that clients will receive the data sufficiently fast
	   and you do not have to worry about buffering for clients
	   receiving at different data rates.  You may use blocking
	   i/o for all communication tasks in the assignment.

	   Input sent from the client via the TCP connection is read
	   from the socket and immediately discarded.

       -D: This option is used to specify a duration that the program
	   shall run for.  After this time is elapsed the program
	   shall terminate automatically.  The time interval shall be
	   specified (as an integer) in seconds.


The program shall be orderly terminated when the user sends a SIGINT
(equivalent to pressing Control-C) and then print, as a final action,
the time elapsed since its invocation, the number of received packets,
and the total number of bytes received.  This could look like:

    Duration: 123.123456 seconds, received 57 packets, 4530 bytes total

This information shall also be output if the program terminates after
a duration specified using -D.
	  


Useful functions:
-----------------
    Command line parsing
	getopt ()

    Time:
	gettimeofday ()

    Signal handling:
        signal ()

    Sockets:
	gethostbyhame (), getaddrinfo ()
	inet_aton (), inet_addr (), inet_ntoa ()
	socket (), bind (), setsockopt (), close ()
	recvfrom (), sendto (), poll ()


Output
------
Example for short form output:

14:09:00.123456 134.102.218.59:40000 -> 134.102.218.58:47000 79 Bytes



A hexdump could look like that:
(remember that packet sizes are not multiples of 16 bytes and the last
 line of your hexdump still should be readable)

Offset  Individual bytes in hexadezimal                    Characters (*)
------  -----------------------------------------------  ----------------
000000: 00 01 02 00 41 42 43 09 00 64 00 00 00 00 30 39  ....ABC..d....09
000010: 00 00 0a 0d                                      ....

(*) Characters with the ASCII codes in the range from 32 to 127 should
    be printed, all other represented by a ".".